- Welcome to the start of our second day of the
Forest Ecosystem Monitoring Cooperative conference,
focused on revealing a changing forested landscape.
Today, we are diving into what we can learn
through an intensive study and manipulation
in a small well-studied area.
Our ability to manage forest in the face of all
the overlapping intersecting challenges
is gonna require both depth and breadth.
We're gonna need depth in our understanding
of why ecosystem function the way that they do
and breadth to understand how that functioning
varies across the landscape.
As a region, we're fortunate to have so many diverse efforts
tracking both sides of this coin, both depth and breadth.
And so today I'm excited to have with us an expert
to walk us through one of our longest running experimental
sites in the Northeast.
I invite all of us to begin to ponder
how to take decades of research forward,
so that we can address monitoring
and landscape-scale management issues that await us.
I'm pleased to welcome Dr. Charles Driscoll
to the FEMC conference this year.
Charles Driscoll is a distinguished and university professor
at Syracuse University.
His work focuses on the effects of disturbances
on forests, urban freshwater and marine ecosystems,
including air pollution, such as acid and mercury deposition
land use and climate pollution,
let me say and climate change.
His current research focuses on the recovery
of use of forest watersheds,
from acidic deposition, co-benefits of carbon dioxide
emissions controls, ecosystem restoration,
and responses to climate change, harmful algal blooms,
atmospheric deposition, watershed and surface water
transport and biotic exposures to mercury.
It's a quite a range.
Dr. Driscoll's talk is titled;
Effects of Changing Atmospheric Deposition and Climate
on the Structure and Function of a Northern forest,
long-term measurements and experiments
from the Hubbard Brook Experimental Forest in New Hampshire.
Charlie, thank you for being here today
and thank you to all of our attendees
for joining us on a second day.
And we look forward to your talk.
- Thanks Jim, and good morning
so as Jim said, I'm gonna talk a little bit
about Hubbard Brook,
a little bit of background information on the site.
It's an intensive study site.
It's been going since the 1950s, and talk a little bit
about our research approach and observations.
I'm gonna highlight long-term measurements
and experiments.
We also do other types of approaches,
but in the interest of time,
I'm gonna just focus on those two.
And I'm gonna touch on two major issues that we focus on,
air pollution effects and climate change,
and then I'll close.
So undoubtedly, many of you are aware of Hubbard Brook.
It's a US Forest Service intensive study site.
It's about 3000 hectares.
It's located in Central New Hampshire.
It was established in the fifties to investigate the effects
of forest harvesting on water supplies in New England.
It was a lot of conflicting information at the time.
So it was thought to be in a place where experiments
could be done and long-term measurements could be made.
So the focus was primarily on water resources hydrology.
Then in the sixties, Bob Pierce
invited Gene Likens and Herbert Bormann, Noye Johnson to
take their students and their research to Hubbard Brook,
and that was the start of the Hubbard Brook Ecosystem Study.
So it was initiated in the early sixties
and it started with measurements of precipitation chemistry
and stream chemistry and some initial experiments.
And so Hubbard Brook was a site where acid rain
was first reported in North America,
and it's been since the late seventies,
a National Science Foundation
funded long-term Ecological Research Site.
So this is what the Hubbard Brook Valley looks like.
It's a sort of a bowl-shape area if you haven't been there.
There are a series of experimental watersheds
and most of the work is done at the experimental watersheds
but valley wide surveys are also prominent
within the research portfolio.
So this is a list of the experimental watersheds.
They don't really have any exciting names,
they're just numbered.
They're moderately sized
ranging from about 10 to about 70 hectare.
And some of them have had experiments done
but some of them are referenced.
So I'm gonna show you results
from two reference watersheds,
watershed six which is a biogeochemical reference watershed
and watershed three,
which is a hydrologic reference watershed.
So you might ask why Hubbard Brook
has two reference watersheds.
I can't really give you a good answer for that,
it's just sort of a throwback
to what has been done historically.
I'm also gonna talk a little bit about an experiment
that we did in the started in the late nineties
and continue today, what we did
at Calcium silicate in addition to watershed one.
So the backbone of work at Hubbard Brook
is the small watershed approach.
And this was the approach that was initiated
by Bob Pierce colleagues back,
when Hubbard Brook was started.
And it's a fairly simple idea.
And so within areas of the landscape, such as shown here
measurements of precipitation are made and
a gauging station is put in,
which is anchored to a bedrock
to try to collect all the water
or most of the water that feeds the system.
So water balances can be done
measuring the precipitation inputs in the stream outputs
and then, by difference evapotranspiration could be obtained
and what Bormann and Likens did
to expand that was to take these hydrologic measurements
and superimpose on those chemical measurements.
But since that time, additional things have been done.
There've been vegetation surveys, microbial assays.
There's a lot of work done on
insects and birds by colleagues, largely from government
and Wesley college, other institutions, Buffalo Smithsonian.
So we try to take an ecosystem approach.
So this is just the slide showing you a design
of a bulk precipitation collector, much like the ones that
were done used back in the early sixties
the first to measure acid rain.
So the chemical inputs as well as the water inputs,
to the system and then a gauging station at the base
of the watershed.
So this is the, what the gauging station of watershed six.
So a little bit of background information about the forest.
It is underlined by resistant bedrock, low weathering rates.
The area was glaciated like much of the region.
About 14,000 years ago, the soils are rather a civics
Spodsosols, although there is variability depending
on their position in the landscape.
And we've learned quite a bit
about that in, in recent years, thanks to Scott Bailey
from the Forest service and colleagues.
The forest is largely Northern hardwood, of the conifers
at higher elevation and humid continental climate.
So this is a slide to show you a soil pit.
So you can see that there is a upper thick organic horizon
which goes into the mineral soil and then
down into seed horizon material, relatively shallow soils.
I've got a slide here to show you a landscape
of what one of the experimental watersheds would look
like if you, if you could do a cutout
from the gauging station up to the top of the watershed.
So you can see the, the outlay of bedrock
until here and the approximate height of the canopy.
So generally we have hardwoods in the
in the lower elevation and then there's a transition zone.
And then it creates into canopy or forest at the
at the crest at the highest elevations within the watershed.
This is a very poor quality picture
of what the forest looks like.
And part of it, this is a little bit perchy
than we usually see, but it's, you can get a sense.
And this is a map of the forest distribution.
So I mentioned that we do surveys across the whole Valley.
We also try to extrapolate work
at Hubbard Brook for the greater Northern forest region.
So this is color coded by the types of vegetation,
the reddish color which is the bulk
of the area is Northern hardwoods.
You can see Hemlocks prominent by the base of the watershed
along the, the channel of the main Hubbard Brook.
And then the greens you can see are Spruce Fir forest
at the highest, highest elevation.
So we've been fortunate that there've been some very
important vegetation surveys at Hubbard Brook dating back
to Bob Whitaker's work in the sixties at that time.
This shows you a above ground biomass over time.
This is for watersheds six.
We also had sort of Valley wide surveys.
So you can see that early in the study
the forest was strongly grading growing pretty strongly
and then starting in the early eighties
that growth leveled off.
And in recent years it's been declining growth.
There've been some fairly significant climatic
events that have happened over the last 20 or so years.
There was a significant ice storm.
That may be some of you remember in 1998.
I'll talk a little bit about that
towards the end of the talk.
And then more recently, about five years ago
there was a significant microburst then both
of those impacted the forest somewhat.
And so we've surveyed the distribution of vegetation
at the sites.
This shows sort of a course level distribution
of species.
The upper panel represents basal area of canopy trees
and then the lower panel represents sapling density.
And so these are color coded just to try to highlight some
of the nature observations.
The red represents sugar maple.
So you can see sugar maple is pretty prominent
in watershed six have a very strong dynamic.
It was growing relatively well early in the study.
And then in the early eighties, it's really been declining
and that growth has been replaced largely by American beach.
And you can see beeches suffered from peak scale
like much of the Northern forest.
So you can see a large increase
in saplings starting in the 1980s.
This slide show you what they a strange look
like at Hubbard Brook.
So the headwater streams
and the experimental watersheds are generally first
and the second order streams.
So I mentioned early on that Hubbard Brook is part
of the long term ecological research program funded
by the NSF involves I think 28 sites that are found
throughout the US and some areas outside the US
there are two sites in Antarctica.
And so this is a conceptual model that we've
developed to sort of set the
the underpinning for the work that's done at Hubbard Brook.
There are three major drivers of change that we look at
and our major theme is a disturbance
within the Northern forest system.
And some of those disturbances are atmospheric deposition
and climate change and then changing biota
and changing biota is sort of a catch all
for a number of types of disturbances.
It could result from forest harvesting.
It could result in
the input of path, pests
so a variety of things.
And so those, those disturbances can influence
and change what we call the geo-physical template
of the watersheds.
So this represents the, the soil
the overall structure of the, of the watershed.
And then, a lot of the focus research is done
on looking at functional responses
and these include vegetation, biogeochemistry, hydrology
and food web, so insects, birds,
mammals
large herbivores are all included in the
in the food web work.
And some of these have feedbacks
particularly in terms of changing biota and
and influencing this geo-physical template that we focus on.
So to try to answer our research questions
we try to use a variety of tools.
We conduct experiments.
We do long-term measurements, we do modeling exercises
and we do gradient studies.
So I'm gonna transition now to talking a little bit about
one of our focuses services in that air pollution
or acid rain.
And that's been a big theme for the work at Hubbard Brook
for a long time, and continues to be a theme, although the
acid rain has changed markedly over the, over the decades.
And we've been fortunate enough to capture
that what Hubbard Brook was
measurements were first made at Hubbard Brook.
They were the values were relatively high
that was near the peak of acid rain.
And then the change started in 1970s with the
1970 Clean Air Act amendments.
And it's had changed over time with various rules that have
have been implemented and continue today.
So this is a slide showing you some of that change.
The y-axis shows the volume way
to concentrations of sulfate and nitrate.
These are annual values in precipitation in bulk
precipitation at Hubbard Brook.
And along the x-axis is the driver
which are the national, these are national scale emissions
of sulfur dioxide and nitrogen oxide.
And you can see this a pretty strong relationship.
So these highest values represent the earliest
measurements that Hubbard Brook.
And then with time, the emissions have declined.
And within there's a very strong, direct relationship
between the sulfate concentrations and to a lesser extent
the nitrate concentrations and precipitation
pretty remote area at Hubbard Brook.
So this is a real national success story
and it's continuing to play out.
I've got a dash line to show you what, back in the day
back in the seventies and eighties
we projected back would be a pre-industrial
or circa 1870 values of, of sulfated nitrate.
So we're approaching those values now and
there's discussion about reducing
air pollution even further.
There was some rollbacks in the Trump administration,
but, you know I'm guardedly optimistic that there'll be some
improvements in air quality, largely focusing
on carbon mitigation from the electric electricity sector.
So one of these has shown
for a couple of years ago, the clean power plan
which was an Obama administration proposal
they give you an idea of what might be achieved.
And we could even go further
with some ambitious proposals that are proposed today.
So not only would we be seeing changes in precipitation
but we've seen changes in stream water chemistry.
So this is a fairly complicated figure
but it gives me an opportunity to brag
about some of the long-term records
which go well beyond when I started at Hubbard Brook.
So this is a panel showing you
for watershed six, the water budget, the sulfate budget
and the lower panel is the nitrate budget.
I'm not going to talk right now about water.
I'll wait, talk a little bit more about that later
but if we look at the sulfate and nitrate
we can see some evidence of the response
of ecosystem to those changes in inputs
from acid rain that I talked about a few minutes ago.
So I'm showing you the annual volume
or the annual flux, excuse me, for sulfate here.
And the precipitation values are shown in red
and the stream values are shown in sort of yellowish green.
And you can see that they were high back when
the measurements were started in the sixties
and they have really gone down quite a bit today
and continue to go down.
The nitrate story's a little more complicated.
The values were stuck really sort of increasing
in the early sixties and then leveled off.
And then with air pollution controls in the late nineties
and early two thousands, largely directed towards nitrate
you can see that they've had dropped down
and you can see that there's a, the green
which is the stream water chemistry.
You can see a longterm decrease, but unlike sulfate
it doesn't follow the pattern maybe somewhat
but not particularly closely.
And in particular there are these large peaks
that we see interspersed throughout the record
but I'll talk a little bit about those later.
So we've seen, also affects
one of the effects that we've seen that is on other
aspects of chemistry, Cations, calcium magnesium
aluminum is a biggie because aluminum was a toxic substance.
And so this is a plot of some of these cations
calcium magnesium sodium potassium is a function of the
flocks of sulfate plus nitrate.
So sulfate plus nitrate represents strong acid anions
largely input from the atmosphere
and they've changed over time.
And so in order for a solution to maintain
its charge balance there has to be mobilization
of these cations to meet the charges associated with these
anions and negative chargers.
And so you can see some corresponding relationship
that pretty strong relationships for all these cations.
Although the nature of the relationship is different.
They're all strongly correlated.
Their fluxes are strongly correlated
with the Fluxes of anions.
And so the, a triangle upside down triangles, potassium
that's a strong nutrients, strongly retained by the forest
but the other cations, magnesium in green, sodium in blue,
and particularly calcium in red
are pretty strongly correlated
with the inputs of those anions.
So as we add in acid rain that leaches
out these nutrient Cations and the source
of the available nutrient cations is largely
exchange sites from soil.
And so this over time, over decades depletes these cations.
And so the amount of available Cations critically calcium
is less available today than it was back,
you know, decades ago.
And that has implications for the ecosystem.
There are two species
of trees that have been particularly impacted by acid rain.
The first is red spruce.
A lot of the great work was done actually in Vermont
working out the mechanisms for red spruce
it's a little bit complicated.
And so what the thinking is is
that high levels of acid rain interact
with a canopy and reach membrane background, calcium
and that predisposes the tree, the needles
of the trees to low temperatures and freezing damage.
There's also a, a soil component of that as well
because some of the calcium is supplied from from soil.
sugar maple make those a little bit different sugar.
Maple requires large amounts of calcium.
And so its effect is largely soil mediated.
It could also be influenced by aluminum high concentrations
of aluminum can impair water uptake
in a nutrient uptake from, for sugar maple.
So we were interested in really sort of sorting out
the role of calcium in this process.
And so back in 1999, we initiated an experiment.
One of the watersheds at Hubbard Brook watershed
one where we added calcium silicate
or Wollastonite to the watershed
and to try to put back what we thought was the quantity
of calcium that had been leached historically
from acid rain.
So the base saturation, we guesstimate, well, we, we
we measure today is 10%
and we thought historically, it was about 20%.
So we added enough calcium to try to bring that back
to what historical values.
So it wasn't really a, a fertilizer treatment
but it was really an experiment to look
at the impacts of lost calcium.
So the theoretical calculation is
for the watershed size is about 30 times
but being a good engineer, I don't know
if Jim said that I was an engineer
but I'm actually an engineer by training.
I added a 50% safety factor.
And so we actually had a little bit more 45 tons
and it was applied by, by helicopter.
I've got to fly to illustrate this.
So this shows the whole operation.
The material was mined in Gouverneur in New York
at Wollastonite mine, it was ground up to relatively
small particle size of 10 microns
hard to apply that directly to the forest
because it would be dispersed all over the place.
So we shipped it to Illinois where it was pelletized looks
like BB size shape particles that was then shipped back
to New Hampshire where it was applied by helicopter.
This material that binds these particles
up is a water-soluble binder.
So when it hits the forest floor and contexts
and moisture falls apart, and you can see sort of a white
particles here on these litter surfaces.
and this is a slide showing you the helicopter operation.
And these sacks are one ton sacks that were dispersed
over about a day and a half in
the fall of 1999.
So we saw really immediate impacts on the stream.
But one thing that actually surprised me was
the impacts that we saw
on the vegetation relatively quickly.
We saw a actually visible changes
in ground level vegetation shortly after the treatment
and the canopy trees, which, you know, are old
the forest was cut, you know, in the early 1900s.
So these are, these are big trees.
It responded relatively quickly.
So this is the time series we're making ongoing
measurements every five years.
I showed you part of this slide earlier.
This is above ground biomass and the watershed six
which is our reference watershed is shown
in blue and watershed one is shown in red.
And then time of the treatment is
is shown here in a sort of a green dash.
And so you can see the measurements
since that time has shown a large increase
in above ground biomass, which I find, I find it astounding.
I showed that the magnitude of the response
This shows you the tree species that's,
so the upper panel is what I showed you before.
We talked about sugar, maple declining
and this is for the lower panels for watershed one.
And so you can see that initial decline
the first value before the treatment
and the initial value after the treatment.
And since that time sugar maple has grown
like gangbusters is doing very well.
So it's completely turned around the sugar maple declined
by adding that calcium back to the system.
Other things that have been found in the study
some work by Gary Holly and colleagues
from UVM who were interested in red spruce impacts
looked at the changes in winter injury at the last night
treated watershed compared to a reference watershed
and observe great declines in
in winter injury from the, from the calcium treatment.
So if we look at what's going on with respect to the
the tree species in the most recent measurements
we measure these every five years.
These are six of the major tree species from watershed six
and this is their relative growth rate.
So we can really see this as sort of a tale of two species
sugar maple continues to do poorly.
This is not in the Wollastonite treatment of the
the reference watershed.
So this, these measurements
the trees did not experience any added calcium.
The sugar maple continues to do poorly
probably the poorest growth of any
of the species, the major species that we monitor.
In contrast red spruce is growing like gangbusters
it's right at the highest elevation.
A lot of the early projections were that these hinder
for species would not do well under changing climate
but in deep red spruce and balsam fir have done very well.
The growth is, is accelerating.
And so, and so I think that there are
two factors that play here.
One is we have alleviated acid rain largely
because of the emission controls.
So the driver that would leach that membrane bound calcium
is no longer there and second it's got in warmer.
So the freezing injury, the very cold temperatures
we no longer see those at Hubbard Brook.
So that's a good transition into climate for me.
But before I do that, I wanna show you another slide
which is intriguing and something that we're looking into.
We've been also measuring the soils at Hubbard Brook
and particularly in watershed one.
And these are data from my colleague, Chris Johnson
And who's been measuring the, the soils
including the forest floor.
So this is forest full of carbon.
I've got a, or he's got a bunch
of data from a bunch of sites
but the watershed one data are shown in green.
And you can see the here that there's a marked decrease
in the fourth floor carbon
and we've also measured it for watershed six.
That's shown in the upside down triangles that are red.
And for years it was relatively flat
some year-to-year variation
but the most recent measurements have shown large decreases
in forest floor carbon in, in watershed six as well.
And so superimposed on this or some other long-term studies
of forest floor carbon one is from a Beech
a forest in Western part of Germany.
Soling and you can see that early on, it was increasing
but the most recent measurement shows a decline.
And then for us in the Czech Republic.
So both forest in Germany
and Czech Republic were heavily impacted by acid rain
which greatly reversed in the late eighties, early nineties.
And you can see the the forest floor carbon
is decreasing very markedly.
So is there some sort of acid rain impact that we're seeing?
So this is an area of, we're not sure what the answer
to this is, but this is an area of ongoing work.
So for the last half, I'd like to transition over
to some climate work, looking at some of our observations.
So we do, as I mentioned
we do longterm measurements and I'll show you some of our
some of our, our interesting measurements.
And we do experiments
not so much watershed level experiments
but more plot level experiments.
And do we do modeling and we're looking at
chronic conditions as well as extreme events.
So just a couple of metrics that we've seen over time
which probably won't be a surprise to you.
This is a mean annual temperature.
This is for three longterm measurements
in New England Hanover, which is an extremely long record.
Well, over a hundred years is shown in gray,
Mount Washington which is a little bit longer
than Hubbard Brook is shown in blue and Hubbard Brook
one of our, our gauging stations has shown a black
and they all show the same thing.
Although different periods of record there's been
as you probably are well aware, a
a pretty significant increase in air temperature
but there's a lot of year to year variability.
But the pattern is sort of remarkably similar
across these three sites, which is
which is interesting in its own, right?
Looking across the seasons at Hubbard Brook
we have a number of meteorological stations sort
of showing data for, for these.
So this is the slope of the annual change
in air temperature and this is by season.
So I've got the four seasons,
fall, winter, spring and summer season.
And then on the far right is the annual and each one of
these colored bars represents a different gauging station.
If it's double cross hatched, it's significant
that the 0.05 level, if it's single cross hatch
it's a significant at the a 0.1 level.
So as I said, there are annual increases in air temperature
and their records are probably most strong
for the winter period.
So we see very clear increases
in temperature over the winter.
Probably the next strongest is the summer.
Although not certainly not as strong as the
as we see in the winter, the spring probably comes in next
then fall probably the,
the least clear trends but showing you an increasing
pattern, and so generally the summer seasons are quite,
excuse me, these shoulder seasons are quite variable
and that's of considerable interest to all of us in here.
I'll talk a little bit more about that later in the lecture.
If we look across the time series here of air temperature
this is for one of the meteorological stations
for Hubbard Brook.
The annual temperature is shown in triangles.
You can see the long-term increase
but a considerable year to year variability.
And the slope represents the change of bent
over the period of record, which extends into the fifties
and then below I've got the winter temperature.
And so you can see the rate here is considerably greater
than the annual temperature.
And you can also see that there's a lot more variability
across this although certainly a statistically
significant trend.
So we've seen that because of this pattern
we've seen sort of manifestations of that.
We back in the day we had a lot of really
cold temperatures at Hubbard Brook.
I know I probably sound like your grandfather here,
but that's indeed true.
The record shows that.
So these very very low temperature classes,
we just are not seeing them to the degree that we saw them
back in the fifties and sixties, they really declined
substantially and so you can see this these temperature
classes and their changes in frequency.
The very low temperature classes are not as abundant.
Those observations are not as abundant
as they were back in the day.
We also see quite a bit of change in precipitation
although it's a highly variable record
but it is a increasing record.
This is the annual precipitation showing you
the time series and you can see there's a lot
of ups and downs, but there's a
statistically significant upward pattern.
And that's consistent with other long records
in New England, this is the Hubbard Brook record here
superimposed on the record at St. Johnsbury,
which is considerably longer and,
so you can see a similar pattern
although the absolute value of the quantity
of precipitation is quite a bit higher at Hubbard Brook
but a lot of year to year, very deadly
but definitely a significant, significant upward trend.
So if you look at across seasons like we did
for temperature, it's very different for precipitation.
Basically we see virtually all of our increase
in precipitation quantity is occurring in the summer months.
So it's not what I would have anticipated
but that's indeed what the record shows.
So not only the, the quantity of precipitation
but the characteristics of precipitation are changing.
We're getting a lot less inputs as snow.
And these cold temperatures are also changing the duration
and the amount of snow pack that we've seen in the record.
So these are three measures of snow pack from snow courses
at Hubbard Brook, the upper panel is maximum snow depth.
The minimum panel is snow water content
and the lowest panel is the ration of snow cover.
And so you can see all
of these measurements are, are decreasing.
And so we just don't have as much snow pack
which is disappointing to me as we did back
in the day at Hubbard Brook.
So I told you I would get to the, the water cycle
and here it is.
So early on in the talk, we
we talked about the small watershed approach
and measurements of precipitation inputs and stream outputs.
Precipitation inputs are shown in red.
The stream outputs are shown in blue
and the difference represents estimated
of evapotranspiration and that is shown in green.
So these are annual water budgets over the,
over the period of record
for Hubbard Brook for watershed three.
And you can see there's a lot of year to year variability
but the stream flow corresponds very strongly
with precipitation inputs, not surprising.
And for most of the record, the estimates
of evapotranspiration have been remarkably consistent.
Although showing a long-term decrease
There's some funky behavior in last few years
that we're scratching our heads on.
You can see by my parents that I scratch my head a lot
but we're seeing initially it starting
in the early two thousands, a decrease
in this estimated ET, and then more recently an increase.
And we have no idea why we're having this strange behavior
after decades of rather consistent response and in ET.
So some things about the precipitation
something known as the center volumes.
So this is the date in which half of the water
leads the watershed at at Hubbard Brook in watershed three
that center data precipitation of stream water, excuse me
is going earlier and earlier and earlier.
And that's a manifestation of less precipitation
as snow and Moore's rain and less snowpack accumulation.
So it's cumulating over a shorter period.
So the water is moving through more within the
Within the early part of the season
And then changes in frequency
which are manifestation of the increases in precipitation.
So this is much like I showed you in the frequency
of occurrence for a temperature, but this is for flow.
So the left side part are, are low flow events.
The high values are high flow events
and this is change in their frequency.
So the reason why this is on a logarithmic scale is
because the range of flows for streams in New England is
is really it's goes over several orders of magnitude.
So in a nutshell, what we're seeing is many
many fewer, very, very low flow events like we would see
in the summer and many, many more, very high flow events.
So this is a measure of the peak flows
for our hydrologic reference watershed watershed three
and they are increasing.
So enroll flow is not only increasing
but the highest flows are also increasing.
And if we look across these high flow events
these are the 20th ice flow events on record.
I would think that they would be a certain type
of event that they're not there all sorts
of different types of flow events.
There are rain on snow events.
There are snow melt events.
Some of them are hurricane events.
Some of them are convective storms.
So it's a hodgepodge of different types
of hydrologic events that are producing these very
very high flow paths.
Okay, so I've got a couple of things
that I would like to close with just as teasers to you.
And some of the things that we're thinking
about and working on.
So the first is what's known as the Vernal Transition.
And so this was identified way back then
by Herbert Bormann as being an important occurrence.
And the idea here, this is a, this is a time series
of monthly values of April, May and June.
And I'm showing you three things here.
I'm showing you the shaded area shows the accumulation
of snow pack and loss of snowpack.
The dashed black line shows you the
emergence of buds from the canopy and then the development
of the full canopy a few weeks later.
And then the red represents soil temperature,
surface soil temperature.
So under snowpack in April, the temperature is low
it's cold, there's snow on the ground.
And so it's near zero values, but slightly above freezing
because of microbial activity in the, in the forest floor.
Then as snow is snow pack leaves, because of the occurrence
you know, the, the emergence of the dark forest floor
it's very effective at absorbing solar radiation.
So you see a very short period of time.
There's this huge increase in surface soil temperature.
And that continues through this period until the
canopy floor develops, and then it levels off.
And so this time period between the loss
of snowpack and the emergence of the canopy
we call the Vernal Transition.
So there's a period of very sharp change.
It's a phenological event
and temperatures change very quickly
and a lot goes on within the forest.
So we think it's, it's an important
it's an important occurrence.
And we thought that for a long time
but we've been looking at the time series of this
and we see, as I told you, before that the snow pack is
occurring earlier, earlier, earlier
and so as the canopy development.
So these periods of these are, are shifting.
And if we look at the, the difference over time, the
open squares represents the last day
of snow pack and the record at Hubbard Brook.
And see, you can see this is
although there's a lot of year to year variability.
The overall trend is it's marching earlier, earlier,
earlier, as we discussed a few minutes ago
and this is the full development
of the canopy shown in the dark circles.
And you can see that's also getting earlier
and earlier and earlier, but it's not keeping paths.
Hey, excuse me for the snow pack.
So this
Vernal Transition period
is not in sync between the loss of snow pack
and the emergence, full emergence or the canopy.
So it's getting, it's getting a larger
and larger discrepancy and we're interested
in what are the implications of this.
The other thing I wanna touch on briefly at the end
is this crazy behavior for nitrate.
A lot of Biogen chemists are focused on nitrate
Maybe you could even argue a little bit too much
or nitrogen a little bit too much.
I showed you this earlier, but this is a blowup
of the nitrate flux record.
So again, precipitation is shown in sort of the
yellowish color and the stream is shown in, in red.
And you can see that in general, there's been
a longterm decrease in nitrate, but there is this sort
of strange behavior where there were large increases
particularly ramping up from the sixties to the eighties
back in the day, that was thought to be due to acid rain.
And it was considered to be a phenomenon known
as nitrogen saturation, but then starting
in the late seventies, that's reversed.
And you can see these little blips over time.
And so we're not sure what these are, but we have
some ideas what may be causing this episodic behavior.
And we're also interested in what is
driving this long-term decline.
It's undoubtedly linked to inputs of atmospheric nitrogen
deposition, but the pattern is not coincidence with it.
The decrease in deposition really
didn't start until the late 1990s and is grown
prominently since that time.
It really started back in the, in the seventies
and it's continued over a much longer period.
So we're interested in why, what is that?
What is driving that?
And it's consistent with other patterns that we see
with that we've been measuring dating back
to Jerry Melillo in 1970.
So made some of them first microbial assays.
We have measurements of nitrogen mineralization
and nitrification and a continuing today with
Peter Groffman's measurements.
And you can see that there are long-term decreases
in these microbial assays
within the soil at, at Hubbard Brook.
So there's a decrease in the cycling
of nitrogen that's been going on for a long time.
These are some soil data.
The upper panel represents the pool of nitrogen
within the forest floor, and that is not changing
but the carbon to nitrogen ratio shown below is changing.
So the carbon to nitrogen ratio is showing
a longterm increase.
We also have other assays, these are gas flux assays
the upper panel shows carbon dioxide fluxes
from soil that does not show any change
although a lot of variability
but the nitrous oxide fluxes are showing very
strong statistically significant decreases
where initially they were a source to the atmosphere
and now they're transitioning to, to be a sink.
So what's driving this?
When we don't, we really don't know.
As I mentioned, initially, we thought it was a
there have been a manifestation of increases in acid rain.
But back in the seventies
there was also a significant drought in New England.
And that could have been a factor for this large increase.
And there were also some insects eruptions that occurred
at this time, which may have been responsible.
Those are the slides just showing you this prominent drought
in New England.
The event in the late eighties
we think may have been a soil freezing event.
And that has spawned a number of research projects
looking at the role of snow pack development in
in winter below ground processes.
So we've done a series of experiments
such as illustrated here, where we shoveled snow
I've not done a lot of shoveling
but some of my graduate students and technicians have
where we expose the soil and that facilitates a freezing.
So you can see these comparisons
of a soil temperature between the control plot
in which is above freezing and the
shoveled plot in red which induces soil freezing and that
results in mortality reduces nutrient uptake
by plants and can cause a Hulsman leeches of nitrate.
So it demonstrated that snow eventually but
but it's not clear that that translates
to the watershed level.
That's still an open question for us.
The one that we know very clearly that
was the '98 Ice Storm, which I mentioned previously
this was a big event and so we run the widespread
in Northern new Orleans and Southern Canada
caused a lot damage, a number of fatalities
we've done measurements, detail measurements, characterizing
that and we've also recently completed some experimental
work that we've done experimental treatments
different levels of icing events and looking at the
at the response of the appliance to those treatments.
So I just want to close with just this one idea
which is sort of the overarching idea
for a project that we just had funded
from the national science foundation data.
And Hubbard Brook this the lead on that?
This is really to try to understand what's
driving this long term pattern of nitrogen retention.
And so we call this phenomena Forest Oligiotrophication
So the, the system is becoming less and less nutrient rich
or more and more nutrient poor.
And we think it's partially driven by
changes ad flows in carbon
So we hypothesized that there could be increased uptake
of carbon by plants,
which then drive
carbon flows, below ground,
and that could facilitate immobilization of
of nitrogen and decreases in mineralization and decrease
available nitrogen which we think will likely feed back
to the, to the trees and cause an increase in reabsorption
of nitrogen which then will have a cascade of
decreases nitrogen and litter fall and then
an ongoing Oligiotrophication of that
So this, we wanna sort of pass this idea and make a series
of measurements but we're also interested
in mode of the drivers.
And we have a couple ideas of what it could be.
It could be a fertilization effect of episodic CO2.
It could be a lengthening of the growing season we know
from nitrogen so there's greater growing season
is getting longer and longer.
So there's greater inputs of carbon
over that longer growing season
we also think it could be doing a terrific period for that.
The changes in the Phosphorus cycles of soils.
We know that it, it it's
it's changing the dynamics of, of nitrogen
but is it translating to the whole watershed process?
And it could also be a manifestation
of recovering from acid rain.
So the trees are less stressed.
There could be some changes in carbon and airflow
and changes in microbial activity that could be driving.
So we think that there is, we have a lot of reference
that there's changes in the internal supply of nitrogen
and that's manifested through decreases in stream nitrate.
So we think that this may be driven by climate effects.
It could be due to, as I mentioned
CO2 uptake could be a lengthy new, the growing season.
It could be some changes in the processing of nitrogen
in soil, and there could be a linkage
to recovery from, from acid rate.
So this is something that we're just starting to work on.
And we're, we're excited about this project.
So to close, I know I've covered a lot
of material and I appreciate your patience
but maybe just to summarize a chart to show you
that we have these longterm measurements and experiments.
And I think they provided a lot
of insight into ecosystem function.
And they're very important in terms
of generating hypotheses and for testing models
of our understanding of ecosystem, pattern and process.
So clearly we have received the inputs of, of acid rain.
We've seen changes in precipitation chemistry
and string chemistry, and we seen the effects of that.
And those effects that, that manifested in changes
in soil changes in stream chemistry and impacting the health
of two prominent tree species, sugar maple and red spruce
so that the clients and acid inputs we'd have
seen really remarkable recovery yet in red spruce
but not sugar maple, because that's a soil mediated
because of a very slow weathering rates from soil.
The exchangeable calcium has not come back.
And so sugar Maples remains.
It continues to be challenged.
So superimpose on these air pollution impacts.
We've seen changing patterns of, of climate increases
in air temperature and precipitation stream discharge.
We've seen decreases in precipitation occurring
as snow and decreases in snowpack accumulation
which seems to affect soil processing during winter.
And these are consistent with regional observations
and future climate projections.
And so one of our ongoing hypothesis is climate
change has really driven the, the nitrogen cycle
the changes in the nitrogen cycle
but that's something we're investigating.
So I appreciate your patience.
And if we have time for questions
I'm happy to try to answer your questions.
- Thank you, that was a great talk.
And we do have a couple of questions rolling in already.
So we'll definitely be keeping you busy
for a little bit longer here.
- No problem.
- First is we had a question from
Neil Camon.
- [Neil] My question has to do with the water loading.
So you've been documenting changes
in water loading to the watershed
and changes in stream flow as a result of that?
- Yes,
- [Neil] One of the things that we've noticed
So our researchers in Lake Champlain have attempted
to do the same thing at the whole Lake watershed scale
looking at all the major river basins
and we're documenting same kind
of changes in precipitation.
And frankly the same kinds of changes in event types
both on the low end and the high end
but what we're not seeing is a real change
in water loading of any kind, which is real interesting.
You would expect that some point or another
you would see those that
that congruence just like your research showed.
So I'm wondering if you have any thoughts as to the
maybe the scale or difference between something the size
of Hubbard Brook relative to something the size
of Lake Champlain and whether there's, you know
some kind of big watershed attenuation factor
that we should try and account for.
- Yeah, I would, you know, I would have to think a little
bit about this, but I would, you know
I guess my first guess might be some something
that is resulting in some change in storage.
So at Hubbard Brook, we see a change
in storage through changes in snowpack, but
because we try to account for the water year in June
we try to minimize the storage impact, but there are some
some of the variation that I showed you is differences
in storage, sort of moisture,
but if you see a long-term trend
you wouldn't think that that storage would change.
So something has to be going on if it is indeed storage
or if there is another loss mechanisms.
So the watersheds at Hubbard Brook are pretty watertight
there is certainly deep seepage, but
but we think it's minimal
from the measurements that, that we have.
But in your case probably those large basins,
probably do have that, but yeah.
So I don't know.
And you don't, you don't see any change
in terms of Lake stage or anything like that.
- [Neil] The Lake stage certainly varies with waterflow
kind of on a day to day, week to week
and season to season basis, I'm not sure actually
that we've looked at that specifically.
- Yeah, yeah.
Yeah, I don't know, I have to think about that.
- Anyway, it's interesting.
I focused right in on that when you were presenting it.
Thank you so much.
- Okay, very good.
- Thanks Neil.
We had a couple come in from the chat.
There was one for
from David brand says peak flows are increasing.
Should we be revisiting our best management practices?
Should we Forest the tenders, be focusing more on
optimal conservation practices to protect water quality
and to enhance flood resilience?
- Yeah, I think that's a great, it's a great point.
So locally here,
I'm dealing with harmful algal blooms.
I'm not sure to what extent that's a problem
throughout New England but it's a prominent problem
in upstate New York.
And we think that that's partially driven
by these very large events
which are increasing erosion and increasing nutrient supply.
And then of course with water increases
in water temperature, it becomes problematic.
So yes, we, we are locally.
We were doing a lot of remediation projects and looking
at best practices to try to improve infiltration and to
and to eliminate these very, very high damaging flow events.
One interesting observation
which is a little bit
challenging that I didn't focus on this in the lecture
is that, you know we're seeing these increases in flow,
but we are not seeing that the forest
is really doing a great job at attenuating
these very high flow events, so you might expect
that with increases in temperature that you might anticipate
an increase in evaporative losses
but we are not really seeing that.
So I think we have to think about, you know
flow paths of water and thinking about riparian zones
and thinking about wetlands
in terms of attenuating
water trying to increase storage and promote infiltration
because we're, I don't think we're going to get a lot
of benefit from additional evapotranspiration
at least not over the short-term from these large events.
It's not clear why that is, maybe
the events are so intense that the trees can't
really process that water as rapidly as we would like.
But, so I think we have to really think carefully
about best practices and,
but that is really a critical concern.
- Thanks, there is a comment and a question about fire.
One pointing out that longer Vernal Transitions correspond
to a longer wildland prescribed fire burn window
is the potential upside, I think for management.
And then a question about any pre or post colonization
fire history at Hubbard Brook that you all have or track?
- Well, we always call the forests at Hubbard Brook asbestos
forest, cause they're really are are quite wet
and quite resistant to fire.
We have had some
episodes of
fire.
And so that, you know, that is an interesting question.
There was a relatively large fire, I think it was in 2017
in the lower Appalachians that really
created a lot of problems.
So usually we don't worry a lot about fire
but I think that given the, the longer growing season, they
the loss of snow pack earlier
and earlier potential increases in drought stress.
I would not be surprised if a fire was not more prominent
in the future, as strange as that, if that seems
and we do that, we have done, you know
where there've been fires in the, in the region
we've gone out regionally and done some, some measurements
but not really focusing at all at the experimental forest.
- Great, thank you.
From Jerry Carlson, he's asking,
Do you have coincidental time measures of soil organism
abundance and complexity, and could they play a large role
in the variations that you have found?
- Yeah, that's well out of my area of expertise
but we do have a lot of interesting information
on invertebrates,
and birds.
And there are large changes and we try to integrate the
the food chain work with the Biogeochemical
and the vegetation work.
We need to do a better job at it
but we are doing more of that
but there have been large declines in insect populations
up at the forest, and there've been some fairly large shifts
in bird species as well.
So yes, we are tracking that.
I can't really speak to that with any great authority.
So I'm just going to leave it at that
not embarrass myself in front of you all.
(Jim laughs)
- Thanks and good to know that that's something
that's out there and being looked at
as this larger part of the study.
Sarah Nelson shares great talk is always
for the investigation of climate on potential forest
electrification.
You slide mentioned an elevational gradient.
Are you collecting climate data
on an elevation gradient at Hubbard Brook?
- Yes, yeah, well, Sarah, good to hear from you.
Hope you're doing well.
We, so we, as I mentioned early on taught
we use a variety of approaches.
And so at Hubbard Brook
there's quite an elevational gradient.
And so there's about a two degree difference between
the lower part of the Valley and the, the highest elevation.
And so we've set in a series
of plots that we've been doing measurements of,
you know sort of physical measurements, as well as
soil measurements associated plant measurements.
So we're trying to take advantage of this gradient as
you know, a speciAL gradient that we could then
infer temporal patterns.
So the amount of snowpack accumulation is much greater
up at higher elevations than it is at lower elevations.
And so we've had these plots
in place and we are taking advantage
of those in the, in the current study.
Another interesting thing that we're doing is
and maybe some of you know, this
but when the
leaf litter inputs
generally the nitrogen content of that leaf litter changes
over the period of, of leaf loss from the canopy.
So we've been capturing cohorts of, of litter at time,
through the leaf fall season
and where we're collecting that. and we will do a series
of litter bags that will have different levels
of nitrogen to try to look
at this resorption phenomenon and try to try to track
try to track that down and understand that better.
So where we're trying to take advantage
of different aspects, spatial
and temporal aspects of the forest
and try to understand, you know, what's going on.
So it's, I didn't emphasize in the talk
but it's an important approach to do quick to do
great in studies as well and taking advantage
of the spacial variations.
- Great, thank you.
Breck Bowden added or asked, what is your explanation
for the strong decrease in nitrogen oxide over time
decreasing nitrification decreasing denitrification or both
- Brac, you should be able to answer that question better
than me, you know, much more about those processes
than I do, but I mean, they're coincidence with
with certainly decreases in nitrification
I showed you those sort of longterm data we don't have as
you know, like if we don't have really good assays about
about denitrification, but the soils
our soil moisture is increasing.
I actually, because I was worried about time
I pulled out a slide of soil moisture.
We have long-term measurements in soil moisture
and soil moisture is increased bigly during the summer
months because that's when all the rain is coming in.
So it,
you know it's not really clear to me what's going on.
That's something that we're hoping to look into
into this current study.
- I wanted to ask if you wanted to pontificate
or just extrapolate and think about, is there any monitoring
that we are not currently doing that we need to be doing
at the larger landscape scale to compliment
or confirm the information you're seeing at Hubbard Brook?
Like if there's one thing that we were going to focus
on at a landscape scale, what might we consider?
- Yeah, that's a really good question.
And I worry a lot about monitoring because, you know
as you know, probably much better
than I monitoring is really under siege.
We're losing more and more monitoring stations all the time
really fundamental measurements, you know
stream flow measurements, things like that.
And that's really critically important
in terms of understanding these processes.
You know, I'm involved in, I am heavily involved
in the NADP, National Atmospheric Deposition Program
and there's a bit of a crisis within that organization
because so many States have pulled out their support
for atmospheric deposition monitoring just at a point when
where we're implementing changes or they're in the case
of the Trump administration, rollbacks and rules.
So are we really characterizing the response
and the effectiveness of these management actions?
So that's one example, but there are lots of examples.
There've been closures of a lot of USGS gauging stations
which is really in my mind, a tragedy
we need the women with these increases in extreme events.
We need those data more
than ever to make intelligent decisions.
So I think it's a, it's a wide spread
it's a widespread problem, and it's hard to point a finger
on any one, but there's lots of examples.
And so I think we just have to make the case
that it's critically important and it's really
giving us measures of what our environment is doing
and how we need to respond to them.
- Thanks.
And one last question we'll sneak in here at the end,
from Dan McKinley, how much of a role can steep headwaters
like Hubbard Brook play in flood attenuation
and how can it be improved?
- Yeah, that's a good question.
You know, I don't, I don't really know.
It's so different than the larger, a larger landscape.
you know, I don't really know what we can do
in terms of informing attenuation other than informing
the drivers, the magnitude of the change.
And so what, what is coming down the plaque
I think we have to think further down slope to think
about effective attenuation or mitigation options
because certainly the rain is increasing.
It's becoming more intense and it's just going
down these high elevation sites and heading into the
into the larger tributaries
and causing and causing problems.
So I think probably lower elevation areas
where there's more developed land
provide opportunities for smart management
but I think we certainly know that it's a problem
and we can try to characterize the problem
but I'm afraid in terms of offering solutions.
it doesn't look like
at least from what I can, you know, what I can see anyway