- I'm happy to introduce Jordon Tourville,
a PhD candidate at SUNY-ESF,
College of Environmental Science and Forestry,
conducting research with the Appalachian Mountain Club.
Jordon will be presenting Forests on the Move:
Tracking Climate Change in Montane Forest of the Northeast.
- Okay, thanks everyone for coming to this talk.
I'd like to share a bit about a collaborative project
between the Dovciak Lab at SUNY-ESF, the Sussman Foundation
and the Appalachian Mountain Club represented here
by David Publicover and Sarah Nelson.
Looking at the spatial dynamics, demographics,
and drivers behind treeline and treeline locations
on several slopes across the Northeast in montane systems.
If we run out of time for questions at the end,
please feel free to email me at the address here.
So here we go.
So to start with very basic definitions,
what are treelines?
Treelines are the locations usually on montane slopes,
where trees can no longer maintain a tall, upright form
generally due to harsh climatic conditions.
And because the location of treelines are
so tightly linked either directly
or indirectly with climate factors.
These are thought to serve as sort of a barometer
for climate change.
Several groups including the Melanie Harsch Lab Group
have surveyed treeline locations on a global scale,
particularly in this meta-analysis here.
And they found that over half of the treelines on slopes
that they studied were advancing consistent
with warming trends,
with the other half not advancing significantly.
So of course, the situation is a little more complicated
'cause treelines in general are not all created equally.
That same group identified
four discreet archetypal treeline forms.
There's the diffuse form that's generally seen
as a gradual decrease in the density
and height of trees as one increases in elevation.
There's abrupt form that forms a very hard barrier,
or a hard line to growth of tall, upright trees.
An island form,
which is represented by small clusters of trees
as one increases in elevation.
And the Krummholz form,
which it seems generally an increase
of very small high class trees as we go up in elevation.
Now, the key takeaway here is
that not all these forms are driven by the same factors.
The abrupt, island and Krummholz forms are generally thought
to be limited more by seedling mortality
or adult tree dieback,
where the diffuse form is more growth limited.
And so increasing temperatures as a result
of climate change could drive an increase
in the diffuse form more so than these other forms.
So keep this in mind as we move through the talk,
it'll be a very important point later on.
Now, the Appalachian Mountain Club has
sort of investigated treeline location
and treeline shifts in the past.
So in the '90s and the early aughts, there was this effort
to map alpine vegetation communities on Katahdin
and this is a product from that effort.
However, the last couple of decades have been some
of the hottest on record,
and so we all thought it would be a very good idea
to revisit this question of shifting treelines
and see if we could see a climate change signal,
and perhaps this would serve as a baseline
for understanding the threats posed
to very sensitive alpine vegetation.
And so going into this particular project,
we had two very well-educated assumptions.
One being that treeline locations are sensitive
to changes in climate,
and that changes in climate itself can be modulated
by topography.
And so three goals that we had here
for this particular project was
to first quantify changes in treeline position
on alpine peaks in the Northeast
over the last several decades
using a spatial analysis approach.
Our second goal being to assess the demographic structure
of trees at treeline,
and the third would be to determine variables
that could potentially explain treeline positions.
And so, corresponding
with these three goals are three hypotheses.
The first being that treelines in the Presidential Range
and Katahdin specifically had shifted up slope
over most recent decades consistent with warming.
The second that the diffused treeline form
that I'd discussed earlier is more sensitive to change,
and we would pick up a signal of this.
And the third is that significant variation
in treeline could be potentially explained by measured
and available climatological and topographical variables,
temperature, slope aspect, et cetera.
And so as I move through this talk,
each slide will be represented
by one of these three hypotheses,
and you'll see that in the top left.
And so our study areas include the Presidential Range
sitting hear on the left,
and Katahdin on the right and their location in the region.
These images are
from the National Agriculture Imagery Program
or NAIP from 2018,
and are false-color near infrared images.
And as I move through the talk,
whenever I show these two ranges side-by-side,
I'll denote them with P or K just for clarification.
And so to actually quantify treeline advance we started
with physical copies or hard copies of images
from both the Presidentials and Katahdin.
The images were from 1978 for the Presidentials,
1991 for Katahdin.
These were scanned, geo and orthorectified
and mosaicked into one image for both.
From their treeline position
it was manually delineated in ArcGIS.
And for the old imagery,
a total of 200 sample points were randomly placed
along this newly created feature.
This process was repeated for new imagery.
And from there we play sample points
on the closest line segment in the new imagery
to old imagery points.
So we have 200 paired points of old
and new imagery, essentially.
From there, we rasterized the features,
multiplied the whole thing by a two meter DEM of the region,
and the mean difference between old
and new sample points correspond
to the change in treeline position.
Additionally, for each sample point,
we extracted the particular treeline form
of that area given information from imagery,
from field photos and from field notes taken by me.
LGS GIS work was supplemented with field work.
A total of 5,400 meter belt transects were put
on the landscape, encompassing the transition
from closed canopy to open alpine areas
through the transition of treeline increasing in elevation.
And for each of the full transect,
for every tree that intersected the transect line,
species ID was recorded.
Three classes of basal diameter going
from small to large were recorded.
Same thing for height and the particular treeline form
of the four discreet forms I discussed earlier
was accounted for.
At each 20 meter interval, a GPS point was taken,
slope, aspect, elevation
and soil depth to bedrock using metal soil probe was
also taken. (clears throat)
To assess potential drivers of treeline change,
I extracted aspect, slope, and hillshade
from a two meter DEM.
These points here represent locations
of HOBO Data Loggers recording hourly soil
and air temperatures
with the oldest being installed in 2007.
These were used to calculate mean monthly temperature across
that time period and annual accumulated growing degree days
from which I interpolated maps based on values
from the HOBO Data Loggers.
In addition, I was able
to downscale annual precipitation data
from a PRISM data source,
and all of these served to serve as predictors
in a multiple linear regression
with the amount of elevation change in treeline serving
as the response.
The note, this was only for the Presidentials.
We did not have this information for Katahdin sadly.
So going into the results,
last panel on the left is showing all areas above treeline
as they were from the old imagery,
Presidentials on top, Katahdin on the bottom.
This middle panel is showing all areas above treeline
for the new imagery.
It's kind of hard to see differences
between the two just looking at it like this,
but this last panel here is highlighting areas
where tree cover has advanced over recent years
from old to new imagery.
And in some areas it is actually quite extensive.
But looking just at the numbers,
we did find that there was an increase in elevation
of treelines about 12 meters for the Presidentials
and eight for Katahdin.
When we convert this to a rate,
this comes out to about 0.3 meters per year
in terms of advance which is very much consistent
with other published values
from slopes in the Northern Hemisphere,
so that does track very well.
This does lead to a small decrease
in the area above treeline,
which has potential implications for sensitive alpine plants
that do grow at these elevations.
A very interesting result comes out
when we look at soil depth data.
So here I'm showing the mean number
of trees per 20 meter interval on our transects.
So all trees between zero and 20, 20 and 40, et cetera.
As it relates to soil depth,
we see a very strong decrease in the number of trees
as the amount of substrate decreases.
And as we go up in elevation along our transect,
we noticed that soil depth decreases very steeply.
Basically, what we take away from this is
that substrate is very, very limiting
in terms of tree growth in these areas,
which is not surprising given
that a lot of these places have no substrate at all,
they're bare rock.
And so while this may seem like a very obvious relationship,
I think it's very important to highlight things like this
as they serve as sort of the upper limit
for which trees can advance.
Looking strictly at treeline forms
between those four forms identified earlier,
we see indeed that the diffuse form
has experienced a greater change.
Elevation shift in treelines is more significantly sailed
than the other forms.
In order to cue and this is true
for both the Presidentials and Katahdin.
Looking at the demographics of treeline,
we do find that tree density does indeed decrease
as we go up on our transects.
This is not terribly surprising.
Here, I'm showing a tree density
between the four different main forms
and between the three basal diameter classes.
We see that there really isn't
that much of a difference between the four forms
in terms of basal diameter,
but when we look at height,
we do see some differences start to emerge.
For instance, in the Krummholz form we see an increase
of small high class trees
as we increase in elevation along our transects.
In the diffuse form we see an increase
in taller size class trees or high class trees.
And so I think these results are very important
because they sort of serve as these indicators
of different treeline forms that can be assessed
with very, very simple transect based methods
and could be very useful for studies in the future
that look at different treeline forms
and how that relates to changing treeline position.
In terms of drivers of change,
our model explained a small amount of variation
about 15%, excuse me.
And we found that both slope
and growing degree days did have significantly different
than zero coefficients in this model
with slope having a positive
and growing degree days having a negative coefficient.
And when we actually plot that out
for accumulated growing degree days,
we find areas that are generally cooler higher elevations
with fewer accumulated growing degree days
do experience a greater change in treeline position
as opposed to hotter lower elevation areas,
which potentially could indicate
that these areas are already
at a critical threshold in terms of growth.
So any increase in temperatures leading
to an increase in growing degree days could serve
to alleviate this fundamental limit on growth
as it relates to temperature.
However, it'd be very helpful for us in the future
to be able to assess how much change
has occurred in temperatures across different elevations.
Currently, we don't have those data,
but that would be very helpful to have.
And looking at slope, we see an increase
of treeline advance as we increase our slope,
which could just mean
that a smaller distance between our sample points
in areas with higher slope leads
to a greater change in elevation.
So to wrap everything up,
in terms of our first hypothesis, this was supported,
we did find a significant upslope advance
of treelines in these two ranges.
For our second hypothesis, this was also supported.
There were greater upslope shifts
of diffuse treelines in particular,
which again, are more limited to temperature
and maybe more sensitive or responsive to climate change.
Secondarily, we also saw an important role.
Soil to depth is serving as that upper limit
for where trees can advance to.
We also note
that a demographic structure can predict a treeline form,
which can be useful as a management and a monitoring tool.
And our third hypothesis was also supported,
growing degree days climate and slope typography,
both together explain a significant although a small amount
of variation in treelines.
And of course, it would be helpful moving forward
to understand the rate of change
in particular climate variables to see how
that itself affects the amount of treeline advance
that we've documented here.
And so with that, I would just like
to acknowledge the people, agencies and funding
that made this possible.
The Sussman Foundation and the NSF in particular.
The AMC, Baxter State Park in particular.
The US Forest Service.
And of course,
I'd like to point out in particular Eben
from Baxter State Park and Georgia and Patrick
from AMC that really helped out
with accessing the HOBO data.
And so with that if we have any time left,
I'd be happy to take any questions that anyone has.
- It looks like we have a question already
from Tim Howard in the chat.
"That final figure looks like you had treeline loss,
downslope movement in some places.
Can you comment on those situations,
anything special about them?"
- It's hard to say,
I don't have any particular mechanisms
as to why that happened.
It didn't occur that much.
I think disturbance probably plays a role in those areas,
if any major mechanism can be seen.
There are some places where there have been some rock slides
that knock back growth.
So if I had to guess,
it would probably have to do with something like that.
- Yeah, I have a question, it's Charlie Cogbill.
- [Moderator] Yes. Go ahead.
- Okay. Charlie Cogbill.
I wonder if you've looked at any of the historical imagery
from our time previous to your...
I guess, it's 1978 or there's at least 100 here,
very good chronology of sequences of photographs
both in the Presidentials and at Katahdin
that could drive your change back
into various previous trends.
And there have been a few studies
that have been done that shown
that there's a lot of infilling
of Krummholz in those photographs,
but very little elevation will advance.
- Right. I would certainly love to look at those images.
We did have someone approach us
with some imagery from, I believe it was 1943
that we'd like to look at,
but so far it hasn't been done,
but definitely we're open to looking at older imagery,
and running the similar kinds of analysis.
- Well, there is a gallery of photographs
that have been accumulated for the Franconia Range
and for the Presidentials within the US Forest Service.
And there's ongoing work on Katahdin right now
about retaking photographs
that were taken in the '60s looking
at changes on the table land there.
So let's keep in touch.
- Yeah. for sure. (laughs)
I'd be very interested.
- All right. I think we have time for one more question.
Sarah Nelson asked,
"Did you test Tmean on the models or just Tmean?
Sometimes Tmean is informative
as it indicates freezing nights, et cetera."
- Yeah. I did, not only Tmean,
but Tmax and then I did monthly values as well for each.
They're highly correlated with Tmean.
So they weren't adding more information essentially.
Yeah, I did.
And these very similar results uphold.
- Okay. So it looks like that's all the time we have.
It sounds like if people want to ask you questions,
they can email you.
And thank you so much for being here
to talk at the conference.
- Yeah. Thanks everyone.
I'm more than happy to respond to any emails.
(clears throat)