- Welcome to the forest pests and diseases tract
at the FEMC 2020 conference.
I will introduce Laura Hancock.
She is with the University of Massachusetts Amherst
and will be presenting
about trait variation and long-term population dynamics
of the invasive garlic mustard
across three forest microhabitats.
- So broadly I want to start my presentation
by talking about some of the theoretical concepts
which underpin much of my study
that I'll be presenting today,
and will hopefully help us
to better understand how certain pests
and invasive species impact our forest ecosystems,
thus allowing us to better protect them.
And so understanding connections between habitats,
especially those of differing suitability or quality,
could be critical to predicting other species ability
to adapt to and persist in new areas and habitats,
whether those be species we want to see in habitats
or species we don't want to see in habitats,
such as invasive species.
And so I'm specifically talking about,
and what my study focuses on,
is source-sink dynamics and metapopulation structure,
specifically in relation to invasion mechanisms
and how source-sink dynamics may play a role
in invasion success of certain species.
And so the picture
on the right is sort of a visual depiction
of what source-sink dynamics is
for anyone who's unfamiliar with that theoretical concept.
So those circles, which are labeled sources
and have the little sun icons
to represent high quality habitat,
which has a lot of light availability say
for plants that need a lot of light,
and then there are these green circles
that are labeled sinks,
which have these little trees and shading.
So maybe indicating lower quality habitat
that's really shaded,
not a lot of resource availability in terms of light.
And so those arrows that are connecting the sources
and the sinks represent propagule dispersal
from those areas.
And so as you can see,
the sources are having allowing propagules,
because sources are such high quality habitat,
species produce a lot of propagules.
So say seeds, if we're talking about plants,
which is what I'm going to focus on,
and those seeds or propagules will disperse
out into sinks.
And so those sinks have populations
which aren't necessarily self-sustaining.
They aren't producing a lot
of propagules in and of themselves.
And so those source populations
or those source habitats are supporting populations growing
in these sink habitats.
And so this can happen on various spatial scales
from landscape level to micro in terms of meters,
depending on whatever is biologically relevant
for the species of interest.
And so this theory
and this theoretical underpinning is particularly relevant
to one species of interest,
I'm sure to possibly many
of you and many people who are interested
in conserving New England forest.
Specifically I'm talking about garlic mustard.
And so you can see garlic mustard growing in both pictures.
And the picture
on the left is sort of the traditional habitat or areas,
the source areas
that we would think of garlic mustard growing.
So sort of these highlight availability environments
that are along anthropogenically disturbed areas,
so parking lots, roads, trails,
places that humans are fairly
consistently disturbing the area and present.
However, within over the past few decades,
garlic mustard has been growing
in much less disturbed areas,
which with less light availability,
and the exact mechanism
or reason for this is really unknown.
But so the picture
on the right shows garlic mustard completely invading
what is generally an intact forest understory,
intact meaning less anthropogenically disturbed.
It's not next to,
right next to a road or a parking lot
or something like that.
And so broadly we know
that garlic mustard is found across most of New England
and much of the United States and North America in general.
So anywhere from the Pacific Northwest
through much of the Midwest,
and as you can see most of New England.
However, maps like this,
which is what you traditionally see
for sort of these range maps of species of interest,
aren't showing that the scale which I was talking about,
which relates to source-sink dynamics
or habitat suitability,
this is just simply showing like broadly geographically
where a species is located,
but within the counties,
which is sort of the the boundaries of geographic
where the species is found
and how this map is like broken out,
this is simply showing like broadly it's found in a county.
It's not showing the habitat types
or the habitats that garlic mustard is found in
within those counties.
And so a little bit more,
and so I'm sort of interested in pulling that apart.
And so a little bit more background on my study species.
So garlic mustard
or Alliaria petiolata is also called "jack by the hedge"
in its native range,
and so garlic mustard is native
to Eurasia and Northern Africa,
and its native range Alliaria petiolata
or garlic mustard usually grows in hedgy areas.
So sort of along those forest edges,
along those areas of sort of dense vegetation,
and often in anthropogenically-disturbed areas.
So it was first recorded in the US circa 1868 in New York.
And it's particularly problematic and of interest
to many New England conservationists and land managers
because it disrupts above
and below ground community composition.
So it disrupts native diversity
and it does this in ways that we can see
and in ways that we can't see,
and this species does this because it disrupts associations
between arbuscular mycorrhiza fungi found in this whale
and native species that often critically depend
on that AMF for survival.
And so the impacts
of especially the microbial disruption can last for years,
and there's even some evidence to show
that areas can't ever recover
to a point of pre invaded status.
But this could obviously vary depending on areas
and lots of factors.
And the picture
on the right just generally shows very broadly
where garlic mustard is found in its native range,
so Eurasia, and in its invaded range, North America,
and it also shows, which I want to specifically call out,
sort of the morphological structures.
So garlic mustard is a small herbaceous ground cover plant.
It has a biennial life cycle.
So its entire life cycle takes place
within two growing seasons or two summers
to over two years before it dies.
Right before it dies, it naturally,
or right before it naturally senescence,
it releases a large amount of propagules,
anywhere between a thousand to tens of thousands of seeds,
but because this will be relevant
to some of my analyses later on that I'm going to present,
I wanted to call out the fact
that garlic mustard has various distinct sort
of morphological structures
throughout its various life stages,
and I specifically wanted to call out
in its first growing season
it has this basil rosette structure,
and then in its second growing season,
its second year of life before natural senescence,
it produces these flowering stocks or reproductive stocks
which house the siliques or fruits which have seeds in them.
And so those siliques are those sort
of narrow yellowy green tissues.
And so specifically with my research project,
I was interested in broadly seeing
how source-sink dynamics plays a role
in A. petiolata success in forest understories
and how that relates to long-term success
and possible management strategies.
And so the specific research questions I asked to get at
that broader interest was does garlic mustard performance,
biomass, allocation, growth and reproductive capacity differ
across forest growth microhabitats?
Which I will show you pictures of what I mean
by these microhabitats in just a second.
And two are there demographic differences
among populations of garlic mustard growing
across microhabitats and over time?
So specifically I was trying to get
at demographic differences
across multiple complete life cycles,
meaning two year periods.
Okay, and so to answer these questions,
I needed to identify focal microhabitats and areas
where I could establish long-term monitoring plots.
And when I say I would establish them,
I really mean my advisor.
So I did this project in conjunction the coauthor
of this talk who originally established these sites.
Otherwise I would not have been able
to do a long-term monitoring project
within a master's degree.
So specifically what I'm talking about,
forest microhabitats, I mean the forest edge,
the forest understory
and sort of an intermediate transition zone between the two.
And so these pictures are either of
or right next to my actual sampling locations,
which I'll show a little bit more information
about in a second,
but so the picture on the left shows
what I mean by forest edge.
So it's an area that's right at the forest edge
where the tree cover,
where there's a high canopy cover,
but most of the area actually has highlight availability.
It's not over shaded by the canopy.
It's mostly herbaceous ground cover,
very little leaf litter,
and generally a lot of anthropogenic activities.
So close to people's yards and your roads,
and so generally there's some sort
of activity happening there.
These intermediate transition zones are areas right
on the other side of that tree line.
So generally there's a mix of herbacious
and woody groundcover.
There's leaf litter on the ground.
There's oftentimes some amount of anthropogenic activity
or evidence of trash or some sort of disturbance.
And there's semi-mature to mature trees creating some amount
of canopy cover and reducing light availability.
And so when I say forest understory,
the picture on the right,
I'm generally talking about areas
that are past these intermediate zones,
where there is little
to no evidence of anthropogenic activity,
there's high leaf litter, very little ground cover,
especially herbaceous ground cover,
and the ground cover
that is there is usually native tree seedlings
that are just starting to grow,
and there's generally mature trees
with a lot of canopy cover and low light availability.
And so specifically to get at both the spatial
and like temporal sampling scheme
that I briefly mentioned earlier,
I established long-term management plots
at the Harvard Forest,
for anyone who is familiar with that site,
and measured various environmental variables,
individual plant performance, and population demography,
both between 2003 and 2006,
so I'll refer to that as sampling period one,
and between 2015 and 2016, sampling period two.
And so I just want to briefly call out that one
of the responses I'm about to show you largely depends
on that sort of life stage,
that various life stage structure that I mentioned earlier.
So you can measure based on survival proportions
between those different life stages
through seedlings, rosettes, and reproductive adults.
You can get at specific rate of replacements
or how an individual
on average contributes to the next generation.
And so I'm happy to answer questions
about that more in just a second,
but so the first result I want
to show you is light availability
across these microhabitats.
So just confirming that
because light is really important
for garlic mustard growth and reproduction,
we know this from various studies,
that these edge habitats do have more light.
So theoretically they are higher quality habitat.
And that the intermediate sites
and forest sites have less light availability,
although it does not represent a gradient
from edge to understory,
the intermediate and forest sites have the same amount
of light availability across two separate metrics,
both photosynthetically active radiation and canopy cover.
And so I'm going to show you some plant performance
and trait variation data just really quickly.
And so by plant performance
and trait variation I measured multiple metrics
for growth, such as height of second-year plants,
and harvested adult plants right before natural senescence,
dried the tissues in drying ovens,
and got various metrics for total biomass,
reproductive biomass, and things
like reproductive to vegetative tissue ratios,
because that can help us get at reproductive capacity
outside of just simple seed production.
And so for the graph, before I show you the actual data,
I just wanted to orient you to the axes.
So the top panel is adult height.
So just simply the height from the bottom of the plant
to the tip top of the reproductive stalk
for an adult second second-year plant.
The middle panel is the number
of fruits produced per plant, just simple count numbers.
And then the bottom panel is the number
of branches per plant.
So just another metric for general growth.
And then along the x-axis, you have the edge,
the intermediate, and the forest microhabitats.
And so all of this data is pooled across all
of those sampling periods that I had mentioned earlier.
And as you can see, not surprisingly,
sort of what we were expecting,
'cause we were expecting these edge microhabitats
to be these source populations based simply
on light availability and the fact
that they're more anthropogenically disturbed.
The results do confirm that the edge microhabitats,
when we pulled all of this data across years,
showed that plants across all
of these metrics were the largest and perform the best.
And generally the forest plants performed the worst.
Although in one metric for adult height,
the intermediate and forest plans performed about the same.
So moving on to per plant biomass,
total biomass is this top panel on the left,
reproductive biomass, meaning the entire biomass
of fruiting tissue, seeds, and any flowering tissues
that were harvested,
and then reproductive to vegetative biomass ratios.
So simply all vegetative tissues compared
to all reproductive tissues,
which I just mentioned what that was a second ago.
And so again, you see the same sort of trend
for total and reproductive biomass,
where the edge plants were the largest, performed the best,
the intermediate sites had intermediate-sized plants,
and then the forest sites had the smallest plants.
What's particularly interesting,
and what I want to call your attention to
on the graph on the right,
is that the edge
and the intermediate plants actually allocated
about the same amount of tissue
or about the same amount of resources
to reproductive tissues.
However, the forest plants showed decreased allocation
to reproductive tissues,
possibly indicating that
this environment was really low quality
and the plants were stressed out
before they could get to that second year
to be able to produce seeds and reproductive tissues.
And so this is sort of showing the exact same story,
that plants in the edge across time.
So the x-axis now shows 2004 and 2016.
So bringing in a little bit
of that temporal component is showing
that again the edge plants produced the most number
of seeds consistently across time,
although there is some variation.
The intermediate plants produced an intermediate amount
of seeds and the forest plants produced the fewest,
and you can see the P values were significant
for both the effect of microhabitat,
the effect of year,
and the interactive effect of microhabitat and year.
And so just to quickly go over some
of the demographic responses that I measured,
I'm going to show you a graph that is plant density
for various life stages.
So just the number of plants per square meter.
And then population growth rate or lambda
for anyone who's familiar with that analysis.
So similarly following the trends that I just showed you
for individual plant performance,
most of the life stages
for especially seedling density and rosette density,
so those earlier life stages which take place
within the first growing season
and at the very beginning of the second growing season,
plants had the highest densities in the edge,
which you can see by this gray circle,
both in sampling period one,
which was any data collected between 2003 and 2006,
and for sampling period two,
which was data collected in 2015 and 2016.
However, the fruiting adult density,
this story isn't quite as clear
but there was significant declines
for all microhabitat types in sampling period two.
I think that's probably
because sampling period two unfortunately was
during some pretty dry conditions.
So I think just generally plants weren't able to survive
in those really dry hot conditions
in any of the microhabitats.
So the last piece of data that I'm going to show you
before wrapping up is population growth rate,
and so the population growth rate
or lambda is essentially a rate of replacement,
an average rate of replacement for an individual,
meaning one individual
in the current generation is going to produce
how many individuals on average in the next generation.
So going off of that logic,
a stable population size would mean
that one plant in this generation would produce
on average one plant in the next generation,
but if they're producing more than one plant
in the next generation,
the populations are predicted to grow,
and if they're going to produce slightly less
or way less than one individual in the next generation,
the population sizes are predicted to decrease.
So what's really important is that anything above
this horizontal green line means
that populations are still predicted to increase in size
and anything below that one means
that populations are predicted to decrease,
and so what we found looking across space and time
between sampling period one in the early 2000s
and sampling period two in 2015-2016
and across microhabitats,
so the edge, the lighter gray, the intermediate,
the medium gray triangles,
and in the forest these darker gray squares,
was that the intermediate populations
are stable across time.
The edge populations have the highest rate of replacement,
and even though they have declined between sampling periods,
their individual rate
of replacement is still well above one.
So the populations are still predicted to increase.
However, the forest populations,
while sampling in sampling period one they were above one,
have significantly declined far below one
and are predicted to decrease.
And I specifically just want to call out using the last
like 10 seconds that I have
that these declines between the edge
and forest populations track really closely,
which is sort of a little bit more evidence
that the edge microhabitat could be sourcing propagules
into the forest understory.
So just in sort of summary, the edge microhabitat,
it was found to be the most optimal
for plant performance growth and fecundity
across multiple metrics.
Forest plants show depressed growth and performance
with less allocation to reproduction.
Garlic mustards are still growing
in the forest edge and intermediate microhabitats,
but they are predicted to decline in the forest sites.
And source-sink population dynamics
are likely playing a role
into garlic Mustard's incursion into New England forests
at least within this one tract of land
that I was looking at.
And because of that,
management efforts should consider invasion
across these heterogeneous habitats,
because like I mentioned earlier
and just sort of reiterating that graph
from the introduction,
source sites are producing propagules,
which then go into these sink sites.
So decreasing propagule pressure
within source sites could lead
to extinctions of populations
or at least decreases in population size
within the sink sites.
And so with that acknowledgement,
so I just want to thank my lab
and I'm happy to take any questions.
- [Facilitator] You have a question from John Cart.
So what do I do if I'm managing a small riparian site
with an established infestation?
- So that's a great question.
I do not specifically study sort of management responses,
so I'm not sure
if I would be the best person to answer that,
but I would say based on
what my little experience I guess is with that,
my lab mate's much more focused
on sort of management efforts
and the implications for garlic mustard populations.
So what is effective management strategies?
But within the context of what I presented,
I would say that largely it has to do
with what your goals are.
So if it's to eradicate populations
versus decrease the population size,
if you're trying to bring back native species
versus simply decrease the presence
of garlic mustard in an area.
And I think it also depends on, well,
there's a lot of factors that have been shown
to impact sort of invasion success.
So the source-sink dynamics
that I was presenting would just be one mechanism
by which garlic mustard could be
possibly successful invading say forest understories,
or various other habitat types,
but I think to be able to specifically answer your question,
I would need sort of site specific information.
I'm not sure if that really answers your question,
but that's sort of what I can tell you I think based
on the information that I have.
- [Facilitator] Another question,
which native species would do best
at competing against garlic mustard?
- That's a great question.
So again, that's sort of outside of what I study.
So I really do like population sort of population dynamics
of species, but I would say that,
so since garlic mustard is really good at disrupting
like I said that AMF, the arbuscular mycorrhizal fungi,
associations with native plant species,
garlic mustard is non-mycorrhizal.
So it doesn't need those associations.
It would likely be species that are not going to be overly,
that would not be dependent on AMF,
that would not be overly browsed by say deer,
because it's been shown
that deer browsing also helps to contribute
to garlic mustard's invasion success because especially
like ungulates will preferentially browse native species
over garlic mustard,
which just increases its competitive ability.
So I would say species that fall
into those sorts of categories,
not attractive to deer browsers or other native herbivores,
not dependent on AMF associations.
Hopefully that answers your question.
- [Facilitator] Great.
Another similar question
in the very few remaining minutes,
do you notice any other invasive species that seem
to grow well alongside garlic mustard?
- Yes, that's a great question.
So specifically in the areas that I was in, no.
Garlic mustard was pretty much
the most pervasive invasive species.
I know in other areas I've seen,
especially Japanese knotweed,
I'll see garlic mustard growing right along areas
where just Japanese knotweed is in a massive monoculture.
Also multiflora rose,
I'll see that in association with garlic mustard
in at least some of the areas
that I've sort of anecdotally surveyed,
but outside of that, no.
But there is definitely evidence
to show that garlic mustard does grow in areas
with various other invasive species.
But I can't necessarily tell you more specific information
about if the invasive species,
if there are specific invasive species
it grows with commonly.