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Harvard Forest Symposium Abstract 2018

  • Title: Trait variation and long-term metapopulation dynamics of the invasive Alliaria petiolata across three growth microhabitats
  • Primary Author: Laura Hancock (University of Massachusetts - Amherst )
  • Additional Authors: Kristina Stinson (University of Massachusetts - Amherst ); Julia Wheeler (University of Massachusetts - Amherst )
  • Abstract:

    Alliaria petiolata (garlic mustard) is an invasive herb in North America that is known to impact above and below ground forest understory community composition and function. Though largely restricted to disturbed edge microhabitats in its home range, garlic mustard invades intact forest understories – a novel microhabitat type – in its new range. Different environmental conditions between the edge, understory, and intermediate zone between the two could select for different traits, leading to divergence in characteristics and demography over time. Patch dynamics, such as source-sink dynamics, could also be affecting A. petiolata’s ability to invade and persist in microhabitats of lower quality. Long-term monitoring of active invasions is rare and can provide insight into the invasion process. In this study, we conducted a metapopulation field survey at three sites at the Harvard Forest (Petersham, MA) from 2003-2006 (sampling period 1), and again in 2015 and 2016 (sampling period 2) to evaluate trait variation and long-term population dynamics between the three microhabitat types. Specifically, our study investigates whether: 1) functional and fitness-related traits (height, number of branches, and number of fruits per plant) differ between microhabitat types, 2) biomass allocation (total biomass, reproductive biomass, and root:shoot) of second year plants differ between microhabitats, 3) the microhabitats support similar densities (plants/sq. m) of seedlings and fruiting adults, 4) densities of plants are increasing, stable, or in decline in each microhabitat type between sampling periods, and 5) whether all populations are self-sustaining, e.g. population growth rates (lambda; λ) are above or equal to 1 in both sampling periods. Our results show that plants in the edge were significantly (p<0.05) taller, branchier, and produced more fruits than plants in the intermediate and forest patches. Plants in the intermediate microhabitat had significantly more branches and fruits, but were not taller than, plants in the forest. Biomass allocation was also significantly different between plants in the microhabitats for total biomass, and reproductive biomass, with plants in the edge having the most total and reproductive biomass. However, root:shoot did not differ between the microhabitats. Seedling density was highest in the edge microhabitat than in the forest and intermediate microhabitats which had similar densities, during both sampling periods. Reproductive adult densities were generally similar between all microhabitat types in sampling period 1, though the intermediate patches supported slightly more fruiting adults. In sampling period 2, all microhabitats showed a decline in the number of adult plants they supported. The intermediate patch showed the steepest decline and no longer supported the highest density of plants but was intermediate between edge and forest. Population growth rates calculated from sampling period 1, showed that the edge, intermediate, and forest populations were increasing (λ>1). However, growth rates calculated from sampling period 2 showed a steep decline in the forest understory populations, indicating that those populations are in decline (λ<1). The edge and intermediate populations both still have λ>1, but the population growth rates in the edge have declined between sampling periods. Since edge and intermediate microhabitats had a higher density of adult plants which produced more fruit and had larger reproductive biomass, we speculate that the edge populations, and possibly the intermediate populations, may be helping to sustain the forest populations.

  • Research Category: Physiological Ecology, Population Dynamics, and Species Interactions; Invasive Plants, Pests & Pathogens; Conservation and Management