Research

Global Change & BiodiversityScreen Shot 2015-01-20 at 8.49.09 AM

Can we protect biodiversity in order to buffer against the negative effects of climate change?  We know from past experiments that this is possible in some instances (e.g. drought in grasslands), but what about disturbances that push primary producers in entirely different ways?  We are using an extreme flooding event that occurred in 2013 at the Jena Biodiversity Experiment (Germany) to assess how high diversity communities are more capable of resisting change and recovering following this change.

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This research was covered by Hudson Valley NPR.  You can learn more about it by listening to the radio segment here.

Screen Shot 2015-01-07 at 11.23.51 AMLocal-scale changes in plant diversity: reassessments and implications for biodiversity–ecosystem function experiments

The recent meta-analysis by Vellend et al. (2013) was an important step towards understanding average changes in local-scale plant biodiversity. The meta- analysis included studies that have assessed changes in biodiversity at small spatial scales and provided evidence that, on average, biodiversity is not changing. Past work has indicated that while biodiversity is decreasing globally but increasing regionally (Sax & Gaines 2003), it may be decreasing, neutral, or increasing locally (Vellend et al. 2013). The abundance of observational studies examining changes in biodiversity at small scales makes a local-scale biodiversity meta-analysis both

important and timely. We agree with the need for such an assessment, yet we think it may be helpful to clarify the underlying logic and reassess the data in several ways.

Screen Shot 2015-01-07 at 11.34.01 AMTemperate and boreal tree seedling growth in response to increased temperature, leaf litter, and drought

Species will respond individually to climate change and this poses a challenge for modeling climate–vegetation dynamics using broader taxonomic or biogeographical classifications. Additionally, responses to climate and environmental conditions may shift with ontogeny, further complicating efforts to understand the likely rates and directions of vegetation change. We measured emergence, leaf-out rate, growth, and survival of first-year seedlings in response to warming, precipitation regime shifts, and seedbed condition (leaf litter presence/absence). We grouped species into three levels of organization (species-specific, biome-level and broad taxonomic group) and hypothesized that most metrics of seedling performance would be best described by species-specific models, as even similar species may respond in vastly different ways to global change. Results showed that the species-specific model was the best fit for emergence and development rates, whereas growth and survival could be captured through broader groupings, with the broadleaf temperate group exhibiting the greatest growth and conifers the shortest survival times. The sign and magnitude of response to climate and seedbed condition varied with treatment combinations and metric of performance. For example, seedlings grew more in response to warming, but conditions too dry or too wet limited this positive response. Also, warmer temperatures generally increased emergence, development, and growth, but decreased survival, whereas leaf litter presence decreased emergence and slowed development, but increased survival. The results presented here are for first-year seedlings and in many cases the responses are different from other studies using older plants. Future research and climate vegetation modeling needs to assess performance at multiple development stages and determine where key bottleneck phases for population growth occur for individual species.

Facilitation in extreme environments

Screen Shot 2015-01-07 at 11.39.19 AMLiving close to your neighbors – the importance of both facilitation and competition in plant communities

Recent work has demonstrated that competition and facilitation likely operate jointly in plant communities, but teasing out the relative role of each has proven difficult. Here we address how competition and facilitation vary with seasonal fluctuations in environmental conditions, and how the effects of these fluctuations change with plant ontogeny. We planted three sizes of pine seedlings (Pinus strobus) into an herbaceous diversity experiment and measured pine growth every two weeks for two growing seasons. Both competition and facilitation occurred at different times of year between pines and their neighbors. Facilitation was important for the smallest pines when environmental conditions were severe. This effect decreased as pines got larger. Competition was stronger than facilitation overall and outweighed facilitative effects at annual time scales. Our data suggest that both competition and the counter-directional effects of facilitation may be more common and more intense than previously considered.

Screen Shot 2015-01-07 at 11.42.32 AMComplex facilitation and competition in a temperate grassland: loss of plant diversity and elevated CO2 have divergent and opposite effects on oak establishment

Encroachment of woody vegetation into grasslands is a widespread phenomenon that alters plant community composition and ecosystem function. Woody encroachment is often the result of fire suppression, but it may also be related to changes in resource availability associated with global environmental change. We tested the relative strength of three important global change fac- tors (CO2 enrichment, nitrogen deposition, and loss of herbaceous plant diversity) on the first 3 years of bur oak (Quercus macrocarpa) seedling performance in a field experiment in central Minnesota, USA. We found that loss of plant diversity decreased initial oak survival but increased overall oak growth. Conversely, elevated CO2 increased initial oak seedling survival and reduced overall growth, especially at low levels of diversity. Nitrogen deposition surprisingly had no net effect on survival or growth. The magnitude of these effects indicates that long- term woody encroachment trends may be most strongly associated with those few individuals that survive, but grow much larger in lower diversity patches. Further, while the CO2 results and the species richness results appear to describe opposing trends, this is due only to the fact that the natural drivers are moving in opposite directions (decreasing species richness and increasing CO2). Interestingly, the mechanisms that underlie both patterns are very similar, increased CO2 and increased species richness both increase herbaceous biomass which (1) increases belowground competition for resources and (2) increases facilitation of early plant survival under a more diverse plant canopy; in other words, both competition and facilitation help determine community composition in these grasslands.

Liana competition & facilitation

Screen Shot 2015-01-07 at 11.12.17 AMUnique competitive effects of lianas and trees in a tropical forest understory

Lianas are an important component of tropical forests, contributing up to 25 % of the woody stems and 35 % of woody species diversity. Lianas invest less in struc- tural support but more in leaves compared to trees of simi- lar biomass. These physiological and morphological differences suggest that lianas may interact with neighboring plants in ways that are different from similarly sized trees. However, the vast majority of past liana competition studies have failed to identify the unique competitive effects of lia- nas by controlling for the amount of biomass removed. We assessed liana competition in the forest understory over the course of 3 years by removing liana biomass and an equal amount of tree biomass in 40 plots at 10 sites in a second- ary tropical moist forest in central Panama. We found that growth of understory trees and lianas, as well as planted seedlings, was limited due to competitive effects from both lianas and trees, though the competitive impacts varied by species, season, and size of neighbors. The removal of trees resulted in greater survival of planted seedlings compared to the removal of lianas, apparently related to a greater release from competition for light. In contrast, lianas had a species-specific negative effect on drought-tolerant Dipteryx oleifera seedlings during the dry season, potentially due to competition for water. We conclude that, at local scales, lianas and trees have unique and differential effects on understory dynamics, with lianas potentially competing more strongly during the dry season, and trees competing more strongly for light.

Screen Shot 2015-01-07 at 11.18.17 AMLianas have a greater competitive effect than trees of similar biomass on tropical canopy trees

Lianas (woody vines) reduce growth and survival of host trees in both temperate and tropical forests; however, the relative strength of liana-tree competition in comparison to tree-tree competition remains unexplored. When controlling for biomass, lianas may have greater competitive effects than trees because the unique morphology of lianas allows them to reach the forest canopy at relatively small stem diameters and deploy a substantial crown above their host. We tested the hypothesis that lianas have a greater negative effect on canopy trees than do trees of similar biomass with a liana- and tree sapling- cutting experiment in a seasonal tropical moist forest in Panama. The response of canopy trees to the cutting treatments was characterized as the change in their daily water use by measuring their sap velocity before and after cutting. We compared the responses of canopy trees around which a similar biomass of either lianas or tree saplings had been cut to control trees with no cutting. Liana cutting increased canopy- tree sap velocity by ;8% from before to after cutting relative to control trees during the dry season. In contrast, canopy-tree sap velocity did not respond to tree cutting, probably because trees with biomass similar to lianas were confined to the forest understory. We observed a similar pattern of sap velocity changes during the wet season, but treatment differences were not significant. Our results demonstrate that release from liana competition, but not tree competition, resulted in increased water transport in canopy trees, and suggests that relative to their biomass, lianas have greater competitive effects on canopy tree performance than do competing trees.

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