What we do

Our research combines large-scale observational studies across biogeographic regions, field experiments, functional and phylogenetic approaches, and advanced statistics and modeling to address environmental issues and to explore fundamental questions in ecology. We are broadly focused on two main themes: 1) Community assembly - mechanisms across local, landscape, and biogeographic scales; and 2) Global change biology - applying community assembly theory to environmental issues. We also explore other topics of interest including herbivory, disturbance and plant hemiparasites 

RESEARCH PROGRAMS

 -COMMUNITY ASSEMBLY-

Integrating mechanisms across local, landscape, and biogeographic scales 

 

A key challenge at the interface of ecology and conservation is to understand the mechanisms by which communities assemble and lead to the diversity observed in nature. These mechanisms range from the continental-scale influence of temperature and moisture, to the more localized influence of micro-climate, soil fertility and biotic interactions. In the Spasojevic lab, we integrate processes across multiple scales, from continental-scale biogeographic processes to local interactions among coexisting species, to better elucidate mechanisms of community assembly. Our approach focuses on using quantitative examinations of taxonomic, functional and phylogenetic diversity coupled with spatial and environmental data to elucidate the role of multiple interacting mechanisms.

 

 

Below are a few of our recent projects

 - GLOBAL CHANGE BIOLOGY - 

Applying community assembly theory to environmental issues 

 

Community assembly theory provides a powerful framework for understanding how multiple global change factors are influencing individual species, changing plant communities and altering ecosystem function. For example, predicting species distributions under future climate scenarios has been strengthened by including both continental scale processes and landscape scale processes. A key goal of the Spasojevic lab is to integrate multiple ecological processes to understand two key questions related to global change: 1) what are the mechanisms underlying ecosystem resilience (why do some ecosystems have an irreversible response to global change while others recover); and 2) how biotic interactions influence which species may be able to survive climatic warming through natural dispersal or assisted relocation.

 

Below are a few recent projects.

How do plants and microbes contribute to plant taxonomic beta-diversity?

 

Abiotic and biotic processes are both hypothesized to play and important role in community assembly and the maintenance of biodiversity. However, fundamental gaps remain in our understanding of the contribution of biotic processes to spatial variation in community composition (β-diversity). In a high-elevation landscape unit that extends from established alpine tundra vegetation to near-barren subnival soils associated with glacial retreat we examined the abiotic and biotic drivers of the assembly of plant taxonomic β-diversity. Using variation partitioning analysis, we asked how microbial composition, plant composition, environmental factors and spatial structure measured between 2007 and 2008 contribute to the assembly plant β-diversity in 2015. We found that biotic processes (microbes, plants) and abiotic processes (environment, space) jointly contributed to plant β-diversity. Importantly, however, we found that microbial composition, environmental factors, and space contributed more to plant β-diversity at lower densities (fewer than 100 plants per plot), while plant composition, spatially structured microbes, and plant-microbe associations contributed more to plant β-diversity at higher densities (greater than 100 plants). Our study highlights the importance of incorporating biotic interactions into studies of β-diversity and suggests that plants and microbes may play important roles as drivers of β-diversity in plant communities

In Preperation: Ecology

Project in collaboration with: Emily C. Farrer, Dorota Porazinska, Jane G. Smith, Andrew J. King, Stephen Schmidt and Katharine N. Suding 

When does intraspecific trait variation contribute to functional beta-diversity?

 

Intraspecific trait variation (ITV) is hypothesized to play an important role in community assembly and the maintenance of biodiversity. However, fundamental gaps remain in our understanding of how ITV contributes to mechanisms that create spatial variation in the functional-trait composition of communities (functional β-diversity). We examined how ITV contributes to functional β-diversity and environmental filtering in woody plant communities in a temperate forest in the Missouri Ozarks, USA. We found that functional β-diversity that included ITV increased with spatial extent and decreased with spatial grain, suggesting stronger environmental filtering within spatially extensive landscapes that contain populations locally adapted to different habitats. Our study suggests that mean trait values may mask the strength of assembly mechanisms such as environmental filtering, especially in landscape-scale studies that encompass strong environmental gradients and locally adapted populations. Our study highlights the utility of integrating ITV into studies of functional β-diversity to better understand the ecological conditions under which trait variation within and among species contributes most strongly to patterns of biodiversity across spatial scales.

In Press: Journal of Ecology

Project in collaboration with: Jonathan Myers

Using functional diversity to test the tolerance hypothesis for plant species richness along gradients in R.H. Whittaker's data. 

The physiological tolerance hypothesis proposes that plant species richness is highest in warm and/or wet climates because a wider range of functional strategies can persist under such conditions. Recent advances in trait-based ecology offer a new way to test this hypothesis. In a reanalysis of R.H. Whittaker’s (1960) data from the Siskiyou Mountains (Oregon, USA), Grace et al. (2011) demonstrated that herb species richness declined from lush north-facing slopes to harsh south-facing slopes. We used these same data and data on plant height and three foliar traits related to resource acquisition in a structural equation model to ask if ‘functional diversity’ (modeled as a latent property reflected in the joint responses of functional richness, evenness, and dispersion) can explain the effect of climate on species richness. We found that the data were consistent with this theoretical specification and supported the inference that functional diversity mediates the effect of topography on species richness: i.e., lush north slopes support communities with greater functional diversity and therefore greater species richness than harsh south slopes. Our results support the physiological tolerance hypothesis for climatic gradients in species richness. 

Journal of Ecology 102:447-455. 

Project in collaboration with: Susan Harrison, Ellen I. Damschen and James B. Grace

 

Scaling up the diversity-resilience relationship with trait databases and remote sensing data: the recovery of productivity after wildfire?

 

Understanding the mechanisms underlying ecosystem resilience - why some systems have irreversible response to disturbances while others recover - is critical for conserving biodiversity and ecosystem function in the face of global change. Despite the widespread acceptance of a positive relationship between biodiversity and resilience, empirical evidence for this relationship remains fairly limited in scope and localized in scale. We combined tools used in large scale studies of biodiversity (remote sensing, trait databases) with theoretical advances developed from small scale experiments to ask if the functional diversity within a range of woodland and forest ecosystems influences the recovery of productivity after wildfires across the four-corners region of the United States. Using structural equation modeling, we found that functional diversity in regeneration traits (fire tolerance, fire resistance, resprout ability) was a stronger predictor of the recovery of productivity after wildfire than the functional diversity of seed mass or species richness. Our study provides some of the first direct empirical evidence for functional diversity increasing resilience at large spatial scales. Our approach highlights the power of combining theory based on local scale studies with tools used in studies at large spatial scales and trait databases to understand pressing environmental issues.

In Press: Global Change Biology

Project in collaboration with: Christie A. Bahlai, Bethany A. Bradley, Bradley J. Butterfield, Mao-Ning Tuanmu, Seeta Sistla, Ruscena Wiederholt and Katharine N. Suding

Above- and belowground biotic interactions facilitate plant movement into cooler environments 


One of the most significant unanswered questions about biotic responses to climate change is how interspecific interactions will influence the ability of species to colonize cooler climatic refugia. Either facilitation or inhibition of such colonization could arise from aboveground plant-plant interactions and/or belowground plant-soil feedbacks. We transplanted three endemic plant species into cooler topographic locations (higher elevations, north-facing slopes) than those they currently inhabit in the rugged Siskiyou Mountains (OR, USA), where the climate has warmed by 2°C in recent decades. Two species enjoyed higher success in cooler locations, and this success was enhanced by aboveground effects of the surrounding plant community, as shown by neighbor removal treatments. For one of these species, its success was also facilitated by the higher soil organic matter in cooler sites. These results are a novel experimental demonstration of two important factors that may buffer climate change impacts on plants: facilitation and rugged topography. 

Ecology Letters 17:700-709. 

Project in collaboration with: Susan Harrison

Changes in alpine vegetation over 21 years: are patterns across a heterogeneous landscape consistent with predictions?


One significant unanswered question about biotic responses to climate change is how plant communities within topographically complex landscapes will respond to climate change. Alpine plant communities are strongly influenced by topographic microclimates which can either buffer or compound the effects of more regional climatic changes. Here, we analyzed species changes over 20+ years in a complex alpine landscape with pronounced gradients in microtopography and consequently large variation in temperatures, snow depths, and nitrogen availability across small (10 m) scales. Using data from long-term monitoring plots from six community types, we asked how species composition and functional diversity changed over time in these different areas of the landscape, and whether fine-scale heterogeneity allowed species to move in response to temporal changes in the environment. 

Ecosphere 4(9):117.  

Project in collaboration with: Bill Bowman, Hope C. Humphries, Tim Seastedt and Katie Suding

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