Understanding the causes and consequences of changes in biodiversity is critical to management and conservation in an era of global changes such as climate change, biological invasions, and habitat loss. I examine the long-term impacts of global changes on plant communities and ecosystem functions. Specifically, I determine: 1) patterns of changes in communities in responses to global change, 2) mechanisms that structuring communities, and 3) consequences of such changes in terms of ecosystem functions. I use observational studies, manipulative experiments, ecoinformatics, statistical modeling, and interdisciplinary collaborations to test basic ecological theory and provide applied information for land management and conservation.

Long-term plant community dynamics

Climate change, fire regime shifts, and biological invasions are three of the greatest challenges of our time. Their direct effects are difficult to predict; their interactions are poorly understood; their indirect effects on species interactions are almost entirely unknown. I am interested in how global changes and species interactions shape plant communities and how plant communities have responded to global changes. To achieve this goal, a long-term perspective is needed since many ecological responses take decades to unfold.

I am using a unique historical dataset (John Curtis, Vegetation of Wisconsin, the 1950s) to determine how have plant communities changed over the past 50+ years in responses to environmental changes such as climate change, land use change, biological invasions, and fire suppression. Local understory diversity of pine barrens in Wisconsin has increased over time despite climate change and fire suppression. However, their beta diversity has dramatically decreased, suggesting server biotic homogenization (Li and Waller, Ecology, 2015). Functional diversity of pine barrens has similar changes as species diversity. More importantly, climate change and fire suppression interacted to affect changes in functional diversity at these site, highlighting the need to study multiple environmental changes simultaneously (Li and Waller, Diversity and Distributions, 2017). Across the state of Wisconsin, the proportion of random species associations have increased over time (Li and Waller, Global Ecology and Biogeography, 2016). These studies have applied relevance for conservation of endangered fire-maintained habitats such as pine barrens and prairie.

Relevant publications:

  • Daijiang Li and Donald Waller. 2015. Drivers of observed biotic homogenization in pine barrens of central Wisconsin. Ecology. 96:1030–1041
  • Daijiang Li and Donald Waller. 2016. Long-term shifts in the patterns and underlying processes of plant associations in Wisconsin forests. Global Ecology and Biogeography. 25: 516–526.
  • Daijiang Li and Donald Waller. 2017. Fire exclusion and climate change interact to affect long-term changes in the functional composition of plant communities. Diversity and Distributions. 23: 496-506.
  • Kris Verheyen, Pieter De Frenne, …, Daijiang Li, …, Markus Bernhardt-Romermann. 2017. Combining biodiversity resurveys across regions to advance global change research. Bioscience. 67: 73-83.

What controls the responses of communities?

There is an emerging consensus that both deterministic (e.g. habitat filtering, biotic interactions) and stochastic (e.g. dispersal, ecological drifts, random speciation and extinction) mechanisms control responses of communities to disturbances. Using the long-term dataset from Wisconsin, I have found evidence to support this argument (Li and Waller, Global Ecology and Biogeography, 2016). Environmental filtering, biotic interactions and dispersal limitation all appear to affect plant community structure. However, the influence of dispersal limitation increased in fragmented forests. Therefore, fragmentation may make it even more difficult to predict responses of communities to global change.

Functional traits affect how environmental conditions filter species into communities and how species compete, mechanistically linking fundamental ecological processes to community structure. But not every trait interacts with every environmental variable, which justifies the need to statistically study trait-environment interactions. Using simulations, I have demonstrated the need to account for phylogenetic relationships among species using phylogenetic generalized linear mixed models (PGLMMs) when testing trait-environment interactions (Li and Ives, Methods in Ecology and Evolution, in press). Phylogenies play an important role in community ecology by providing information about evolutionary relationships among species; phylogenetic community structure thus also provide insights about mechanisms underlying community assembly. Consequently, functional traits and phylogenies have been increasly used to study mechanisms of community assembly (in different ways as the trait-environment interactions mentioned above). However, we know very little about whether information provided by traits and phylogenies overlap and, if so, how much. Such answer is a necessary first step to further look at mechanisms. With collaborators, I have developed a statistical framework to solve this question and applied it to two empirical datasets (Li, Ives, and Waller, New Phytologist, 2017). My work builds a solid start point for a better predictive ecology, which in turn can offer guidance for conservation practices in wildlife management.

Relevant publications:

  • Daijiang Li and Donald Waller. 2016. Long-term shifts in the patterns and underlying processes of plant associations in Wisconsin forests. Global Ecology and Biogeography. 25: 516–526.
  • Daijiang Li, Anthony R. Ives, and Donald Waller. 2017. Can functional traits account for phylogenetic signal in community composition? New Phytologist. 214: 607-618.
  • Daijiang Li and Anthony Ives. 2017. The statistical need to include phylogeny in trait-based analyses of community composition. Methods in Ecology and Evolution. In press.

What are the consequences of global changes on ecosystem functions?

Global changes have led to widespread changes in community structure and composition, which may not be able to preserve pre-settlement functions. Comparing with species richness, species evenness (i.e. the relative abundance of species) can be changed far more easily. For example, biological invasions alter species relative abundances before they out competed any native species. However, impacts of changes in species evenness on ecosystem function received much less attentions. I investigated the impacts of changes in species evenness caused by species invasions on litter decomposition, one of the most important function of ecosystems. I have found that changes in species evenness have non-additive effects on litter decomposition (Chen et al., Plos One, 2013) and the directions depend on which plant functional group is dominant (Li, Peng, and Chen, Plant and Soil, 2013). This research has important conservation implications for invasive species management —which invasive species is the dominant one matters. If we cannot remove all invasive species, make sure that the dominant one is the one has the least impacts on ecosystem functions.

Relevant publications:

  • Daijiang Li, Shaolin Peng, Baoming Chen. 2013. The effects of leaf litter evenness on decomposition depend on which plant functional group is dominant. Plant and Soil. 365:1-2, 255-266.
  • Baoming Chen, Shaolin Peng, Carla M. D’Antonio, Daijiang Li, Wentao Ren. (2013). Non-Addiitive Effects on Decomposition from Mixing Litter of the Invasive Mikania micrantha H.B.K. with Native Plants. PLoS One. 8(6): e66289.