Research
My research uses plant physiological measurements to understand shifts in plant communities and their cascading effects on ecosystem-level processes in response to changing environmental drivers.
Tree dynamics in African savannas
Savanna trees evolved under frequent and interacting disturbances, including fire, drought, and herbivory by large mammals. Each of these pressures independently exert selective pressures on how plants allocate carbon among growth, defense against herbivory, and carbon storage. Tradeoffs in C allocation across these pools can influence competitive dynamics among species across rainfall and disturbance gradients. In this project, we are characterizing the physiological differences of the dominant Acacia species at Mpala Research Centre, which collectively account for ~60% of tree cover across the site. Our goal is to link species-specific physiological strategies to changes in tree growth and abundance in response to herbivore exclusion and drought, using data from a long-term herbivore exclusion experiment.
Impacts of herbivore exclusion on savanna tree physiology
Mpala Research Centre, Kenya
Collaborators: Dr. Tyler Coverdale (Notre Dame) and Dr. Jason McLachlan (Notre Dame)
Savanna tree cover generally increases with mean annual rainfall (MAR), but we lack a mechanistic understanding of whether this increase is driven by rainfall intensity, frequency, or longer growing seasons associated with MAR. To disentangle these factors, we are conducting an in situ rainfall manipulation experiment to isolate the effects of rainfall intensity, deep soil water availability, and growing season length on the growth of tree saplings and grasses. We aim to (1) identify the key resource axes that differentiate trees from grasses and (2) predict changes in tree cover under novel rainfall regimes. Our results suggest that differences in rainfall regimes take time to manifest but drive long-term impacts on savanna structure by reducing the time required for trees to escape from disturbance-induced bottlenecks. Furthermore, species-specific physiological responses to rainfall determine which tree species escape and which do not, shaping the long-term composition of savanna communities.
Impacts of rainfall regimes on savanna tree-grass interactions
Wits Rural Facility, South Africa
PIs and collaborators: Dr. Ricardo Holdo (PI; UGA), Dr. Jesse Nippert (PI; KSU), Dr. Michael Belovitch, Jules NeSmith (project manager), Wayne Twine (WRF), Justice Ndhala (on-site manager)
Repeated tree clearing to mitigate bush encroachment
Mthimkhulu Game Reserve, South Africa
The increase of woody plants in semi-arid savannas has reduced herbaceous biomass and diversity, altered habitat structure, and modified savanna ecohydrology. The goal of this project is to assess whether repeated shrub clearing is a viable and effective management practice to reduce shrub cover and increase herbaceous biomass and herbivore presence in a mopane-dominated savanna. Three 60 x 60 m plots have been repeatedly cleared all mopane trees greater than 4 m tall since 2015 at Mthimkhulu Game Reserve. It took 6 years of repeated clearing to reach 50% tree mortality, but soil moisture, grass biomass, and herbivore visitation increased after three years. Our results suggested repeated and targeted management is needed for long-term reductions in woody cover but restoring a grass-dominated ecosystem is possible. This project is in collaboration with the South African Environmental Observation Network and the Mthimkhulu Community.
PIs and collaborators: Dr. Tony Swemmer (SAEON), Dr. Jesse Nippert (KSU), and
Peace Nkuna (project manager)
Control plot
Cleared plot
Woody encroachment in tallgrass prairie
Physiological strategies of clonal encroaching shrubs
Konza Prairie Biological Station, Manhattan, KS
After the conversion of grasslands to row crop agriculture, woody encroachment poses one of the greatest threats to grassland conservation. This phenomenon is driven by the expansion of native shrubs, but not all species are expanding at the same rate. In this study, we characterized the growth and resource-use strategies of the most abundant encroaching shrubs at Konza Prairie. The focal species spanned an order of magnitude in abundance. We found that the two most rapidly encroaching species had contrasting physiological strategies: tall species that form dense thickets, shade out grasses, and suppress fire vs. shorter, drought-tolerant species that coexist with grasses. Our study highlights the diversity of encroaching shrubs in mesic grasslands and suggests an equally diverse management approach may be required to combat their spread. By understanding the
physiological strategies of these species, we can better predict which species are likely to increase under future climate and land-use scenarios and develop more targeted management strategies to mitigate encroachment.
Collaborators: Dr. Jesse Nippert (KSU), Dr. Zak Ratajczak (KSU), Greg Tooley (CSU)
Combined effects of herbicide, brush clearing, and fire on woody encroachment
Konza Prairie Biological Station, Manhattan, KS
Clonal shrubs encroaching into grasslands pose a significant management challenge because of their ability to resprout following fire or brush clearing. In this experiment, we evaluated the effectiveness of herbicide application, mechanical brush clearing, and annual prescribed fire in reducing woody cover. While treatments effectively decreased the abundance of non-resprouting trees, clonal shrubs reestablished. In areas where fire was intense enough to kill shrub buds, shrubs declined and forbs became dominant. These results suggest repeated management is required to achieve long-term reductions in shrub cover, and that perennial grasses may not quickly reestablish following shrub removal.
Collaborators: Dr. Rory O'Connor (USDA), Dr. Meghan Avolio (John Hopkins), Dr. Kevin Wilcox (UNCG), Dr. Kim Komatsu (UNCG), Dr. Sally Koerner (UNCG)
Grassland Macroecology
Grass-dominated ecosystems cover ~25% of the terrestrial surface, harbor tremendous amounts of biodiversity, and help regulate global carbon and water cycling. Despite their widespread distribution, grasses are underrepresented in trait databases, limiting our ability to synthesize and predict changes in these widely distributed but often overlooked ecosystems. Grasses are typically grouped into C3 or C4 functional types, which ignores important diversity within the C4 functional group. The grassland macroecology group is developing a novel framework to group grass function based on evolutionary lineage, rather than photosynthetic type. This project has connected me with highly collaborative group of grassland ecophysiologists, expanded my understanding of evolutionary ecology, and provided me with mentorship opportunities with MSc students.
Project PIs and students: Dr. Jesse Nippert (KSU), Dr. Dan Griffith (OSU), Dr. Chris Still (OSU), Dr. Brent Helliker (UPenn), Dr. Stephanie Pau (UC-Berkley), Dr. Bill Riley (Lawrence Berkely National Labs), Ryan "the elder" Slapikas (FSU), Ryan "the younger" Donnelly (KSU), Adam Abdullahi (UPenn), Che-Ling Ho (UPenn)