Growth and flux responses to environmental gradients in tallgrass prairie
Konza Prairie Biological Station: Photo credit: Judd Patterson
Generally, ecologists have environmental data at the leaf scale, at the scale of the watershed using eddy covariance towers (100s of meters), and at regional scales (using satellite imagery). However, much of the within-watershed spatial variability in ecosystems (and tallgrass prairie in particular) occurs at length scales of 20 to 50 m. To assess this missing scale of water and energy flux, we have operated landscape sensor networks that can obtain continuous measurements of key environmental variables (including VPD, soil moisture, air/soil/canopy temperature, windspeed / direction, and irradiance) since 2008.
Working with Nate Brunsell (Geography - KU) and Troy Ocheltree (Range and Forest Stewardship - CSU), our goals for this project are: (1) Increase our ability to predict the environmental drivers of energy / water flux in the community (dominant grasses and forbs) (2) document environmental variability and drivers of biological responses along topographic gradients, and (3) scale the physiological responses of these plant communities to the landscape energy and water balance
Watershed 4B on Konza (Sept. 2011). Reddish patches are 'shrub islands' of Cornus drummondii
Over the last 150 years, and the last 50 years in particular, woody plants have expanded into long-standing grasslands and savannas on every continent except Antarctica—a phenomenon known as woody encroachment. Woody encroachment has a negative impact on plant diversity in North America and in some cases, leads to desertification. We’re studying how internal dynamics of shrub-grass competition interact with the external forcing of global change, in order to understand tree-grass competition in general, and how the balance of trees vs. grasses will change in the future.
To guide our research, we use a conceptual framework built from aspects of theoretical savanna ecology, stable-state theory, and coexistence theory. We’re applying these perspectives to a diverse group of data-types, including long-term data-sets of grass/shrub cover, physiological measurements of plant form and function, and vital rates of shrubs (i.e. demography). Currently, Rory O'Connor is investigating the mechanisms of woody seedling establsihment to identify how the interplay of fire and grass competition regulates woody seedling survival. Rory has established a field experiment on Konza where rough-leaf dogwood seedlings are grown under varying above- and below-ground competition and growth is assessed at varying fire intervals.
In the Spring 2016, we will begin a new field project to manipulate rainfall amount (and water availability) on established shrub islands. This project (sh-RaMPs) will investigate shrub physiology, demography, and growth responses to drought (50-75% growing season rainfall reduction).
A few years ago, our own Teall Culbertson initiated a project on Konza Prairie to determine the drinking water sources of the captive bison herd. Her results showed a reliance by this herd on ephemeral water sources (puddles, wallows) and water from forage [published in Ecosphere in Feb. 2013]. Using this novel isotopic approach, we are now working with Tony Swemmer (SAEON - Ndlovu Node) and Michelle Henley (Save the Elephants .ORG). Michelle is one of the foremost experts of elephant ecology, and has many GPS-collared individuals. Working together, we hope to collect data for the next several years (2013-17) to create a detailed picture of resource availability, use, and patterns of movements for elephants in northeast South Africa.
The most under-studied aspect of ecology is the below-ground world of plant roots. While we all acknowledge the sweeping significance of roots on ecosystem processes, quantifying root abundance, form, and function is extremely difficult. Thus, we tend to ignore everything going on belowground!
Since the research of Weaver in the 1930's-50's, we have known about the occurrence of deep roots in the mesic tallgrass prairies of North America. However, we now know that most of these grasses rely heavily on surface roots (0-10cm) for water uptake, regardless of deep roots. Thus, why do tallgrass species have deep roots, what is the distribution of roots across the soil profile, and how does root class (length / width) change by depth and according to managment activities (burning / grazing)?
In addition to asking questions of root type and distribution, we want to know how the types of roots produced impacts the potential hydraulic capacity of the whole plant. For example, what is the capacity of roots produced at great depths (over a meter deep) to deliver water to the surface plant? Our preliminary data suggests that deep roots have severely reduced stele / cortex ratio. Coupled with the fact that there is exponentionally less root biomass at depth than surface soils, most grasses in this system functionally cannot rely on deep soil moisture to support the aboveground plant water demand.
In Mapungubwe National Park, in northern South Africa, riparian forests along the Limpopo River have contracted, and extensive stands of closed-canopy forest once dominated by Acacia xanthophloea, Faidherbia albida, Ficus sycomorus have been replaced by savanna. Water availability for these forests has changed over time, with altered flow in the Limpopo River. Since 2012, we have worked with Tony Swemmer (SAEON - Ndlovu Node) and Tim O'Connor (SAEON) to understand the spatial and temporal patterns of water-use among these trees.
Our results to date show that species most suseptible to mortality have a greater reliance on water from unsaturated soils and /or are more suseptible to elephant damage. In addition, we have preliminary data that suggests that expansion of the sub-canopy / small tree Croton megalobotrys (feverberry) may have a novel source-water strategy compared to coexisting tree species. Thus, multiple interacting factors are having species-specific impacts in the Mapungubwe riparian forests, with consequences for habitat availability, hydrology, and biodiversity in this system.
Drought is a pervasive force that structures ecosystems. The consequences of drought in grassland communities and ecosystems is well-documented. Less emphasis has focused on describing the ability of individual species to persist through drought.
In this experiment, we are working with Mark Ungerer (Biology -KSU) to examine the physiological, morphological, and genomic variability during drought in paired grass congeners with C3 or C4 photosynthetic pathways. We are using pairs of species (in the genera Festuca and Paspalum) for dry-down experiments designed to evaluated physiological and gene expression changes during water stress, as well as basic leaf and root anatomical differences that may contribute to differences in drought tolerance. This research will constitute a portion of Seton Bachle's MS thesis. Data collected during the dry-down experiment include basic gas exchange parameters, stress-related chlorophyll fluorescence, thermal imaging of shoot material, root architecture and root type characterization and leaf water potentials.
A fundamental challenge for ecology is to explain the apparently stable coexistence of trees and grasses in savannas and woodlands, which are poised in the continuum between forests (which are completely tree-dominated) and grasslands (which lack trees). Why do trees not displace grasses in these environments? Water availability is the likely factor constraining tree cover, but the mechanisms driving this constraint are still unknown. The goal of this proposal is to comprehensively tackle this problem from an ecohydrological perspective. This requires that we resolve how trees and grasses a) use and b) partition soil moisture as a function of depth across a wide range of environmental conditions.
We are working with Ricardo Holdo at the University of Missouri and Tony Swemmer (SAEON - South Africa) to address three primary research objectives: (1) characterize the spatiotemporal partitioning of soil moisture by trees and grasses across climatic and edaphic gradients, (2) quantify the strength of tree-grass competition in tropical savannas, and (3) identify key interspecific ecohydrological tradeoffs of tree and grass rooting niches.