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), 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). Our first goal is to test whether the spatial and temporal patterns of woody encroachment fit the construct of bi-stability. If woody encroachment is governed by stable-state dynamics, what mechanisms (i.e. positive/negative feedbacks) result in bi-stability? Where and when do tipping points occur (i.e. what is the resilience to press and pulse pressures, solo and in tandem)? By answering these questions, we hope to develop generalizable models of shrub-grass bi-stability, which can be used to answer theoretical and applied questions.
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), Michelle Henley (Save the Elephants .ORG) and Craig from Olyphants West. 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 (2013-15) 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 and Rob Taylor (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.
Recognizing which traits are the best predictors of relative abundance in well-watered and water-stressed situations will aid in the prediction of plant community structure under altered temperature-precipitation regimes. The predicted impacts of climate change on grassland systems worldwide increases the importance of understanding how native grasses are likely to respond to future changes in water availability and air temperatures.
Working with Joe Craine, we have been conducting anatomical and physiological measurements taken on hundreds of species of herbaceous plants (locally from the tallgrass prairie as well as global representatives of the family POACEAE) grown from seed in a growth chamber. Gas exchange measurements are made under optimal light, temperature, and humidity conditions. All plants were exposed to a dry-down period and were monitored until stomatal conductance was zero. At this point, water potential (Ψcrit) was measured and the plants were harvested to measure root length, diameter, volume, and mass, leaf area, leaf tissue density, root tissue density, and root to shoot ratio.
Climate change, atmospheric chemistry changes, and invasive species are altering plant communities in natural areas of the Colorado Front Range, but more rapid and dramatic community transformations are believed underway as a result of native consumer activities interacting with these other change factors. Research proposed here addresses two important questions: 1) What are the mechanisms that allow for winter annual species to become a dominant life form along the Colorado Front Range, and 2) do these new plant species alter the role of prairie dogs from that of a keystone species to one of an ecosystem transformer?
Working with Tim Seastedt (CU- Boulder) and Laurel Hartley (CU-Denver), we are examining if if the combination of expanded growing season and wetter winter conditions favors the emergence of a ‘new’ group of dominant plants on the Front Range, the winter annuals, and manipulations will document the consequences and feedbacks of these plants on the native perennial species. The emergence of a new temporal niche, occupied by species new to the region, is hypothesized to cause a cascade of biogeochemical feedbacks that sustain the abundance of this new group.Thus, global changes result in a ‘keystone species’ functioning as an ‘ecosystem transformer’, with grasslands being converted to shrublands or other novel ecosystems. While effects to date appear strongest along the urban-wildlands corridor, the expansion of global change factors into the shortgrass steppe and mixed grass prairies may expand the region where ‘desertification effects’ of prairie dogs are anticipated.
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 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.