General objectives for this project:

This project is a collaborative effort with Jay Ham (Atmospheric Science- CSU) and Nate Brunsell (Geography - KU). Currently we have environmental data at the leaf scale, at the scale of the eddy covariance towers (100s of meters), and at watershed or regional scales. However, much of the within-watershed spatial variability on tallgrass prairie occurs at length scales of 20 to 50 m. Thus, we installed a sensor network that can obtain continuous measurements of key environmental variables at 40 m intervals across a watershed.

Our goals for this project are: (1) Increase our ability to predict the environmental drivers of the community (dominant grasses and forbs) (2) Document environmental variability along a topographic gradient , and (3) begin to scale the physiological responses of these plant communities to the fluxs of water and carbon at the watershed level (using eddy covariance data).

Variations in topography, soil type, vegetative cover, and species composition may cause spatial variability in the surface energy balance across the landscape. Quantifying this variation is necessary to estimate carbon and water balances at the same areal scale used to make land management decisions (i.e., field or watershed scale). Furthermore, measures of spatial variability aid in the interpretation of physiological responses of the grassland community, topo-edaphic variability in primary productivity, improve hydrologic modeling, and enhance the interpretation of data from remote sensing platforms and flux towers.

A 10-station sensor network was deployed in May, 2008 to measure the spatial variation in the surface energy balance in tallgrass prairie. Stations in the sensor network were distributed at 30- to 50-m intervals between two eddy covariance towers in an annually burned, ungrazed watershed (one tower at an upland and lowland topographic position, respectively). Measurements at each station included: air and soil temperature, relative humidity, wind speed, surface temperature, soil heat flux, and soil water content. Data were accessible real-time using a wireless network. Ancillary bi-weekly measurements conducted over the summer 2008 included canopy reflectance, canopy size, LAI, plant heights, leaf water potentials, and instantaneous measurements of soil and leaf gas exchange. Data from the sensor network and flux data from the eddy covariance towers were coupled with a numerical modeling technique to approximate latent heat and sensible heat fluxes at each station in the network.

Google Earth image showing the distribution of the 10 sensor platforms along a topographic gradient on the Konza Prairie.

Results from the summer 2008 and 2009 suggest:

(1) Considerable spatial variability is present across a relatively homogenous landscape (a single watershed) as illustrated by the spatial scale and temporal stability of the surface energy balance,

(2) Plant physiological responses to environmental variability vary by species, location, and time of summer, reinforcing the specificity of micro-site environmental conditions as a key driver of plant community structure in this grassland.

With continued research on this project we hope to answer several questions, including:

•  what is the specific mechanism by which grasses are affecting the subdominant community to regulate their occurrence/productivity?

•  how do environmental factors interact to promote or mitigate this response?