Changes in site ecohydrology following the conversion of mesic grassland to shrubland

Background/Question/Methods

Shrub encroachment of grasslands is a transformative ecological process by which native woody species increase in cover and frequency and replace the herbaceous community.  The mechanisms driving these changes and the consequences of woody encroachment have been well-documented for semi-arid grasslands, but less is known about this process in mesic grasslands. Our research presented here assesses the ecohydrological consequences of woody encroachment by measuring differences in source-water use between grasses and shrubs, leaf-level flux (transpiration), canopy growth and productivity, and landscape flux (ET) by comparing annually-burned (no encroachment) and infrequently burned (encroached) grassland sites at the Konza Prairie Biological Station in eastern KS, USA.  Sampling at Konza Prairie occurred at regular intervals during the summer from 2009-2011.  Source-water differences were assessed using the stable isotopic signatures of water in plant xylem and soil.  Differences in growth and productivity were assessed with bi-weekly clipping of aboveground biomass and measurements of LAI.  Canopy-level fluxes were assessed using the eddy covariance technique with flux towers present on both encroached and un-encroached grassland sites.

Results/Conclusions

Source-water analysis using stable isotopes showed that the dominant encroaching shrub in this grassland (Cornus drummondii) had a primary reliance on groundwater, in direct contrast to the native herbaceous community that relied on surface soil moisture.  Canopy development and productivity varied dramatically between sites, as LAI values of ~5-6 are reached by early June in shrub-encroached versus LAI values of 4-5 in native grassland by the end of August.  Comparisons of productivity across years varied as a function of seasonal climate (rainfall, air temperature, VPD) in the grassland site, while patterns of shrub growth were decoupled from ambient environmental conditions.  Landscape water-use efficiency (assessed as the change in carbon gain by the change in latent energy) was higher in woody encroached sites, compared to native grassland suggesting that the greater rooting depth by shrubs alters ecohydrology compared to grassland without encroachment. Groundwater reliance by the encroaching shrub species may buffer this growth form from forecast climate variability, reducing susceptibility to future droughts compared to coexisting herbaceous species.