%0 Journal Article %J PLoS ONE %D 2013 %T Evidence of physiological decoupling from grassland ecosystem drivers by an encroaching woody shrub %A Jesse B. Nippert %A Ocheltree, T.W. %A Orozco, G.L. %A Z. Ratajczak %A Ling, B. %A Skibbe, A.M. %K Ecosystems %K Grasslands %K Grazing %K Leaves %K Plant physiology %K Seasons %K Shrubs %K Water resources %X

Shrub encroachment of grasslands is a transformative ecological process by which native woody species increase in cover and frequency and replace the herbaceous community. Mechanisms of encroachment are typically assessed using temporal data or experimental manipulations, with few large spatial assessments of shrub physiology. In a mesic grassland in North America, we measured inter- and intra-annual variability in leaf δ13C in Cornus drummondii across a grassland landscape with varying fire frequency, presence of large grazers and topographic variability. This assessment of changes in individual shrub physiology is the largest spatial and temporal assessment recorded to date. Despite a doubling of annual rainfall (in 2008 versus 2011), leaf δ13C was statistically similar among and within years from 2008-11 (range of −28 to −27‰). A topography*grazing interaction was present, with higher leaf δ13C in locations that typically have more bare soil and higher sensible heat in the growing season (upland topographic positions and grazed grasslands). Leaf δ13C from slopes varied among grazing contrasts, with upland and slope leaf δ13C more similar in ungrazed locations, while slopes and lowlands were more similar in grazed locations. In 2011, canopy greenness (normalized difference vegetation index – NDVI) was assessed at the centroid of individual shrubs using high-resolution hyperspectral imagery. Canopy greenness was highest mid-summer, likely reflecting temporal periods when C assimilation rates were highest. Similar to patterns seen in leaf δ13C, NDVI was highest in locations that typically experience lowest sensible heat (lowlands and ungrazed). The ability of Cornus drummondii to decouple leaf physiological responses from climate variability and fire frequency is a likely contributor to the increase in cover and frequency of this shrub species in mesic grassland and may be generalizable to other grasslands undergoing woody encroachment.

%B PLoS ONE %V 8 %P 81630 - %G eng %U https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0081630 %M KNZ001606 %R 10.1371/journal.pone.0081630 %0 Journal Article %J Ecosphere %D 2013 %T Identifying the water sources consumed by bison: implications for large mammalian grazers worldwide %A Jesse B. Nippert %A Culbertson, T.S.F. %A Orozco, G.L. %A Ocheltree, T.W. %A Helliker, B.R. %X

The sources of drinking water consumed by grazers vary over time and may be highly selective, similar to choices in diet. Water sources consumed by large grazers in natural populations are not typically measured directly. Instead, consumption is inferred based on animal proximity to water sources. Here, we analysed the stable isotopic signature of water (δ18O and δD) extracted from fecal samples from a herd of bison in mesic grassland as a direct estimation of the water sources consumed over time. Bison at this site have their choice of a range of habitats and drinking water sources. Potential source-water samples measured had a large range of isotopic signatures, allowing the isotopic composition of water from bison fecal samples to be proportionally estimated based on varying sources. Results indicate bison have low reliance on multiple streams on site; rather, the majority of water consumed was from rainfall-fed sources (puddles and wallows) and from forage. Our research suggests that source-water analysis from fecal samples is a robust technique when samples from large grazers can be collected soon after production. These results have implications for analyses of the foraging patterns and landscape utilization by this and other large grazers, because hotter and drier future conditions are likely to reduce the frequency and amount of rainfall-fed puddles available for consumption in many grassland systems worldwide.

%B Ecosphere %V 4 %P 23 - %G eng %U https://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/ES12-00359.1 %M KNZ001554 %R 10.1890/ES12-00359.1