Master of Environmental Studies Capstone Projects

Document Type

Thesis or dissertation

Date of this Version

12-2010

Comments

Presented to the Faculties of the University of Pennsylvania in Partial Fulfillment of the Requirements for the Degree of Master of Environmental Studies 2010.

Abstract

Stable carbon and nitrogen isotope ratios (δ13C and δ15N) and C:N are evaluated in individuals of three species (Festuca lenensis, Potentilla acaulis, and Pontentilla sericea) occurring across an elevation gradient along a south-facing slope. δ13C is a common proxy for water use efficiency (WUE) in plants, C:N is a proxy for nitrogen indicate variation in δ13C values (and hence WUE) that is significantly correlated with elevation (and thus water availability) in F. lenensis and P. acaulis, but show no such correlation in P. sericea. Variation in C:N (and thus NUE) is significantly correlated with elevation (and total soil nitrogen) in P. sericea only. That no species simultaneously increased bot WUE and NUE suggests a trade-off between the two. The apparent plasticity in WUE seen in F. lenesis and P. acaulis may explain their abundance along the slope, whereas P. sericea - which had the highest WUE - is significantly more abundant where soil moisture levels are low. δ15N results indicate variation in the isotope ratio that is significantly correlated with elevation in all three species. Locations higher on the slope, at the drier end of the aridity gradient, show higher levels of soil nitrate, a greater abundance of lichens and legumes, and decreased plant δ15N. These results are consistent with previous work showing strong topographic effects on local N-cycles, and also the potential that the upper slope is playing a crucial role in bringing nitrogen into the system.

A better understanding of these results, and how legume and lichen abundance will be affected by future increases in temperature and increases in grazing pressure, will help us predict the future plant community composition in the region. The future distribution of these important grazing species will be impacted by their physiological response to different soil moisture levels and the availability of soil nitrogen.

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Date Posted: 15 February 2011