Photochemistry and Photobiology, 2004, 79(5): 404–410 Symposium-in-Print UV Radiation Effects on Plant Growth and Forage Quality in a Shortgrass Steppe Ecosystem{

Levels of UV were manipulated in a native shortgrass steppe using open-sided structures with tops that either passed or blocked wavelengths shorter than ~370 nm. Precipitation was controlled to create a drought or a very wet year. Subplots were either nondefoliated or defoliated to simulate grazing by livestock, which is the primary land use. Plant community productivity and forage quality were assessed in response to the two climate change variables (UV, precipitation) and grazing stress. Productivity and seasonal standing biomass of the dominant grass species were negatively affected by passing versus blocking UV, but only in the dry year. Another species was negatively affected by passing UV in the wet year, indicating the potential for future shifts in species composition. Forage quality for ruminants increased when UV was passed compared with blocked, as determined by in vitro digestible dry matter, depending on species and precipitation. Nitrogen concentrations and soluble and fiber components of vegetation also displayed some UV effects, but they were generally small and depended on species, season or amount of precipitation (or all). Grazing treatment had large positive effects on current-year productivity only in the wet year and some small positive effects on quality in both wet and dry years. Interactions between UV and grazing treatment were not observed. INTRODUCTION Past and future climate change includes many variables such as CO2 enrichment, increased temperatures, and altered precipitation and surface UV radiation levels. These changes in climate and abiotic conditions can interact with current uses of, and biotic stressors to, ecosystems. Alterations in surface UV radiation may result from ozone reduction or changes in atmospheric particulates and cloudiness (or both). Fewer than 5% of the studies of UV effects on plants have been conducted under field conditions and most of these used agricultural croplands (1). The shortgrass steppe of North America is native grassland situated at a relatively high altitude (1.65 km), where semiarid conditions result in a sparse canopy and high photosynthetically active radiation (PAR) intensities at ground level. Grazing by cattle is the primary land use of shortgrass steppe, and droughts are frequent (2). We studied potential interactions between UV, grazing and precipitation on productivity, decomposition and arthropod consumers of a native shortgrass steppe in Colorado. This report focuses on primary productivity of the community, and the quality of the material produced for ruminants during 2 years of study in which precipitation was manipulated to create a very wet and a drought year. UV can affect ecosystems directly and indirectly. Direct effects can be physiological damage to plants, consumers and microorganisms, increased production of secondary compounds that screen UV and photodegradation effects on decomposition of plant litter (1,3). Indirect effects are feedbacks on ecosystem structure and function through many pathways, which include altered competitive relationships among species, biogeochemical cycles and carbon budgets. The direct effect of UV on plant production is generally negative and small (4). However, ecosystems are complex and positive production responses to UV have been reported. UV can in some cases reduce drought stress in plants and increase plant production through several potential water conservation and stress tolerance mechanisms (5–8), although the interactive effects of the two stresses have also been reported to be neutral (9–11) or negative (12,13). Drought-tolerant species may sometimes also be more tolerant of UV radiation (14,15). {Posted on the website on 28 February 2004 *To whom correspondence should be addressed: Forest, Range and Watershed Stewardship Department, Colorado State University, Fort Collins, CO 80523, USA. Fax: 970-491-2156; e-mail: daniel.milchunas@ colostate.edu Abbreviations: ANPP, aboveground net primary production; ANOVA, analysis of variance; IVDDM, in vitro digestible dry matter; PAR, photosynthetically active radiation. 2004 American Society for Photobiology 0031-8655/04 $5.00þ0.00