UV Radiation Effects on Plant Growth and Forage Quality in a Shortgrass Steppe Ecosystem¶

Photochemistry and Photobiology, May 2004 by Milchunas, Daniel G, King, Jennifer Y, Mosier, Arvin R, Moore, John C, Et al

ABSTRACT

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.

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 $5.00+0.00

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).

Of particular economic and ecological significance for grazing systems is the potential for indirect negative effects of UV through reduced plant tissue quality for consumers due to enhanced production of plant secondary compounds. An increased production of secondary compounds with UV can be common and relatively large compared with general production responses (4), and reductions in soluble protein content have also been reported (16,17). Again, however, positive responses to UV in tissue quality parameters may also occur (18,19). Most studies on the effects of UV on herbivores have been directed at arthropods (20-23), with relatively little focus on large ruminant herbivores. Ruminants differ from invertebrate and other mammalian herbivores because of microbial processing of forage in the rumen. Ruminants cannot always increase intake in response to poor quality forage but may become bulk-limited when fiber components are high and nutrients such as nitrogen are low. Rate and extent of digestion and physical breakdown can restrict passage from the rumen, reducing further intake. Soluble and fiber components and nitrogen concentrations of forages are important factors in the capacity of a rangeland to sustain domestic and wild ruminant species. Hemicellulose and cellulose of the fibrous fraction are potentially digestible but rate limiting, the degree of which depends on encrustation by lignin, the availability of nitrogen and the rate of particle-size breakdown (24). Lignin is virtually indigestible by the ruminant. Rate of passage in the ruminant is complex, but in general and in most cases, the higher the digestibility and the lower the lignin, the higher the rate of passage. The digestibility of a forage or diet integrates over other mineral nutrient compositions or limitations as well as over concentrations, ratios and structure of carbon and nitrogen in an index of the potential for microbial breakdown but does not account for physical breakdown processes such as mastication. In this study, we assess forage production and quality responses to UV, grazing and precipitation treatments through analyses of soluble and fiber fractions, nitrogen concentrations and in vitro digestibility of plant species growing in a native shortgrass steppe ecosystem.