Temperature, moisture and initial litter chemistry are factors that influence decomposition rates of plant litter in terrestrial systems. However, litter decay in arid and semi-arid ecosystems are often faster than models predict and photodegradation has been suggested as an additional factor in decomposition. Photodegradation involves the direct and indirect breakdown of plant litter components and lignin, a major constituent of the secondary cell wall, has been implicated as a potential target. Lignin strongly absorbs ultraviolet radiation (“UV;” 280-400nm) and it is unknown if initial concentrations can influence photodegradation. We collected Artemisia tridentata (sagebrush) leaves at ten sites along a 1000-m altitudinal gradient in the semi-arid Bighorn Mountains in Wyoming. Litter was then exposed to 3.2 kJ m-2 d-1 of biologically effective ultraviolet-B radiation (280-320 nm) for 310 d in a laboratory setting. We suspected that plants growing along this gradient would have marked differences in initial leaf chemistry (cellulose, hemicellulose, lignin, C:N and soluble UV-absorbing compounds) which could potentially influence litter decay.
Results/Conclusions
Epidermal transmittance of UV in sagebrush leaves decreased from 20 to 12% along the 1000-m gradient and there was a concomitant increase in bulk-soluble UV-absorbing compounds. There were no differences in initial concentrations of cellulose or hemicellulose, however initial lignin concentrations and C:N ratios decreased from 14 to 9% and 2.6 to 1.9, respectively, over the 1000-m gradient. Mass loss was negatively correlated with elevation and this relationship became more apparent over time. After 310 d, litter had lost between 5.98 to 9.44% of its initial mass. Changes in cellulose, hemicellulose and lignin concentrations were subtle; these constituents were typically 1-6% lower than initial values. It appears that differences in initial litter quality can influence photodegradation of sagebrush litter.