Bromus tectorum litter alters photosynthetic characteristics of biological soil crusts from a semiarid shrubland

Invasion by the exotic annual grass Bromus tectorum has increased the cover and connectivity of fine litter in the sagebrush steppes of western North America. This litter tends to cover biological soil crusts, which could affect their metabolism and growth. To investigate this possible phenomenon, biological soil crusts dominated by either the moss Bryum argenteum or the lichen Diploschistes muscorum were covered with B. tectorum litter (litter treatment) or left uncovered (control treatment) and exposed to natural field conditions. After periods of five and ten months, we removed the litter and compared the photosynthetic performance of biological soil crusts from the two treatments. Litter induced photosynthetic changes in our samples. In both B. argenteum and D. muscorum, biological soil crusts that had been covered with litter for ten months had lower rates of gross photosynthesis and lower chlorophyll content than control samples. Similarly in both biological soil crust types, litter reduced the rate of dark respiration. For D. muscorum, the reduction in dark respiration fully compensated for the decrease in gross photosynthesis, resulting in similar values of net photosynthesis in the two treatments. In contrast, for B. argenteum, net photosynthesis was four-times greater in the control than the litter treatment. Also under litter cover, D. muscorum showed three common adaptations to shade conditions: a decrease in the light compensation point, in the light intensity needed to achieve 95% of maximal net photosynthesis, and in the chlorophyll a/b ratio. None of these changes was apparent in B. argenteum. Overall, our results indicate that photosynthetic responses to the presence of litter varied among species of the crust biota and that the litter can reduce the photosynthetic capacity of biological soil crusts. These results help to explain field observations of decreases in biological soil crust cover and changes in biological soil crust composition with increases in litter cover, and suggest that the landscape-wide invasion by B. tectorum may have substantial effects on biological soil crust performance and therefore their capacity to function in semiarid shrublands.