Interactions between photodegradation and litter thickness in a California annual grassland

In annual grasslands that experience a mediterranean-type climate, the synchrony between plant senescence and high UV-B levels over summer results in high litter UV-B exposure. We examined the decomposition of both shaded and sun-exposed litter over summer and inferred the relative effects of photodegradation and microbial action from changes in mass loss and litter chemistry. The attenuation of photodegradation with litter layer thickness and the carry-over effects of summer litter exposure on wet season decomposition were also assessed. Over summer, mass loss from grass and forb litter exposed to ambient sunlight ranged from 8-10%, whereas lignin decreases in grass litter were approximately 20%. For all shading treatments, mass losses from grass litter over summer were also approximately 8%; however, lignin decreased significantly only in the low shade treatments. Aboveground production of annual grasses nearly quadrupled in response to the combined effects of N addition, elevated atmospheric CO₂, increased precipitation and warming. The estimated proportion of annual grass litter exposed to high UV-B ranges from 80% under ambient conditions to less than 20% in plots receiving the combined global change treatments. After one year of decomposition in the field, mass losses from grass leaves exposed to sunlight over summer were more than double mass losses from summer-shaded leaves. These results reveal an important role of sun exposure over summer in accelerating litter decomposition in these grasslands and provide evidence that changes in the quantity of litter deposition may modulate the importance of photodegradation integrated across the litter layer.