Litter Decomposition in a California Annual Grassland: Interactions Between Photodegradation and Litter Layer Thickness

In annual grasslands that experience a mediterranean-type climate, the synchrony between plant senescence and peak solar radiation over summer results in high litter sun exposure. We examined the decomposition of both shaded and sun-exposed litter over summer and inferred the effects of photodegradation from changes in mass loss and litter chemistry. The carry-over effects of summer litter exposure on wet season decomposition were also assessed, and the attenuation of photodegradation with litter layer thickness was used to estimate the proportion of grass litter lignin susceptible to photodegradation under different treatments of a factorial global change experiment. Over summer, mass loss from grass and forb litter exposed to ambient sunlight ranged from 8% to 10%, whereas lignin decreased in grass litter by approximately 20%. After one year of decomposition, mass losses from grass leaves exposed to sunlight over summer were more than double the mass losses from summer-shaded leaves. When shade litter layer thickness was varied, mass losses over summer for all treatments were also approximately 8%; however, lignin decreased significantly only in the low shade treatments (0–64 g m⁻² of shade litter). 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 grass litter lignin experiencing full photodegradation ranged from 100% under ambient conditions to 31–62% in plots receiving the combined global change treatments. These results reveal an important role of sun exposure over summer in accelerating litter decomposition in these grasslands and provide evidence that future changes in the quantity of litter deposition may modulate the influence of photodegradation integrated across the litter layer.