UV radiation and reduced rainfall: Legacy effects on litter and first-year decomposition in two Mediterranean evergreen shrubs

Climatic models for the Mediterranean Basin predict higher ultraviolet (UV) radiation and drier conditions by century’s end, potentially altering litter traits, early-stage decomposition, and nutrient cycling. This study investigates how UV radiation (UV-B and UV-A) and reduced rainfall influence leaf litter characteristics and its decomposition in two evergreen sclerophyllous Mediterranean species, Arbutus unedo and Phillyrea angustifolia, growing in a natural shrubland. A field litterbag decomposition experiment was conducted using litter from plants growing in eighteen 9 m 2 plots arranged in three blocks. For six years, plots experienced one of three UV radiation conditions: ambient UV, without ambient UV-B, and without ambient UV, combined with two rainfall levels (natural or reduced). Treatments altered the initial leaf litter chemistry. In A. unedo, UV-B exposure decreased the leaf carbon/nitrogen (C/N) ratio (due to higher N) under natural rainfall, while in P. angustifolia , mainly UV-A radiation increased this ratio. Over the studied decomposition period (374 days), leaf litter mass loss in both species was limited, but P. angustifolia litter lost more mass (11%) than A. unedo litter (3%), likely due to a lower C/N ratio and a higher microbial activity. UV-A and UV-B radiation affected early litter decomposition in both species by altering recalcitrant compounds and/or microbial biomass, though effects on A. unedo depended on the rainfall amount. Overall, UV radiation appeared to inhibit rather than stimulate decomposition, thus not supporting a predominant role of UV radiation-induced photodegradation. Hence, future climate change may exacerbate present UV and drought effects on litter decomposition, slowing down decomposition rates and enhancing C accumulation in Mediterranean ecosystems. • Legacy UV radiation and/or drought effects on litter quality are species-specific • Drought reduced positive UV-B radiation effects on A. unedo initial litter quality • Microbial response was weak in both species, particularly in A. unedo • UV radiation, mainly UVA, or drier conditions, reduced litter mass loss in P angustifolia • No evidences of UV radiation-driven litter photodegradation in either species