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

Abstract

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 m2 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

Cl-R-H was the recipient of a PhD grant from the Senescyt-UdG co-financed grant program (Contract Nº AR2Q-000421–2016). This work was funded by the Spanish Government “Ministerio de Ciencia Innovación y Universidades – MCIN” (Ref. CGL2014–55976-R, Ref. PID2021–123735OB16C2/MCIN/AEI/10.13039/501100011033/FEDER, UE; and Ref. PCI2020–120702–2/AEI/ 10.13039 /17 501.100.011.033) and the University of Girona (Ref. ASING2011/3 and Ref. MPCUdG2016). SOLIPLANT have been recognised as consolidated research group by the Generalitat de Catalunya (2017SGR0055 and 2021SGR0047). AMR acknowledges the funding from the AGAUR-ICREA Academia award (Catalan Institution for Research and Advanced Studies, ref. 2024 ICREA 00144). Open Access funding provided thanks to the CRUE-CSIC agreement with Elsevier

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