Multiyear impacts of partial throughfall exclusion on Buxus sempervirens in a Mediterranean forest

Abstract

Aim of study: We examined the impact of sustained partial throughfall exclusion on the functional performance of Buxus sempervirens L. in the understory of a Mediterranean evergreen forest. We further considered whether any impacts of throughfall exclusion were affected by light availability.Area of study: The study was conducted in the south of France.Material and methods: Several leaf physiological and branch structural traits were measured along a light gradient after seven years from the onset of a throughfall exclusion experiment (TEE). The results were analysed along with annual growth and survival data.Main results: Plant mortality was nil in both the throughfall exclusion and control treatments. Stem diameter growth was reduced by 39% in plants subjected to throughfall exclusion, but this difference was only significant at the p = 0.10 significance level. Leaf physiology remained unaffected by the TEE, but small changes were evident in branch structural traits in high light microsites following throughfall exclusion; branches had lower wood density in the TEE plot, and more biomass was partitioned to leaves relative to stems.Research highlights: These changes do not seem to reflect an acclimatory response that would enhance drought tolerance. Instead, we suggest that these drought effects might exacerbate vulnerability to xylem cavitation in the more open microsites. Reduced growth and increased vulnerability to drought may indicate an incipient decline in plant vitality following TEE. The extension of observations to the whole-plant level and longer periods will elucidate the consequences of these observations for plant fitness, and permit verification of the positive effect of shade on Buxus sempervirens under increased drought.Key words: common box; drought susceptibility; shade; rainfall manipulation; abiotic stress; long-term study.