Nitrogen and phosphorus resorption efficiencies change under drought and shrub encroachment in a Mediterranean ecosystem

Mediterranean ecosystems, such as the savannah-type cork oak (Quercus suber) woodlands, are hotspots for climate change, as the highest impacts are forecasted for the Mediterranean region, mainly by more frequent and intense severe droughts. These ecosystems are also threatened by shrub encroachment, which might further decrease tree water availability and affect ecosystem functioning and resilience. Nevertheless, the combined effects of drought and shrub encroachment on ecosystems have seldom been investigated. A precipitation manipulation and shrub removal experiment was established in a cork oak woodland located in SE Portugal and invaded by the native shrub gum rockrose (Cistus ladanifer). Here we present and discuss the combined effects of drought and shrub encroachment on litterfall production of cork oak trees, an evergreen species, over two contrasting years, a wet year (2018) and a dry year (2019) and assess the nitrogen and phosphorus resorption efficiencies from senescent to green leaves.

A previous study reported significant increases in cork oak’s nitrogen resorption efficiency in response to drought. Our preliminary results also indicate changes in nitrogen and phosphorus resorption efficiencies. An increase in nutrient resorption efficiency is likely to mitigate the limitation in nutrient uptake by the roots during drought, improving tree fitness in the short-term. However, it will probably exert a negative feedback on the nitrogen and phosphorus cycles in the long-term which might affect the ecosystem functioning under the forecasted droughts.