Biogeosciences | 2019
Drought reduces tree growing season length but increases nitrogen resorption efficiency in a Mediterranean ecosystem
Abstract
Abstract. Mediterranean ecosystems are hotspots for climate change, as the\nhighest impacts are forecasted for the Mediterranean region, mainly by more\nfrequent and intense severe droughts. Plant phenology is a good indicator of\nspecies responses to climate change. In this study, we compared the spring\nphenology of cork oak trees (Quercus suber), an evergreen species,\nover 2 contrasting years, a mild year\xa0(2004) and a dry year\xa0(2005), which\nwas the most severe drought since records exist. We evaluated the timing of\noccurrence, duration, and intensity of bud development, budburst, shoot\nelongation, trunk growth, and leaf senescence (phenophases) and assessed the\nnitrogen resorption efficiency from senescent to green leaves. The\ntemperature was the main driver of budburst. Nevertheless, water had the main\nrole of constraining all the other phenophases by strongly reducing the growing\nseason length ( − 48\u2009%) and consequently the tree growth. Basal area\nincrement was the most affected growth variable ( − 36\u2009%), although\nit occurred at a similar rate in the 2 years. Shoot elongation was also\nreduced ( − 21\u2009%), yet elongation occurred at a higher rate in the dry\nyear compared to the mild year. Leaf senescence during the bulk period was higher\nin the dry year, in which leaves were shed at the same rate over a longer\nperiod. Nitrogen concentrations in green and senescent leaves were affected\nby drought and nitrogen resorption efficiency increased remarkably\n( + 22\u2009%). Our results highlight the importance of studying different\nphenological metrics to improve our understanding of the ecosystem s responses\nto climate change. The faster dynamics observed in shoot elongation, while\nall other phenophases developed at the same rate, indicate that leaf area\ndevelopment is privileged in cork oak. Water availability was the main driver\nof spring growth in this Mediterranean ecosystem; however, growth may be\naffected by complex interplays between precipitation and temperature, such as\nhigher temperatures during dry winters or heatwaves during spring, that are\nlikely to result in water stress. Longer studies are needed to disentangle\nthose interplays. Finally, a higher nitrogen resorption efficiency in\nresponse to drought appears to be an adaptive trait that mitigates the\nlimitation in nitrogen uptake by the roots during drought and contributes to\nimproving tree fitness in the short term but will probably exert a negative\nfeedback on the nitrogen cycle in the long term, which might affect the\necosystem functioning under the forecasted droughts.