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https://doi.org/10.5194/bg-2018-393
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Research article 11 Sep 2018

Research article | 11 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Drought reduces tree growing season length but increases nitrogen resorption efficiency in a Mediterranean ecosystem

Raquel Lobo-do-Vale1, Cathy K. Besson2, Maria C. Caldeira1, Maria M. Chaves3, and João S. Pereira1 Raquel Lobo-do-Vale et al.
  • 1Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal
  • 2Instituto Dom Luiz, Centro de Geofísica da Universidade de Lisboa, Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
  • 3Laboratório de Ecologia Molecular, ITQBNOVA, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal

Abstract. Mediterranean ecosystems are hotspots for climate change, as the highest impacts are forecasted for the Mediterranean region, mainly by more frequent and intense severe droughts. Plant phenology is a good indicator of species' responses to climate change. In this study, we compared phenology of cork oak trees (Quercus suber), an evergreen species, over two contrasting years, including the most severe drought (2005) since records exist. We evaluated not only the timing of occurrence of the vegetative phenophases in spring (bud development, budburst, shoot elongation, trunk growth and leaf senescence), but also their duration and intensity. We also quantified nitrogen in green and senescent leaves and assessed the nitrogen resorption efficiency. Temperature was the main driver for budburst. Nevertheless, water had a main role constraining all the other phenophases, by strongly reducing the growing season length (−48%) and consequently tree growth. Basal area increment was the most affected growth variable (−38%), although the rate of increase remained similar among years. Shoot elongation was reduced by −21% yet elongation occurred at a higher rate in the dry as compared to the mild year. Leaf senescence during the bulk period was higher in the dry year, in which leaves were shed at the same rate over a longer period. Nitrogen concentration in green and senescent leaves were affected by drought and cork oak remarkably increased the nitrogen resorption efficiency (+22%), which appears to be an adaptive trait that mitigates the limitation in nitrogen uptake by the roots during drought. Water availability was the main driver of the growing season length in this Mediterranean ecosystem, although it may also be affected by complex interplays between precipitation and temperature. Our results highlight the importance of studying different phenological metrics to improve our understanding of the ecosystems responses to climate change. The faster dynamics observed in shoot elongation, in contrast with the other phenophases, are indicative that cork oak privileges leaf area development, while all other phenophases develop at the same rate. Finally, a higher nitrogen resorption efficiency in response to drought may clearly improve tree fitness in the short-term, but will probably exert a negative feedback on the nitrogen cycle in the long-term which might affect the ecosystem functioning under the forecasted droughts.

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By comparing the cork oak tree vegetative phenology in two contrasting precipitation years, in a Mediterranean ecosystem, we showed the critical role of water availability in extending the length of the growing season and determining tree growth. The observed higher transfer of nitrogen from senescent to green leaves in response to drought might compensate the limited nitrogen uptake by the roots. Our results contribute to improve our understanding of the ecosystems responses to climate change.
By comparing the cork oak tree vegetative phenology in two contrasting precipitation years, in a...
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