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 the spring
phenology of cork oak trees (Quercus suber), an evergreen species,
over 2 contrasting years, a mild year (2004) and a dry year (2005), which
was the most severe drought since records exist. We evaluated the timing of
occurrence, duration, and intensity of bud development, budburst, shoot
elongation, trunk growth, and leaf senescence (phenophases) and assessed the
nitrogen resorption efficiency from senescent to green leaves. The
temperature was the main driver of budburst. Nevertheless, water had the main
role of constraining all the other phenophases by strongly reducing the growing
season length (−48 %) and consequently the tree growth. Basal area
increment was the most affected growth variable (−36 %), although
it occurred at a similar rate in the 2 years. Shoot elongation was also
reduced (−21 %), yet elongation occurred at a higher rate in the dry
year 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 concentrations in green and senescent leaves were affected
by drought and nitrogen resorption efficiency increased remarkably
(+22 %). Our results highlight the importance of studying different
phenological metrics to improve our understanding of the ecosystem's responses
to climate change. The faster dynamics observed in shoot elongation, while
all other phenophases developed at the same rate, indicate that leaf area
development is privileged in cork oak. Water availability was the main driver
of spring growth in this Mediterranean ecosystem; however, growth may be
affected by complex interplays between precipitation and temperature, such as
higher temperatures during dry winters or heatwaves during spring, that are
likely to result in water stress. Longer studies are needed to disentangle
those interplays. Finally, a higher nitrogen resorption efficiency in
response to drought appears to be an adaptive trait that mitigates the
limitation in nitrogen uptake by the roots during drought and contributes to
improving 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.
Supplementary material to "Drought reduces tree growing season length but increases nitrogen resorption efficiency in a Mediterranean ecosystem"
