Dampening effects of long‐term experimental drought on growth and mortality rates of a Holm oak forest

Abstract

Forests respond to increasing intensities and frequencies of drought by reducing growth and with higher tree mortality rates. Little is known, however, about the long-term consequences of generally drier conditions and more frequent extreme droughts. A Holm oak forest was exposed to experimental rainfall manipulation for 13 years to study the effect of increasing drought on growth and mortality of the dominant species Quercus ilex, Phillyrea latifolia, and Arbutus unedo. The drought treatment reduced stem growth of A. unedo (-66.5%) and Q. ilex (-17.5%), whereas P. latifolia remained unaffected. Higher stem mortality rates were noticeable in Q. ilex (+42.3%), but not in the other two species. Stem growth was a function of the drought index of early spring in the three species. Stem mortality rates depended on the drought index of winter and spring for Q. ilex and in spring and summer for P. latifolia, but showed no relation to climate in A. unedo. Following a long and intense drought (2005-2006), stem growth of Q. ilex and P. latifolia increased, whereas it decreased in A. unedo. Q. ilex also enhanced its survival after this period. Furthermore, the effect of drought treatment on stem growth in Q. ilex and A. unedo was attenuated as the study progressed. These results highlight the different vulnerabilities of Mediterranean species to more frequent and intense droughts, which may lead to partial species substitution and changes in forest structure and thus in carbon uptake. The response to drought, however, changed over time. Decreased intra- and interspecific competition after extreme events with high mortality, together with probable morphological and physiological acclimation to drought during the study period, may, at least in the short term, buffer forests against drier conditions. The long-term effects of drought consequently deserve more attention, because the ecosystemic responses are unlikely to be stable over time.Nontechnical summaryIn this study, we evaluate the effect of long-term (13 years) experimental drought on growth and mortality rates of three forest Mediterranean species, and their response to the different intensities and durations of natural drought. We provide evidence for species-specific responses to drought, what may eventually lead to a partial community shift favoring the more drought-resistant species. However, we also report a dampening of the treatment effect on the two drought-sensitive species, which may indicate a potential adaptation to drier conditions at the ecosystem or population level. These results are thus relevant to account for the stabilizing processes that would alter the initial response of ecosystem to drought through changes in plant physiology, morphology, and demography compensation.

The Response of Soil CO2 Efflux to Water Limitation Is Not Merely a Climatic Issue: The Role of Substrate Availability

“…In ecosystems facing severe spells of drought, plant assimilation is strictly dependent on water availability [42][43][44][45], whose duration controls the length of the growing season [46][47][48]. In this study, photosynthesis showed a strong seasonal variation, as confirmed by the significant correlation of Anet Table A3. SIR and total El-FAME were both significantly mostly correlated with SR h , with the exception of carbohydrates and amino acids, and fungi and protozoa respectively (Table 4).…”

Decline in holm oak coppices (Quercus ilex L. subsp. ballota (Desf.) Samp.): biometric and physiological interpretations

“…However, fewer studies have attempted to identify the underlying functional problem in "weak" individuals. Most of the studies dealing with the decline of holm oak at individual tree level have focused mainly on characterizing the effects or symptoms of decay in the aerial part of the stools, both at leaf and xylem level (Tognetti et al, 1998;Camarero et al, 2004;Corcuera et al, 2004), sometimes through water exclusion experiments (Limousin et al, 2009;Barbeta et al, 2013;Pérez-Ramos et al, 2013). However, because holm oak is a species with a strong resprouting capacity and as most of the stools have been managed in a coppice system over a long period of time, it would seem reasonable to assume that the cause of decay of some stools may be at least partially explained by factors relating to the root system .…”

The effects of thinning intensity and tree size on the growth response to annual climate in Cedrus atlantica: a linear mixed modeling approach

“…Indeed, 60 to 80 % of the French metropolitan territory may experience a Mediterranean type climate by the end of the twenty-first century (Roman-Amat 2007). The acclimation of existing forest stands to increasing drought relies on different physiological adjustments, among which leaf area reduction seems of particular importance over the long term (Martin-StPaul et al 2013), and may be related to changes in stem density (Barbeta et al 2013). Thinning operation, which modulates stem density and stand vertical structure, can thus accelerate this stand acclimation and has been reported to improve tree resistance to drought stress (Misson et al 2003) and post-drought resilience (Martín-Benito et al 2008;Sohn et al 2013).…”