To die or not to die: early warnings of tree dieback in response to a severe drought

Abstract

Summary1. Some disturbances can drive ecological systems to abrupt shifts between alternative stages (tipping points) when critical transitions occur. Drought-induced tree death can be considered as a nonlinear shift in tree vigour and growth. However, at what point do trees become predisposed to drought-related dieback and which factors determine this (tipping) point? We investigated these questions by characterizing the responses of three tree species, silver fir (Abies alba), Scots pine (Pinus sylvestris) and Aleppo pine (Pinus halepensis), to a severe drought event. 2. We compared basal area increment (BAI) trends and responses to climate and drought in declining (very defoliated and dying) vs. non-declining (slightly or not defoliated) trees by using generalized additive mixed models. Defoliation, BAI and sapwood production were related to functional proxies of tree vigour measured at the onset and end of the drought (non-structural carbohydrate concentrations, needle N content and C isotopic discrimination, presence of wood-inhabiting fungi). We evaluated whether early warning signals (increases in synchronicity among trees or in autocorrelation and standard deviation) could be extracted from the BAI series prior to tree death. 3. Declining silver fir and Scots pine trees showed less growth than non-declining trees one to three decades, respectively, before the drought event, whereas Aleppo pines showed growth decline irrespective of tree defoliation. At the end of the drought period, all species showed increased defoliation and a related reduction in the concentration of sapwood soluble sugars. Defoliation was constrained by the BAI of the previous 5 years and sapwood production. No specific wood-inhabiting fungi were found in post-drought declining trees apart from blue-stain fungi, which extensively affected damaged Scots pines. Declining silver firs showed increases in BAI autocorrelation and variability prior to tree death. 4. Synthesis. Early warning signals of drought-triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for Scots and Aleppo pines. Longer growth and defoliation series, additional vigour parameters and multi-species comparisons are required to understand and predict drought-induced tree death.

Prior height, growth, and wood anatomy differently predispose to drought-induced dieback in two Mediterranean oak speciesk

“…This allows comparing growth and wood anatomy between trees showing different sensitivity to drought and contrasting defoliation degrees. Here, we follow this approach by analyzing the responses of two Mediterranean oak species (holm oak, Portuguese oak) to the exceptionally warm and dry 2012 year which caused widespread forest dieback in eastern Spain (Camarero et al 2015). The holm oak (Quercus ilex subsp.…”

“…The SPEI varies from negative to positive values corresponding to dry and wet periods, respectively. The SPEI monthly values were calculated for the study site considering the period 1960-2013 and 1-18-month-long scales following Camarero et al (2015). The SPEI accounts for the negative effect of warmer temperatures on water availability by statistically modeling cumulative water balances.…”

Drought Influence over Radial Growth of Mexican Conifers Inhabiting Mesic and Xeric Sites

“…Droughts reduce radial growth and alter the hydraulic conductivity of trees [6][7][8], with a negative impact on carbon sequestration [9,10] and forest productivity [11,12]. Moreover, given that aridity is expected to intensify in the future in many forested areas, this may have negative and long-term implications on forest productivity [7,13].…”

“…These effects of drought stress on growth and productivity may differ between forest type and tree species [7]. Some studies have argued that the radial growth of a tree species growing under xeric habitats is more affected by drought than those from mesic habitats [14].…”

Major Changes in Growth Rate and Growth Variability of Beech (Fagus sylvatica L.) Related to Soil Alteration and Climate Change in Belgium

“…Dying trees usually have lower growth rates, higher growth variability and an increased response to climate [66]. As the dieback and mortality of individual beech trees is still scarce and scattered in Belgium, we assume that beech has coped with global change up to now, but the question is: for how long?…”