Forecasting Forest Vulnerability to Drought in Pyrenean Silver Fir Forests Showing Dieback

Abstract

Forest dieback is manifested as widespread loss of tree vigor, growth decline and high mortality rates. Forest dieback is becoming increasingly frequent and extended, particularly in drought-prone regions. This is the case of the southwestern Spanish Pyrenees, where keystone species such as Silver fir reach their xeric and southern distribution limits. While dieback of this species has been widely documented in this area, we still lack methodologies to forecast the vulnerability of these forests in response to increasing drought stress so as to anticipate their potential dieback in the future. Here we study multiple features of Silver fir forests and trees to evaluate whether previous growth rates and their growth trends are valid predictors of forest dieback. Further, we validate our methodology revisiting two Silver fir sites sampled two decades ago. The defoliation degree was strongly related with radial growth, and growth trends differed between moderately to highly defoliated trees and non-defoliated trees. Forests showing dieback, i.e., those in which 25% of the sampled trees showed defoliation > 50%, were located at low elevation and received less rainfall in summer than forests showing no dieback. Trees showing high defoliation presented lower growth rates than nondefoliated trees. Moreover, we ratified that defoliation has increased considerably over the last two decades in one of the two revisited sites, but we were unable to accurately forecast growth trends in both sites, particularly in the site not showing dieback. The retrospective assessment of growth rates and trends offers valuable information on the vulnerability of Silver fir trees to drought. However, we are still far from being able to forecast the vulnerability of Silver fir forests to increasing drought. A systematic monitoring of growth across a wide tree-ring network of sites might provide valuable information to advance in this direction.

Silver Fir Decline in Pure and Mixed Stands at Western Edge of Spread in Croatian Dinarides Depends on Some Stand Structure and Climate Factors

“…Crown defoliation at both study sites showed a strong negative correlation with increment, which supports the findings of Erdle and MacLean [42], Bigler et al [45], and Gazol et al [67], who found differences in growth trends between trees with higher and lower degrees of defoliation.…”

Post-glacial re-colonization and natural selection have shaped growth responses of silver fir across Europe

“…Against the popular conviction that provenances from the southern part of species distribution are potentially more able to withstand forthcoming climate conditions, we observed a reduced growth performance of the individuals from the Pyrenees, which are likely linked to their poor genetic pool due to isolation processes rather than natural selection processes. As a consequence, numerous episodes of tree mortality and forest decline have been observed and projected in the Pyrenees region (Camarero et al, 2015;Gazol et al, 2015Gazol et al, , 2020Sánchez-Salguero et al, 2017). In contrast, assisted gene flow using provenances from the Balkan lineage might be considered as they showed the best longterm growth performance.…”

The role of nutritional impairment in carbon‐water balance of silver fir drought‐induced dieback

“…Silver fir is a keystone montane and subalpine conifer widely distributed across Europe that reaches its southern and xeric distribution limit in the Spanish Pyrenees (Caudullo et al, 2017). Given that the tree growth sensitivity to climate is expected to increase under climate change scenarios at these rear‐edge populations (Gazol et al, 2020; Sánchez‐Salguero et al, 2017), advancing the understanding of drought‐induced dieback of silver fir populations near its rear edge is fundamental for understanding how it will respond to further warming and drying climate trends in other European regions. In this study, we aim at (i) analyzing the long‐ and short‐term growth trends of declining and non‐declining silver fir trees showing contrasting vigor (estimated as different crown defoliation); (ii) characterizing their foliar carbon and oxygen isotope ratios and nutrient composition as a time‐integrated proxy of water and carbon balance and nutritional status (iii) and relating growth, isotopic and elemental composition data to each other and to intraspecific trait variability and soil properties in order to outline mechanistic linkages among them.…”