Drought-induced oak decline in the western mediterranean region: An overview on current evidences, mechanisms and management options to improve forest resilience

Increased forest vulnerability is being reflected as more widespread and severe drought-induced decline episodes. In particular, the Mediterranean area is revealing a high susceptibility to phenomena of loss in tree vitality across species. Within tree species, oaks (Quercus spp.) are experiencing extensive decline in many countries. However, in the wake of the so-called âoak decline phenomenonâ, the attention on these species has generally been limited. In this paper, we review the current available literature on oakdecline cases reported within the Mediterranean Basin, with particular remark for those occurred in Italy and Spain. More specifically our main aims were to: (i) provide an update on the patterns and mechanisms of decline by focusing on tree-ring and wood-anatomical variables; (ii) provide some hints for improving the resistance and resilience of oak stands experiencing decline. Our review reveals that drought is reported as the main driver triggering oak decline within the Mediterranean Basin, although other causes (i.e., increasing temperature, pathogens attack or excessive stand density) could exacerbate decline. In most reported cases, drought induced a substantial reduction of growth and changes in some wood anatomical properties. Indeed, growth decline prior death is also indicated as an early-warning signal of impending death. In ring-porous oak species, declining trees were often characterized by a very low production of latewood and a decrease in lumen area of the widest earlywood vessels, suggesting a potential reduction of hydraulic conductivity. Moreover, hydraulic dysfunction is reported as the main cause of decline. Finally, regarding management actions that should be considered for improving the resilience of declining stands and preserve the species-specific stand composition, it could be useful to shorten the rotation period of coppice stands or promoting their gradual conversion towards high forests, and favoring more drought-resistant species should also be considered. In addition, regeneration prior to regeneration cuts should be improved by anticipating seed dispersal or by planting oak seedlings obtained from local germoplasm

Evaluating growth and intrinsic water-use efficiency in hardwood and conifer mixed plantations

Abstract Key message Juglans, Fraxinus, Quercus and Pinus species seem to better maximize the carbon–water ratio providing useful indications on species selection for forestry plantations in areas with increasing drought risk. Abstract Maximizing carbon sequestration for a given water budget is extremely important in the contest of climate change in the Mediterranean region, which is characterized by increasing temperatures and rising water stress. This issue is fundamental for plantation stands, where limited water availability during the growing season reduces CO2 assimilation and, consequently, tree growth. In this study, the main objective was to investigate the performances in terms of carbon–water balance of conifer (Pinus halepensis and Cupressus sempervirens) and hardwood (Quercus robur, Juglans regia, Fraxinus excelsior and Populus spp.) mixed plantations. To this aim, we used carbon isotope signatures to evaluate the intrinsic water-use efficiency (iWUE) and the species-specific relationship between basal area increments (BAI) and iWUE. At the species level, the highest iWUE values corresponded to the lowest carbon accumulation in terms of BAI, for water-saving species such as Cupressus. Conversely, Populus had the lowest iWUE and the highest BAI accumulation. Juglans, Fraxinus, and Pinus showed the most balanced ratio between BAI and iWUE. Overall, no clear correlation of iWUE and BAI was evident within all species, except for Populus and Cupressus. Considering projected aridification and increased temperatures that will negatively impact the growth, our data suggest that Pinus, for conifers, and Quercus, Juglans, Fraxinus for hardwood species should be preferred when choosing species for forestry plantation, as they performed better in terms of BAI and iWUE ratio

Drought and Phytophthora Are Associated With the Decline of Oak Species in Southern Italy

Forest decline induced by climate change is a global phenomenon that affects many tree species, mainly in drought-prone areas as the Mediterranean region. In southern Italy, several oak species have shown decline symptoms and elevated mortality since the 2000s due to drought stress. However, it remains to be answered whether decline occurred alone or whether a pathogen was also involved. To this aim, we compared two coexisting oak species in a forest located in southern Italy which are assumed to be less (Quercus cerris) and more tolerant to drought (Quercus pubescens). We sampled fifteen couples of neighboring declining (D) and non-declining (ND) trees of both species. Wood cores were taken from all trees to perform dendrochronological analyses to detect the decline onset and link it to potential climatic drivers. Carbon isotope ratios (d13C) were analyzed in wood of the two vigor classes to compare their water-use efficiency. Phytophthora presence was also assessed in soil samples from ten D-ND couples of trees per species. The oak species most affected by drought-induced decline in terms of leaf shedding and mortality was Q. cerris, i.e., the least tolerant to drought. In both species, the D trees showed a reduced growth rate compared with ND trees from 2000 onward when drought and warming intensified. Q. pubescens showed higher growth sensitivity to precipitation, temperature and drought than Q. cerris. This sensitivity to climate was magnified in D trees whose growth decreased in response to warm and dry conditions during the prior winter and the late summer. The Q. pubescens D trees were more efficient in their water use than ND trees before the growth divergence between D and ND trees amplified. In the studied area, Phytophthora quercina was isolated from 40% of the sampled trees, and tended to be more frequent amongst ND than amongst D trees. Our data suggests that droughts and warm summer conditions triggered oak decline. The high prevalence of P. quercina in the studied area warrants further study as a potential predisposing factor

Rinnovazione dell’abete bianco in boschi della regione mediterranea: un modello logico.

Questa nota riporta una sintesi dei principali fattori che influenzano la rinnovazione naturale dell’abete bianco (Abies alba Mill.), con particolare attenzione a microclima luminoso, disponibilità idrica, competizione e rapporti interspecifici. Le possibili interazioni fra questi fattori, in relazione ai processi di rinnovazione naturale delle abetine di abete bianco della regione oro-mediterranea, sono quindi rappresentate in un semplice modello logico

Size Matters a Lot: Drought-Affected Italian Oaks Are Smaller and Show Lower Growth Prior to Tree Death

Hydraulic theory suggests that tall trees are at greater risk of drought-triggered death caused by hydraulic failure than small trees. In addition the drop in growth, observed in several tree species prior to death, is often interpreted as an early-warning signal of impending death. We test these hypotheses by comparing size, growth, and wood-anatomy patterns of living and now-dead trees in two Italian oak forests showing recent mortality episodes. The mortality probability of trees is modeled as a function of recent growth and tree size. Drift-diffusion-jump (DDJ) metrics are used to detect early-warning signals. We found that the tallest trees of the anisohydric Italian oak better survived drought contrary to what was predicted by the theory. Dead trees were characterized by a lower height and radial-growth trend than living trees in both study sites. The growth reduction of now-dead trees started about 10 years prior to their death and after two severe spring droughts during the early 2000s. This critical transition in growth was detected by DDJ metrics in the most affected site. Dead trees were also more sensitive to drought stress in this site indicating different susceptibility to water shortage between trees. Dead trees did not form earlywood vessels with smaller lumen diameter than surviving trees but tended to form wider latewood vessels with a higher percentage of vessel area. Since living and dead trees showed similar competition we did not expect that moderate thinning and a reduction in tree density would increase the short-term survival probability of trees.This research was financially supported by the project “Alarm of forest mortality in Southern Italy” (Gorgoglione Administration, Basilicata Region, Italy) and by the CGL2015-69186-C2-1-R project (Spanish Ministry of Economy).Peer Reviewe

Long-term temporal relationships between environmental conditions and xylem functional traits: A meta-analysis across a range of woody species along climatic and nitrogen deposition gradients

The objectives of this study were to provide a quantitative description of the long-term effects of environmental variability on xylem functional traits, in order to better assess xylem capacity to change in response to climate change. Twenty-six sites throughout the world, primarily in Europe, were chosen where results from long-term measurements of anatomical traits were previously published. Published data on long-term xylem anatomy (conduit size and density) and ring width variability were compiled across a range of tree species, which was subsequently related to variability in temperature, precipitation and nitrogen deposition rates across the study sites using generalized additive models and Bayesian methods. We found some appreciable relationships between xylem traits (conduit area Ac and conduit density Dc) and environmental variables; whereas combined trait indices (lumen fraction: Ac × Dc and vessel composition: Ac/Dc) were found to be rather constant across a wide range of environmental conditions and to be decoupled from tree growth rates. Overall, results suggested xylem traits coordinated towards a homeostasis in xylem function, which appeared to act across a wide range of environmental conditions. Results showed also nitrogen deposition was associated with xylem traits and vessel composition: Increased nitrogen availability due to nitrogen deposition might facilitate construction of a xylem structure efficient for water transport, and concurrently provide capacity to withstand the risks of drought-induced embolism

Oak forest decline in Southern Italy: the study case of Gorgoglione forest

Il declino delle foreste indotto dai cambiamenti climatici è un fenomeno che interessa diverse specie in Italia, tra cui anche quelle del genere Quercus. Recentemente è iniziato in Basilicata uno studio su un soprassuolo misto di Q. cerris e Q. pubescens che presenta evidenti sintomi di deperimento. Dopo aver censito le piante secondo una classificazione sintomatologica basata su scala visuale, sono state selezionate 15 coppie di piante sane e deperienti, da cui sono state prelevate carote legnose e campioni fogliari. L’obiettivo è stato delineare, tramite indagine dendrocronologica, dendro-anatomica e isotopica, il profilo delle piante in deperimento identificando il periodo in cui si è verificato il declino, gli effetti sui parametri dendro-anatomici strettamente legati alla vulnerabilità xilematica e la risposta ecofisiologica. I pattern di accrescimento evidenziano una riduzione del tasso di crescita nelle piante deperienti rispetto a quelle sane, a partire dal 2000, coinciso con il susseguirsi di una serie di annate particolarmente siccitose (meno di 400 mm di pioggia annuale). In tale periodo si evidenzia anche, nelle piante deperienti, un forte calo di produzione del legno tardivo ed una riduzione nella produzione di vasi di conduzione nel legno primaverile che, con molta probabilità, ne ha compromesso la funzionalità idraulica predisponendole al declino. Contrariamente a quanto atteso, le analisi isotopiche del carbonio e dell’azoto a livello fogliare, non evidenziano particolari differenze nella risposta ecofisiologica tra piante sane e deperienti. In conclusione, i primi risultati di questo studio confermerebbero l’ipotesi che gravi episodi di siccità contribuiscono ad innescare fenomeni di deperimento nel genere Quercus spp., che possono manifestarsi anche dopo alcuni anni, accentuati da attacchi di insetti e patogeni di debolezza

Apical dominance ratio as an indicator of the growth conditions favouring Abies alba natural regeneration under Mediterranean environment

As reported in several studies, the presence of Abies alba Mill (silver fir) has been declining throughout its natural range over a large part of mountainous belt in the Mediterranean area. In such a context, regeneration establishment depends mainly on the occurrence of a suitable combination of water and light availability. Thus, before planning any forest management practice, it is essential to investigate on the optimal microclimate conditions influencing the success of natural regeneration of silver fir. To this aim, changes in growth and photosynthesis together with C, N and O isotope compositions have been investigated on silver fir naturally recruited saplings, growing in mixed stands with Fagus sylvatica on Apennine mountains (southern Italy). The apical dominance ratio (ADR, ratio between apical shoot length and length of first whorl lateral twigs) has been used as an indicator for microclimate conditions in which saplings grow. Based on the range of ADR values (i.e. from 0.10 to 1.30), saplings were distributed in four classes. As expected, increases in height, root collar diameter and radial growth correspond to enhancing ADR values, gaining the optimal conditions in class IV. This latter also displayed the best performance in terms of maximum CO2 assimilation at saturating light (Amax) and water-use efficiency as assessed by carbon isotope discrimination analysis. Conversely, class I and II seem to display the highest performance in terms of CO2 respiration rate (Rd) and absolute water loss saving as assessed by the application of oxygen isotopes. We conclude that, in relatively mild Mediterranean areas, forest managers should promote silvicultural treatments favouring light conditions and migration of saplings towards class IV of ADR. This class represents the optimal microclimate for regeneration establishment of silver fir