Individual differences in the access to deep soil water pools may explain the differential damage among coexisting, conspecific trees as a consequence of drought-induced dieback. We addressed this ...issue by comparing the responses to a severe drought of three Mediterranean oak species with different drought tolerance, Quercus pubescens L. and Quercus frainetto Ten., mainly thriving at xeric and mesic sites, respectively, and Quercus cerris L., which dominates at intermediate sites. For each species, we compared coexisting declining (D) and non-declining (ND) trees. The stable isotope composition (δ2H, δ18O) of xylem and soil water was used to infer a differential use of soil water sources. We also measured tree size and radial growth to quantify the long-term divergence of wood production between D and ND trees and non-structural carbohydrates (NSCs) in sapwood to evaluate if D trees presented lower NSC values. The ND trees had access to deeper soil water than D trees except in Q. frainetto, as indicated by significantly more depleted xylem water values. However, a strong δ2H offset between soil and xylem water isotopes observed in peak summer could suggest that both tree types were not physiologically active under extreme drought conditions. Alternative processes causing deuterium fractionation, however, could not be ruled out. Tree height and recent (last 15-25 years) growth rates in all species studied were lower in D than in ND trees by 22 and 44%, respectively. Lastly, there was not a consistent pattern of NSC sapwood concentration; in Q. pubescens, it was higher in ND trees while in Q. frainetto, the D trees were the ones exhibiting the higher NSC concentration. We conclude that the vulnerability to drought among conspecific Mediterranean oaks depends on the differential access to deep soil water pools, which may be related to differences in rooting depth, tree size and growth rate.
Forest dieback because of drought is a global phenomenon threatening particular tree populations. Particularly vulnerable stands are usually located in climatically stressing locations such as xeric ...sites subjected to seasonal drought. These tree populations show a pronounced loss of vitality, growth decline, and high mortality in response to extreme climate events such as heat waves and droughts. However, dieback events do not uniformly affect stands, with some trees showing higher symptoms of drought vulnerability than other neighboring conspecifics. In this study, we investigated if trees showing different vulnerabilities to dieback showed lower growth rates (Grs) and higher sensitivities to the climate in the past using dendroecology and the Vaganov-Shashkin (VS) process-based growth model. We studied two
Pinus pinaster
stands with contrasting Grs showing recent dieback in the Iberian System, north-eastern Spain. We compared coexisting declining (D) and non-declining (ND) trees with crown defoliation values above and below the 50% threshold, respectively. The mean growth rate was lower in D than in ND trees in the two stands. The two vigor classes showed a growth divergence prior to the dieback onset and different responsiveness to climate. The ND trees were more responsive to changes in spring water balance and soil moisture than D trees, indicating a loss of growth responsiveness to the climate in stressed trees. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. The presented comparisons indicated different stand vulnerabilities to drought contingent on-site conditions. Further research should investigate the role played by environmental conditions and individual features such as access to soil water or hydraulic traits and implement them in process-based growth models to better forecast dieback.
Drought constrains tree growth in regions with seasonal water deficit where growth decline can lead to tree death. This has been observed in regions such as the western Mediterranean Basin, which is ...a climate-warming hotspot. However, we lack information on intra- and inter-specific comparisons of growth rates and responses to water shortage in these hotspots, considering tree species with different drought tolerance. We sampled several sites located in north-eastern Spain showing dieback and high mortality rates of three pine species (Pinus sylvestris, Pinus pinaster, Pinus halepensis). We dated death years and reconstructed the basal area increment of coexisting living and recently dead trees using tree ring data. Then, we calculated bootstrapped Pearson correlations between a drought index and growth. Finally, we used linear mixed-effects models to determine differences in growth trends and the response to drought of living and dead trees. Mortality in P. sylvestris and P. pinaster peaked in response to the 2012 and 2017 droughts, respectively, and in sites located near the species’ xeric distribution limits. In P. halepensis, tree deaths occurred most years. Dead trees showed lower growth rates than living trees in five out of six sites. There was a strong growth drop after the 1980s when climate shifted towards warmer and drier conditions. Tree growth responded positively to wet climate conditions, particularly in the case of living trees. Accordingly, growth divergence between living and dead trees during dry periods reflected cumulative drought impacts on trees. If aridification continues, tree drought mortality would increase, particularly in xeric distribution limits of tree species.
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 (
) and more tolerant to drought (
). 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 (d
C) were analyzed in wood of the two vigor classes to compare their water-use efficiency.
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
, 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.
showed higher growth sensitivity to precipitation, temperature and drought than
. 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
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,
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
in the studied area warrants further study as a potential predisposing factor.
Seasonal patterns of wood formation (xylogenesis) remain understudied in mixed pine–oak forests despite their contribution to tree coexistence through temporal niche complementarity. Xylogenesis was ...assessed in three pine species (
Pinus cembroides
,
Pinus leiophylla
,
Pinus engelmannii
) and one oak (
Quercus grisea
) coexisting in a semi-arid Mexican forest. The main xylogenesis phases (production of cambium cells, radial enlargement, cell-wall thickening and maturation) were related to climate data considering 5–15-day temporal windows. In pines, cambium activity maximized from mid-March to April as temperature and evaporation increased, whereas cell radial enlargement peaked from April to May and was constrained by high evaporation and low precipitation. Cell-wall thickening peaked from June to July and in August–September as maximum temperature and vapour pressure deficit (VPD) increased. Maturation of earlywood and latewood tracheids occurred in May–June and June–July, enhanced by high minimum temperatures and VPD in
P. engelmannii
and
P. leiophylla
. In oak, cambial onset started in March, constrained by high minimum temperatures, and vessel radial enlargement and radial increment maximized in April as temperatures and evaporation increased, whereas earlywood vessels matured from May to June as VPD increased. Overall, 15-day wet conditions enhanced cell radial enlargement in
P
.
leiophylla
and
P. engelmannii
, whereas early-summer high 15-day temperature and VPD drove cell-wall thickening in
P. cembroides
. Warm night conditions and high evaporation rates during spring and summer enhanced growth. An earlier growth peak in oak and a higher responsiveness to spring–summer water demand in pines contributed to their coexistence.
Climate extremes such as cold spells are becoming more frequent as climate variability increases. However, few studies have evaluated the impacts of winter cold spells on forest cover, tree growth ...and leaf and sapwood non-structural carbohydrate (NSC) concentrations. We analyzed changes in tree cover using remote sensing data and compared the radial growth of coexisting and defoliated Pinus halepensis trees and non-defoliated P. halepensis and Pinus pinaster trees. We also compared NSC concentrations in leaves and sapwood of defoliated and non-defoliated P. halepensis and Quercus ilex trees. In January 2021, a rapid drop in temperatures led to minimum values (−21.3 °C) in eastern Spain and triggered canopy defoliation in several planted (P. halepensis) and native (Q. ilex) tree species. The cold spell led to a decrease in forest cover in the most defoliated stands and reduced radial growth of defoliated P. halepensis and sapwood NSC concentrations in P. halepensis and Q. ilex, particularly starch. Prior to the cold spell, defoliated P. halepensis trees significantly (p < 0.05) grew more (2.73 ± 1.70 mm) in response to wetter winter conditions than non-defoliated P. halepensis (2.29 ± 1.08 mm) and P. pinaster (1.39 mm) trees. Those P. halepensis individuals which grew faster at a young age were less resilient to the winter cold spell in later years. The study stands showed a high recovery capacity after the cold spell, but the Mediterranean drought-avoiding P. halepensis was the most affected species.
We still lack information on the long-term growth responses to climate of relict tree populations, which often persist in topoclimatic refugia. To fill that research gap, we studied three relict cork ...oak (Quercus suber) populations located in northern Spain using dendrochronology. The sites were subjected to humid (Zarautz), continental (Bozoó) and xeric (Sestrica) climate conditions. Cool–wet conditions during the current spring enhanced growth in Bozoó and Sestrica, whereas wet conditions in the previous October enhanced growth in Zarautz. In this site, growth also increased in response to dry conditions in the prior winter linked to high North Atlantic Oscillation indices. Correlations between the precipitation summed from the previous September to the current May peaked at the driest site (Sestrica). The strongest growth responses to drought severity were also found at this site, where growth negatively responded to 9-month early-summer droughts, followed by the continental Bozoó site, where growth was constrained by 1-month July droughts. Growth declined in response to 6-month January droughts in the wettest site (Zarautz), where cork oak was vulnerable to previous late-summer to autumn drought stress. Despite warmer and drier spring conditions that would negatively impact cork oak at the Bozoó and Sestrica sites, trees from these populations could tolerate further aridity.
Recent drought-induced dieback alters forest dynamics, which are also shaped by past management. In western Pyrenean silver fir (Abies alba) stands, dieback concurs in space and time with the ...legacies of past management, but the impacts on forest growth, structure and composition are unknown. We aim to disentangle how dieback interacts with the legacies of past human use and modulates the recent dynamics of silver fir forests. To this end, we sampled eleven silver fir forests across wide climatic gradients and included declining and non-declining sites. We measured radial growth, structure, composition, understory cover and type and amount of deadwood. Silver fir growth declines in response to late-summer drought. In declining sites, most defoliated stands showed the lowest silver fir density and were those where growth depended more on water availability. Tree death enhanced the cover of dominant understory plants such as Buxus sempervirens. Past management activities leave an imprint in the growth of silver fir, such as releases due to past logging, but also affect the number of stumps and snags and the current tree density. A more extensive monitoring will be required to fully disentangle the multiple influences of past management legacies and current climate change on forest dynamics.
Assessing and reconstructing the impacts of defoliation caused by insect herbivores on tree growth, carbon budget and water use, and differentiating these impacts from other stresses and disturbances ...such as droughts requires multi-proxy approaches. Here we present a methodological framework to pinpoint the impacts of pine processionary moth (
Thaumetopoea pityocampa
), a major winter-feeding defoliator, on tree cover (remote-sensing indices), radial growth and wood features (anatomy, density, lignin/carbohydrate ratio of cell walls, δ
13
C and δ
18
O of wood cellulose) of drought-prone pine (
Pinus nigra
) forests in north-eastern Spain. We compared host defoliated (D) and coexisting non-defoliated (ND) pines along with non-host oaks (
Quercus faginea
) following a strong insect outbreak occurring in 2016 at two climatically contrasting sites (cool-wet Huesca and warm-dry Teruel). Changes in tree-ring width and wood density were analyzed and their responses to climate variables (including a drought index) were compared between D and ND trees. The Normalized Difference Infrared Index showed reductions due to the outbreak of –47.3% and –55.6% in Huesca and Teruel, respectively. The D pines showed: a strong drop in growth (–96.3% on average), a reduction in tracheid lumen diameter (–35.0%) and lower lignin/carbohydrate ratios of tracheid cell-walls. Both pines and oaks showed synchronous growth reductions during dry years. In the wet Huesca site, lower wood δ
13
C values and a stronger coupling between δ
13
C and δ
18
O were observed in D as compared with ND pines. In the dry Teruel site, the minimum wood density of ND pines responded more negatively to spring drought than that of D pines. We argue that multi-proxy assessments that combine several variables have the potential to improve our ability to pinpoint and reconstruct insect outbreaks using tree-ring data.
Background and Objectives—Coexisting tree and shrub species will have to withstand more arid conditions as temperatures keep rising in the Mediterranean Basin. However, we still lack reliable ...assessments on how climate and drought affect the radial growth of tree and shrub species at intra- and interannual time scales under semi-arid Mediterranean conditions. Materials and Methods—We investigated the growth responses to climate of four co-occurring gymnosperms inhabiting semi-arid Mediterranean sites in northeastern Spain: two tree species (Aleppo pine, Pinus halepensis Mill.; Spanish juniper, Juniperus thurifera L.) and two shrubs (Phoenicean juniper, Juniperus phoenicea L.; Ephedra nebrodensis Tineo ex Guss.). First, we quantified the intra-annual radial-growth rates of the four species by periodically sampling wood samples during one growing season. Second, we quantified the climate–growth relationships at an interannual scale at two sites with different soil water availability by using dendrochronology. Third, we simulated growth responses to temperature and soil moisture using the forward, process-based Vaganov‒Shashkin (VS-Lite) growth model to disentangle the main climatic drivers of growth. Results—The growth of all species peaked in spring to early summer (May–June). The pine and junipers grew after the dry summer, i.e., they showed a bimodal growth pattern. Prior wet winter conditions leading to high soil moisture before cambium reactivation in spring enhanced the growth of P. halepensis at dry sites, whereas the growth of both junipers and Ephedra depended more on high spring–summer soil moisture. The VS-Lite model identified these different influences of soil moisture on growth in tree and shrub species. Conclusions—Our approach (i) revealed contrasting growth dynamics of co-existing tree and shrub species under semi-arid Mediterranean conditions and (ii) provided novel insights on different responses as a function of growth habits in similar drought-prone regions.
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