Drought-tolerant tree species with high growth rates and a good capacity for carbon storage in woody tissues (dense wood) are searched for due to aridification. Deciduous, ring-porous tree and shrub ...species could show such drought tolerance and growth traits, thus representing good candidates for climate-smart rewilding. However, we still do not know the long-term growth rates of these species and how they respond to drought, particularly in climate change hotspots such as the Mediterranean Basin. We analysed these issues at the site and individual levels in two ring-porous, deciduous species (Pistacia terebinthus, Celtis australis) using dendroecology and wood anatomy. The ring width, earlywood vessel diameter, vessel density (VD) and area (%) were measured in two focal sites, one per species, and then growth data were compared with two secondary sites to test if site-to-site synchrony changed through time. Ring-width indices (RWI) and the hydraulic diameter (Dh) of earlywood were calculated. Growth rates (ring width), Dh and vessel area were higher in C. australis (1.03−2.26 mm, 269 μm, 33.9 %) than in P. terebinthus (0.57−0.72 mm, 146 μm, 21.5 %). Consequently, VD was higher in P. terebinthus than in C. australis (104 vs. 61 vessels mm−2). The ring width and Dh were more coupled in P. terebinthus (r = 0.43) than in C. australis (r = 0.32). RWI series of the focal and secondary sites have been synchronized since the 1990s as temperatures rose. Precipitation during the growing season (May, June) enhanced growth and VD of both species. P. terebinthus was more responsive to a drought index than C. australis. The two study species show high growth rates and tolerate drought being thus suitable candidates for climate-smart rewilding.
•Drought tolerance of two Mediterranean, ring-porous, deciduous species was assessed.•Celtis australis showed wider rings and earlywood vessels than Pistacia terebinthus.•P. terebinthus basal area increment increased, whereas it decreased in C. australis.•Spring precipitation enhanced growth and earlywood vessel density in both species.•P. terebinthus was more responsive to drought than C. australis.
Riparian forests from semi-arid regions with saline soils are dominated by facultative phreatophytes such as tamarisks (Tamarix spp.). Tamarisks tolerate drought and salt stress by using shallow and ...deep soil water sources depending on precipitation and flow variability in ephemeral or permanent streams. Therefore, their radial growth and wood anatomy would differently respond to precipitation and drought severity depending on stream ephemerality. To assess these responses, tree-ring-width, earlywood anatomy and sapwood nutrient concentrations were measured in five riparian tamarisk stands located in semi-arid north-eastern Spain. These sites experienced different aridity degrees and were located near ephemeral (e.g., Lanaja-dry site) or permanent streams in intensively irrigated agricultural areas (e.g., Valcuerna). Tree rings were widest in Peñaflor (2.94 mm) and narrowest in the driest Lanaja-dry site (1.11 mm), where tree-to-tree growth coherence was higher than in the other sites. Wet conditions during the water year enhanced growth, particularly in the most responsive Lanaja-dry site and in Lopín, whereas elevated flow increased the earlywood hydraulic diameter in Valcuerna. In the Lanaja-dry site, Ca, Cu, K and Mn wood concentrations were higher than in Valcuerna, but C and N concentrations were lower. Elevated Ca wood concentrations correspond to higher drought stress in the Lanaja-dry site, whereas higher N concentrations reflect eutrophication due to extensive use of agricultural fertilizers in Valcuerna. In sites near permanent streams such as Valcuerna, depth to groundwater may be a more robust proxy of growth than in drier sites located near more ephemeral streams such as Lanaja-dry site where tamarisk growth is constrained by 12-month long spring droughts.
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•Riparian tamarisk forests from a semi-arid region in NE Spain were studied.•Growth and earlywood vessel area responded to winter-spring precipitation and flow.•Growth responsiveness to drought increased near ephemeral streams.•Wood Ca and N increased near ephemeral and permanent streams, respectively.•Tamarisk tree-rings record information on drought severity and agricultural fertilization.
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•Mediterranean riparian forests are negatively impacted by drought.•Growth responses to climate and drought of four tree species were investigated.•Growth was reduced by drought and ...low spring-summer river flow.•Populus nigra and Fraxinus angustifolia were the most responsive species to drought.•Ash earlywood anatomy, and wood C and O isotopes also responded to drought.
Mediterranean riparian forests have been altered by past use and are also negatively impacted by climate and hydrological droughts. However, we lack data on their historical changes in extent combined with multi-proxy, long-term assessments of tree growth and leaf gas exchange responses to climate, drought severity and river flow. These evaluations must also consider their current stand structure and the amount of lying deadwood which are proxies of river dynamics and forest maturity. To fill these research gaps, we studied four riparian tree species (Populus alba, Populus nigra, Fraxinus angustifolia and Ulmus minor) inhabiting a Mediterranean riparian forest located in north-eastern Spain. We quantified and analyzed: stand structure; lying deadwood; radial growth; relationships between growth, climate variables, the SPEI drought index and river flow; stable C (δ13C) and O (δ18O) isotopes in wood of P. alba, P. nigra and F. angustifolia; and earlywood anatomy in F. angustifolia. Mature sites were dominated by P. nigra and F. angustifolia and showed the highest amount of decayed lying deadwood. Radial growth was reduced by drought and low spring-summer river flow. We found the highest growth responses to 3- (P. nigra, r = 0.62; P. alba, r = 0.46) or 12-month SPEI (F. angustifolia, r = 0.54; U. minor, r = 0.53). The coordinated decrease in δ18O and intrinsic water-use efficiency (WUE) in P. alba and P. nigra could involve an increase in stomatal conductance rate. P. alba and P. nigra were more enriched in δ18O than F. angustifolia, suggesting the former used more enriched shallow groundwater in dry periods. The F. angustifolia WUE and P. nigra δ18O series were positively and negatively correlated with the SPEI, respectively. The F. angustifolia hydraulic diameter decreased in response to drought, whereas its vessel density and WUE were positively associated. Overall, P. nigra and F. angustifolia were the species most responsive to drought.
Several dieback episodes triggered by droughts are revealing the high vulnerability of Mediterranean forests, manifested as declines in growth, increased defoliation, and rising mortality rates. ...Understanding forest responses to such climate extreme events is of high priority for predicting their future vegetation dynamics. We examined how remotely sensed measures of vegetation activity (NDVI, Normalized Difference Vegetation Index) and radial growth (BAI, basal area increment) responded to climate extreme events. We considered tree (Pinus sylvestris, Quercus pubescens, Quercus frainetto) and shrub (Juniperus phoenicea) populations from Italy and Spain showing recent dieback phenomena. Two components of drought, namely elevated atmospheric demand (VPD, vapor pressure deficit) and low soil moisture were analyzed in nearby stands showing or not showing dieback symptoms. Dieback stands exhibited lower NDVI values than non-dieback stands. NDVI and BAI were positively related in all sites except for the dieback stand of Q. frainetto that was negatively related. Such NDVI-BAI linkages were related to specific time windows, which could be useful for identifying when climatic conditions have the greatest influence on vegetation. Growth decline occurred in response to increasing VPD, but responses differed among species. J. phoenicea was the most negatively impacted by higher VPD, whereas oaks responded to soil moisture. A high VPD was related to stronger growth reduction in dieback P. sylvestris trees regardless of soil moisture changes. We highlighted that coupling between proxies of forest productivity (NDVI, BAI) allows better understanding and forecasting of drought-induced dieback phenomena in forests and shrublands. Scaling up from tree to stand levels might be feasible when using the maximum growing season NDVI, which can be applied for retrospective modeling of the impact of drought stress on forest productivity and tree growth.
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•NDVI and growth were positively related in all sites, except for the dieback site of Q. frainetto that was negatively related.•J. phoenicea and P. sylvestris growth showed a strong dependence on vapor pressure deficit.•Q. pubescens and Q. frainetto growth depended on soil moisture.•Coupling radial growth and NDVI at multiple Spatio-temporal scales improves dieback assessment.
Old-growth mountain forests represent an ideal setting for studying long-term impacts of climate change. We studied the few remnants of old-growth forests located within the Pollino massif (southern ...Italy) to evaluate how the growth of conspecific young and old trees responded to climate change. We investigated two conifer species (Abies alba and Pinus leucodermis) and two hardwood species (Fagus sylvatica and Quercus cerris). We sampled one stand per species along an altitudinal gradient, ranging from a drought-limited low-elevation hardwood forest to a cold-limited subalpine pine forest. We used a dendrochronological approach to characterize the long-term growth dynamics of old (age > 120 years) versus young (age < 120 years) trees. Younger trees grew faster than their older conspecifics during their juvenile stage, regardless of species. Linear mixed effect models were used to quantify recent growth trends (1950–2015) and responses to climate for old and young trees. Climate sensitivity, expressed as radial growth responses to climate during the last three decades, partially differed between species because high spring temperatures enhanced conifer growth, whereas F. sylvatica growth was negatively affected by warmer spring conditions. Furthermore, tree growth was negatively impacted by summer drought in all species. Climate sensitivity differed between young and old trees, with younger trees tending to be more sensitive in P. leucodermis and A. alba, whereas older F. sylvatica trees were more sensitive. In low-elevation Q. cerris stands, limitation of growth due to drought was not related to tree age, suggesting symmetric water competition. We found evidence for a fast-growth trend in young individuals compared with that in their older conspecifics. Notably, old trees tended to have relatively stable growth rates, showing remarkable resistance to climate warming. These responses to climate change should be recognized when forecasting the future dynamics of old-growth forests for their sustainable management.
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•We characterized radial growth dynamics in old-growth forests.•Climate-growth relations of young and old trees were assessed.•Younger trees grew faster than their older conspecifics.•Growth sensitivity to climate differed between young and old trees.•Old trees showed stable growth rates and were resilient to climate warming
Drought stress causes forest dieback that is often explained by two interrelated mechanisms, namely hydraulic failure and carbon starvation. However, it is still unclear which functional and ...structural alterations, related to these mechanisms, predispose to dieback. Here we apply a multi-proxy approach for the characterization of tree structure (radial growth, wood anatomy) and functioning (δ13C, δ18O and non-structural carbohydrates (NSCs)) in tree rings before and after drought-induced dieback. We aim to discriminate which is the main mechanism and to assess which variables can act as early-warning proxies of drought-triggered damage. The study was tailored in southern Italy in two forests (i.e., San Paolo (SP) and Oriolo (OR)) where declining and non-declining trees of a ring-porous tree species (Quercus frainetto Ten.) showing anisohydric behavior coexist. Both stands showed growth decline in response to warm and dry spring conditions, although the onset of dieback was shifted between them (2002 in SP and 2009 in OR). Declining trees displayed a sharp growth drop after this onset with reductions of 49% and 44% at SP and OR sites, respectively. Further, contrary to what we expected, declining trees showed a lower intrinsic water-use efficiency compared with non-declining trees after the dieback onset (with reductions of 9.7% and 5.6% at sites SP and OR, respectively), due to enhanced water loss through transpiration, as indicated by the lower δ18O values. This was more noticeable at the most drought-affected SP stand. Sapwood NSCs did not differ between declining and non-declining trees, indicating no carbon starvation in affected trees. Thus, the characterized structural and functional alterations partially support the hydraulic failure mechanism of dieback. Finally, we show that growth data are reliable early-warning proxies of drought-triggered dieback.
•Crown damage is a good predictor of mortality after a Diplodia outbreak.•Diplodia sapinea can kill both vigorous and weak trees.•Pinus sylvestris is more likely to die than P. nigra, P. pinea or P. ...halepensis.
Diplodia shoot blight is an emergent forest disease in Europe caused by Diplodia sapinea. The short-term impacts of the pathogen on tree physiology are well known, but its capacity to cause mortality has been poorly documented.
We compared the survival of four pine species affected by Diplodia shoot blight following a hailstorm: Pinus sylvestris, P. nigra, P. pinea and P. halepensis. In the case of P. sylvestris, survival in the hail-affected sites was compared with survival in other sites affected by Diplodia shoot blight with no hailstorm records. Mortality and crown condition were recorded over two years. Dendrochronological analyses were conducted to assess growth responses to drought and to test the influence of radial growth before the outbreak of Diplodia shoot blight on survival. The endophytic community, as well as the abundance of D. sapinea, was quantified by metabarcoding and qPCR respectively, and were correlated with crown damage.
Crown damage was the best predictor of mortality across species. Pinus sylvestris experienced a much higher rate of mortality than P. nigra, P. pinea and P. halepensis. Two years after the outbreak, P. halepensis was the only species that could recover crown condition. Mortality was found to be unrelated to radial growth prior the outbreak. Drought responses did not correlate with mortality differences across pine species or sites. In the case of P. sylvestris, mortality was initially higher amongst diseased trees in areas affected by hailstorms than in sites not affected by hailstorms, however it tended to equalise after two years. Amongst P. sylvestris trees, crown damage correlated with a higher abundance of the pathogen. Signs of competition amongst endophytes were observed between non-defoliated and defoliated Scots pine trees following hailstorms.
Our study shows that D. sapinea can cause a significant mortality to P. sylvestris. The legacy effects of crown damage can last for at least two years after an outbreak. During this time, crown damage is a good predictor of survival, and can help managers decide which trees remove after an outbreak.
Anticipating future impacts of climate warming and aridification on drylands requires understanding how coexisting woody plant species respond to climate variability. However, we lack knowledge of ...the growth resilience capacity of Mediterranean shrubs. Do coexisting trees and shrubs differ in their response to climate? Do coexisting shrub species have comparable post‐drought growth resilience?
This study was conducted in two Mediterranean shrublands with sparse trees in semi‐arid north‐eastern Spain. We selected sites situated in formerly agricultural or grassland areas in two regions subjected to semi‐arid Mediterranean climate conditions. We sampled six shrubs' species (Juniperus phoenicea, Juniperus oxycedrus, Pistacia lentiscus, Pistacia terebinthus, Rhamnus lycioides and Rhamnus alaternus) and one tree species (Pinus halepensis) to measure their radial growth and to reconstruct their past growth patterns using dendrochronology. We quantified climate–growth relationships of trees and shrubs, as well as the growth resilience capacity after drought events of the six shrubs.
Growth patterns differed between species but a prevalent trade‐off between growth and longevity was found. Growth responses to climate were comparable between species but differed between sites. Most species responded positively to precipitation and negatively to temperature. The most negative correlations of growth with drought severity were found at the driest Valcuerna site. Shrubs differed in their resilience capacity. Across sites, species were more resistant and resilient in Alcubierre, the wettest site. The shrubs P. terebinthus and R. lycioides were more resistant and resilient than the rest of species, which required longer growth recovery times and accumulated stronger growth reductions, particularly J. phoenicea.
Synthesis. These results highlight the importance of drought as a driver of growth in Mediterranean scrublands. Growth resilience strategies differed between species with junipers being less resilient to drought than other coexisting shrubs. Further research should investigate how this difference in post‐drought resilience is related to functional traits, particularly those related with plant water‐use strategies.
Resumen
Anticiparse a los futuros impactos del calentamiento climático y la aridificación sobre los ecosistemas arbustivos de zonas secas exige comprender cómo responden las plantas leñosas que coexisten a la variabilidad climática. Sin embargo, desconocemos la capacidad de resiliencia del crecimiento radial en arbustos mediterráneos. ¿Difieren los árboles y arbustos coexistentes en su respuesta al clima? ¿Tienen las especies de arbustos que coexisten una capacidad de recuperación del crecimiento tras la sequía comparable?
Este estudio se llevó a cabo en dos matorrales mediterráneos con árboles dispersos en la zona semiárida del nordeste de España. Seleccionamos lugares situados en antiguas zonas agrícolas o de pastizales en dos regiones sometidas a condiciones climáticas mediterráneas semiáridas. Se tomaron muestras de madera de seis especies arbustivas (Juniperus phoenicea, Juniperus oxycedrus, Pistacia lentiscus, Pistacia terebinthus, Rhamnus lycioides y Rhamnus alaternus) y una arbórea (Pinus halepensis) para medir su crecimiento radial y reconstruir sus patrones de crecimiento mediante dendrocronología. Cuantificamos las relaciones clima‐crecimiento de árboles y arbustos, así como la capacidad de recuperación del crecimiento tras eventos de sequía de los seis arbustos.
Los patrones de crecimiento difirieron entre especies. Se encontró una relación negativa entre tasas de crecimiento y longevidad. Las respuestas del crecimiento al clima fueron similares entre especies, pero difirieron entre sitios. La mayoría de las especies respondieron positivamente a las precipitaciones y negativamente a la temperatura. Las correlaciones más negativas del crecimiento con la severidad de la sequía se encontraron en el lugar más seco (Valcuerna). Los arbustos difirieron en su capacidad de resistencia. Los arbustos fueron más resistentes y resilientes en el sitio más húmedo (Alcubierre). Los arbustos P. terebinthus y R. lycioides fueron más resistentes y resilientes que el resto de especies, que requirieron mayores tiempos de recuperación del crecimiento y acumularon mayores reducciones de crecimiento, particularmente J. phoenicea.
Síntesis. Nuestros resultados ponen de manifiesto la importancia de la sequía como determinante del crecimiento en los matorrales mediterráneos. Las estrategias de resistencia al crecimiento difieren entre especies, siendo las especies de Juniperus menos resistentes a la sequía que otros arbustos coexistentes. En futuras investigaciones se deberá estudiar cómo esta diferencia en la resiliencia tras la sequía está relacionada con los rasgos funcionales de las plantas, en particular los relacionados con las estrategias de uso del agua.
These results highlight the importance of drought as a driver of growth in Mediterranean scrublands. Growth resilience strategies differed between species with junipers being less resilient to drought than other coexisting shrubs. Further research should investigate how this difference in post‐drought resilience is related to functional traits, particularly those related with plant water‐use strategies.
Cork is one of the main non-timber forest products in the world. Most of its production is concentrated in the Iberian Peninsula, a climate change hotspot. Climate warming may lead to increased ...aridification and reduce cork production in that region. However, we still lack assessments of climate-cork relationships across ample geographical and climatic gradients explicitly considering site aridity. We quantified cork growth by measuring cork ring width and related it to climate variables and a drought index using dendrochronology. Four cork oak (Quercus suber) forests located from north eastern Spain to south western Morocco (31.5–41.5° N) and subjected to different aridity levels were sampled. Warm conditions in spring to early summer, when cork is formed, reduced cork width, whereas high precipitation in winter and spring enhanced it. The response of cork to increased water availability in summer peaked (r = 0.89, p = 0.00002) in the most arid and continental site considering 14-month long droughts. A severe drought caused a disproportionate loss of cork production in this site, where for every five-fold decrease in the drought index, the cork-width index declined by a factor of thirteen. Therefore, site aridity determines the responses of cork growth to the soil water availability resulting from accumulated precipitation during winter and spring previous to cork growth and until summer. In general, this cumulative water balance, which is very dependent on temperature and evapotranspiration rate, is critical for cork production, especially in continental, dry sites. The precipitation during the hydrological year can be used as a proxy of cork production in similar sites. Assessments of climate-cork relationships in the western Mediterranean basin could be used as analogues to forecast the impacts of aridification on future cork production.
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•Climate warming is expected to advance leaf unfolding and enhance tree growth.•We compared long-term series of leaf phenology and estimated wood formation.•Long-term leaf and wood formation were not ...coupled.•A longer growing season does not necessarily leads to larger radial growth.
Climate warming is expected to lengthen the growing season of tree species and enhance radial growth rates. Alternatively, a longer growing season could not lead to improved radial growth if wood production depends more on growth rate than on growing season length. We test these ideas by comparing leaf phenology data and the estimated start and end dates of wood formation predicted by the VS-Lite growth model. We analyzed long-term series of leaf unfolding and fall dates and reconstructed radial growth of two pine species under contrasting climatic conditions: Scots pine (Pinus sylvestris) in a Russian boreal site and Aleppo pine (Pinus halepensis) in a Spanish Mediterranean site. On average, leaf onset occurred in days 99 and 163 in P. halepensis and P. sylvestris, respectively, about 40 days earlier than the estimated start date of wood formation. The onset of leaf unfolding advanced 2.1 days per decade in P. sylvestris in response to warmer May temperatures. Radial growth was enhanced by warm-wet spring-summer conditions in P. sylvestris and by wet soil conditions from prior winter up to current summer in P. halepensis. In this species the growing season length and the radial growth rate were not coupled because the growing season length shortened during cool-wet periods whereas growth rates increased. In P. sylvestris leaf onset was delayed during years with low growth rates suggesting a potential coupling between warmer spring conditions, earlier leaf onset and enhanced growth whenever soil water content is high enough. Overall, we show that longer growing seasons do not necessarily imply higher radial growth rates.
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