Climate and Soil Microsite Conditions Determine Local Adaptation in Declining Silver Fir Forests.

Ongoing climatic change is threatening the survival of drought-sensitive tree species, such as silver fir (Abies alba). Drought-induced dieback had been previously explored in this conifer, although the role played by tree-level genetic diversity and its relationship with growth patterns and soil microsite conditions remained elusive. We used double digest restriction-site-associated DNA sequencing (ddRADseq) to describe different genetic characteristics of five silver fir forests in the Spanish Pyrenees, including declining and non-declining trees. Single nucleotide polymorphisms (SNPs) were used to investigate the relationships between genetics, dieback, intraspecific trait variation (functional dendrophenotypic traits and leaf traits), local bioclimatic conditions, and rhizosphere soil properties. While there were no noticeable genetic differences between declining and non-declining trees, genome-environment associations with selection signatures were abundant, suggesting a strong influence of climate, soil physicochemical properties, and soil microbial diversity on local adaptation. These results provide novel insights into how genetics and diverse environmental factors are interrelated and highlight the need to incorporate genetic data into silver fir forest dieback studies to gain a better understanding of local adaptation.

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

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ18 O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower δ18 O than ND trees. The uncoupling between iWUE and δ18 O, together with the positive correlations between P and K leaf concentrations and δ18 O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies.

Las poblaciones del límite xérico de distribución de las especies de árboles son particularmente vulnerables al decaimiento forestal inducido por sequía. Este es el caso de los bosques de abeto (Abies alba) situados en el suroeste de Europa. Si bien los patrones de decaimiento provocado por sequía del abeto se han explorado previamente, falta información sobre el papel que desempeña el deterioro nutricional a pesar de sus interacciones potenciales con los balances de agua y carbono de los árboles. En este estudio, hemos realizado un análisis comparativo del crecimiento radial, la eficiencia intrínseca del uso del agua (iWUE), los isótopos de oxígeno (δ18O) y las concentraciones de nutrientes en hojas de árboles decaídos (DD) y no decaídos (ND) en cuatro abetares de los Pirineos españoles. También evaluamos las relaciones entre la predisposición al decaimiento, la variación de rasgos intraespecíficos (densidad de la madera y rasgos de las hojas) y las propiedades físico-químicas de la rizosfera. El inicio de la disminución del crecimiento en los árboles DD ocurrió hace más de dos décadas y posteriormente mostraron una baja resiliencia de crecimiento frente a las sequías. Los árboles DD presentaron concentraciones foliares consistentemente más bajas de nutrientes como P, K, Cu y Ni que los árboles ND. Los fuertes efectos del estado de los nutrientes foliares en los índices de resiliencia del crecimiento respaldan el papel clave que desempeña la nutrición mineral en el funcionamiento y el crecimiento de los árboles antes, durante y después de la sequía. En contraste, la variabilidad en la densidad de la madera y los rasgos morfológicos de las hojas, así como las propiedades del suelo, mostraron una relación débil con el estado nutricional de los árboles y la respuesta del crecimiento a la sequía. En los sitios más cálidos y de baja elevación, los árboles DD mostraron relaciones clima-crecimiento más fuertes y un δ18 O más bajo que los árboles ND. El desacoplamiento entre iWUE y δ18 O, junto con las correlaciones positivas entre las concentraciones foliares de P y K y δ18 O, apuntan a fuentes de agua más profundas del suelo/lecho rocoso y un desacoplamiento vertical entre la absorción de nutrientes y agua en los árboles DD. Este estudio proporciona información novedosa sobre los mecanismos que impulsan el decaimiento del abeto y destaca la necesidad de incorporar la nutrición de los árboles en los estudios de muerte regresiva del bosque.

Modeling Climate Impacts on Tree Growth to Assess Tree Vulnerability to Drought During Forest Dieback.

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.

ENSO and NAO affect long-term leaf litter dynamics and stoichiometry of Scots pine and European beech mixedwoods.

Litterfall dynamics (production, seasonality and nutrient composition) are key factors influencing nutrient cycling. Leaf litter characteristics are modified by species composition, site conditions and water availability. However, significant evidence on how large-scale, global circulation patterns affect ecophysiological processes at tree and ecosystem level remains scarce due to the difficulty in separating the combined influence of different factors on local climate and tree phenology. To fill this gap, we studied links between leaf litter dynamics with climate and other forest processes, such as tree-ring width (TRW) and intrinsic water-use efficiency (iWUE) in two mixtures of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) in the south-western Pyrenees. Temporal series (18 years) of litterfall production and elemental chemical composition were decomposed following the ensemble empirical mode decomposition method and relationships with local climate, large-scale climatic indices, TRW and Scots pine's iWUE were assessed. Temporal trends in N:P ratios indicated increasing P limitation of soil microbes, thus affecting nutrient availability, as the ecological succession from a pine-dominated to a beech-dominated forest took place. A significant influence of large-scale patterns on tree-level ecophysiology was explained through the impact of the North Atlantic Oscillation (NAO) and El Niño-Southern Oscillation (ENSO) on water availability. Positive NAO and negative ENSO were related to dry conditions and, consequently, to early needle shedding and increased N:P ratio of both species. Autumn storm activity appears to be related to premature leaf abscission of European beech. Significant cascading effects from large-scale patterns on local weather influenced pine TRW and iWUE. These variables also responded to leaf stoichiometry fallen 3 years prior to tree-ring formation. Our results provide evidence of the cascading effect that variability in global climate circulation patterns can have on ecophysiological processes and stand dynamics in mixed forests.