Tree-ring δ13C and δ18O, leaf δ13C and wood and leaf N status demonstrate tree growth strategies and predict susceptibility to disturbance

Comparative Foliar Atmospheric Mercury Accumulation across Functional Types in Temperate Trees

Vegetation assimilation of atmospheric gaseous elemental mercury (GEM) represents the largest dry deposition pathway in global terrestrial ecosystems. This study investigated Hg accumulation mechanisms in deciduous broadleaves and evergreen needles, focusing on how ecophysiological strategies─reflected by δ13C, δ18O, leaf mass per area, and leaf dry matter content-mediated Hg accumulation. Results showed that deciduous leaves exhibited higher total Hg (THg) concentrations and accumulation rates (THgrate), which were 85.3 ± 17.7 and 110.0 ± 0.3% higher than those in evergreen needles. The two tree types exhibited distinct ecophysiological strategies: deciduous broadleaves, with higher stomatal conductance and photosynthetic rates, rapidly adjust stomata to changes in meteorological and pollutant factors, playing a key role in controlling THgrate. In contrast, evergreen needles featured stable stomatal control, highlighting the direct positive effect of GEM on their THgrate. Precipitation and wind speed negatively influenced foliar THgrate. Correlations between PM2.5, NO2, and THgrate in evergreen needles suggested synergistic patterns between atmospheric Hg and pollutants. This study underscores distinct GEM accumulation mechanisms across tree functional types and emphasizes the importance of species-specific foliar ecophysiological strategies. An empirical model linking THgrate with ecophysiological, meteorological, and atmospheric pollution factors was provided, contributing to the refinement of foliar Hg accumulation models.

Mistletoe-induced carbon, water and nutrient imbalances are imprinted on tree rings

Mistletoes are xylem-tapping hemiparasites that rely on their hosts for water and nutrient uptake. Thus, they impair tree performance in the face of environmental stress via altering the carbon and water relations and nutritional status of trees. To improve our understanding of physiological responses to mistletoe and ongoing climate change, we investigated radial growth, stable carbon and oxygen isotopic signals, and elemental composition of tree rings in silver fir (Abies alba Mill.) and Scots pine (Pinus sylvestris L.) forests infested with Viscum album L. We compared temporal series (1990-2020) of basal area increment (BAI), intrinsic water-use efficiency (iWUE), oxygen isotope composition (δ18O), nutrient concentrations and stoichiometric ratios between non-infested (NI) and severely infested (SI) fir and pine trees from populations located close to the xeric distribution limit of the species in north-eastern Spain. The SI trees showed historically higher growth, but the BAI trend was negative for more than three decades before 2020 and their growth rates became significantly lower than those of NI trees by the mid-2010s. Mistletoe infestation was related to an enhanced sensitivity of radial growth to vapour pressure deficit (atmospheric drought). The SI trees showed less pronounced iWUE increases (fir) and lower iWUE values (pine) than NI trees. The lower tree-ring δ18O values of SI trees may be the result of several superimposed effects operating simultaneously, including leaf-level evaporative enrichment, source water isotopic signals, and anatomical and phenological differences. We observed a deterioration of potassium (K) nutrition in tree-ring wood of both species in SI trees, along with accumulation of manganese (Mn). We suggest that such nutritional patterns are driven by the indirect effect of mistletoe-induced drought stress, particularly in pine. The combined analyses of different physiological indicators imprinted on tree rings provided evidence of the progressive onset of carbon, water and nutrient imbalances in mistletoe-infested conifers inhabiting seasonally dry regions.

Nutritional changes in trees during drought-induced mortality: A comprehensive meta-analysis and a field study

Both macronutrients and micronutrients are essential for tree growth and development through participating in various ecophysiological processes. However, the impact of the nutritional status of trees on their ability to withstand drought-induced mortality remains inconclusive. We thus conducted a comprehensive meta-analysis, compiling data on 11 essential nutrients from 44 publications (493 independent observations). Additionally, a field study was conducted on Pinus sylvestris L. trees with varying drought-induced vitality loss in the "Visp" forest in southern Switzerland. No consistent decline in tree nutritional status was observed during tree mortality. The meta-analysis revealed significantly lower leaf potassium (K), iron (Fe), and copper (Cu) concentrations with tree mortality. However, the field study showed no causal relationships between nutritional levels and the vitality status of trees. This discrepancy is mainly attributed to the intrinsic differences in the two types of experimental designs and the ontogenetic stages of target trees. Nutrient reductions preceding tree mortality were predominantly observed in non-field conditions, where the study was conducted on seedlings and saplings with underdeveloped root systems. It limits the nutrient uptake capacity of these young trees during drought. Furthermore, tree nutritional responses are also influenced by many variables. Specifically, (a) leaf nutrients are more susceptible to drought stress than other organs; (b) reduced tree nutrient concentrations are more prevalent in evergreen species during drought-induced mortality; (c) of all biomes, Mediterranean forests are most vulnerable to drought-induced nutrient deficiencies; (d) soil types affect the direction and extent of tree nutritional responses. We identified factors that influence the relationship between tree nutritional status and drought survival, and proposed potential early-warning indicators of impending tree mortality, for example, decreased K concentrations with declining vitality. These findings contribute to our understanding of tree responses to drought and provide practical implications for forest management strategies in the context of global change.

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.

Effects of Water Deficits on Prosopis tamarugo Growth, Water Status and Stomata Functioning

The effect of water deficit on growth, water status and stomatal functioning of Prosopis tamarugo was investigated under controlled water conditions. The study was done at the Antumapu Experiment Station of the University of Chile. Three levels of water stress were tested: (i) well-watered (WW), (ii) medium stress intensity (low-watered (LW)) and (iii) intense stress (non-watered (NW)), with 10 replicates each level. All growth parameters evaluated, such as twig growth, specific leaf area and apical dominance index, were significantly decreased under water deficit. Tamarugo twig growth decreased along with twig water potential. The stomatal conductance and CO₂ assimilation decreased significantly under the water deficit condition. Tamarugo maintained a high stomatal conductance at low leaf water potential. In addition, tamarugo reduced its leaf area as a strategy to diminish the water demand. These results suggest that, despite a significant decrease in water status, tamarugo can maintain its growth at low leaf water potential and can tolerate intense water deficit due to a partial stomatal closing strategy that allows the sustaining of CO₂ assimilation in the condition of reduced water availability.

Relationships between Leaf Anatomy and Physiological Functioning of Southern US Oak Species Differing in Flood Tolerance

Research Highlights: Bottomland oaks receive less attention than upland species, however their adaptations to flooding and summer water stress will extend our understanding of the oak genus and links between physiology and leaf anatomy. Background and objectives: Determining links between leaf anatomy and physiology can aid in parameterizing dynamic global vegetation models for oak systems, therefore we sought to (1) compare leaf anatomic, nutrient, and physiological parameters for bottomland oaks differing in flood tolerance, (2) determine correlations across parameters and determine which anatomic and nutrient parameters best predict photosynthetic capacity metrics, and (3) compare these data with reported literature values for oaks across the globe. Materials and Methods: We measured CO2 response curves (A/Ci) on leaves from Nuttall, Shumard, swamp chestnut, water and white oak seedlings planted in the Southeastern United States (US) and estimated stomatal size and density, epidermal cell size, vein density, leaf mass per area (LMA) and nitrogen (N) concentrations. Principal component analysis among these leaf anatomic and nutrient parameters was used to determine the best predictors of photosynthetic parameters including Rubisco-limited carboxylation rate (VCmax) and electron transport limited carboxylation rate (Jmax). Results: We found that although physiological parameters were similar, flood-tolerant oaks had lower leaf N concentrations and larger, more infrequent stomata than less flood-tolerant species. Leaf epidermal properties were correlated with N concentrations and a principal component capturing this correlation as well as principal components correlated with mesophyll conductance and leaf carbon concentrations were found to best explain variation in VCmax and Jmax. These Southeastern US oaks exhibited similar leaf physiological parameters and LMA as oaks reported in the literature but differed in leaf epidermal and stomatal properties as well as leaf N concentrations increasing the reported range of these parameters within the oak genus. Conclusions: Therefore, leaf anatomy and nutrient parameters as opposed to physiology differed across flood tolerance and between bottomland oaks and broader literature values.

A multi-proxy assessment of dieback causes in a Mediterranean oak species

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.