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Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes., (© 2022. The Author(s).)

Background and Aims The structure of a forest stand, i.e. the distribution of tree size features, has strong effects on its functioning. The management of the structure is therefore an important tool in mitigating the impact of predicted changes in climate on forests, especially with respect to drought. Here, a new functional–structural model is presented and is used to assess the effects of management on forest functioning at a national scale. Methods The stand process-based model (PBM) Castanea was coupled to a stand structure module (SSM) based on empirical tree-to-tree competition rules. The calibration of the SSM was based on a thorough analysis of intersite and interannual variability of competition asymmetry. The coupled Castanea–SSM model was evaluated across France using forest inventory data, and used to compare the effect of contrasted silvicultural practices on simulated stand carbon fluxes and growth. Key Results The asymmetry of competition varied consistently with stand productivity at both spatial and temporal scales. The modelling of the competition rules enabled efficient prediction of changes in stand structure within the Castanea PBM. The coupled model predicted an increase in net primary productivity (NPP) with management intensity, resulting in higher growth. This positive effect of management was found to vary at a national scale across France: the highest increases in NPP were attained in forests facing moderate to high water stress; however, the absolute effect of management on simulated stand growth remained moderate to low because stand thinning involved changes in carbon allocation at the tree scale. Conclusions This modelling approach helps to identify the areas where management efforts should be concentrated in order to mitigate near-future drought impact on national forest productivity. Around a quarter of the French temperate oak and beech forests are currently in zones of high vulnerability, where management could thus mitigate the influence of climate change on forest yield [ABSTRACT FROM AUTHOR]

Abstract: In addition to global warming, the frequency and the intensity of droughts will probably increase in central and southern Europe. Resulting climate changes and soil water deficits could alter tree growth, according to sensitivity of each species. The aim of this study was to compare the growth response of three European species (Fagus sylvatica, Quercus petraea and Pinus sylvestris) to climatic variations and soil water deficits in the same temperate forest. Three mature stands per species were sampled to obtain earlywood, latewood and total ring chronologies from 1960 to 2007. Climate–growth relationships were established by bootstrapped correlations and response function analysis. Monthly bioclimatic regressors were simulated by a physiological water balance model that used daily climatic data and stand parameters to estimate soil water deficits. Our results highlighted a common sensitivity to precipitation from May to July for the dominant tree growth of the three species but also differences in the species vulnerability to climate and soil water deficits. Beech was the most sensitive species to the climatic conditions of the current growing season. Beech growth was positively correlated with precipitation from May to July and negatively with maximal temperatures in June and July. Oak growth was negatively correlated with minimal temperatures in the previous August and September and positively with precipitation in the previous October and December during pointer years. This led to long-term consequences for growth, probably due to carbon reserve depletion. Pine growth was positively influenced by warm December but was also vulnerable to maximal temperatures and soil water deficits from June to August. The climate in August only influenced the pine growth probably because the growing season of pine was longer than that of the deciduous species. For both oak and pine, latewood was the component that was most sensitive to climatic variations and soil water deficits. According to the study findings, an increase in the frequency and the intensity of droughts could affect the three species. Maximum summer temperatures could have negative impacts for beech and pine growth. Dry and warm autumns could lead to long-term consequences and decrease the oak growth. [Copyright &y& Elsevier]

Abstract: Consequences of climate warming on tree phenology are readily observable, but little is known about the differences in phenological sensitivity to temperature between species and between populations within a species. The aim of the present study is to compare phenological sensitivities to temperature of seven woody species between each other and within-species between two geographical areas using both altitudinal and temporal gradients (Abies alba, Acer pseudoplatanus, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Ilex aquifolium and Quercus petraea). The timing of leaf unfolding was monitored (i) over 2 years along two altitudinal gradients in the Pyrénées mountains (six species), and (ii) over 22 years in Fontainebleau forest (four species). Three species were present in both areas which allowed us to compare their phenological sensitivity to temperature over altitudinal and temporal gradients. Along altitudinal gradients, we observed for all species an advance in leaf unfolding with decreasing elevation, ranging from 11 to 34 days 1000m−1 for beech and oak, respectively. Across the temporal gradient, we found significant advances in leaf unfolding for oak (−0.42daysyear−1) and ash (−0.78daysyear−1) since 1976, whereas no significant advance was observed for beech and hornbeam. For both gradients and for all species, significant correlations were found between leaf unfolding dates and temperature, except for beech in the temporal study. Moreover, we highlighted that phenological sensitivity to temperature was very similar between the two geographically separated populations (Pyrénées and Fontainebleau forests). Thus, oak had the strongest sensitivity (−7.48 and −7.26days°C−1 in altitudinal and temporal gradient, respectively) and beech had the lowest (−2.09 and −2.03days°C−1). Our results suggest that population sensitivity to global warming might be stable for a given species, in spite of its possible local adaptation. [Copyright &y& Elsevier]

Abstract: Vegetation phenology is the chronology of periodic phases of development. It constitutes an efficient bio-indicator of impacts of climate changes and a key parameter for understanding and modelling vegetation-climate interactions and their implications on carbon cycling. Numerous studies were devoted to the remote sensing of vegetation phenology. Most of these were carried out using data acquired by AVHRR instrument onboard NOAA meteorological satellites. Since 1999, multispectral images were acquired over the whole earth surface every one to two days by MODIS instrument onboard Terra and Aqua platforms. In comparison with AVHRR, MODIS constitutes a significant technical improvement in terms of spatial resolution, spectral resolution, geolocation accuracy, atmospheric corrections scheme and cloud screening and sensor calibration. In this study, 250 m daily MODIS data were used to derive precise vegetation phenological dates over deciduous forest stands. Phenological markers derived from MODIS time-series and provided by MODIS Global Land Cover Dynamics product (MOD12Q2) were compared to field measurements carried out over the main deciduous forest stands across France and over five years. We show that the inflexion point of the asymmetric double-sigmoid function fitted to NDVI temporal profile is a good marker of the onset of green-up in deciduous stands. At plot level, the prediction uncertainty is 8.5 days and the bias is 3.5 days. MODIS Global Land Cover Dynamics MOD12Q2 provides estimates of onset of green-up dates which deviate substantially from in situ observations and do not perform better than the null model. RMSE values are 20.5 days (bias -17 days) using the onset of greenness increase and 36.5 days (bias 34.5 days) using the onset of greenness maximum. An improvement of prediction quality is obtained if we consider the average of MOD12Q2 onset of greenness increase and maximum as marker of onset of green-up date. RMSE decreases to 16.5 days and bias to 7.5 days. [Copyright &y& Elsevier]

Abstract: Modelling phenology is crucial to assess the impact of climate change on the length of the canopy duration and the productivity of terrestrial ecosystems. Focusing on six dominant European tree species, the aims of this study were (i) to examine the accuracy of different leaf phenology models to simulate the onset and ending of the leafy season, with particular emphasis on the putative role of chilling to release winter bud dormancy and (ii) to predict seasonal shifts for the 21st century in response to climate warming. Models testing and validation were done for each species considering 2 or 3 years of phenological observations acquired over a large elevational gradient (1500m range, 57 populations). Flushing models were either based solely on forcing temperatures (1-phase models) or both on chilling and forcing temperatures (2-phases models). Leaf senescence models were based on both temperature and photoperiod. We show that most flushing models are able to predict accurately the observed flushing dates. The 1-phase models are as efficient as 2-phases models for most species suggesting that chilling temperatures are currently sufficient to fully release bud dormancy. However, our predictions for the 21st century highlight that chilling temperature could be insufficient for some species at low elevation. Overall, flushing is expected to advance in the next decades but this trend substantially differed between species (from 0 to 2.4 days per decade). The prediction of leaf senescence appears more challenging, as the proposed models work properly for only two out of four deciduous species, for which senescence is expected to be delayed in the future (from 1.4 to 2.3 days per decade). These trends to earlier spring leafing and later autumn senescence are likely to affect the competitive balance between species. For instance, simulations over the 21st century predict a stronger lengthening of the canopy duration for Quercus petraea than for Fagus sylvatica, suggesting that shifts in the elevational distributions of these species might occur. [Copyright &y& Elsevier]

Abstract: In the context of climate change, the amount of carbon allocated to soil, particularly fresh litter, is predicted to increase with terrestrial ecosystem productivity, and may alter soil carbon storage capacities. In this study we performed a 1-year litter-manipulation experiment to examine how soil CO2 efflux was altered by the amount of fresh litter. Three treatments were applied: litter exclusion (E), control (C, natural amount: 486gm−2) and litter addition (A, twice the natural amount: 972gm−2). Litter decomposition rate was not affected by fresh litter amount. However, the addition or exclusion of fresh litter quickly increased or decreased total soil CO2 efflux (F S) significantly, but the relative contribution of fresh litter to total soil respiration remained unchanged between the C and A treatments, as determined by laboratory measurements. Variation in F S among treatments was not related to modification of its temperature sensitivity which was not affected by fresh litter amount (Q 10: 3.5 for E, 3.2 for C, 3.6 for A). While litter exclusion was the main cause of the F S decrease in the E treatment, only 68% of F S was directly attributable to litter addition in the A treatment. The remaining 32% of F S in the A treatment was related to a real priming effect that appeared to be a long-lasting phenomenon. This priming effect lasting over 1 year may be related to a continuous release of organic compounds from litter to soil because of the progressive decomposition of leaf litter. Q 10 estimates and isotopic data lead to the hypothesis that the priming effect corresponded to the activation of the whole soil system. As a consequence, the increase in ecosystem productivity may lead, via an increase in the amount of litter, to an increase in carbon turnover in soil. Further labelling experiments involving high-frequency carbon stable isotope measurements of CO2 efflux would help to clarify the relative importance of bulk soil and rhizosphere in the priming effect. [Copyright &y& Elsevier]