Maxime Cailleret

Drought-induced decline and mortality of silver fir differ among three sites in Southern France

ContextIn the Mediterranean area, numerous decline and mortality processes have been reported during recent decades, affecting forest dynamics. They are likely due to increases in summer drought severity and therefore especially affect drought-sensitive species, such as silver fir (Abies alba Mill.).Aims and methodsTo understand the relationships between tree growth, crown condition and mortality probability, radial growth trends of healthy, declining (showing crown damages) and dead trees were compared using tree-ring analysis. Factors involved in determining this mortality were also examined at the plot and tree level using altitudinal gradients on three contrasted sites in southeastern France.ResultsIndividuals with higher inter-annual variability in growth were more prone to dieback. At two sites, dead trees displayed lower growth rates over their entire lifetime, while, on the last site, their juvenile growth rate was higher. Trees with crown damage had higher growth rates than healthy trees on one site, and their radial growth trends over time always differed from those of dead trees. Mortality and crown damage were little related to altitude, but strongly differed between sites and among plots underlining the importance of local edaphic and topographic conditions.ConclusionThese results suggest that the relationships among mortality probability, crown condition and growth can differ among sites, and highlight the impact of soil conditions and the need to assess them in tree mortality studies.

Future ecosystem services from European mountain forests under climate change

Summary 1.Ecosystem services (ES) from mountain forests are highly relevant for human societies. ES with a direct economic support function (e.g. timber production), regulatory services (e.g. protection from natural hazards) and cultural services (e.g. recreation) are likely to be affected strongly by a rapidly changing climate. To evaluate whether adverse climate change effects on ES can be counteracted by adapting management, dynamic models and indicator-based assessments are needed. 2.We applied a forest dynamic model in case study areas of four European mountain regions and evaluated the future supply of four ES - timber production, carbon sequestration, biodiversity, and protection against natural hazards - using state-of-the-art ES indicators. Forest dynamics were simulated under three management scenarios (no management, business-as-usual, and alternative management) and five climate change projections for selected representative stand types in each region. We analysed potential trade-offs and synergies between ES, and evaluated future changes among regions, forest stands, climate and management scenarios. 3.Impacts of climate change on the provision of multiple ES were found to be highly heterogeneous and to depend on the region, site, and future climate. In the absence of large-scale natural disturbance (not considered), protection services, carbon stock and deadwood abundance (proxy for biodiversity) benefitted from no management in all regions. Negative impacts of climate change were evident for the provision of multiple ES but limited to the most severe climate scenarios and low-elevation stands. Synergies and trade-offs between the majority of ES were found to be sensitive to the choice of management strategy and – in some regions – to climate change. 4.Synthesis and applications. Management regimes in European mountain forests should be regionally adapted to stand and site conditions. Although in some cases alternative management regimes may be more suitable than current management for supporting multiple ecosystem services, adaptation options should be evaluated carefully at the local scale due to the highly different magnitude of the impacts of climate change in different regions and along elevation gradients. This article is protected by copyright. All rights reserved.

The agony of choice: different empirical mortality models lead to sharply different future forest dynamics

Dynamic models are pivotal for projecting forest dynamics in a changing climate, from the local to the global scale. They encapsulate the processes of tree population dynamics with varying resolution. Yet, almost invariably, tree mortality is modeled based on simple, theoretical assumptions that lack a physiological and/or empirical basis. Although this has been widely criticized and a growing number of empirically derived alternatives are available, they have not been tested systematically in models of forest dynamics. We implemented an inventory-based and a tree-ring-based mortality routine in the forest gap model ForClim v3.0. We combined these routines with a stochastic and a deterministic approach for the determination of tree status (alive vs. dead). We tested the four new model versions for two Norway spruce forests in the Swiss Alps, one of which was managed (inventory time series spanning 72 years) and the other was unmanaged (41 years). Furthermore, we ran long-term simulations (-400 years) into the future under three climate scenarios to test model behavior under changing environmental conditions. The tests against inventory data showed an excellent match of simulated basal area and stem numbers at the managed site and a fair agreement at the unmanaged site for three of the four empirical mortality models, thus rendering the choice of one particular model difficult. However, long-term simulations under current climate revealed very different behavior of the mortality models in terms of simulated changes of basal area and stem numbers, both in timing and magnitude, thus indicating high sensitivity of simulated forest dynamics to assumptions on tree mortality. Our results underpin the potential of using empirical mortality routines in forest gap models. However, further tests are needed that span other climatic conditions and mixed forests. Short-term simulations to benchmark model behavior against empirical data are insufficient; long-term tests are needed that include both nonequilibrium and equilibrium conditions. Thus, there is the potential to greatly improve the robustness of future projections of forest dynamics via more reliable tree mortality submodels.

Accurate modeling of harvesting is key for projecting future forest dynamics: a case study in the Slovenian mountains

Maintaining the provision of multiple forest ecosystem services requires to take into consideration forest sensitivity and adaptability to a changing environment. In this context, dynamic models are indispensable to assess the combined effects of management and climate change on forest dynamics. We evaluated the importance of implementing different approaches for simulating forest management in the climate-sensitive gap model ForClim and compared its outputs with forest inventory data at multiple sites across the European Alps. The model was then used to study forest dynamics in representative silver fir–European beech stands in the Dinaric Mountains (Slovenia) under current management and different climate scenarios. On average, ForClim accurately predicted the development of basal area and stem numbers, but the type of harvesting algorithm used and the information for stand initialization are key elements that must be defined carefully. Empirical harvesting functions that rigorously impose the number and size of stems to remove fail to reproduce stand dynamics when growth is just slightly under- or overestimated, and thus should be substituted by analytical thinning algorithms that are based on stochastic distribution functions. Long-term simulations revealed that both management and climate change negatively impact conifer growth and regeneration. Under current climate, most of the simulated stands were dominated by European beech at the end of the simulation (i.e., 2150 AD), due to the decline of silver fir and Norway spruce caused mainly by harvesting. This trend was amplified under climate change as growth of European beech was favored by higher temperatures, in contrast to drought-induced growth reductions in both conifers. This forest development scenario is highly undesired by local managers who aim at preserving conifers with high economic value. Overall, our results suggest that maintaining a considerable share of conifers in these forests may not be feasible under climate change, especially at lower elevations where foresters should consider alternative management strategies.

Research frontiers for improving our understanding of drought-induced tree and forest mortality

Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land-atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global monitoring network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.

Climate Change Impact on Tree Architectural Development and Leaf Area

The response of forests to the forecasted increase in climate stress occurrence is considered a key issue in climate change scenarios [1]. Although forest productivity increased in most ecosystems during the 20th century [2,3], a review by Allen et al. [4] underlined an emerging trend of heat and drought induced forest decline and dieback at global scale. Several and generally combined physical and biological causes contribute to observed tree decline or die-off [4-7]. Apart extensive insect outbreaks [8], understanding the respective role of hydraulic failure and carbon starvation due to excessive or long lasting water stress is one of the major research goal in order to predict forest response to climate change [9].

Drought induced tree mortality – a tree‐ring isotope based conceptual model to assess mechanisms and predispositions

Drought-induced tree mortality is likely to increase in future as climate models forecast increased frequency of drought events together with higher air temperatures (Dai, 2013; Allen et al., 2015). Besides the presence of inciting (e.g. heat and drought events) and contributing factors (e.g. opportunistic biotic agents such as bark beetles), predisposition of particular species or individuals of a given species is considered as central for understanding why some trees survive while others succumb to drought (Manion, 1981; McDowell et al., 2008; Voltas et al., 2013; Gessler et al., 2016; Martin-StPaul et al., 2017). This is also crucial for simulating tree mortality in dynamic vegetation models (Meir et al., 2015). Predisposing factors are assumed to be related to long-term climatic stressors (Voltas et al., 2013), prevailing longterm nutrient supply (Gessler et al., 2016), water use strategies (Hentschel et al., 2014), tree height and interspecific and intraspecific competition (Grote et al., 2016) and pests, pathogens or air pollution (Allen et al., 2010). Even though plants growing in dry environments may have the broadest hydraulic safety margins (Choat et al., 2012; MartinStPaul et al., 2017), drought events force these and also the carbon safety margins to points where trees may be at risk of physiological failure or failure to defend against biotic attacks (McDowell, 2011). Hydraulic and symplastic failure, and strong reduction in carbon pools (also called ‘carbon starvation’) have been postulated as the two main, nonexclusive physiological mechanisms leading to tree mortality (Adams et al., 2017), in strong interaction with biotic agents (Anderegg et al., 2015). Hydraulic failure summarizes all aspects of cellular desiccation causing cessation of symplastic biochemical functioning, and disruption of water transport through xylem embolism (McDowell et al., 2011). Carbon starvation describes the situation when the carbon demand for maintenance of cellular and defensivemetabolism is not sufficiently met owing to low carbohydrate supply from photosynthesis and storage (McDowell et al., 2008). Recently, Adams et al. (2017) showed that xylem hydraulic failure was ubiquitous across taxa, while carbon starvation was not universal but still common for gymnosperms (see alsoMart ınez-Vilalta et al., 2016) probably after long-term moderate drought stress (McDowell et al., 2008). Moreover, hydraulic function and carbohydrate metabolism are strongly linked and thus theremight be interdependencies between hydraulic failure and carbon starvation (McDowell, 2011; Sevanto et al., 2014). Even though hydraulic failure might occur independent of carbon starvation, many cases have been observed where carbon balance and hydraulics were both impaired (Adams et al., 2017). The carbon starvation–hydraulic failure concept as applied here is rather a continuum with relatively stronger influence of the one or the other process on mortality. Trees’ predisposition to carbon starvation or to hydraulic failure (Fig. 1a)may be indicated by specific syndromes of traits (Anderegg et al., 2016) reflecting different strategies to face drought (Pivovaroff et al., 2016) modified by differences in local resource availability. The main approach we have chosen for our conceptual model is a conspecific synchronic comparison of growth and tree ring isotopic signals between later dying and surviving trees over longer time periods from the same stand aiming at understanding mortality mechanisms and the respective predisposition in a given environmental context. We do not claim our results to be generalizable for a given species as they will not only be affected by species specific traits but also by tree individual (intraspecific) trait differences and by local site conditions. A key strategy for dealing with drought concerns the coupling between xylem resistance to embolism and stomatal response to drought (Martin-StPaul et al., 2017). On the one side, conservative/stress resistant strategies through early stomatal closure during drought are often seen as predisposition to carbon deficiency. On the other side, acquisitive/competitive strategies with high treelevel hydraulic conductance and resource use are favourable for the development of individuals under normal conditions, but are assumed to increase the risk of xylem embolism and consequently mortality when water supply is restricted or atmospheric water demand is high (see Mart ınez-Vilalta & Garcia-Forner, 2017). Moreover, trait combinations that primarily allow acclimation to environmental factors other than drought might have distinct effects on the drought response. For instance, acclimation to high nutrient availabilitymay increase tree sensitivity to hydraulic failure e.g. through a low ratio of root area to leaf area, while trait combinations favoured under lownutrient supplymay facilitate the carbon starvation trajectory (Gessler et al., 2016). While these trait differences mainly occur between species, more risky or more conservative resource use strategies have also been observed within populations of a given species (Hentschel et al., 2014). Adams et al. (2017) synthesized the physiologicalmechanisms of drought induced tree mortality with a multi species approach taking into account studies that assessed percentage of loss of xylem hydraulic conductivity (PLC) and changes in nonstructural carbohydrates (NSCs) before mortality. Despite their potentially high ability to describe the mechanisms of tree mortality the determination of these physiological indicators is costly and labour intensive, and prone to methodological artefacts (e.g. Cochard et al., 2013; Quentin et al., 2015). These methods are not applicable to a large number of species and sites yet. Old grown

Calibration data for empirical mortality models of 18 European tree species

The dataset comprises > 90 000 records from inventories in 54 strict forest reserves in Switzerland and Lower Saxony / Germany along a considerable environmental gradient. It was used to develop parsimonious, species-specific mortality models for 18 European tree species based on tree size and growth as well as additional covariates on stand structure and climate. Inventory data Measurements had been conducted repeatedly on up to 14 permanent plots per reserve for up to 60 years with re-measurement intervals of 4 - 27 years. The permanent plots vary in size between 0.03 and 3.47 ha. The inventories provide diameter measurements at breast height (DBH) and information on the species and status (alive or dead) of trees with DBH ≥ 4 cm for Switzerland and ≥ 7 cm for Germany. We excluded three permanent plots where at least 80 % of the trees died during an interval of 10 years, and mortality could be clearly assigned to a disturbance agent. Mortality in the remaining stands was rather low, with a mean annual mortality rate of 1.5 % and strong variation between plots from 0 to 6.5 % (assessed for trees of all species with DBH ≥ 7 cm). Data selection We only used data from permanent plots with at least 20 trees per species to obtain reliable plot-level mortality rates even for species with low mortality rates (about 5 % during 10 years), and selected tree species occurring on at least 10 plots to cover sufficient ecological gradients. This led to a dataset of 197 permanent plots and 18 tree or shrub species: Abies alba Mill., Acer campestre L., Acer pseudoplatanus L., Alnus incana Moench., Betula pendula Roth, Carpinus betulus L., Cornus mas L., Corylus avellana L., Fagus sylvatica L., Fraxinus excelsior L., Picea abies (L.) Karst, Pinus mugo Turra, Pinus sylvestris L., Quercus pubescens Willd., Quercus spp. (Q. petraea Liebl. and Q. robur L.; not properly differentiated in the Swiss inventories), Sorbus aria Crantz, Tilia cordata Mill. and Ulmus glabra Huds.. Predictors of tree mortality We considered tree size and growth as key indicators for mortality risk. Radial stem growth between the first and second inventory and DBH at the second inventory were used to predict tree status (alive or dead) at the third inventory. To this end, the annual relative basal area increment (relBAI) was calculated as the compound annual growth rate of the trees basal area. Additional covariates on stand structure and climate comprise mean annual precipitation sum (P), mean annual air temperature (mT), the mean and the interquartile range of DBH (mDBH, iqrDBH), basal area (BA) and the number of trees (N) per hectare. Further information For further information, refer to Hulsmann et al. (in press) How to kill a tree – Empirical mortality models for eighteen species and their performance in a dynamic forest model. Ecological Applications.