Satellite data reveal differential responses of Swiss forests to unprecedented 2018 drought

Satellite-based evidence of recent decline in global forest recovery rate from tree mortality events

Climate-driven forest mortality events have been extensively observed in recent decades, prompting the question of how quickly these affected forests can recover their functionality following such events. Here we assessed forest recovery in vegetation greenness (normalized difference vegetation index) and canopy water content (normalized difference infrared index) for 1,699 well-documented forest mortality events across 1,600 sites worldwide. By analysing 158,427 Landsat surface reflectance images sampled from these sites, we provided a global assessment on the time required for impacted forests to return to their pre-mortality state (recovery time). Our findings reveal a consistent decline in global forest recovery rate over the past decades indicated by both greenness and canopy water content. This decline is particularly noticeable since the 1990s. Further analysis on underlying mechanisms suggests that this reduction in global forest recovery rates is primarily associated with rising temperatures and increased water scarcity, while the escalation in the severity of forest mortality contributes only partially to this reduction. Moreover, our global-scale analysis reveals that the recovery of forest canopy water content lags significantly behind that of vegetation greenness, implying that vegetation indices based solely on greenness can overestimate post-mortality recovery rates globally. Our findings underscore the increasing vulnerability of forest ecosystems to future warming and water insufficiency, accentuating the need to prioritize forest conservation and restoration as an integral component of efforts to mitigate climate change impacts.

Satellite Observations Reveal a Positive Relationship Between Trait-Based Diversity and Drought Response in Temperate Forests

Climate extremes such as droughts are expected to increase in frequency and intensity with global change. Therefore, it is important to map and predict ecosystem responses to such extreme events to maintain ecosystem functions and services. Alongside abiotic factors, biotic factors such as the proportion of needle and broadleaf trees were found to affect forest drought responses, corroborating results from biodiversity-ecosystem functioning (BEF) experiments. Yet it remains unclear to what extent the behavior of non-experimental systems at large scales corresponds to the relationships discovered in BEF experiments. Using remote sensing, the trait-based functional diversity of forest ecosystems can be directly quantified. We investigated the relationship between remotely sensed functional richness and evenness and forest drought responses using data from temperate mixed forests in Switzerland, which experienced an extremely hot and dry summer in 2018. We used Sentinel-2 satellite data to assess aspects of functional diversity and quantified drought response in terms of resistance, recovery, and resilience from 2017 to 2020 in a scalable approach. We then analyzed the BEF relationship between functional diversity measures and drought response for different aggregation levels of richness and evenness of three physiological canopy traits (chlorophyll, carotenoid/chlorophyll ratio, and equivalent water thickness). Forest stands with greater trait richness were more resistant and resilient to the drought event, and the relationship of trait evenness with resistance or resilience was hump-shaped or negative, respectively. These results suggest forest functional diversity can support forests in such drought responses via a mixture of complementarity and dominance effects, the first indicated by positive richness effects and the second by negative evenness effects. Our results link ecosystem functioning and biodiversity at large scales and provide new insights into the BEF relationships in non-experimental forest ecosystems.

The ant and the grasshopper: Contrasting responses and behaviors to water stress of riparian trees along a hydroclimatic gradient

Riparian trees are particularly vulnerable to drought because they are highly dependent on water availability for their survival. However, the response of riparian tree species to water stress varies depending on regional hydroclimatic conditions, making them unevenly vulnerable to changing drought patterns. Understanding this spatial variability in stress responses requires a comprehensive assessment of water stress across broader spatial and temporal scales. Yet, the precise ecophysiological mechanisms underlying these responses remain poorly linked to remotely sensed indices. To address this gap, the implementation of remote sensing methods coupled with in situ validation is essential to obtain consistent results across diverse spatial and temporal contexts. We conducted a multi-tool analysis combining multispectral and thermal remote sensing indices with in situ ecophysiological measurements at different temporal scales to analyze the responses of white poplar (Populus alba) to seasonal changes in drought along a hydroclimatic gradient. Using this approach, we demonstrate that white poplars along the Rhône River (France) exhibit contrasting responses and behaviors during drought depending on the latitudinal context. White poplars in a Mediterranean climate show rapid stomatal closure to reduce water loss and maintain high minimum water potential levels, although this results in a decrease in remotely sensed greenness. Conversely, white poplars located upstream in a temperate climate show high transpiration and stable greenness but lower minimum water potential and water content. A site in the middle of the gradient has intermediate responses. These results demonstrate that white poplars along a climate gradient can have a range of responses to drought along the iso/anisohydricity continuum. These results are important for future climatic conditions because they show that the same species can have different mechanisms of drought resilience, even in the same river valley. This raises questions regarding how these riparian tree populations will respond to future climatic and hydrological conditions.

Responses of stem growth and canopy greenness of temperate conifers to dry spells

Dry spells strongly influence biomass production in forest ecosystems. Their effects may last several years following a drought event, prolonging growth reduction and therefore restricting carbon sequestration. Yet, our understanding of the impact of dry spells on the vitality of trees' above-ground biomass components (e.g., stems and leaves) at a landscape level remains limited. We analyzed the responses of Pinus sylvestris and Picea abies to the four most severe drought years in topographically complex sites. To represent stem growth and canopy greenness, we used chronologies of tree-ring width and time series of the Normalized Difference Vegetation Index (NDVI). We analyzed the responses of radial tree growth and NDVI to dry spells using superposed epoch analysis and further explored this relationship using mixed-effect models. Our results show a stronger and more persistent response of radial growth to dry spells and faster recovery of canopy greenness. Canopy greenness started to recover the year after the dry spell, whereas radial tree growth remained reduced for the two subsequent years and did not recover the pre-drought level until the fourth year after the event. Stem growth and canopy greenness were influenced by climatic conditions during and after drought events, while the effect of topography was marginal. The opposite responses of stem growth and canopy greenness following drought events suggest a different impact of dry spells on trees´ sink and source compartments. These results underscore the crucial importance of understanding the complexities of tree growth as a major sink of atmospheric carbon.

The energy-water limitation threshold explains divergent drought responses in tree growth, needle length, and stable isotope ratios

Predicted increases in extreme droughts will likely cause major shifts in carbon sequestration and forest composition. Although growth declines during drought are widely documented, an increasing number of studies have reported both positive and negative responses to the same drought. These divergent growth patterns may reflect thresholds (i.e., nonlinear responses) promoted by changes in the dominant climatic constraints on tree growth. Here we tested whether stemwood growth exhibited linear or nonlinear responses to temperature and precipitation and whether stemwood growth thresholds co-occurred with multiple thresholds in source and sink processes that limit tree growth. We extracted 772 tree cores, 1398 needle length records, and 1075 stable isotope samples from 27 sites across whitebark pine's (Pinus albicaulis Engelm.) climatic niche in the Sierra Nevada. Our results indicated that a temperature threshold in stemwood growth occurred at 8.4°C (7.12-9.51°C; estimated using fall-spring maximum temperature). This threshold was significantly correlated with thresholds in foliar growth, as well as carbon (δ13 C) and nitrogen (δ15 N) stable isotope ratios, that emerged during drought. These co-occurring thresholds reflected the transition between energy- and water-limited tree growth (i.e., the E-W limitation threshold). This transition likely mediated carbon and nutrient cycling, as well as important differences in growth-defense trade-offs and drought adaptations. Furthermore, whitebark pine growing in energy-limited regions may continue to experience elevated growth in response to climate change. The positive effect of warming, however, may be offset by growth declines in water-limited regions, threatening the long-term sustainability of the recently listed whitebark pine species in the Sierra Nevada.

Soil moisture determines the recovery time of ecosystems from drought

Recovery time, the time it takes for ecosystems to return to normal states after experiencing droughts, is critical for assessing the response of ecosystems to droughts; however, the spatial dominant factors determining recovery time are poorly understood. We identify the global patterns of terrestrial ecosystem recovery time based on remote sensed vegetation indices, analyse the affecting factors of recovery time using random forest regression model, and determine the spatial distribution of the dominant factors of recovery time based on partial correlation. The results show that the global average recovery time is approximately 3.3 months, and that the longest recovery time occurs in mid-latitude drylands. Analysis of affecting factors of recovery time suggests that the most important environmental factor affecting recovery time is soil moisture during the recovery period, followed by temperature and vapour pressure deficit (VPD). Recovery time shortens with increasing soil moisture and prolongs with increasing VPD; however, the response of recovery time to temperature is nonmonotonic, with colder or hotter temperatures leading to longer recovery time. Soil moisture dominates the drought recovery time over 58.4% of the assessed land area, mostly in the mid-latitudes. The concern is that soil moisture is projected to decline in more than 65% regions in the future, which will lengthen the drought recovery time and exacerbate drought impacts on terrestrial ecosystems, especially in southwestern United States, the Mediterranean region and southern Africa. Our research provides methodological insights for quantifying recovery time and spatially identifies dominant factors of recovery time, improving our understanding of ecosystem response to drought.

Immediate and carry-over effects of late-spring frost and growing season drought on forest gross primary productivity capacity in the Northern Hemisphere

Forests are increasingly exposed to extreme global warming-induced climatic events. However, the immediate and carry-over effects of extreme events on forests are still poorly understood. Gross primary productivity (GPP) capacity is regarded as a good proxy of the ecosystem's functional stability, reflecting its physiological response to its surroundings. Using eddy covariance data from 34 forest sites in the Northern Hemisphere, we analyzed the immediate and carry-over effects of late-spring frost (LSF) and growing season drought on needle-leaf and broadleaf forests. Path analysis was applied to reveal the plausible reasons behind the varied responses of forests to extreme events. The results show that LSF had clear immediate effects on the GPP capacity of both needle-leaf and broadleaf forests. However, GPP capacity in needle-leaf forests was more sensitive to drought than in broadleaf forests. There was no interaction between LSF and drought in either needle-leaf or broadleaf forests. Drought effects were still visible when LSF and drought coexisted in needle-leaf forests. Path analysis further showed that the response of GPP capacity to drought differed between needle-leaf and broadleaf forests, mainly due to the difference in the sensitivity of canopy conductance. Moreover, LSF had a more severe and long-lasting carry-over effect on forests than drought. These results enrich our understanding of the mechanisms of forest response to extreme events across forest types.

Towards precision forestry: Drought response from remote sensing-based disturbance monitoring and fine-scale soil information in Central Europe

Prolonged drought and susceptibility to biotic stressors induced an extensive calamity in Norway spruce (Picea abies (L.) Karst.) and widespread crown defoliation in European beech (Fagus sylvatica L.) in Central Europe. For future management decisions, it is crucial to link changes in canopy cover to site conditions. However, current knowledge on the role of soil properties for drought-induced forest disturbance is limited due to the scarcity and low spatial resolution of soil information. We present a fine-scale assessment on the role of soil properties for forest disturbance in Norway spruce and European beech derived from optical remote sensing. A forest disturbance modeling framework based on Sentinel-2 time series was applied on 340 km² in low mountain ranges of Central Germany. Spatio-temporal information on forest disturbance was calculated at 10 m spatial resolution in the period 2019-2021 and intersected with high-resolution soil information (1:10,000) based on roughly 2850 soil profiles. We found distinct differences in disturbed area, depending on soil type, texture, stoniness, effective rooting depth and available water capacity (AWC). For spruce, we found a polynomial relationship between AWC (R² = 0.7) and disturbance, with highest disturbed area (65 %) for AWC between 90 and 160 mm. Interestingly, we found no evidence for generally higher disturbance on shallow soils, although stands on the deepest soils were significantly less affected. Noteworthy, sites affected first did not necessarily exhibit highest proportions of disturbed area post-drought, indicating recovery or adaptation. We conclude that site- and species-specific understanding of drought impacts benefits from a combination of remote sensing and fine-scale soil information. Since our approach revealed which sites were affected first and most, it qualifies for prioritizing in situ monitoring activities to most vulnerable stands in acute drought conditions as well as for developing long-term strategies for reforestation and site-specific risk assessment for precision forestry.

Long-term soil water limitation and previous tree vigor drive local variability of drought-induced crown dieback in Fagus sylvatica

Ongoing climate warming is increasing evapotranspiration, a process that reduces plant-available water and aggravates the impact of extreme droughts during the growing season. Such an exceptional hot drought occurred in Central Europe in 2018 and caused widespread defoliation in mid-summer in European beech (Fagus sylvatica L.) forests. Here, we recorded crown damage in 2021 in nine mature even-aged beech-dominated stands in northwestern Switzerland along a crown damage severity gradient (low, medium, high) and analyzed tree-ring widths of 21 mature trees per stand. We aimed at identifying predisposing factors responsible for differences in crown damage across and within stands such as tree growth characteristics (average growth rates and year-to-year variability) and site-level variables (mean canopy height, soil properties). We found that stand-level crown damage severity was strongly related to soil water availability, inferred from tree canopy height and plant available soil water storage capacity (AWC). Trees were shorter in drier stands, had higher year-to-year variability in radial growth, and showed higher growth sensitivity to moisture conditions of previous late summer than trees growing on soils with sufficient AWC, indicating that radial growth in these forests is principally limited by soil water availability. Within-stand variation of post-drought crown damage corresponded to growth rate and tree size (diameter at breast height, DBH), i.e., smaller and slower-growing trees that face more competition, were associated with increased crown damage after the 2018 drought. These findings point to tree vigor before the extreme 2018 drought (long-term relative growth rate) as an important driver of damage severity within and across stands. Our results suggest that European beech is less likely to be able to cope with future climate change-induced extreme droughts on shallow soils with limited water retention capacity.

European beech dieback after premature leaf senescence during the 2018 drought in northern Switzerland

During the particularly severe hot summer drought in 2018, widespread premature leaf senescence was observed in several broadleaved tree species in Central Europe, particularly in European beech (Fagus sylvatica L.). For beech, it is yet unknown whether the drought evoked a decline towards tree mortality or whether trees can recover in the longer term. In this study, we monitored crown dieback, tree mortality and secondary drought damage symptoms in 963 initially live beech trees that exhibited either premature or normal leaf senescence in 2018 in three regions in northern Switzerland from 2018 to 2021. We related the observed damage to multiple climate- and stand-related parameters. Cumulative tree mortality continuously increased up to 7.2% and 1.3% in 2021 for trees with premature and normal leaf senescence in 2018, respectively. Mean crown dieback in surviving trees peaked at 29.2% in 2020 and 8.1% in 2019 for trees with premature and normal leaf senescence, respectively. Thereafter, trees showed first signs of recovery. Crown damage was more pronounced and recovery was slower for trees that showed premature leaf senescence in 2018, for trees growing on drier sites, and for larger trees. The presence of bleeding cankers peaked at 24.6% in 2019 and 10.7% in 2020 for trees with premature and normal leaf senescence, respectively. The presence of bark beetle holes peaked at 22.8% and 14.8% in 2021 for trees with premature and normal leaf senescence, respectively. Both secondary damage symptoms occurred more frequently in trees that had higher proportions of crown dieback and/or showed premature senescence in 2018. Our findings demonstrate context-specific differences in beech mortality and recovery reflecting the importance of regional and local climate and soil conditions. Adapting management to increase forest resilience is gaining importance, given the expected further beech decline on dry sites in northern Switzerland.

Soil-plant interactions modulated water availability of Swiss forests during the 2015 and 2018 droughts

Central Europe has been experiencing unprecedented droughts during the last decades, stressing the decrease in tree water availability. However, the assessment of physiological drought stress is challenging, and feedback between soil and vegetation is often omitted because of scarce belowground data. Here we aimed to model Swiss forests' water availability during the 2015 and 2018 droughts by implementing the mechanistic soil-vegetation-atmosphere-transport (SVAT) model LWF-Brook90 taking advantage of regionalized depth-resolved soil information. We calibrated the model against soil matric potential data measured from 2014 to 2018 at 44 sites along a Swiss climatic and edaphic drought gradient. Swiss forest soils' storage capacity of plant-available water ranged from 53 mm to 341 mm, with a median of 137 ± 42 mm down to the mean potential rooting depth of 1.2 m. Topsoil was the primary water source. However, trees switched to deeper soil water sources during drought. This effect was less pronounced for coniferous trees with a shallower rooting system than for deciduous trees, which resulted in a higher reduction of actual transpiration (transpiration deficit) in coniferous trees. Across Switzerland, forest trees reduced the transpiration by 23% (compared to potential transpiration) in 2015 and 2018, maintaining annual actual transpiration comparable to other years. Together with lower evaporative fluxes, the Swiss forests did not amplify the blue water deficit. The 2018 drought, characterized by a higher and more persistent transpiration deficit than in 2015, triggered widespread early wilting across Swiss forests that was better predicted by the SVAT-derived mean soil matric potential in the rooting zone than by climatic predictors. Such feedback-driven quantification of ecosystem water fluxes in the soil-plant-atmosphere continuum will be crucial to predicting physiological drought stress under future climate extremes.

Radial Growth of Trees Rather Than Shrubs in Boreal Forests Is Inhibited by Drought

Of all forest biomes, boreal forests are experiencing the most significant warming. Drought caused by warming has a dramatic impact on species in boreal forests. However, little is known about whether the growth of trees and shrubs in boreal forests responds consistently to warming and drought. We obtained the tree-ring width data of 308 trees (Larix gmelinii and Pinus sylvestris var. mongolica) and 133 shrubs (Pinus pumila) from 26 sites in northeastern China. According to the climate data from 1950 to 2014, we determined three extreme drought years (1954, 1967, and 2008). The response difference of radial growth of trees and shrubs in boreal forests to drought was compared using resilience index, moving correlation and response analysis. The results showed that high temperature (mean and maximum temperature) in previous and current growing seasons promoted the growth of P. pumila, but inhibited the growth of trees. On the contrary, wetter conditions (higher PDSI) promoted tree growth but were not conducive to P. pumila growth in high latitudes. Moving correlation analysis showed similar results. In addition, water deficit was more likely to inhibit P. pumila growth in low latitudes. The drought resistance of P. pumila was stronger than that of L. gmelinii and P. sylvestris var. mongolica. Therefore, the growth loss and recovery time of P. pumila during drought was less than those of trees. We concluded that L. gmelinii and P. sylvestris var. mongolica are more prone to growth decline than P. pumila after the drought caused by climate warming. In the future climate warming, shrub growth may benefit more than trees. Our findings are of great significance in predicting the future changes in ecosystem composition and species distribution dynamics in extreme climate susceptible areas.