Legacy Effects Research Articles

Prairie restoration promotes the abundance and diversity of mutualistic arbuscular mycorrhizal fungi.

Predicting how biological communities assemble in restored ecosystems can assist in conservation efforts, but most research has focused on plants, with relatively little attention paid to soil microbial organisms that plants interact with. Arbuscular mycorrhizal (AM) fungi are an ecologically significant functional group of soil microbes that form mutualistic symbioses with plants and could therefore respond positively to plant community restoration. To evaluate the effects of plant community restoration on AM fungi, we compared AM fungal abundance, species richness, and community composition of five annually cultivated, conventionally managed agricultural fields with paired adjacent retired agricultural fields that had undergone prairie restoration 5-9 years prior to sampling. We hypothesized that restoration stimulates AM fungal abundance and species richness, particularly for disturbance-sensitive taxa, and that gains of new taxa would not displace AM fungal species present prior to restoration due to legacy effects. AM fungal abundance was quantified by measuring soil spore density and root colonization. AM fungal species richness and community composition were determined in soils and plant roots using DNA high-throughput sequencing. Soil spore density was 2.3 times higher in restored prairies compared to agricultural fields, but AM fungal root colonization did not differ between land use types. AM fungal species richness was 2.7 and 1.4 times higher in restored prairies versus agricultural fields for soil and roots, respectively. The abundance of Glomeraceae, a disturbance-tolerant family, decreased by 25% from agricultural to restored prairie soils but did not differ in plant roots. The abundance of Claroideoglomeraceae and Diversisporaceae, both disturbance-sensitive families, was 4.6 and 3.2 times higher in restored prairie versus agricultural soils, respectively. Species turnover was higher than expected relative to a null model, indicating that AM fungal species were gained by replacement. Our findings demonstrate that restoration can promote a relatively rapid increase in the abundance and diversity of soil microbial communities that had been degraded by decades of intensive land use, and community compositional change can be predicted by the disturbance tolerance of soil microbial taxonomic and functional groups.

Improving crop salt tolerance through soil legacy effects

Soil salinization poses a critical problem, adversely affecting plant development and sustainable agriculture. Plants can produce soil legacy effects through interactions with the soil environments. Salt tolerance of plants in saline soils is not only determined by their own stress tolerance but is also closely related to soil legacy effects. Creating positive soil legacy effects for crops, thereby alleviating crop salt stress, presents a new perspective for improving soil conditions and increasing productivity in saline farmlands. Firstly, the formation and role of soil legacy effects in natural ecosystems are summarized. Then, the processes by which plants and soil microbial assistance respond to salt stress are outlined, as well as the potential soil legacy effects they may produce. Using this as a foundation, proposed the application of salt tolerance mechanisms related to soil legacy effects in natural ecosystems to saline farmlands production. One aspect involves leveraging the soil legacy effects created by plants to cope with salt stress, including the direct use of halophytes and salt-tolerant crops and the design of cropping patterns with the specific crop functional groups. Another aspect focuses on the utilization of soil legacy effects created synergistically by soil microorganisms. This includes the inoculation of specific strains, functional microbiota, entire soil which legacy with beneficial microorganisms and tolerant substances, as well as the application of novel technologies such as direct use of rhizosphere secretions or microbial transmission mechanisms. These approaches capitalize on the characteristics of beneficial microorganisms to help crops against salinity. Consequently, we concluded that by the screening suitable salt-tolerant crops, the development rational cropping patterns, and the inoculation of safe functional soils, positive soil legacy effects could be created to enhance crop salt tolerance. It could also improve the practical significance of soil legacy effects in the application of saline farmlands.

Effects of experimental drought and plant diversity on multifunctionality of a model system for crop rotation

In low-diversity productive grasslands, modest changes to plant diversity (richness, composition and relative abundance) may affect multiple ecosystem functions (multifunctionality), including yield. Despite the economic importance of productive grasslands, effects of plant diversity and environmental disturbance on multifunctionality are very rarely quantified. We systematically varied species richness, composition, and relative abundance of grassland ley communities and manipulated water supply (rainfed and drought) to quantify effects of diversity and environmental disturbance on multifunctionality. We then replaced the grassland leys with a monoculture crop to investigate ‘follow-on’ effects. We measured six agronomy-related ecosystem functions across one or both phases: yield, yield consistency, digestibility and weed suppression (grassland ley phase), legacy effect (effect on follow-on crop yield), and nitrogen fertiliser efficiency (full rotation). Drought reduced most ecosystem functions, although effects were species- and function-specific. Increased plant diversity affected mean performance, and reduced variation, across the six functions (contributing to multifunctional stability). Multifunctionality index values across a wide range of mixture diversity were higher than the best monoculture under both rainfed and drought conditions (transgressive over-performance). Higher-diversity, lower-nitrogen (150N) mixtures had higher multifunctionality than a low-diversity, higher-nitrogen (300N) grass monoculture. Plant diversity in productive grasslands is a practical farm-scale management action to mitigate drought impacts and enhance multifunctionality of grassland-crop rotation systems.

LEGACY EFFECT OF EXERCISE TRAINING TRIALS ON COGNITION IN OLDER ADULTS: SYSTEMATIC REVIEW AND META-ANALYSIS

INTRODUCTION & AIMS Exercise training trials consistently show efficacy for improving cognitive outcome in older adults, especially when the intervention is supervised. However, whether this improvement in cognition is sustained long-term (i.e. a legacy effect) is unknown. While this is likely impacted by long-term adherence to exercise, the legacy effect of exercise training trials on cognition has not been established. Therefore, the aim of this study was to review the literature and investigate the legacy effect of an exercise trial on cognitive function in older adults. METHODS A systematic review of the literature published in English was conducted in PubMed, Scopus, CINAHL, SPORTdiscus, Web of Science and Cochrane Central Register of Controlled Trials electronic databases. Study screening and data extraction were undertaken by two independent researchers. The main inclusion criteria were randomised controlled trial design with non-exercise control, older adult cohort (average age ≥50 years), supervised exercise intervention and cognitive assessments at post-trial and follow up (≥3 months). Pairwise random-effects restricted maximum likelihood meta-analysis estimated the standardised mean difference (Hedges’ g) between intervention and control groups for each cognitive function outcomes at post-trial and follow-up. RESULTS A total of 12,539 articles were screened with 21 trials (n=2,504 participants aged 75.2±6.4 and mostly (56%) female) included in a meta-analysis. After 5.2±3.2 months, a legacy effect was seen with a small-to-moderate between-group effect for the cognitive domain of executive function favouring the exercise interventions (SMD=0.30, 95% CI 0.11 to 0.49; p=0.002). No effect was observed for other cognitive domains. The Cochrane risk-of-bias tool for randomized trials (RoB2) tool identified that 73.7% of the trials had “some concerns” and 26.3% had high risk for bias. CONCLUSION In older adults who received an exercise intervention there was a small-to-moderate legacy effect for improved executive function compared to controls, five months after trial completion.

Inferred drought-induced plant allocation shifts and their impact on drought legacy at a tropical forest site.

While droughts predominantly induce immediate reductions in plant carbon uptake, they can also exert long-lasting effects on carbon fluxes through associated changes in leaf area, soil carbon, etc. Among other mechanisms, shifts in carbon allocation due to water stress can contribute to the legacy effects of drought on carbon fluxes. However, the magnitude and impact of these allocation shifts on carbon fluxes and pools remain poorly understood. Using data from a wet tropical flux tower site in French Guiana, we demonstrate that drought-induced carbon allocation shifts can be reliably inferred by assimilating Net Biosphere Exchange (NBE) and other observations within the CARbon DAta MOdel fraMework. This model-data fusion system allows inference of optimized carbon and water cycle parameters and states from multiple observational data streams. We then examined how these inferred shifts affected the duration and magnitude of drought's impact on NBE during and after the extreme event. Compared to a static allocation scheme analogous to those typically implemented in land surface models, dynamic allocation reduced average carbon uptake during drought recovery by a factor of 2.8. Additionally, the dynamic model extended the average recovery time by 5 months. The inferred allocation shifts influenced the post-drought period by altering foliage and fine root pools, which in turn modulated gross primary productivity and heterotrophic respiration for up to a decade. These changes can create a bust-boom cycle where carbon uptake is enhanced some years after a drought, compared to what would have occurred under drought-free conditions. Overall, allocation shifts accounted for 65% [45%-75%] of drought legacy effects in modeled NBE. In summary, drought-induced carbon allocation shifts can play a substantial role in the enduring influence of drought on cumulative land-atmosphere CO2 exchanges and should be accounted for in ecosystem models.

Climate change critically affects the status of the land-system change planetary boundary

The planetary boundaries framework defines a safe operating space for humanity. To date, these boundaries have mostly been investigated separately, and it is unclear whether breaching one boundary can lead to the transgression of another. By employing a dynamic global vegetation model, we systematically simulate the strength and direction of the effects of different transgression levels of the climate change boundary (using climate output from ten phase 6 of the Coupled Model Intercomparison Project models for CO2 levels ranging from 350 ppm to 1000 ppm). We focus on climate change-induced shifts of Earth’s major forest biomes, the control variable for the land-system change boundary, both by the end of this century and, to account for the long-term legacy effect, by the end of the millennium. Our simulations show that while staying within the 350 ppm climate change boundary co-stabilizes the land-system change boundary, breaching it (>450 ppm) leads to critical transgression of the latter, with greater severity the higher the ppm level rises and the more time passes. Specifically, this involves a poleward treeline shift, boreal forest dieback (nearly completely within its current area under extreme climate scenarios), competitive expansion of temperate forest into today’s boreal zone, and a slight tropical forest extension. These interacting changes also affect other planetary boundaries (freshwater change and biosphere integrity) and provide feedback to the climate change boundary itself. Our quantitative process-based study highlights the need for interactions to be studied for a systemic operationalization of the planetary boundaries framework.

Declined benefit of earlier spring greening on summer growth in northern ecosystems under future scenarios

Declined benefit of earlier spring greening on summer growth in northern ecosystems under future scenarios

Divergent responses of subtropical evergreen and deciduous forest carbon cycles to the summer 2022 drought

Ongoing shifts in climate, especially extreme drought, is posing considerable threats to the forest carbon uptake worldwide. In China, summer 2022 was the warmest and driest since the beginning of meteorological measurements. This study synthesized the tower-based carbon fluxes and climate data from two subtropical evergreen and deciduous forest ecosystems to investigate the effects of such summer drought. Interestingly, the net ecosystem production (NEP = −NEE) only exhibited a slight decrease at the deciduous forest while it even enhanced at the evergreen forest during the summer 2022 drought. Further analysis revealed that although reductions in gross primary productivity (GPP) and ecosystem respiration (R e) were found at both sites, larger decrease in R e than GPP at the evergreen forest led to stronger NEP compared to the previous year. However, the NEP of two forest ecosystems sharply reduced in the following 2023, which can be ascribed to the legacy effects of the summer 2022 drought. The results of multiple linear regression revealed that soil water content (SWC) was recognized as the primary driver of GPP and R e, and downwelling shortwave radiation (R g) regulated the variability of NEP during the summer 2022. Across these forest carbon fluxes including GPP, R e and NEP, the deciduous forest exhibited larger resistance, whereas the evergreen forest showed stronger resilience. All analyses emphasized the diverse adaptive strategies among vegetation types, which acted an important role in assessing ecosystem carbon sequestration in face of future climate change.

Soil health indicators reveal that past dairy manure applications create a legacy effect

AbstractUnderstanding the long‐term effects of manure applications on the soil microbial component in semiarid climates will be key to sustain essential processes that affect their productivity and soil health. In this paper, soil health indicators encompassed both selected chemical and biological indicators. From 2004 to 2009, solid dairy manure treatments were applied to plots at cumulative rates of 0, 134, and 237 dry Mg ha−1 (34–56 dry Mg ha−1 year−1) in a randomized complete block with three replicates. Soil samples were taken from each manure rate in the spring of 2020 at 0–15 and 15–30 cm. Eleven years after manure applications ceased, many of the soil chemical and biological indicators were different between the manure and control treatments. In general, soil organic carbon and biological indicators were significantly greater in the 134 and 237 Mg ha−1 treatments as compared to the 0 Mg ha−1 treatment.

Heathland management affects soil response to drought

Abstract Drought can affect ecosystem functioning by altering plant–soil interactions, posing a significant threat to vulnerable ecosystems like heathlands. In heathlands, ongoing nitrogen deposition increases the dominance of fast‐growing grasses over the slow‐growing shrub Calluna vulgaris. These changes above‐ground can influence soil dynamics and heathlands' responses to drought. Here, we assessed whether the legacy effects of drought on heathland soils depended on mowing times as a management practice commonly used to regenerate Calluna and decrease the abundance of fast‐growing grasses. Using a long‐term field experiment, we investigated if soil response to drought differed under Calluna and grasses, as well as under Calluna plants of different growth stages through different mowing times. We hypothesized that drought would decrease soil C and nutrient pools underneath grasses and younger Calluna plants through its impact on soil microbial communities. Our results show that long‐term drought decreased soil C but only underneath grasses and old Calluna, while under young Calluna, soil C increased under drought when compared to control conditions. Bacterial and fungal community composition differed between drought and control and were affected by the growth stage of Calluna but not by plant growth strategies. Furthermore, drought and Calluna growth stage‐induced changes in bacterial communities directly affected total soil C and indirectly by reducing microbial C, which was positively related to total soil C. Synthesis and applications. Understanding ecosystem response to drought is crucial for maintaining biodiversity and mitigating climate change feedbacks. Heathlands are threatened by high nitrogen deposition, a lack of management and an increasing frequency of drought. Our results emphasize the importance of frequent mowing (decadal) as a management practice that promotes a younger and more Calluna‐dominated plant community for reducing soil C losses under future climate change‐induced drought conditions. The active management of heathlands is essential not only for keeping above‐ground vegetation dynamics, but also for maintaining below‐ground soil nutrient and carbon pools.

Legacy effects of early cotyledon removal on the growth, carbon and nitrogen storage, and drought response of Quercus variabilis seedlings

Predation of cotyledons can reduce short-term survival and growth in emerging oak seedlings, potentially affecting species recruitment. However, the physiological basis of reduced seedling performance after early cotyledon predation and the mid-term consequences of cotyledon predation on seedling drought tolerance remain poorly understood. We performed an experiment where we simulated predation by systematically removing cotyledons from Quercus variabilis seedlings at seven growth stages during emergence in the first growing season. We assessed survival, growth, and nitrogen (N) and non-structural carbohydrate (NSC) storage. Subsequently, in the second growing season, we assessed drought tolerance in seedlings with cotyledons removed at four growth stages in the previous season, and subjected seedlings to two drought levels to assess overall performance. During the first growing season, seedling survival showed less sensitivity to cotyledon removal compared to growth, N and NSC storage, which were significantly reduced when cotyledons were removed within the first 4 days after emergence. Early cotyledon removal (4 days after emergence) in the first growing season enhanced drought tolerance in the second growing season by reducing leaf embolism vulnerability, but it did not affect leaf drought tolerance traits such as the osmotic potential or the modulus of elasticity. Early cotyledon removal had a lasting effect on seedling growth and N and NSC storage building in the second growing season. Additionally, it exacerbated the negative consequences of drought on seedlings growth and N storage building, but it did not influence survival or NSC storage. Our findings evidence that early cotyledon removal in emerging oak seedlings has significant short- and medium-term repercussions on growth and capacity to store N and NSC, and it hinders the capacity of seedlings to respond to drought despite early cotyledon removal increases leaf drought tolerance. These functional legacies can influence oak seedling recruitment.

Legacy effects of premature defoliation in response to an extreme drought event modulate phytochemical profiles with subtle consequences for leaf herbivory in European beech.

Extreme droughts can have long-lasting effects on forest community dynamics and species interactions. Yet, our understanding of how drought legacy modulates ecological relationships is just unfolding. We tested the hypothesis that leaf chemistry and herbivory show long-term responses to premature defoliation caused by an extreme drought event in European beech (Fagus sylvatica L.). For two consecutive years after the extreme European summer drought in 2018, we collected leaves from the upper and lower canopy of adjacently growing drought-stressed and unstressed trees. Leaf chemistry was analyzed and leaf damage by different herbivore-feeding guilds was quantified. We found thatdrought had lasting impacts on leaf nutrients and on specialized metabolomic profiles. However, drought did not affect the primary metabolome. Drought-related phytochemical changes affected damage of leaf-chewing herbivores whereas damage caused by other herbivore-feeding guilds was largely unaffected. Drought legacy effects on phytochemistry and herbivory were often weaker than between-year or between-canopy strata variability. Our findings suggest that a single extreme drought event bears the potential to long-lastingly affect tree-herbivore interactions. Drought legacy effects likely become more important in modulating tree-herbivore interactions since drought frequency and severity are projected to globally increase in the coming decades.

Grasses and shrubs have decoupled legacy effects related to plant nutrition: the interplay of symbiotic fungi and nutrients in soil

Grasses and shrubs have decoupled legacy effects related to plant nutrition: the interplay of symbiotic fungi and nutrients in soil

Water level drawdown induces a legacy effect on the seed bank and retains sediment chemistry in a eutrophic clay wetland

The lack of extreme water level fluctuations in managed, non-peat forming wetland ecosystems can result in decreased productivity through the loss of heterogeneity of these ecosystems. Stochastic disruption, such as a water level drawdown, can effectively reverse this effect and return the wetland to a more productive state, associated with higher biodiversity through new vegetation development. Yet, aside from the effect on vegetation dynamics, little is known about longer-term effects (30 years) of a water level drawdown, hereafter referred to as legacy effects, and how this may impact future water level drawdowns.Here, we aim to unravel the legacy effects of a water level drawdown, stand alone and along a water level gradient, on seed bank properties and nutrient availability in a eutrophic clay wetland. To identify these, we studied the hydrologically managed nature reserve Oostvaardersplassen in the Netherlands. Here, one section was subjected to a multi-year water level drawdown and another section was kept inundated. We determined seed bank properties in both areas, spatially and along a soil elevation gradient (20 cm). Nutrient availability was measured by taking sediment samples along the water level gradient and through experimental manipulation of the water level in an indoor mesocosm experiment.Germination was higher in locations with a water level drawdown history, especially at relatively high elevations. Additionally, the proportion of pioneer species in the seed bank was higher in the water level drawdown area. Overall, nutrient concentrations were higher compared to other aquatic systems. Nutrient availability was higher in the inundated area and did not respond to the water level gradient. We conclude that 30 years after an induced water level drawdown there is no depletion of nutrients, while we still observe a legacy effect in the number of viable seeds in the seed bank.

Assessing the influence of climate on the growth of green ash trees from five Plant Hardiness Zones growing in a range-wide provenance test near the species’ northern range limit

Green ash is threatened with extirpation from emerald ash borer (EAB) attack. Because green ash has an extensive range, its restoration could require both breeding for EAB resistance and possible adaptation to a variety of environmental gradients. We assessed the growth and climate sensitivity of green ash from five Plant Hardiness Zones (PHZs) growing in a range-wide provenance test in Vermont. Although there tended to be greater growth among trees from the warmest PHZ (7), differences were rarely statistically distinguishable. For trees from all PHZs, growth was positively correlated with precipitation in the current year and negatively correlated with precipitation the year before (a possible legacy effect). Growth was negatively associated with temperature the year of ring formation but positively associated with temperatures the year before. Growth was often positively correlated with winter snow but was negatively associated with spring or fall snow for the warmest PHZs. Climate correlations for PHZ 3 were unusual in that: 1) only positive correlations were detected, 2) no legacy effects were noted, and 3) despite being from the coldest region, no correlations with snow were found. Growth increased over time for the warmest PHZs during a period of simultaneous increases in temperature and precipitation.

Seasonal aridity regulates drivers and temporal variability of wood phenology: A meta-analysis of dendrometer monitoring data across the Northern Hemisphere

The secondary growth phenology of trees, also known as wood phenology, plays a vital role in assessing the timing of carbon sequestration in forests. However, its spatio-temporal variability and determinants in the Northern Hemisphere remain debated. Here, we presented a meta-analysis using secondary growth phenology data based on dendrometer monitoring of 59 tree species and 84 sites to explore the drivers and temporal connections of phenology across seasonal arid and humid regions in the Northern Hemisphere. We found spring temperatures and precipitation co-regulated the start of secondary growth (SOG) in spring arid regions, and SOG advanced with the increase in spring temperatures and precipitation. However, SOG was only influenced by spring temperatures in spring humid regions. The end of secondary growth (EOG) indicated strong correlations with autumn temperatures only in autumn arid regions but not in humid regions. Interestingly, the earlier SOG promoted advancement of the time of maximum secondary growth rate (MOG) and EOG both in seasonal humid and arid regions, while the end of secondary growth in the previous year (prEOG) negatively affected SOG only in seasonal arid regions. The legacy effects of previous phenological events on subsequent ones should be integrated into phenological prediction models to help accurately assess global carbon, water, and energy cycles.

Tree post-drought recovery: scenarios, regulatory mechanisms and ways to improve.

Efficient post-drought recovery of growth and assimilation enables a plant to return to its undisturbed state and functioning. Unlike annual plants, trees suffer not only from the current drought, but also from cumulative impacts of consecutive water stresses which cause adverse legacy effects on survival and performance. This review provides an integrated assessment of ecological, physiological and molecular evidence on the recovery of growth and photosynthesis in trees, with a view to informing the breeding of trees with a better ability to recover from water stress. Suppression of recovery processes can result not only from stress damage but also from a controlled downshift of recovery as part of tree acclimation to water-limited conditions. In the latter case, recovery processes could potentially be activated by turning off the controlling mechanisms, but several obstacles make this unlikely. Tree phenology, and specifically photoperiodic constraints, can limit post-drought recovery of growth and photosynthesis, and targeting these constraints may represent a promising way to breed trees with an enhanced ability to recover post-drought. The mechanisms of photoperiod-dependent regulation of shoot, secondary and root growth and of assimilation processes are reviewed. Finally, the limitations and trade-offs of altering the photoperiodic regulation of growth and assimilation processes are discussed.

Drought legacies in mixed Mediterranean forests: Analysing the effects of structural overshoot, functional traits and site factors

Previous favorable climate conditions stimulate tree growth making some forests more vulnerable to hotter droughts. This so-called structural overshoot may contribute to forest dieback, but there is little evidence on its relative importance depending on site conditions and tree species because of limited field data. Here, we analyzed remote sensing (NDVI) and tree-ring width data to evaluate the impacts of the 2017 drought on canopy cover and growth in mixed Mediterranean forests (Fraxinus ornus, Quercus pubescens, Acer monspessulanum, Pinus pinaster) located in southern Italy. Legacy effects were assessed by calculating differences between observed and predicted basal area increment (BAI). Overall, the growth response of the study stands to the 2017 drought was contingent on site conditions and species characteristics. Most sites presented BAI and canopy cover reductions during the drought. Growth decline was followed by a quick recovery and positive legacy effects, particularly in the case of F. ornus. However, we found negative drought legacies in some species (e.g., Q. pubescens, A. monspessulanum) and sites. In those sites showing negative legacies, high growth rates prior to drought in response to previous wet winter-spring conditions may have predisposed trees to drought damage. Vice versa, the positive drought legacy found in some F. ornus site was linked to post-drought growth release due to Q. pubescens dieback and mortality. Therefore, we found evidences of structural drought overshoot, but it was restricted to specific sites and species. Our findings highlight the importance of considering site settings such as stand composition, pre-drought conditions and different tree species when studying structural overshoot. Droughts contribute to modify the composition and dynamics in mixed forests.

Diversified grain rotations can be highly and reliably productive in unstable climates

The crop rotation is a farm-scale management choice that dictates agronomic output, ecological impacts, and farm viability. Rotations have become less diverse recently. Re-diversifying may help agricultural systems meet the growing, dynamic demands from society and challenges from climate change. Using a long-term experiment, we tested whether diversified rotations could a) match the productivity (grain yield) of simplified rotations while b) stabilizing productivity against variable weather in the western U.S. Corn Belt. We identified rotation design choices that drive these productivity-stability dynamics. Although more productive rotations were more susceptible to weather (r = 0.63; p < 0.001), this tradeoff was surmountable (p < 0.001): the corn-oat-winter wheat-soybean (Zea mays L., Avena sativa L., Triticum aestivum L., Glycine max (L.) Merr.) rotation was a) no less productive than the highly unstable, corn-soybean rotation (p = 0.11); and b) only marginally less stable than the most stable rotation (p = 0.06). Crop selection and sequencing were critical to this outcome. High productivity was due to a) overyielding of individual crops in diversified rotations that increased with time; b) sequencing to allow higher-yielding winter crops; and c) beneficial crop legacy effects. Stability was highest in rotations that included crops belonging to more weather-niches (r2 = 0.67; p < 0.001). Surprisingly, these weather-niches groups did not correspond to traditional categories like cool- vs warm-season crops. These results suggest four principles for the design, study, and implementation of diversified grain rotations that are stable under erratic weather and are as productive as current standard practices.

Revealing legacy effects of extreme droughts on tree growth of oaks across the Northern Hemisphere

Forests are undergoing increasing risks of drought-induced tree mortality. Species replacement patterns following mortality may have a significant impact on the global carbon cycle. Among major hardwoods, deciduous oaks (Quercus spp.) are increasingly reported as replacing dying conifers across the Northern Hemisphere. Yet, our knowledge on the growth responses of these oaks to drought is incomplete, especially regarding post-drought legacy effects. The objectives of this study were to determine the occurrence, duration, and magnitude of legacy effects of extreme droughts and how that vary across species, sites, and drought characteristics. The legacy effects were quantified by the deviation of observed from expected radial growth indices in the period 1940–2016. We used stand-level chronologies from 458 sites and 21 oak species primarily from Europe, north-eastern America, and eastern Asia. We found that legacy effects of droughts could last from 1 to 5 years after the drought and were more prolonged in dry sites. Negative legacy effects (i.e., lower growth than expected) were more prevalent after repetitive droughts in dry sites. The effect of repetitive drought was stronger in Mediterranean oaks especially in Quercus faginea. Species-specific analyses revealed that Q. petraea and Q. macrocarpa from dry sites were more negatively affected by the droughts while growth of several oak species from mesic sites increased during post-drought years. Sites showing positive correlations to winter temperature showed little to no growth depression after drought, whereas sites with a positive correlation to previous summer water balance showed decreased growth. This may indicate that although winter warming favors tree growth during droughts, previous-year summer precipitation may predispose oak trees to current-year extreme droughts. Our results revealed a massive role of repetitive droughts in determining legacy effects and highlighted how growth sensitivity to climate, drought seasonality and species-specific traits drive the legacy effects in deciduous oak species.