•Deciduous and evergreen trees exhibit significant difference in Td dynamics.•The plant hydraulic model captures the observed Td response to SWC and VPD.•All trees exhibit high HFR under drought, ...while deciduous trees have higher SCR.•The coordination between Gs and plant hydraulics mediates the response to stresses.
The physiological response of plants to water stresses has been a focus in understanding plant-atmosphere feedback. Tropical forests are particularly noteworthy for their remarkable diversity and dynamics. However, a deep understanding of physiological response and associated mortality risk of tropical trees with different leaf phenology under drought is still deficient. In this study, we combined sap flow measurements and a plant hydraulic model to present the water utilization of four canopy trees (two deciduous trees and two evergreen trees) in a Panamanian seasonal tropical forest during the 2016 El Niño drought. The results showed that the transpiration (Td) of deciduous trees rapidly decreased with intensifying soil drought during the dry season, while evergreen trees still maintained high Td and increased with the increase of atmospheric dryness (vapor pressure deficit (VPD)). During the wet season, the Td of both deciduous and evergreen trees was jointly driven by increasing soil moisture (SWC) and reducing VPD. The differential Td pattern during the dry season is closely related to the difference in leaf phenology, with deciduous trees greatly reducing canopy stomatal conductance (Gs) through defoliation, and thereby maintaining leaf water potential (Ψl) and reducing water loss. Additionally, deciduous trees demonstrated a slow increase in the difference between midday (Ψl,md) and predawn (Ψl,pd) leaf water potential (i.e. lower hydraulic sensitivity (σ)) compared to evergreen trees. Evaluation of mortality risk under changing water stresses further indicated that all trees exhibited high hydraulic failure risk (HFR) under decreasing SWC scenarios. Moreover, deciduous trees displayed a much higher stomatal closure risk (SCR) than evergreen trees. These differences in Gs response and σ suggest a key role of the coordination between stomatal regulation and plant hydraulics in mediating physiological response to water stresses. More investigation and mechanism illumination will facilitate the prediction of tropical forest response to drought.
•Climate warming is expected to advance leaf unfolding and enhance tree growth.•We compared long-term series of leaf phenology and estimated wood formation.•Long-term leaf and wood formation were not ...coupled.•A longer growing season does not necessarily leads to larger radial growth.
Climate warming is expected to lengthen the growing season of tree species and enhance radial growth rates. Alternatively, a longer growing season could not lead to improved radial growth if wood production depends more on growth rate than on growing season length. We test these ideas by comparing leaf phenology data and the estimated start and end dates of wood formation predicted by the VS-Lite growth model. We analyzed long-term series of leaf unfolding and fall dates and reconstructed radial growth of two pine species under contrasting climatic conditions: Scots pine (Pinus sylvestris) in a Russian boreal site and Aleppo pine (Pinus halepensis) in a Spanish Mediterranean site. On average, leaf onset occurred in days 99 and 163 in P. halepensis and P. sylvestris, respectively, about 40 days earlier than the estimated start date of wood formation. The onset of leaf unfolding advanced 2.1 days per decade in P. sylvestris in response to warmer May temperatures. Radial growth was enhanced by warm-wet spring-summer conditions in P. sylvestris and by wet soil conditions from prior winter up to current summer in P. halepensis. In this species the growing season length and the radial growth rate were not coupled because the growing season length shortened during cool-wet periods whereas growth rates increased. In P. sylvestris leaf onset was delayed during years with low growth rates suggesting a potential coupling between warmer spring conditions, earlier leaf onset and enhanced growth whenever soil water content is high enough. Overall, we show that longer growing seasons do not necessarily imply higher radial growth rates.
•We established single leaf area model of two broad-leaved species, Tilia amurensis and Betula platyphylla.•The general mode for Tilia amurensis, Betula platyphylla and Pinus koraiensis was to turn ...on the corresponding phenological events when the AAT reaches a certain threshold.•Compared with those with temperature as the independent variable, the phenology models with time as the independent variable had lower goodness of fit.•AAT-based models were used to establish a leaf area index prediction model that reflects the impact of abnormal climate and improves prediction accuracy and reliability.
Global warming is causing substantially earlier spring leaf phenology in temperate‐zone trees. Understanding the drivers and mechanisms behind the observed plant phenological changes is important for predicting future phenological dynamics. Here, leaf phenological dynamic data (including number and area) observed in situ for three tree species (namely, Tilia amurensis, Pinus koraiensis and Betula platyphylla) on Changbai Mountain from 2017 to 2020 were analyzed. We found that none of the tree species showed active accumulated temperature (AAT) differences in phenological events and phase; however, differences in time (i.e., photoperiod) were observed and could be explained by AAT being the primary controller of plant phenology. Moreover, we developed process-based spring leaf phenology models that were fitted and validated using the leaf development process and time series. Compared with the time-based models, the phenology model with AAT as the independent variable is more robust in fitting and predicting leaf area and leaf number. Our novel findings provide evidence of AAT effects on leaf unfolding, whereby when the AAT reaches a certain threshold, the corresponding phenology will be triggered. Therefore, we used AAT-based models and plot survey data to simulate the leaf area index (LAI) dynamics of the tree species studied, which provides a feasible method to understand the complex processes scaling up from the plant and forest‐levels. Our study has provided a new answer to how temperature affects spring leaf phenology in temperate forests, and significantly improved the predictability for leaf development.
Temperature is the primary factor controlling plant phenology. As temperature changes with latitude, leaf phenology in spring always shows a significant latitudinal pattern. However, under asymmetric ...warming at the low and high latitudes, the variability of the spring leaf phenology with latitude is becoming unclear. Based on the 23,094 observations of the leaf unfolding date (LUD) for woody species located in eastern China within latitudes 23–49°N, we analyzed the variability of LUD and its latitudinal sensitivity (Slat, days °N−1, expressed in delayed days per degree in latitude) during 1963–2008. The results showed an earlier LUD at the mid- (−2.2 days decade−1) and high (−2.5 days decade−1) latitude regions, while a stable LUD at the low-latitude regions during 1963–2008. However, the temperature sensitivity of LUD (ST, days °C−1, expressed in advanced days per degree in temperature) remained stable across the latitudes although a slight decreasing trend from 1963 to 2008. As a result, the non-uniform optimal preseason warming with latitude (Tlat, °C °N−1, expressed in the increase of temperature per degree in latitude) decreased Slat from 2.38 (days °N−1) in 1963 to 1.55 (days °N−1) in 2008. Further analyses indicated that the Growing Degree Hours (GDH) played a critical role in these processes, although the Chilling Hours (CH) showed significant variability after 1991. Our results provide evidence that the change in the balance of CH and GDH across latitude induced declining Slat over the last 40 years in eastern China. Furthermore, it may continue under the future climate warming scenarios and ultimately has important consequences on the structure and function of ecosystems.
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•LUD of tree species was advanced at middle and high latitudes in central east China.•The latitudinal sensitivity of LUD (Slat, days per °N) was declined during 1963–2008.•Growing Degree Hours instead of Chilling Hours played a critical role in this process.
Here we report the representation of stomatal regulation of transpiration and CO2 assimilation is key to forecasting terrestrial ecosystem responses to global change. Given its importance in ...determining the relationship between forest productivity and climate, accurate and mechanistic model representation of the relationship between stomatal conductance (gs) and assimilation is crucial. We assess possible physiological and mechanistic controls on the estimation of the g1 (stomatal slope, inversely proportional to water use efficiency) and g0 (stomatal intercept) parameters, using diurnal gas exchange surveys and leaf level response curves of six tropical broadleaf evergreen tree species. g1 estimated from ex-situ response curves averaged 50% less than g1 estimated from survey data. While g0 and g1 varied between leaves of different phenological stages, the trend was not consistent among species. We identified a diurnal trend associated with g1 and g0 that significantly improved model projections of diurnal trends in transpiration. The accuracy of modelled gs can be improved by accounting for variation in stomatal behavior across diurnal periods, and between measurement approaches, rather than focusing on phenological variation in stomatal behavior. Additional investigation into the primary mechanisms responsible for diurnal variation in g1 will be required to account for this phenomena in land surface models.
•Analysis of 49088 phenological observations from 6 tree species in Switzerland.•Leaf unfolding has advanced by up to –3.0 days/decade since 1985.•Leaf colouring was mainly delayed since 1985, ...reaching +4.0 days/decade.•Climate change intensified drought for trees at low and high elevations.•Shifting phenology amplified drying for low-elevation beech, rowan, and sycamore.
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Climate change alters the bioclimatic conditions during the growing period of trees directly, but also indirectly by causing shifts in spring and autumn leaf phenology that lead to changes in the timing and length of the growing period. Several studies researched the ecological consequences of direct climate change effects on bioclimatic conditions during the growing period of trees. However, the complementary and indirect effects through phenological shifts on these conditions have been insufficiently investigated. We analysed 49088 leaf unfolding and leaf colouring dates of six major European tree species from Switzerland, observed between 200 and 1900 m a.s.l. during 1961–2018. We estimated phenological trends, the resulting changes in bioclimatic conditions during the growing period, and the relative contributions of phenological shifts towards these changes. Our results show that climate change advanced leaf unfolding by up to –3.0 days/decade since 1985. Leaf colouring was mainly delayed at low elevations and was advanced or delayed at high elevations with species-specific rates between –3.1 and +4.0 days/decade. While the length of the growing period and growing degree-days increased for most species after 1985, precipitation during the growing period predominantly decreased by up to –43.6 mm/decade. Furthermore, drought intensity during the growing period (based on the number of days with negative water balance) increased significantly for most species, reaching +6.7 days/decade at low elevations. Phenological shifts amplified the trends towards drier conditions by up to +81% at low elevations for beech, rowan, and sycamore, but weakened them by up to –84% at high elevations for beech, rowan, sycamore, and larch. These findings indicate widely increased drought stress, especially at lower elevations. Further, we conclude that future forest net ecosystem productivity in Central Europe will depend strongly on elevation and species composition, despite a general lengthening of the growing period for trees.
During droughts, leaves are predicted to act as ‘hydraulic fuses’ by shedding when plants reach critically low water potential (Ψplant), thereby slowing water loss, stabilizing Ψplant and protecting ...against cavitation-induced loss of stem hydraulic conductivity (K
s).
We tested these predictions among trees in seasonally dry tropical forests, where leaf shedding is common, yet variable, among species. We tracked leaf phenology, Ψplant and K
s in saplings of six tree species distributed across two forests.
Species differed in their timing and extent of leaf shedding, yet converged in shedding leaves as they approached the Ψplant value associated with a 50% loss of K
s and at which their model-estimated maximum sustainable transpiration rate approached zero. However, after shedding all leaves, the Ψplant value of one species, Genipa americana, continued to decline, indicating that water loss continued after leaf shedding. K
s was highly variable among saplings within species and seasons, suggesting a minimal influence of seasonal drought on K
s.
Hydraulic limits appear to drive diverse patterns of leaf shedding among tropical trees, supporting the hydraulic fuse hypothesis. However, leaf shedding is not universally effective at stabilizing Ψplant, suggesting that the main function of drought deciduousness may vary among species.
Climate warming is substantially shifting the leaf phenological events of plants, and thereby impacting on their individual fitness and also on the structure and functioning of ecosystems. Previous ...studies have largely focused on the climate impact on spring phenology, and to date the processes underlying leaf senescence and their associated environmental drivers remain poorly understood. In this study, experiments with temperature gradients imposed during the summer and autumn were conducted on saplings of European beech to explore the temperature responses of leaf senescence. An additional warming experiment during winter enabled us to assess the differences in temperature responses of spring leaf‐out and autumn leaf senescence. We found that warming significantly delayed the dates of leaf senescence both during summer and autumn warming, with similar temperature sensitivities (6–8 days delay per °C warming), suggesting that, in the absence of water and nutrient limitation, temperature may be a dominant factor controlling the leaf senescence in European beech. Interestingly, we found a significantly larger temperature response of autumn leaf senescence than of spring leaf‐out. This suggests a possible larger contribution of delays in autumn senescence, than of the advancement in spring leaf‐out, to extending the growing season under future warmer conditions.
Climate warming is substantially shifting the leaf phenology, but to date the processes underlying leaf senescence and their associated environmental drivers remain unclear. In this study, using experimental temperature gradients and saplings of European beech, we found that warming significantly delayed leaf senescence timing both during summer and autumn warming, with similar temperature sensitivities. Interestingly, we found a significantly larger temperature response of autumn leaf senescence than of spring leaf‐out, suggesting a possible larger contribution of delays in autumn senescence, than of the advancement in spring leaf‐out, to extending the growing season under future climate warming conditions.
Leaf and stem phenology are critical drivers of tree growth patterns in seasonal climates, but the implications for species differences in radial stem growth dynamics are still poorly studied. In our ...study, we compared the leaf phenology and stem phenology with the underlying cell development as well as annual stem growth between five diffuse-porous (DP) and five ring-porous (RP) coexisting angiosperm species in cold and humid temperate forests. Our results show that RP species unfolded leaves later but initiated wood growth earlier compared to DP species. Yet, xylem vessel maturation in the stem started in June and was remarkably synchronized between DP and RP, which implies that species from both functional groups can effectively avoid vessel cavitation potentially triggered by late spring frost. DP species exhibited one peak in stem growth across the growing season reflecting a uniform vessel formation pattern. Instead, RP species exhibited two peaks in stem growth, with the first peak reflecting expansion of early-wood vessels and the second peak related to subsequent fiber and vessel proliferation in the late-wood. In general, species with a greater number of growth days from the start of cambium activity until full lignification of cells exhibited higher annual stem growth, regardless of species group. The observed differences in leaf and stem phenology between DP and RP species are discussed with respect to the adaptation potential of the two functional groups to changing climate conditions in cold and humid temperate forests.
In temperate forests, leaf phenology is a sensitive indicator of climate change and a major regulator of seasonal carbon and water cycling. Many studies have documented large intra-site leaf ...phenology variability across individual trees but conventional approaches for monitoring individual tree-scale leaf phenology are often limited to a small spatial extent and sample size. Recent availability of PlanetScope satellite data with a 3 m spatial resolution, near-daily revisiting frequency, and global coverage provides opportunities to overcome this limitation. It also has the advantage of providing spatially explicit information across large spatial coverages compared with ground methods. However, comprehensive assessments of PlanetScope's capacity and scalability for individual tree-scale leaf phenology monitoring remain lacking. To address this knowledge gap, we propose an approach that integrates 0.1 m resolution airborne imagery and ground phenology records of individual trees with PlanetScope image time series, testing it at six NEON forest sites in eastern North America. We first extracted key phenological metrics at the individual tree scale from PlanetScope satellites and then evaluated the metrics with corresponding phenological metrics derived from ground observations over 2018 and 2019. Our results show that PlanetScope-derived fine-scale land surface phenology is able to 1) characterize significant leaf phenology variability at the individual tree scale across all forest sites and years, with r ranging from 0.21 to 0.42 when comparing PlanetScope-derived individual tree-scale phenological metrics with their ground correspondences. The accuracy is improved at the species level (r = 0.57–0.82) when more PlanetScope pixels are included; and 2) capture relatively more variations in fall phenology but also with larger uncertainties (e.g., r = 0.82 and RMSE = 2.14; species level) relative to spring phenology (r = 0.76 and RMSE = 0.72). Collectively, this study presents a comprehensive evaluation of PlanetScope's capacity for individual tree/species-scale leaf phenology monitoring and highlights the potential of PlanetScope to provide rich fine-scale phenology information to significantly advance the field of plant phenology research.
•A satellite-airborne-ground integration was used to derive tree-scale phenology.•The derived phenological metrics were rigorously evaluated across sites and years.•The tree-scale phenology from PlanetScope agreed well with ground observations.•PlanetScope satellites captured more phenology variations in fall than in spring.•Fine-scale phenology details with large area coverage was mapped by PlanetScope.
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