Accurate monitoring of tropical leaf phenology, such as the leaf-on/off status, at both individual and ecosystem scales is essential for understanding and modelling tropical forest carbon and water ...cycles, and their sensitivity to climate change. The discrepancy between tree-crown size and pixel size (i.e., spatial resolution) across orbital sensors can affect the capability of cross-scale phenology monitoring, an aspect that remains understudied. To examine the impact of spatial resolution on tropical leaf phenology monitoring, we applied a spectral index-guided, ecologically constrained autoencoder (IG-ECAE) to automatically generate a deciduousness metric (i.e., percentage of upper canopy area that is leaf-off status within an image pixel) from simulated VIS-NIR PlanetScope data at a range of resolutions from 3 m to 30 m, as well as from VIS-NIR data of three satellite platforms with the same range of spatial resolutions (3 m PlanetScope, 10 m Sentinel-2, and 30 m Landsat-8). We compared the deciduousness metrics derived from the simulated and satellite data to corresponding measurements derived from WorldView-2 (three sites) and local phenocams (four sites) at five tropical forest sites. Our results revealed that: (1) the IG-ECAE model captured the amount of deciduousness across spatial scales, with the highest accuracy obtained from PlanetScope, followed by Sentinel-2 and Landsat-8; (2) coarser spatial resolutions led to lower accuracies in tropical deciduousness monitoring, as demonstrated by both simulated PlanetScope data across various spatial resolutions and real satellite data; and (3) while not as accurate in capturing fine-scale tropical phenological diversity as PlanetScope, Sentinel-2 provided satisfactory monitoring of deciduousness seasonality at the ecosystem level consistently across all phenocam sites, whereas Landsat-8 failed to do so. Collectively, this study provides a robust assessment for advancing cross-scale tropical leaf phenology monitoring with potential for extension to pan-tropical regions and highlights the impact of spatial resolution on such monitoring efforts.
•Spatial resolution affects phenology monitoring assessed by simulated and real data.•The spatial resolution's impact was evaluated with WorldView-2 and phenocam data.•Coarser resolution reduces phenology monitoring accuracy using deciduousness metric.•Sentinel-2(10 m) nears PlanetScope(3 m)’s ability for ecosystem phenology monitoring.•PlanetScope(3 m) remains vital for characterizing fine-scale phenological diversity.
Forest fragmentation has been increasingly exacerbated by deforestation, urbanization, and agricultural expansion. Monitoring the forest fragments via the lens of tree-crown scale leaf phenology is ...critical to understand tree species phenological responses to climate change and identify the fragment species vulnerable to environmental disturbance. Despite advances in remote sensing for phenology monitoring, detecting tree-crown scale leaf phenology in fragmented forests remains challenging. Simultaneous tracking of key spring phenological events that are crucial to ecosystem functions and climate change responses is also neglected. To address these challenges, we develop a novel tree-crown scale remote sensing phenological monitoring framework to characterize all the critical spring phenological events of individual trees of deciduous forest fragments, with Trelease Woods in Champaign, Illinois as a case study. The novel framework comprises four components: 1) generate high spatiotemporal resolution fusion imagery from multi-scale satellite time series with a hybrid deep learning fusion model; 2) calibrate PlanetScope imagery time series with fusion data using histogram matching; 3) model tree-crown scale phenology trajectory with a Beck logistic-based method; 4) detect a diversity of tree-crown scale phenological events using several phenological metric extraction methods (i.e., threshold- and curve feature-based methods). Combined with weekly in-situ phenological observations of 123 individual trees across 12 broadleaf species from 2017 to 2020, the framework effectively bridges the satellite- and field-based phenological measures for the key spring phenological events (i.e., budswell, budburst, leaf expansion, and leaf maturity events) at the tree-crown scale, particularly for large individuals (RMSE <1 week for most events). Calibration of PlanetScope imagery using multi-scale satellite fusion data in consideration of landscape fragmentation is critical for monitoring tree phenology of forest fragments. Compared to curve feature-based methods, threshold-based phenometric extraction methods demonstrate enhanced capability in detecting spring leaf phenological dynamics of individual trees. Among the phenological events, full leaf out and early leaf expansion events are retrieved with high accuracy using calibrated PlanetScope time series (RMSE from 3 to 5 days and R-squared higher than 0.8). With both intensive satellite and field phenological efforts, this novel framework is at the forefront of interpreting tree-crown scale remotely sensed phenological metrics in the context of biologically meaningful field phenological events in fragmented forest setting.
•Develop a tree-crown scale remote sensing leaf phenological monitoring framework.•Monitor all key spring phenological events of individual trees of forest fragments.•Bridge diverse satellite- and field- spring phenological events of individual trees.•Leaf out and early leaf expansion field events can be retrieved with high accuracy.•Spring tree phenological event detection accuracy varies across tree crown sizes.
•An integrated field and remote sensing approach to partition three phenology rhythms.•Forests rejuvenate canopy in dry seasons and regulate water stress through xylem.•Forests rejuvenate canopy in ...wet seasons and regulate water stress through stomata.•Forests shed and flush leaves alternatively by both xylem and stomatal regulations.
The climatic drivers of leaf phenology and water stress regulation strategies in tropical/subtropical forest biomes are poorly understood on the continental scale. Widespread field observations and remotely sensed plant phenology and physiology data across tropical forest ecosystems at various scales provide new insight into the response of tropical forests to climate change. In this study, we collected seasonal litterfall data from 65 sites across tropical/subtropical America for analysis against remotely sensed photosynthetic and climatic indicators. We proposed an integrated field and remote sensing approach for partitioning three rhythms of forest phenology across tropical and subtropical America in response to various wet- and dry-season variabilities in sunlight, liquid water supply and atmospheric dryness. The results showed that in a dry-season leaf shedding and rejuvenation phenology, trees encounter limited soil water stress, shift to a young canopy by replacing old leaves with new leaves, and regulate water use through the xylem to maximize light capture and productivity. In a wet-season leaf shedding & rejuvenation phenology, trees on the contrary shed and rejuvenate canopy leaves to maximize light use and increase photosynthesis during the sunny wet season. Trees show a dominant stomatal regulation of seasonal water use, resulting in considerable seasonal changes in canopy photosynthesis and transpiration. However, in a dry-season leaf shedding and wet-season rejuvenation phenology, trees shed leaves due to water stresses and grew new leaves to maximize photosynthesis with sufficient water availability. Trees shift from wet-season stomatal regulation to dry-season xylem regulation with fewer canopy leaves. Our findings provide new physiological insight into the mechanism of sunlight availability, liquid water supply and atmospheric dryness in driving leaf phenology and photosynthesis across tropical and subtropical forests.
Deciduous and evergreen trees are usually considered the main coexisting functional groups in seasonally dry tropical forests (SDTF). We compared leaf and stem traits of 22 woody species in the ...Brazilian Caatinga to investigate whether deciduous (DC) and evergreen (EV) species have divergent water-use strategies. Our hypothesis was that DC trees compensate for their short leaf longevity by being less conservative in water use and showing higher variation in the seasonal water potential after leaf shedding. Evergreen species should exhibit a highly conservative water use strategy, which reduces variations in seasonal water potential and the negative effects of desiccation. Our leaf dynamics results indicate that the crown area of DC trees is more sensitive to air and soil drought, whereas EV trees are only sensitive to soil drought. Deciduous species exhibit differences in a set of leaf traits confirming their acquisitive strategy, which contrasts with evergreen species. However, when stomatal traits are considered, we found that DC and EV have similar stomatal regulation strategies (partially isohydric). We also found divergent physiological strategies within DC. For high wood density DC, the xylem water potential (Ψxylem) continued to drop during the dry season. We also found a negative linear relationship between leaf life span (LL) and the transpiration rate per unit of hydraulic conductivity (Λ), indicating that species with high LL are less vulnerable to hydraulic conductivity loss than early-deciduous species. Collectively, our results indicate divergence in the physiology of deciduous species, which suggests that categorizing species based solely on their leaf phenology may be an oversimplification.
Amazon forest leaf phenology patterns have often been inferred from the Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI). But reliable MODIS detection of seasonal ...and interannual leaf phenology patterns has also been questioned and is generally not validated with field observation. Here we compare inter-annual patterns of local-scale upper canopy leaf phenology and demography derived from tower-mounted phenocams at two upland forest sites in the Central Amazon, to corresponding satellite vegetation indices retrieved from MODIS-MAIAC (Multi-Angle Implementation of Atmospheric Correction). We focus on forest response to an unprecedented drought caused by the El Niño of 2015-16. At both sites, multi-year phenocam data showed post-drought shifts in leaf demography. These were consistent with MODIS-MAIAC anomalies in two vegetation indices. Specifically, a precocious leaf flush at both sites during the first two post-drought months, Feb-Mar 2016, caused (1) an anomalous decrease in flushing trees in Jun–Jul of 2016 and (2) an increase of trees with early mature stage leaves (2-4 mo age) in Apr-May-Jun of 2016. At both sites, these two phenological anomalies showed up in MODIS-MAIAC as, respectively, (1) a strong negative anomaly in Gcc (Green chromatic coordinate), which prior work has shown to be sensitive to the abundance of leaves 0-1 mo old, and (2) a strong positive anomaly in EVI, which is sensitive to abundance of leaves 2-4 mo age. A shift to sub-optimal seasonal leaf age mix is expected to change the ecosystem-scale intrinsic photosynthetic capacity for ~18 month after the drought.
•Modis interannual and seasonal variation in tropical forest greenness is debated.•Drought and high temperatures occurred during 2015-16 El Niño in the Central Amazon.•Tower-mounted phenocams and MODIS-MAIAC vegetation indices assessed leaf phenology.•Both detect a cascade of anomalies set off by post-drought precocious leaf flush.•Shifted leaf demography may attenuate photosynthesis seasonality over following year.
Flowering plants predominantly conduct water in tubes known as vessels, with vessel diameter playing a crucial role in plant adaptation to climate and reactions to climate change. The importance of ...vessels makes it essential to understand how and why vessel diameter, plant height, and other ecological factors are interrelated. Although shoot length is by far the main driver of variation in mean vessel diameter across angiosperms, much remains to be understood regarding the factors accounting for the abundant variation around the y-axis in plots of mean species vessel diameter against shoot length. Here, we explore the potential role of porosity types, wood density, leaf phenology, background imperforate tracheary element type, vasicentric tracheids, vascular tracheids, perforation plate type, and successive cambia in causing variation in the y-intercept or slope of the mean species vessel-diameter– and vessel-density–shoot-length associations at the shoot tip and base. We detected numerous cases of ecologically significant variation. For example, latewood vessels of ring porous species scale with a lower slope than earlywood, i.e., latewood vessels are relatively narrow in taller plants. This pattern is likely the result of selection favoring freezing-induced embolism resistance via narrow vessels. Wood density was negatively associated with vessel diameter, with low wood density plants having wider vessels for a given height. Species with scalariform perforation plates scale with a lower shoot base vessel-diameter–shoot-length slope, likely reflecting selection against scalariform plates in wide vessels. In other cases, functional groups scaled similarly. For example, species with successive cambia did not differ from those with conventional single cambia in their mean vessel-diameter–shoot-length scaling, rejecting our prediction that species with successive cambia should have narrower vessels for a given shoot length. They did, however, have fewer vessels per unit shoot cross-sectional area than plants of similar heights, likely because vessels have longer functional lifespans (and therefore are fewer) in species with successive cambia. Our methods illustrate how vessel diameter can be studied taking shoot length into account to detect ecologically important variation and construct theory regarding plant adaptation via the hydraulic system that includes plant size as a vital element.
Climate warming and surface ozone (O3) pollution are important global environmental issues today. However, the combined impacts of air warming and O3 on phenology and its functional traits of urban ...trees are still poorly understood. Here, an experiment was performed to explore the variations of the spring phenological and functional traits in leaves of Populus alba 'Berolinensis' and Forsythia suspensa under ambient air (15.8 °C, 35.7 ppb), increased air temperature (IT, ambient air temperature + 2 °C, 17.9 °C), elevated O3 (EO, ambient air O3 concentrations + 40 ppb, 77.4 ppb), and their combined treatments (17.7 °C, 74.5 ppb). Our results showed that: IT advanced the beginning of leaf bud expansion phase of P. alba 'Berolinensis' and F. suspensa for 6 d and 5 d, respectively, increased leaf unfolding rate, leaf area and dry weight, and enhanced photosynthesis and antioxidative enzyme activities. EO delayed the beginning of leaf bud expansion phase of P. alba 'Berolinensis' for 5 d, decreased leaf area and biomass, and inhibited photosynthesis and caused oxidative damage of plant leaves. Compared to EO, the combined treatment advanced the spring phenophase, increased growth and induced the higher level of photosynthetic rate and antioxidative enzymes activities in plant leaves, which indicated that the positive effects of increased temperature (17.7 °C) alleviated the inhibition of growth and photosynthesis induced by ozone. Our findings can provide a theoretical reference for predicting the adaptation of functional traits of the two trees blossomed early under warming and O3 pollution at spring phenological stage.
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•Increased air temperature advanced spring phenophase, increased leaf areas and leaf unfolding rates of poplar and forsythia.•Elevated ozone delayed leaf budding occurrence and increased oxidative stress in leaves of two urban tree species.•Increased air temperature alleviated the adverse effects of ozone fumigation on leaf functional traits of plants.
The phenology of spring leaf unfolding plays a key role in the structure and functioning of ecosystems. The classical concept of heat requirement (growing degree days) for leaf unfolding was ...developed hundreds of years ago, but this model does not include the recently reported greater importance of daytime than night-time temperature.
A manipulative experiment on daytime vs night-time warming with saplings of three species of temperate deciduous trees was conducted and a Bayesian method was applied to explore the different effects of daytime and night-time temperatures on spring phenology.
We found that both daytime and night-time warming significantly advanced leaf unfolding, but the sensitivities to increased daytime and night-time temperatures differed significantly. Trees were most sensitive to daytime warming (7.4 ± 0.9, 4.8 ± 0.3 and 4.8 ± 0.2 d advancement per degree Celsius warming (d °C−1) for birch, oak and beech, respectively) and least sensitive to night-time warming (5.5 ± 0.9, 3.3 ± 0.3 and 2.1 ± 0.9 d °C−1). Interestingly, a Bayesian analysis found that the impact of daytime temperature on leaf unfolding was approximately three times higher than that of night-time temperatures.
Night-time global temperature is increasing faster than daytime temperature, so model projections of future spring phenology should incorporate the effects of these different temperatures.
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