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Our limited understanding of the impacts of drought on tropical forests significantly impedes our ability in accurately predicting the impacts of climate change on this biome. Here, we investigated the impact of drought on the dynamics of forest canopies with different heights using time‐series records of remotely sensed Ku‐band vegetation optical depth (Ku‐VOD), a proxy of top‐canopy foliar mass and water content, and separated the signal of Ku‐VOD changes into drought‐induced reductions and subsequent non‐drought gains. Both drought‐induced reductions and non‐drought increases in Ku‐VOD varied significantly with canopy height. Taller tropical forests experienced greater relative Ku‐VOD reductions during drought and larger non‐drought increases than shorter forests, but the net effect of drought was more negative in the taller forests. Meta‐analysis of in situ hydraulic traits supports the hypothesis that taller tropical forests are more vulnerable to drought stress due to smaller xylem‐transport safety margins. Additionally, Ku‐VOD of taller forests showed larger reductions due to increased atmospheric dryness, as assessed by vapor pressure deficit, and showed larger gains in response to enhanced water supply than shorter forests. Including the height‐dependent variation of hydraulic transport in ecosystem models will improve the simulated response of tropical forests to drought. [ABSTRACT FROM AUTHOR]

Droughts cause extreme anomalies in tropical forest growth, but the direction and magnitude of tropical forests in response to droughts are still widely debated. Here, we used four satellite‐based canopy growth proxies (CGPs), including three optical and one passive microwave, and in situ fluxes observations from eddy covariance (EC) measurements for a retrospective investigation of the impacts of historical droughts on tropical forest growth from a statistical point of view. Results indicate two opposite directions in drought‐related canopy dynamics across pantropical forests. The canopy of tropical forests with higher CGPs is more vulnerable to drought stress and recovers faster in the post‐drought recovery period. In contrast, the canopy of tropical forests with lower CGPs increases during the drought period and declines in the subsequent recovery period, which is beyond general expectation. In situ measurements from eddy‐covariance flux towers showed that forests with higher gross primary production and latent heat flux decreased photosynthesis and evapotranspiration during the drought period but increased photosynthesis and evapotranspiration faster during the post‐drought recovery period, supporting the findings from satellite observations. Our statistical analysis against climatic factors predicts that higher‐CGPs tress with probably taller and bigger canopies are more responsive to shortage of water availability caused by drought; while lower‐CGPs tress with shorter and smaller canopies are more responsive to sunlight availability and tend to increase their canopy leaves and enhance photosynthesis in sunnier days during the drought period. Our results highlight the differences in tropical forests in responding to drought stress, which are worth incorporated in Earth system models for time‐series evaluations. [ABSTRACT FROM AUTHOR]

Climatic drivers for canopy leaf shedding and flush of evergreen broadleaved forest biome are still unclear at the continental scale across tropical and subtropical region. This imposes a challenge for modeling pantropical photosynthesis seasonality in Earth system models. Here, we examined three potential climatic triggers, vapor pressure deficit–a proxy of atmospheric water deficit, downward shortwave incoming solar radiation–a proxy of sunlight availability, and terrestrial water storage–a proxy of soil water availability observed by the Gravity Recovery and Climate Experiment, by comparing with two satellite phenological proxies–the Enhanced Vegetation Index and Continuous Solar‐induced chlorophyll fluorescence. Results show that tropical leaf phenology varies greatly from equatorial bimodal seasonality to higher‐latitude unimodal seasonality. Sunlight availability dominantly controls the whole seasonal leaf phenology across the pantropical region. Atmospheric dryness is one main type of water stress for leaf phenology during the first half year. However, soil water stress strongly inhibits the first‐half of leaf phenology in tropical Asia and the second‐half of leaf phenology in Congo, but shows rare constraint on the leaf phenology in Amazon. Ignoring these various roles of soil moisture availability and atmospheric dryness in influencing tropical leaf phenology might lead to unexpected uncertainty for predicting the water and carbon cycles of tropical forest ecosystem in Earth system models. Key Points: Sunlight dominantly controls whole seasonal leaf phenology across the pantropical regionAtmospheric dryness is the main type of water stress for leaf phenology during the first half yearSoil water stress is one main type of water stress in the first half year in tropical Asia and the second half of year in Congo [ABSTRACT FROM AUTHOR]

Despite its perennial canopy, the Amazonian tropical evergreen forest shows significant canopy growth seasonality, which has been represented by optical satellite-based observations. In this paper, a new Microwave Temperature–Vegetation Drought Index (MTVDI) based on Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sensors was used to capture the canopy seasonality from 2003 to 2010 in comparison with four climatic dryness indicators (Palmer Drought Severity Index (PDSI), Climatological Water Deficit (CWD), Terrestrial Water Storage (TWS), Vapor Pressure Deficit (VPD)) and two photosynthesis proxies (Enhanced Vegetation Index (EVI) and Solar-Induced chlorophyll Fluorescence (SIF)), respectively. Our results suggest that the MTVDI shows opposite seasonal variability with two photosynthesis proxies and performs better than the four climatic dryness indicators in reflecting the canopy photosynthesis seasonality of tropical forests in the Amazon. Besides, the MTVDI captures wet regions that show green-up during the dry season with mean annual precipitation higher than 2000 mm per year. The MTVDI provides a new way for monitoring the canopy seasonality of tropical forests from microwave signals. [ABSTRACT FROM AUTHOR]

• Leaf shedding of tropical evergreen forests depends on synchrony of rainfall and radiation. • Forests replace old leaves by new one to maximize photosynthesis under rarely water stress. • Forests drop off senescent leaves to avoid hydraulic failure when suffer severe water stress. • Ongoing changes in seasonality of tropical rainfall versus radiation alter future leaf phenology. Despite a perennial evergreen canopy, tropical evergreen forests continuously shed old leaves and grow new leaves at different time across different tropical regions. To examine how water and light availability control the leaf phenology of tropical evergreen forests, we used monthly litterfall mass data from 30 sites in Pan-Asian tropics and solar-induced chlorophyll fluorescence (SIF) as leaf shedding and a proxy for canopy photosynthetic activity and productivity, respectively. Two types of climate were identified depending on the positive and negative corrections between seasonal precipitation (Pre) and shortwave radiation (SW), named as synchronous and asynchronous climates, respectively. In regions where Pre and SW are positively correlated, litterfall and SIF are parabolically greater in the wet season (named as synchronous-unimodal regime), or show bimodal litterfall peaks in the dry and wet seasons (named as synchronous-bimodal regime). In regions where Pre and SW are negatively correlated, litterfall is parabolically greater in the dry-sunny season (named as asynchronous-unimodal regime), or show multiple peaks at the wet season, coincides with relatively high SIF (named as asynchronous-multimodal regime). The adaptive strategies of leaf phenology could be generated as a response to water and light availability balancing the benefits and costs from the plant perspective. On the one hand, plant prioritizes light acquisition by exchanging old, senescent leaves for new, efficient leaves to optimize photosynthesis with sufficient water availability. On the other hand, plant sheds old leaves to minimize maintenance costs and to avoid hydraulic failure under water stress during dry season. Our findings are crucial for assessing both the impacts of future climate change on tropical evergreen forests and the feedbacks of tropical evergreen forests on the terrestrial carbon cycle. This study also suggests that the ongoing changes in tropical rainfall seasonality relative to radiation seasonality may significantly alter canopy leaf phenology under future climate conditions. [ABSTRACT FROM AUTHOR]