• Energy Research

There is a debate concerning the definition and extent of tropical dry forest biome and vegetation type at a global spatial scale. We identify the potential extent of the tropical dry forest biome based on bioclimatic definitions and climatic data sets to improve global estimates of distribution, cover, and change. We compared four bioclimatic definitions of the tropical dry forest biome–Murphy and Lugo, Food and Agriculture Organization (FAO), DryFlor, aridity index–using two climatic data sets: WorldClim and Climatologies at High-resolution for the Earth’s Land Surface Areas (CHELSA). We then compared each of the eight unique combinations of bioclimatic definitions and climatic data sets using 540 field plots identified as tropical dry forest from a literature search and evaluated the accuracy of World Wildlife Fund tropical and subtropical dry broadleaf forest ecoregions. We used the definition and climate data that most closely matched field data to calculate forest cover in 2000 and change from 2001 to 2020. Globally, there was low agreement (< 58%) between bioclimatic definitions and WWF ecoregions and only 40% of field plots fell within these ecoregions. FAO using CHELSA had the highest agreement with field plots (81%) and was not correlated with the biome extent. Using the FAO definition with CHELSA climatic data set, we estimate 4,931,414 km2 of closed canopy (≥ 40% forest cover) tropical dry forest in 2000 and 4,369,695 km2 in 2020 with a gross loss of 561,719 km2 (11.4%) from 2001 to 2020. Tropical dry forest biome extent varies significantly based on bioclimatic definition used, with nearly half of all tropical dry forest vegetation missed when using ecoregion boundaries alone, especially in Africa. Using site-specific field validation, we find that the FAO definition using CHELSA provides an accurate, standard, and repeatable way to assess tropical dry forest cover and change at a global scale.

AbstractMounting evidence suggests that anthropogenic global change is altering plant species composition in tropical forests. Fewer studies, however, have focused on long‐term trends in reproductive activity, in part because of the lack of data from tropical sites. Here, we analyze a 28‐year record of tropical flower phenology in response to anthropogenic climate and atmospheric change. We show that a multidecadal increase in flower activity is most strongly associated with rising atmospheric CO2 concentrations using yearly aggregated data. Compared to significant climatic factors, CO2 had on average an approximately three‐, four‐, or fivefold stronger effect than rainfall, solar radiation, and the Multivariate ENSO Index, respectively. Peaks in flower activity were associated with greater solar radiation and lower rainfall during El Niño years. The effect of atmospheric CO2 on flowering has diminished over the most recent decade for lianas and canopy trees, whereas flowering of midstory trees and shrub species continued to increase with rising CO2. Increases in flowering were accompanied by a lengthening of flowering duration for canopy and midstory trees. Understory treelets did not show increases in flowering but did show increases in duration. Given that atmospheric CO2 will likely continue to climb over the next century, a long‐term increase in flowering activity may persist in some growth forms until checked by nutrient limitation or by climate change through rising temperatures, increasing drought frequency and/or increasing cloudiness and reduced insolation.

A warmer world could extend the growing seasons for plants. Changes in spring phenology have been studied, yet autumn phenology remains poorly understood. Using >500,000 phenological records of four temperate tree species between 1951 and 2013 in Europe, we show that leaf senescence in warm autumns exhibits stronger climate responses, with a higher phenological plasticity, than in cold autumns, indicating a nonlinear response to climate. The onset of leaf senescence in warm autumns was delayed due to the stronger climate response, primarily caused by night-time warming. However, daytime warming, especially during warm autumns, imposes a drought stress which advances leaf senescence. This may counteract the extension of growing season under global warming. These findings provide guidance for more reliable predictions of plant phenology and biosphere–atmosphere feedbacks in the context of global warming. Autumn leaf senescence has later onset, higher phenological plasticity and a stronger climatic response under warm compared to cold autumns. While night-time warming delays senescence, drought induced by daytime warming advances it, which may lead to loss in growing season under global warming.

SummaryPhenological events – defined points in the life cycle of a plant or animal – have been regarded as highly plastic traits, reflecting flexible responses to various environmental cues.The ability of a species to track, via shifts in phenological events, the abiotic environment through time might dictate its vulnerability to future climate change. Understanding the predictors and drivers of phenological change is therefore critical.Here, we evaluated evidence for phylogenetic conservatism – the tendency for closely related species to share similar ecological and biological attributes – in phenological traits across flowering plants. We aggregated published and unpublished data on timing of first flower and first leaf, encompassing ˜4000 species at 23 sites across theNorthern Hemisphere. We reconstructed the phylogeny for the set of included species, first, using the software program Phylomatic, and second, fromDNAdata. We then quantified phylogenetic conservatism in plant phenology within and across sites.We show that more closely related species tend to flower and leaf at similar times. By contrasting mean flowering times within and across sites, however, we illustrate that it is not the time of year that is conserved, but rather the phenological responses to a common set of abiotic cues.Our findings suggest that species cannot be treated as statistically independent when modelling phenological responses.Synthesis. Closely related species tend to resemble each other in the timing of their life‐history events, a likely product of evolutionarily conserved responses to environmental cues. The search for the underlying drivers of phenology must therefore account for species' shared evolutionary histories.

Earlier spring phenology observed in many plant species in recent decades provides compelling evidence that species are already responding to the rising global temperatures associated with anthropogenic climate change. There is great variability among species, however, in their phenological sensitivity to temperature. Species that do not phenologically “track” climate change may be at a disadvantage if their growth becomes limited by missed interactions with mutualists, or a shorter growing season relative to earlier‐active competitors. Here, we set out to test the hypothesis that phenological sensitivity could be used to predict species performance in a warming climate, by synthesizing results across terrestrial warming experiments. We assembled data for 57 species across 24 studies where flowering or vegetative phenology was matched with a measure of species performance. Performance metrics included biomass, percent cover, number of flowers, or individual growth. We found that species that advanced their phenology with warming also increased their performance, whereas those that did not advance tended to decline in performance with warming. This indicates that species that cannot phenologically “track” climate may be at increased risk with future climate change, and it suggests that phenological monitoring may provide an important tool for setting future conservation priorities.

AbstractThe Hawaiian Islands have been identified as a global biodiversity hotspot. We examine the Normalized Difference Vegetation Index (NDVI) using Climate Data Records products (0.05 × 0.05°) to identify significant differences in NDVI between neutral El Niño-Southern Oscillation years (1984, 2019) and significant long-term changes over the entire time series (1982–2019) for the Hawaiian Islands and six land cover classes. Overall, there has been a significant decline in NDVI (i.e., browning) across the Hawaiian Islands from 1982 to 2019 with the islands of Lāna’i and Hawai’i experiencing the greatest decreases in NDVI (≥44%). All land cover classes significantly decreased in NDVI for most months, especially during the wet season month of March. Native vegetation cover across all islands also experienced significant declines in NDVI, with the leeward, southwestern side of the island of Hawai’i experiencing the greatest declines. The long-term trends in the annual total precipitation and annual mean Palmer Drought Severity Index (PDSI) for 1982–2019 on the Hawaiian Islands show significant concurrent declines. Primarily positive correlations between the native ecosystem NDVI and precipitation imply that significant decreases in precipitation may exacerbate the decrease in NDVI of native ecosystems. NDVI-PDSI correlations were primarily negative on the windward side of the islands and positive on the leeward sides, suggesting a higher sensitivity to drought for leeward native ecosystems. Multi-decadal time series and spatially explicit data for native landscapes provide natural resource managers with long-term trends and monthly changes associated with vegetation health and stability.

AbstractTropical forests are hyper‐diverse and perform critical functions that regulate global climate, yet they are also threatened by rising temperatures. Canopy temperatures depart considerably from air temperatures, sometimes by as much as air temperatures are projected to increase by the end of this century; however, canopy temperatures are rarely measured or considered in climate change analyses. Our results from near‐continuous thermal imaging of a well‐studied tropical forest show that canopy temperatures reached a maximum of ~34°C, and exceeded maximum air temperatures by as much as 7°C. Comparing different canopy surfaces reveals that bark was the warmest, followed by a deciduous canopy, flowers, and coolest was an evergreen canopy. Differences among canopy surfaces were largest during afternoon hours, when the evergreen canopy cooled more rapidly than other canopy surfaces, presumably due to transpiration. Gross primary productivity (GPP), estimated from eddy covariance measurements, was more strongly associated with canopy temperatures than air temperatures or vapor pressure deficit. The rate of GPP increase with canopy temperatures slowed above ~28–29°C, but GPP continued to increase until ~31–32°C. Although future warming is projected to be greater in high‐latitude regions, we show that tropical forest productivity is highly sensitive to small changes in temperature. Important biophysical and physiological characteristics captured by canopy temperatures allow more accurate predictions of GPP compared to commonly used air temperatures. Results suggest that as air temperatures continue to warm with climate change, canopy temperatures will increase at a ~40% higher rate, with uncertain but potentially large impacts on tropical forest productivity.