Are forest seed sources being used for ecological restoration in dry tropical forests across three South American countries?

Assessing aboveground carbon density with field observations and biophysical and spectral predictors in the northmost Neotropical dry forest

In the context of ecosystem conservation, tropical dry forests have received little or no attention compared with their next-door neighbors, the tropical rainforests. This lack of conservation effort in tropical dry forests is reflected in the fact that few national parks and biological reserves protect and preserve their natural richness, and there are only a handful of real biological research stations with a mandate to bridge the gap between ecology and conservation biology in these ecosystems. The funding and legal framework developed by international institutions and local governments has been implemented mainly to protect mature forest or pristine national parks located in regions other than dry forests. Several complex reasons may explain the lack of protection afforded to tropical dry forests. One of them is rooted in the romanticized vision that the tropics do not exist beyond the Amazon basin, a vision one finds every day in the scientific and nonscientific literature and in the electronic media. Another reason is the high economic value associated with goods and services that can be extracted from tropical dry forests, which contrasts with the relatively small economic value of tropical rainforests. This exploitation is furthered by local and national governments when they use dry forests not as the last frontier but as the first frontier of economic development. In the Caribbean, Mesoamérica, and northern South America, tropical dry forests are located in the most fertile zones for agroindustry and ecotourism development, and they contain a large proportion of the human population. Thus, only a small proportion of their total area is under some level of conservation. In Mesoamérica and in Venezuela, <1% of this ecologically, socially, and economically essential ecosystem is protected. Tropical dry forests are in dire need of integrated and multidisciplinary conservation research projects aimed at expanding traditional species- and niche-based research; increasing the biological and ecological knowledge base; and including human dimensions, which inevitably underlie how ecosystems change over time. Ecological studies on tropical dry forest succession, degradation, and restoration are few and most of them have been generated from a few sites. Tied to these ecological studies, conservation schemes are necessary that emphasize the tropical dry forest's contribution to environmental services and its value as a forest ecosystem, rather than as range or agricultural land. Rather than solely promoting the conservation of forest patches that will form isolated national parks or reserves, conservation approaches that pay landowners for environmental services must be implemented. Integrated land-management plans that complement the efforts of governments and the private sector must be enforced. The payment for ecosystem services carried out by the Costa Rican National Forest Financing Fund (FONAFIFO) is a good example of the feasibility of such initiatives. Furthermore, efforts toward conservation of tropical dry forests must also address the need to consider ecosystem services provided by mature ecosystems and areas at various successional stages within and outside public lands. In fact, secondary dry forests may be the dominant landscape in the forthcoming years, as land abandoned by local farmers recovers. In Costa Rica alone, almost 50% of the Guanacaste Peninsula is covered by deciduous secondary forests. Payment for ecosystem services in the tropics, however, requires funds that are largely unavailable. For a payment strategy to succeed, it is critical that national environmental authorities display the necessary political will to invest resources and develop the required regulatory framework. But it is also fundamental that the governments of developed countries, as well as multinational agencies, international conservation organizations, and private donors, look beyond humid tropical ecosystems and expand their portfolio of conservation investments toward tropical dry forests. We could uncover a wealth of valuable information by consciously promoting land-use practices that minimize the amount of stress and land-cover conversion carried out around dry forested areas. Such an approach, combined with educational programs that promote bioliteracy in local schools and community decision-making organizations, would highlight the economic and ecological value of the tropical dry forest as an ecosystem. This approach would further uncover the contributions of tropical dry forests to society, a role that far surpasses their value for anything else. Such an approach, however, can only be explored and demonstrated by further investigating how to successfully integrate alternative land uses into the management of dry forest ecosystems. With an increasing population, a long history of land-use change, and free-trade agreements that encourage large agroindustry developments (e.g., extensive cattle ranching, and watermelon and sugarcane plantations in Mexico and Costa Rica) and a significant increase of their exploitation for ecotourism, the future of tropical dry forests, at least in the Caribbean, Mesoamérica, and northern South America, is of great concern. Land-use choices that are made around these forests will continue to influence their rapid degradation regardless of how many studies we conduct to find out what it is needed to promote their conservation. The remaining dry forests, which include numerous rare and endemic species, offer a unique opportunity to learn more about types of interactions between species and the resulting ecological processes. This information, in turn, would be of tremendous help to policy makers responsible for defining sustainable development policies aimed at benefiting threatened species (e.g., Scarlet Macaw [Ara macao], Yellow-shouldered Parrot [Amazona barbadensis], jaguar [Panthera onca], Baird's tapir [Tapirus bairdii], great false vampire bat [Vampyrum spectrum)], guayacán [Guaiacum sanctum]). More studies are also needed to understand the ecological succession of disturbed tropical dry forest left in fragments and its contribution as a stepping stone between fragments and the forest. Because small populations or communities may be recovering in regenerating fragments, these areas should not be considered waste land but rather important elements that connect protected areas. Furthermore, the immediate sharing of this ecological knowledge with local communities would help convince them to invest in the longevity of this ecosystem and to promote the recovery of degraded areas. An additional and highly contentious topic is the conflict between increasing human demands on water resources and the varying water needs of the differing components of the tropical forest matrix. The matrix is composed of deciduous tropical forest, mangroves, wetlands, savannas, and riparian forests. Such biological wealth is currently endangered by human water demands that depend on this limited resource. Increasing and uncontrolled use of limited water resources for irrigation, human consumption, and tourism—a phenomenon that translates into new dams, deviation of rivers, and the use of river discharge during low-flow seasons—jeopardizes the future of tropical dry forest biodiversity. We believe that research and conservation in tropical dry forests must generate conservation models that are promoted with the same fervor as those designed for humid forests. For example, phenological aspects (e.g., leaf shed, flowering, fruiting) have been especially neglected in large-scale studies, downplaying the fact that part of the uniqueness of this ecosystem is its phenological responses to changing environmental conditions. These "preadaptations" may be essential as global-change-coping mechanisms by tropical biotas. Many questions have yet to be answered. But if we are to understand the phenological responses of individual species and the drivers and effects of degradation and land exploitation on the phenological cycles of the ecosystem at a larger scale—including the organisms that depend on them—more comprehensive conservation efforts are necessary. These efforts must be linked to adequate funding levels that promote comprehensive in situ and comparative studies among dry forest ecosystems across the Americas.

Slash-and-burn effects on fine root biomass and productivity in a tropical dry forest ecosystem in México

Little is known about the successional dynamics of insects in the highly threatened tropical dry forest (TDF) ecosystem. For the first time, we studied the response of carabid beetles to vegetal succession and seasonality in this ecosystem in Colombia. Carabid beetles were collected from three TDF habitat types in two regions in Colombia: initial successional state (pasture), early succession, and intermediate succession (forest). The surveys were performed monthly for 13 months in one of the regions (Armero) and during two months, one in the dry and one in the wet season, in the other region (Cambao). A set of environmental variables were recorded per month at each site. Twenty-four carabid beetle species were collected during the study. Calosomaalternans and Megacephalaaffinis were the most abundant species, while most species were of low abundance. Forest and pasture beetle assemblages were distinct, while the early succession assemblage overlapped with these assemblages. Canopy cover, litter depth, and soil and air temperatures were important in structuring the assemblages. Even though seasonality did not affect the carabid beetle assemblage, individual species responded positively to the wet season. It is shown that early successional areas in TDF could potentially act as habitat corridors for species to recolonize forest areas, since these successional areas host a number of species that inhabit forests and pastures. Climatic variation, like the El Niño episode during this study, appears to affect the carabid beetle assemblage negatively, exasperating concerns of this already threatened tropical ecosystem.

Dry forests in Sub-Saharan Africa are of critical importance for the livelihood of the local population given their strong dependence on forest products. Yet these forests are threatened due to rapid population growth and predicted changes in rainfall patterns. As such, large-scale woody cover monitoring of tropical dry forests is urgently required. Although promising, remote sensing-based estimation of woody cover in tropical dry forest ecosystems is challenging due to the heterogeneous woody and herbaceous vegetation structure and the large intra-annual variability in the vegetation due to the seasonal rainfall. To test the capability of Sentinel-2 satellite imagery for producing accurate woody cover estimations, two contrasting study sites in Ethiopia and Tanzania were used. The estimation accuracy of a linear regression model using the Normalised Difference Vegetation Index (NDVI), a Partial Least Squares Regression (PLSR), and a Random Forest regression model using both single-date and multi-temporal Sentinel-2 images were compared. Additionally, the robustness and site transferability of these methods were tested. Overall, the multi-temporal PLSR model achieved the most accurate and transferable estimations (R2 = 0.70, RMSE = 4.12%). This model was then used to monitor the potential increase in woody coverage within several reforestation projects in the Degua Tembien district. In six of these projects, a significant increase in woody cover could be measured since the start of the project, which could be linked to their initial vegetation, location and shape. It can be concluded that a PLSR model combined with Sentinel-2 satellite imagery is capable of monitoring woody cover in these tropical dry forest regions, which can be used in support of reforestation efforts.

Because tropical dry forests have been disturbed by humans more than wetter tropical forests, there is an urgent need for conservation of the remaining tropical dry forests, and thus a large amount of research has also been directed toward their restoration. The development of cost-efficient silvicultural treatments will be critical to achieve sustained use of forest products from the remaining natural dry forests, as well as for restoration efforts. Tropical dry forests present special problems for silviculturists because prolonged dry seasons reduce growth rates and can cause significant tree mortality, especially for seedlings. Coppice systems are particularly important in the regeneration of these forests. Management of both timber and nontimber products is confined largely to managing their extraction, with relatively little application of silviculture. Although reduced-impact logging operations are increasingly applied in tropical forests, postharvest silvicultural treatments have not been integrated in the management of tropical forests, particularly in dry forests. Management in any form is often complex in areas where dry forests have been fragmented and harvested for subsistence products, such as firewood, and are affected by the damaging effects of livestock grazing and wildfire.

Tropical dry forest, especially in Central America, has experienced high rates of deforestation primarily due to conversion to agricultural fields and pastures. Yet little is known about the effects of the conversion of Central American tropical dry forest on the pollination interactions between hummingbirds and native plants. A better understanding of hummingbird-plant communities in transformed tropical dry forest ecosystems can be useful for the development of conservation strategies to maintain hummingbird diversity and plant-pollinator interactions. As a consequence, in this study, we evaluated how habitat conversion and seasonality affect the total number of recorded hummingbird-plant interactions in a tropical dry forest of Central America. We used network analysis to assess the effects of habitat conversion on the structure and dynamics of the plant-hummingbird assemblages at our study sites. Data on hummingbird visitation to native flowering plants were collected along four transects located in patches of tropical dry forest and four transects in agricultural areas. Each transect was visited twice during the dry season and twice during the rainy season. Our data suggest that, at a local scale, seasonality is a stronger predictor than habitat type of the recorded number of hummingbird-plant interactions at our study sites. The lack of differences in the number of interactions with respect to habitat type is probably related to the generalist nature of our studied hummingbird-plant assemblages, allowing plants and hummingbirds to persist and form new interactions in the transformed environment. Our data also suggest that, although hummingbird-plant assemblages can persist in agricultural environments, habitat conversion to agriculture can cause changes in network patterns such as lower interaction diversity, lower partner diversity, and a higher level of generalization, which have negative implications for the conservation of mutualist pollination interactions. Therefore, our data highlight the importance of natural and semi-natural tropical dry forest remnants within agricultural landscapes for the conservation of pollinators and pollination services necessary for the reproduction of native plants.

Relationship between annual rainfall and tree mortality in a tropical dry forest: Results of a 19-year study at Mudumalai, southern India

Tropical forests, spanning wet and dry forest ecosystems, are pivotal in regulating the global carbon cycle and climate through dynamic exchanges of energy, water, and carbon. These ecosystems influence regional and global climate patterns via biogeochemical feedback mechanisms. However, climate change is altering these processes, with rising temperatures intensifying evaporative demand and affecting photosynthetic activity, as indicated by changes in net ecosystem exchange (NEE). Vegetation and biomass variations further impact microclimates, feeding back into heat and water budgets. Understanding the dynamics and meteorological drivers of carbon and water fluxes is essential for comprehending land surface&amp;#8211;atmosphere interactions.This study compares the climatological and ecological functions of tropical wet and dry forests by examining two contrasting sites in the tropical Andes Mountains of southern Ecuador: the montane dry forest (MDF) in the Laipuna Reserve and the montane rain forest (MRF) in the Reserva Biol&amp;#243;gica San Francisco. The MDF is characterized by a deciduous forest and exhibits pronounced seasonality, with distinct dry (June&amp;#8211;December) and wet (January&amp;#8211;May) periods, driven by the inter-hemispheric shift of the Intertropical Convergence Zone (ITCZ). In contrast, the MRF experiences year-round rainfall, sustaining an evergreen lower montane forest type. Eddy-covariance measurements were used to monitor water and carbon fluxes under these contrasting climatic regimes. This comparison provides valuable insights into the differential roles of these ecosystems in regulating the Earth's energy and carbon budgets under changing climatic conditions. The objective of the study is (i) to quantify the magnitude and seasonality of NEE and its partitioned components, gross primary production (GPP), and ecosystem respiration (Reco) And (ii) to identify the meteorological drivers responsible for the variations in carbon exchange within each ecosystem. The results reveal significant variations in NEE in the MDF between wet and dry seasons. During the wet season, the average NEE was -3.9 &amp;#956;mol m&amp;#8315;&amp;#178; day&amp;#8315;&amp;#185;, while in the dry season, it declined substantially to -0.8 &amp;#956;mol m&amp;#8315;&amp;#178; day&amp;#8315;&amp;#185;. In contrast, the MRF demonstrated a consistently higher average NEE of -18 &amp;#956;mol m&amp;#8315;&amp;#178; day&amp;#8315;&amp;#185;. These variations are driven by distinct environmental factors. In the MDF, water availability, regulated primarily by precipitation, is the dominant factor influencing carbon exchange. Conversely, the carbon dynamics in MRF are predominantly governed by energy inputs, with light playing a critical role in driving its NEE.

The study determined the carbon stocks and litter nutrient concentration in tropical forests along the ecological gradient in Kenya. This could help understand the potential of mitigating climate change using tropical forest ecosystems in different ecological zones, which are being affected by climate change to a level that they are becoming carbon sources instead of sinks. Stratified sampling technique was used to categorize tropical forests into rain, moist deciduous and dry zone forests depending on the average annual rainfall received. Simple random sampling technique was used to select three tropical forests in each category. Modified consistent sampling technique was used to develop 10 main 20 m × 100 m plots in each forest, with 20 2 m × 50 m sub-plots in each plot. Systematic random sampling technique was used in selecting 10 sub-plots from each main plot for inventory study. Non-destructive approach based on allometric equations using trees’ diameter at breast height (DBH), total height and species’ wood specific gravity were used in estimating tree carbon stock in each forest. Soil organic carbon (SOC) and litter nutrient concentration (total phosphorus and nitrogen) were determined in each forest based on standard laboratory procedures. The results indicated that, whilst trees in rain forests recorded a significantly higher (p < 0.001) DBH (20.36 cm) and total tree height (12.1 m), trees in dry zone forests recorded a significantly higher (p < 0.001) specific gravity (0.67 kg m−3). Dry zone tropical forests stored a significantly lower amount of total tree carbon of 73 Mg ha−1, compared to tropical rain forests (439.5 Mg ha−1) and moist deciduous tropical forests (449 Mg ha−1). The SOC content was significantly higher in tropical rainforests (3.9%), compared to soils from moist deciduous (2.9%) and dry zone forests (1.8%). While litter from tropical rain forests recorded a significantly higher amount of total nitrogen (3.4%), litter from dry zone forests recorded a significantly higher concentration of total phosphorus (0.27%). In conclusion, ecological gradient that is dictated by the prevailing temperatures and precipitation affects the tropical forests carbon stock potential and litter nutrient concentration. This implies that, the changing climate is having a serious implication on the ecosystem services such as carbon stock and nutrients cycling in tropical forests.

Guanica Forest, with seasonal rainfall averaging 860 mm annually, is among the driest of tropical or subtropical forests studied to date. It is composed of over 12,000 live tree stems per hectare, only 2.3 and 12 percent of which exceed 10 cm DBH or 5 m in height, respectively. Of all stems greater than 2.5 cm DBH, 57 percent are stump or root sprouts, attributable to forest cutting 50 years earlier. The dry winter months induce maximum deciduousness and are reflected in a 50 percent reduction in leaf area index, from approximately 4.3 to 2.1. Although less in magnitude, leaf fall was also observed in the moderately dry midsummer months. Relative to wetter forests, tree species richness and total community biomass is low. Approximately 50 percent of the total live-plant biomass of 89.9 t/ha occurs below ground, a higher proportion than for any other comparable forest measured thus far. IN RECENT YEARS, tropical forests have received unprecedented ecological attention. Interest in these ecosystems has been stimulated, in part, by the alarming rate at which they are being modified or completely destroyed. But interest in them is also attributable to their vast stores of carbon and the potential effects of their disruption on the world's carbon balance. Most studies in the tropics have focused on forests growing in humid climates even though they account for a relatively small portion of the forested tropical landscape. Of the total global extent of tropical forest, Brown and Lugo (1982) estimated that about 25 percent is tropical and subtropical wet and rain forest and 33 percent tropical or subtropical moist forest. The remaining 42 percent is tropical or subtropical dry forest (sensu Holdridge 1967). Unlike humid forest, tropical and subtropical dry forest has been very little studied, particularly with respect to taxonomic composition, stand structure, biomass, primary productivity, and rates of carbon turnover. It is, therefore, difficult at the present time to evaluate the significance of tropical and subtropical dry or seasonal forest relative to the global carbon cycle. Additionally, data regarding biomass and related characteristics (e.g., growth and primary productivity) would be useful in assessing the resource potential of dry forest and in furthering our understanding of forest function, especially with respect to climate and seasonality. In 1981 we initiated a comprehensive, long-term study of structure, primary productivity, and plant succession in a subtropical dry forest in southwestern Puerto Rico. This paper reports on the taxonomic composition (woody plants), structure, and biomass of the forest.

Tropical rain forests as carbon sinks

news and update ISSN 1948‐6596 book review Tackling thorny issues in seasonally dry tropical forests The ecology and conservation of seasonally dry forests in Asia, by William J. McShea, Stuart J. Davies and Naris Bhumpakphan (editors) 2011, Smithsonian Institution Scholarly Press, 418 pp. ISBN: 978‐1‐935623‐02‐1 Price: $69.95 / £44.95 (Hardback); http://www.scholarlypress.si.edu/ Seasonally dry tropical forests: ecology and conservation, by Rodolfo Dirzo, Hillary S. Young, Harold A. Mooney and Gerardo Ceballos (editors) 2011, Island Press, 391 pp.ISBN: 978‐1‐59726‐703‐8/978‐1‐59726‐704‐5 Price: $95.00 (Hardback)/$50.00 (Paperback); http://islandpress.org/ The first difficulty in reviewing (and I presume writing) any book on seasonally dry tropical for‐ ests (SDTF) is to come to some understanding as to what SDTF actually are. I hazard a guess that what is a seasonally dry tropical forest to one per‐ son is a seasonal rain forest to another. I myself have recently treated ‘seasonally dry tropical for‐ est’ formations as various types of ‘seasonal rain forest’ (Ghazoul & Sheil, 2010). What is clear is that aseasonal tropical wet forests grade almost imperceptibly into seasonally dry evergreen for‐ est, variably deciduous forest and, ultimately, xe‐ rophytic woodland at the other extreme. Imposing clear‐cut system definitions is always going to be problematic, but it is the nature (and necessity) of human enquiry to impose some sort of classifica‐ tion system on hopelessly complex realities. In this vein two recent edited books on Neotropical (Dirzo et al.) and Asian (McShea et al.) forests de‐ fine SDTF as tropical forests with predictable, regular dry seasons lasting 4‐6 (Neotropics) or 2‐6 months (Asia), while also recognising variations on this theme. It is not so much the duration or inten‐ sity of the dry season that matters, but rather the seasonal water stress that plants are subjected to, and this is as much a function of local topography, soil and disturbance history as it is of climate. Thus in Asia SDTF can occur alongside wet ever‐ green formations in a mosaic of different vegeta‐ tion types, adding to the complexities of distin‐ guishing and mapping SDTF. Both these books begin similarly, with a couple of chapters on floristic composition and biogeography of seasonally dry tropical forests in each respective region. In the Neotropics Linares‐ Palomino et al. identify close floristic affinities among isolated SDTF regions that are distributed across a formerly hypothesised ‘Pleistocene Arc’ of SDTF that extends from Argentina in the south through Paraguay and across Brazil in the north. The Andean dry forests might represent an exten‐ sion of this arc, although the data on this point are ambiguous. Yet these authors argue that there is little evidence for a wide‐ranging Pleistocene SDTF formation, citing long‐distance dispersal events to explain the distribution of widespread species. The floristic ordination of continental Southeast Asian SDTF conducted by Bunyavejchewin et al. separates formations by elevation, topography, soil chemistry and soil physical features, all of which are related to soil water‐holding capacity, as well as the length of the dry season. A striking feature of SDTF and other tropical forests in Asia is their abundance of trees belonging to the Dip‐ terocarpaceae, yet some formations (e.g. mixed deciduous forests and lower montane forests) are notable by the near absence of dipterocarps. A biogeographic approach might provide some in‐ sights into such patterns because the Asian mixed deciduous and lower montane forests are domi‐ nated largely by families that are thought to have warm‐temperate Eurasian origins (e.g. Fagaceae, Lythraceae), while the dipterocarps originated in the southern hemisphere (Gondwana) and colo‐ nised the Asian tropics via India’s collision with Asia around 50 Mya. A fascinating chapter on the evolution of dry forest gingers by John Kress is unfortunately not placed in the context of the dis‐ tribution of forest formations outlined in the ear‐ lier chapters. The central defining feature – a predictable dry season – shapes phenological, behavioural, © 2012 the authors; journal compilation © 2012 The International Biogeography Society — frontiers of biogeography 3.4, 2012

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.

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.