Dry Tropical Ecosystems Research Articles

Seasonal Differences in Vegetation Susceptibility to Soil Drought During 2001–2021

AbstractDroughts typically exert negative effects on vegetation growth, which largely depend on the timing of drought onset. However, huge inconsistencies exist in the seasonal vegetation response to drought among diverse regions across the globe. Here, using the leaf area index (LAI) and solar‐induced chlorophyll fluorescence (SIF), we quantified the vegetation susceptibility by calculating the coincidence rate between vegetation suppression extremes and soil droughts, and further investigated the spatiotemporal changes of vegetation susceptibility during different seasons from 2001 to 2021. We found the vegetation during summer and dry seasons were most susceptible to soil droughts in the extra‐tropics and tropics, respectively. Temporally, the autumn vegetation susceptibility was strengthening in drought‐susceptible regions of extra‐tropics, albeit with insignificant change during spring, summer and the entire growing season. Both the dry and wet seasons showed evidently increasing vegetation susceptibility on the dry tropical ecosystems, which dominated the enhanced vegetation susceptibility of global drought‐susceptible regions. Our findings determined the spatial pattern of most susceptible seasons to soil droughts across the globe and highlighted the enhanced risk to soil droughts, especially in the dry tropics.

Impact of fire-burn on soil geochemical, microbial biomass and carbon stocks in a dry tropical forest ecosystem

Impact of fire-burn on soil geochemical, microbial biomass and carbon stocks in a dry tropical forest ecosystem

Mapping of soil carbon balances changes in the dry tropical forest ecosystem in Pernambuco Brazil

Maps of soil and vegetation carbon stock dynamics resulting from changes in land use in tropical dry areas are still scarce and virtually absent for the Brazilian Northeast region. The few data available were built on a scale that does not allow their use for decision-making and precision farming applications. Based on soil and land use data, we developed a geographical information system to estimate and map carbon balances in the large (86.135 km2) semiarid region of Pernambuco state, Brazil. Maps of carbon stocks for soil and vegetation for the years 2000 and 2016 were created on the scale of 1: 100000, stratified by land use and soil types. In this period, 28% of the area had decreases in soil and vegetation C stocks, 57% had no significant changes and only 13% had increases. Most of the change was associated with converting the open native forest vegetation (Caatinga) into pastures. The net C loss was 291 million Mg, representing an average loss of 2 Mg C ha-1 year-1. Water bodies, urban areas, and other unclassified uses were not accounted for but amounted to only 2% of the area. Overall, the method proved to be a fast and feasible approach to monitoring carbon balances derived from land use changes on a regional scale.

Coupling groundwater availability and soil ability to delineate appropriate land for agricultural uses in the dry tropical ecosystem of Tchéboa (North Cameroon)

Coupling groundwater availability and soil ability to delineate appropriate land for agricultural uses in the dry tropical ecosystem of Tchéboa (North Cameroon)

Widespread degradation and limited protection of forests in global tropical dry ecosystems

Despite their ecological and socioeconomic importance, the forested areas of tropical dry ecosystems remain among the most threatened biomes worldwide. There has been limited research on the current extent, fragmentation, and conservation status of tropical dry forests on a global scale. In this study, we used the GLAD UMD Forest Height dataset to provide a high-resolution global assessment of the extent, patch size distribution, and protection levels of Tropical and Subtropical Dry Broadleaf Forests (TBDF) and forested areas of the Tropical and Subtropical Grasslands, Savannas, and Shrublands (TSGSS) by ecoregion and country. We find that despite increasing protection, degradation continues to rise with less than ⅓ of the original TBDF and TSGSS currently forested and over 80 % of those forests currently exposed to edge effects. <1 % of the remaining forest patches are large (>10 km2), and <25 % of the remaining forests are located in conservation or protection zones (TBDF:16 %; TSGSS:21 %), with a heavy protection bias in S.E. Asia. These results indicate high levels of degradation and low conservation leading to potentially severe ecological consequences. Systematic and coordinated efforts are needed to reduce the pressure on this ecosystem and ensure the ongoing provisioning of biodiversity and ecosystem services in these ecoregions.

Distribution of the northern pampas cat, Leopardus garleppi, in northern South America, confirmation of its presence in Colombia and genetic analysis of a controversial record from the country

Abstract The common name of pampas cat includes a complex of small Neotropical felid species found in various habitats of South America. Recently several species of this complex were proposed, but there are few records in the northernmost distribution of the continent, and reports of the pampas cat’s presence in Colombia have been ambiguous. Current northern limits of Leopardus garleppi are limited to the Andes of Ecuador. Here we present the northernmost record of the species, validating its presence in Colombia and review previous controversial records from the country. The new record is based on a road-killed specimen in a dry tropical forest ecosystem in the municipality of Mercaderes, Department of Cauca. Our morphological and genetic comparisons also shown that a previous record from Colombia supported by a single skin recently used to described a new species named Leopardus narinensis belong to Leopardus tigrinus. With the information compiled from new localities in Colombia and Ecuador, the known distribution range for L. garleppi is increased towards the northern part of South America into areas of Tropical Dry Forest. The distribution now includes 14 ecoregions from Colombia to Bolivia, and four in Colombia (Eastern Cordillera real montane forests, Northwestern Andean montane forests, Northern Andean paramo, and the Patía Valley dry forests), which correspond to similar ecosystems inhabited by the species in other Andean landscapes. The potential distribution hypothesis showed that the species has high suitability in large areas of southwestern Colombia (Nariño). With the recording of L. garleppi in Colombia, the number of extant felid species in the country has increased to seven.

Carbon stock dynamics in a disturbed tropical forest ecosystem of Central India: Strategies for achieving carbon neutrality

Anthropogenic land use changes in the vicinity of forest catchments are explicitly responsible for the deforestation and degradation of tropical forest thereby altering the carbon (C) balance under rising atmospheric carbon dioxide (CO2). Thus, strategic mitigation measures are required to achieve climate neutrality by 2030. It becomes imperative to understand the impact of land cover conversion on the functioning of tropical forests and to determine the carbon dynamics. The present study examined the land cover changes (LUC) in dry tropical ecosystem of Central India during the last two decades and assessed its influence on the forest biomass, litter crop, and C stocks. Geospatial techniques coupled with ground measurements were employed in the study. The total biomass, vegetative C stocks and soil C were estimated as 64 to 338.3 Mg ha−1, 26.4 to 131.1 Mg C/ha and 24.6 to 50.2 Mg C/ha respectively. In the past decades (2000–2020), about 1851.8 ha of forest constituting 15.1% of the total area under study was lost (mainly to Agriculture) under open mixed forest, moderately mixed forest and mixed teak forest. The study demonstrated that forests of the studied region behave as a potential C source, which accounted for a net loss of 59 gigagram carbon (Gg C) equivalent to 216 Gg CO2eq. Besides, discussed three viable scenarios of restoration to offset C emissions and to achieve C neutral landscape by 2030. The conservation and restoration of 26–85% degraded forests (open mixed forest and moderately mixed dense forests), use of agroforestry on 1/5th of existing agricultural lands, or implementation of regenerative agricultural practices in 30% of the area will compensate for net losses, while attaining C neutrality. Further, the finding of this study helps to understand the climatic extremes as per the commitments of Paris Agreement, 2015 and Glasgow Pact, 2021.

Root and branch hydraulic functioning and trait coordination across organs in drought-deciduous and evergreen tree species of a subtropical highland forest.

Vessel traits are key in understanding trees' hydraulic efficiency, and related characteristics like growth performance and drought tolerance. While most plant hydraulic studies have focused on aboveground organs, our understanding of root hydraulic functioning and trait coordination across organs remains limited. Furthermore, studies from seasonally dry (sub-)tropical ecosystems and mountain forests are virtually lacking and uncertainties remain regarding potentially different hydraulic strategies of plants differing in leaf habit. Here, we compared wood anatomical traits and specific hydraulic conductivities between coarse roots and small branches of five drought-deciduous and eight evergreen angiosperm tree species in a seasonally dry subtropical Afromontane forest in Ethiopia. We hypothesized that largest vessels and highest hydraulic conductivities are found in roots, with greater vessel tapering between roots and equally-sized branches in evergreen angiosperms due to their drought-tolerating strategy. We further hypothesized that the hydraulic efficiencies of root and branches cannot be predicted from wood density, but that wood densities across organs are generally related. Root-to-branch ratios of conduit diameters varied between 0.8 and 2.8, indicating considerable differences in tapering from coarse roots to small branches. While deciduous trees showed larger branch xylem vessels compared to evergreen angiosperms, root-to-branch ratios were highly variable within both leaf habit types, and evergreen species did not show a more pronounced degree of tapering. Empirically determined hydraulic conductivity and corresponding root-to-branch ratios were similar between both leaf habit types. Wood density of angiosperm roots was negatively related to hydraulic efficiency and vessel dimensions; weaker relationships were found in branches. Wood density of small branches was neither related to stem nor coarse root wood densities. We conclude that in seasonally dry subtropical forests, similar-sized coarse roots hold larger xylem vessels than small branches, but the degree of tapering from roots to branches is highly variable. Our results indicate that leaf habit does not necessarily influence the relationship between coarse root and branch hydraulic traits. However, larger conduits in branches and a low carbon investment in less dense wood may be a prerequisite for high growth rates of drought-deciduous trees during their shortened growing season. The correlation of stem and root wood densities with root hydraulic traits but not branch wood points toward large trade-offs in branch xylem towards mechanical properties.

An Invasive Ecological Study on the Flora of Soil Seed Bank and Standing Vegetation Across Diverse Anthropo-ecosystems in Indian Dry Tropics

Plant invasions in tropical ecosystems are being increasingly realized particularly in highly dynamic but fragile dry tropical ecosystems, where there is generally little ecological information on invasions in subterranean vegetation. The present study was carried out to understand the floristic composition of both seed bank and standing vegetation across a range of five diverse anthropo-ecosystems in an urban region in Indian dry tropics. A total of one hundred soil samples (each of size of 25cm×25cm from 0-5 cm and 5-10 cm depth) from five anthropic sites (vegetation of University campus, polluted Kali River bank, Brick kiln, Waste land and Road side) were analyzed for their taxonomic position, life form and bio-geographic origin of the seedling emergents in relation to the flora in standing vegetation. A total of 221 plant species (58% aliens, 34% of aliens of American origin, 75% weedy herbs) in standing vegetation spread over 54 families were recorded in standing vegetation of the study sites with more than 55% representation from eight dominant families led by Fabaceae, Poaceae and Asteraceae followed by Malvaceae, Amaranthaceae and Solanaceae. In contrast, a total of 81 seed bank flora (10 unidentified, 62% aliens, 43% of American origin, 87% herbs) distributed over 32 angiospermic families were recorded. While none of the seed bank vegetation at any site showed significant similarity with its standing vegetation indicating the minor role of seed bank flora in the regeneration of the standing plant communities above ground. However, a considerable similarity among seed banks and standing vegetation at other sites indicated a significant possible role of anthropogenic activities in the urban regions of Indian dry tropics, evinced by the largest proportion of grasses and herbs dominated by exotics, especially of American origin. These aliens through successful naturalization via seed banks may cause homogenization of floristic structure. In conclusion, the present study revealed a heavy scale of intrusion by the alien plants dominated by American elements into not only standing vegetation but also in seed banks across the anthropic sites in urban regions in Indian dry tropics which is likely to alter the standing vegetation floristic structure with a larger abundance of alien flora.

Edaphic properties under Vachellia macracantha in an elevation gradient of dry scrub in southern Ecuador

Understanding plant-soil interactions within an ecosystem can be valuable for land management. At species level, legumes promote biotic interactions and soil carbon (C) and nitrogen (N) conservation within dry tropical ecosystems. The objective of this research was to determine if Vachellia macracantha (Humb. & Bonpl. Ex Willd) Seigler & Ebinger influences the edaphic properties in relation to an elevation gradient and distance from the plant stem. The species under study did not show an influence on the edaphic properties in relation to the distance from the plant stem, however, with respect to the elevation gradient, variation was observed in the texture, bulk density, moisture, pH, and stocks of C and N (p-value <0.0001). Soil organic C stocks were measured from 4.68 kg m−2 (1200 m.a.s.l.) to 9.13 kg m−2 (1800 m.a.s.l.). We consider elevation as a key driver, since it integrates climatic variables (temperature and precipitation), decomposition and net primary productivity and expresses the erosive processes that intervene in the spatial distribution of soil types.

Tree survival and resprouting after wildfire in tropical dry and subhumid ecosystems of Chiquitania, Bolivia

Lack of understanding the negative and positive effects of wildfires poses a challenge to the restoration and management of tropical ecosystems. Although fire can have negative impacts on tropical forests, it is known that many tropical savanna and woodland species are adapted to fire. The objective of this study was to determine the resilience of tree species to wildfire in a seasonally dry region of eastern lowland Bolivia. The study was carried out in four tropical dry Chiquitano ecosystems (Abayoy, Cerrado, Chiquitano dry forest, and sub-humid forest), in the department of Santa Cruz, 6–18 months after wildfires occurred in July - October 2019. At 45 sampling points, in both burned and unburned areas, we established 50-m x 5-m transects to determine tree species composition, number of individuals, resprout types, survival, diameter, and height of individuals that were ≥1 cm diameter at breast height (DBH). We calculated the percentages of surviving and dead stems by resprout categories and used logistic regression to determine the probability of survival and probability of resprouting as a function of pre-burn size. In the four ecosystems, 35–71% of the individuals sampled survived (crown and basal resprouting) after fire, while an additional 18–59% of individuals had only basal resprouting. Tree mortality (no reprouts) ranged from 5 to 11%. In burned areas, larger-diameter trees had a greater probability of survival. In contrast, the smaller-diameter trees had a higher probability of resprouting from the base. In conclusion, the Chiquitano tropical dry ecosystems have a high resilience to wildfire in terms of survival and resprouting. Resprouting strategy appears to be an important survival mechanism in trees, especially small trees, and must be considered in the passive restoration of burned forests.

Opportunities for Integrating Social Science into Research on Dry Forest Restoration: A Mini-Review

Seasonally dry tropical forest ecosystems have been greatly reduced in areas through conversions to alternate land uses such as grazing and crop production. The U.N. Decade on Restoration has focused attention on both restoration globally, and also regional attention on tropical dry forests, as they are excellent candidates for regeneration and reforestation. As such, the science of how we restore dry forests is advancing; however, few studies of dry forest restoration include collaborations with social scientists. This is unfortunate, because restoration projects that embrace a people-centered approach have the highest chances of success. Here, I review recent studies that have incorporated aspects of social science and human dimensions into the study and design of dry forest restoration practices. I focus on three key topics that merit a closer integration of restoration research and practice and social science: (1) recognizing that local people are central to project success, (2) cost benefit or effectiveness analyses that evaluate the relative costs of alternative management strategies, and (3) identification of land-use tradeoffs, synergisms and priority mapping. I conclude that closer collaborations among dry forest restoration researchers and a wider group of partners including social scientists, local communities, environmental educators, and geographers will increase the value of restoration research and the likelihood that such projects achieve multiple ecological and societal benefits.

Nutrient cycling and microbial responses to termite and earthworm activity in soils amended with woody residues in the Sudano-Sahel

Termites and earthworms are the main groups of invertebrates in dry tropical ecosystems and exert significant effects on bioturbation and soil reorganization. The dynamics of microbial communities, enzymes and substrates associated with the flux of energy and nutrients induced by their diverse biogenic structures are still under-investigated in agroecological-based cropping systems. The objective of this study was to determine whether the biogenic structures produced by a grass-woody forager termite (Nasutitermes torquatus) and an endogeic earthworm (Millsonia inermis) shift soil organic C (OC), nutrients, enzyme activities and microbial communities in soils amended with woody residues. The experiment was a randomized block design conducted in a Sudano-Sahelian cropping system that was amended annually with woody residues (3 Mg ha−1) or not amended (control soils) for six consecutive years. Samples were taken from fresh sheetings and casts (produced respectively by N. torquatus and M. inermis) and bulk soils in amended plots as well as from control soils in nonamended plots. Samples were analyzed for soil texture, OC, total nitrogen (N), available phosphorus (P), pH, enzyme activity and microbial composition. The results showed that sheetings and casts altered soil biochemical properties significantly. Sheetings and casts were remarkably enriched in OC (45–55% and 82–94%, respectively) and P (20–55% and 73–130%) compared to bulk soils and control soils. Sheetings and casts substantially stimulated β-glucosidase (153–178% and 158–183%, respectively) and N-acetyl-β-glucosaminidase (43–53% and 84–93%) activity, likely due to gut mucus excretions and favorable physicochemical conditions. Sheetings and casts did not fundamentally alter the microbial diversity but raised soil conditions favorable to selective microbial growth. Overall, casts exhibited higher nutrient content (N, P), enzyme activity, and larger total microbial biomass than sheetings. Both structures stimulated soil microbial communities and enzyme activity associated with the increase of pH, OC and nutrients. Earthworm casts and termite sheetings have ecological significance in OC dynamics in plant residues amended soils.

Herbaceous vegetation under planted woody species on coal mine spoil acts as a source of organic matter

The present study was conducted to examine the status of herbaceous biomass development under rehabilitated coal mine spoil by establishing indigenous woody species in India's northern dry tropical region. Since mining activities for coal and minerals are prime industrial activities in this region; therefore, through these activities, a significant size of dry tropical forest ecosystems is destroying at unprecedented rates. Thus, the primary objective of the present study was to understand the influence of planted woody species on herbaceous biomass development. Estimation of herbaceous biomass was accomplished once in 1995 and 2007 under plantation stand of four native woody species. Among planted woody species, maximum herbaceous biomass was quantified at the 5th-year stand of Tectona grandis and lowest at the 17th-year stand of Dendrocalamus strictus. Family Poaceae contributed the maximum in herbaceous biomass production across all plantation stands. A declining trend was observed in herbaceous biomass production as the age of plantation stand increases, reflecting the impact of stand characteristics such as woody species basal cover and canopy cover on herbaceous vegetation development. Herbaceous biomass was positively correlated with species diversity at both ages of plantation stands, emphasising the facilitative role of species for organic matter build-up in disturbed habitats. However, increased competition between herbaceous species for space, nutrients and decreased penetration of light through the closed canopy of planted woody trees significantly influences herbaceous diversity under plantation stand.

Comparison of Physico-chemical Properties of Soils under Different Forest Types in Dry Tropical Forest Ecosystem in Achanakmar-Amarkantak Biosphere Reserve, India

The present study was conducted to assess the variations in physico-chemical properties of soils under four forest types viz. Mixed Forest, Mixed Sal Forest, Mixed Teak Forest, and Teak Mixed Forests in a typical dry tropical ecosystem of Achanakmar-Amarkantak Biosphere Reserve (AABR), Central India. Soil samples were collected from four types of forest and analyzed for the soil samples were taken at three soil depths, i.e. at 0-20 cm, 20-40cm and 40-60 cm and analysed for pH, EC, Organic C, soil moisture, bulk density, nitrogen (N), phosphorous (P) and potassium (K). The pH of soil was lower (5.43) under Mixed Forest and higher (6.13) in Teak Mixed Forest, while EC in soil was lower (0.04 dS m-1) in Mixed Sal Forest and higher (0.32 dS m-1) under Teak Mixed Forest. Organic C varied from 11.5 Mg ha-1 to 17.8 Mg ha-1 at 0-20 cm soil depth which was highest in soil under Mixed Sal Forest and lowest in Teak Mixed Forest. The Organic C and EC values were decreased with an increase in soil depth, while the bulk density showed reverse trend. Soil nutrients in different depths varied between 160.2 -196.9 kg ha-1, 10.4-17.7 kg ha-1 and 266.4-439.1 kg ha-1 for N, P and K, respectively. The nutrient quantities in soil were higher in Mixed Sal Forest and lower under Teak Mixed Forest, which decreased with soil depth. The paper discusses the possible variations in soil properties in relation to structure and composition of forest type and suggests appropriate management practices for the sustainable development of forest soils in dry tropical ecosystem.

Biomass Production Assessment in a Protected Area of Dry Tropical forest Ecosystem of India: A Field to Satellite Observation Approach

In recent decades, degradation and loss of the world’s forest ecosystems have been key contributors to biodiversity loss and future climate change. This article analyzes plant diversity, biomass, carbon sequestration potential (CSP), and the net primary productivity (NPP) of four vegetation types viz., Dense mixed forest (DMF); Open mixed forest (OMF); Teak plantation (TP), and Sal mixed forest (SMF) in the dry tropical forest ecosystem of central India through remote sensing techniques together with physical ground observations during 2013–2018. The total C storage in trees varied from 16.02 to 47.15 Mg ha−1 in studied vegetation types with the highest in DMF and lowest in OMF. The total C storage in stem wood, branches, and foliage falls in the range of 52.93–78.30%, 9.49–22.99%, and 3.31–12.89% respectively. The total standing biomass varied from 83.77 to 111.21 Mg ha−1 and these variations are due to different vegetation types, with the highest in DMF followed by TP, SMF while the lowest was estimated in OMF. The net primary productivity (NPP) [aboveground (AG) + belowground (BG)] varied from 7.61 to 9.94 Mg ha−1 yr−1 with mean values of 8.74 Mg ha−1 yr−1 where AG shares a maximum contribution of 77.66%. The total biomass production was distributed from 64.09 to 82.91% in AG and 17.08–35.91% in BG components. The present study outlines that the studied forest ecosystem has the substantial potential of carbon sequestration and a great possibility of mitigating local and global climate change.

Restoration benefits of soil nutrient manipulation and weeding in invaded dry and wet tropical ecosystems in Hawaiʻi

Reducing soil nutrient availability has been proposed as a strategy to favor native vs. non‐native invasive plant species and represents a potential alternative to traditional manual removal or chemical control methods. We implemented a field experiment in invaded dry and wet montane Hawaiian ecosystems to test responses of soil and dominant plant species to three soil nutrient treatments (Control = no nutrient manipulation; Carbon = C substrate added to reduce nutrients; Fertilizer = fertilizer added to increase nutrients) and two non‐native plant treatments (Weeds removed; Weeds present) in a fully factorial experiment in each ecosystem over 18 months. Carbon amendments reduced soil inorganic nutrient availability by 60–70% in dry shrubland and 30–50% in wet forest. Fertilizer amendments increased soil inorganic nutrient availability by &gt;20‐fold. Altered nutrient availability did not impact gross mineralization or nitrification rates in either ecosystem. In dry shrubland, neither C amendments nor weed removal altered growth or reproduction, but fertilizer increased woody growth and forb/grass reproduction in both natives and non‐natives. In wet forest, weed removal but not C amendments increased growth and survival of native woody seedlings, while fertilizer decreased native seedling survival and increased non‐native woody seedling growth. Overall, growth and reproduction of native and non‐natives responded similarly to altered nutrient availability, indicating that for the tropical ecosystems and species examined, manipulating nutrient availability does not favor native versus non‐native invasive plants in the first 18 months. In contrast, weed removal had positive effects on native plant growth, likely mediated through changes in other resources.

Nutrient‐use strategy and not competition determines native and invasive species response to changes in soil nutrient availability

Non‐native invasive plants often outcompete native species under high resource availability. Restoration techniques that lower resources may, therefore, create favorable conditions for resource conservative native species over resource exploitative invasive species. Research on this topic has focused on temperate grass and forb‐dominated ecosystems and has rarely been tested for woody species or tropical vegetation. We evaluated growth, resource‐use efficiency (RUE), ecophysiology, and competition (i.e. a relative interaction index based on biomass) of four woody native and two dominant invasive species from Hawaiian wet and dry tropical ecosystems in a greenhouse experiment. Density of plants was constant and species were grown with either a conspecific or the invasive species from that ecosystem type across a gradient of soil nutrient availability. Instantaneous photosynthetic rates varied minimally across nutrient availability. However, both invasive and one native species increased leaf area as nutrients increased, providing more photosynthetic area and increasing total biomass. Nitrogen RUE decreased with increasing nutrient availability for all but one native species, while phosphorus RUE remained constant for all but one native species. Competitive interactions were weak, variable, and not significantly impacted by soil nutrients. Overall, plants categorized as invasive or resource exploitative had larger changes in response variables with increasing soil nutrients compared to those categorized as native or resource conservative. These results suggest that manipulating soil nutrient availability is a potentially viable restoration tool for at least some woody species in tropical ecosystems. However, predicting restoration success requires understanding species‐specific ecophysiological traits determining response to altered environmental conditions.

Friend or foe? The role of biotic agents in drought-induced plant mortality

Plant mortality is a complex process influenced by both biotic and abiotic factors. In recent decades, widespread mortality events have been attributed to increasing drought severity, which has motivated research to examine the physiological mechanisms of drought-induced mortality, particularly hydraulic failure. Drought-based mortality mechanisms are further influenced by plant interactions with biota such as neighboring plants, insect pests, and microbes. In this review, we highlight some of the most influential papers addressing these biotic interactions and their influence on plant mortality. Plant–plant interactions can be positive (facilitation), neutral, or negative (competition), depending on drought intensity and neighbor identity. For example, stand-scale mortality likely increases with basal area (an index of competition). However, the diversity of forest stands matters, as more diverse forests suffer less mortality from drought than species-poor forests. Dense forest stands also increase bark beetle attack frequency, which can exacerbate drought stress and mortality, particularly for fast-growing species with lower defense allocation. In some cases, however, drought stress can alleviate biotic attack, depending on feedbacks between plant and pest physiology. Finally, plant interactions with beneficial microorganisms can increase drought tolerance, reduce the likelihood of mortality, and even extend plant distributions into drier habitats. Our review suggests more work is needed in natural herbaceous plant communities as well as dry tropical ecosystems where mortality mechanisms are less understood. Overall, relatively few studies directly link biotic interactions with the physiological mechanisms of mortality. Simultaneous manipulations of biotic interactions and measurements of physiological thresholds (e.g., xylem cavitation) are needed to fully represent biotic interactions in predictive models of plant mortality.

Threatened Neotropical seasonally dry tropical forest: evidence of biodiversity loss in sap-sucking herbivores over 75 years.

Tropical forests cover 7% of the earth's surface and hold 50% of known terrestrial arthropod species. Alarming insect declines resulting from human activities have recently been documented in temperate and tropical ecosystems worldwide, but reliable data from tropical forests remain sparse. The sap-sucking tribe Athysanini is one herbivore group sensitive to anthropogenic perturbation and the largest within the diverse insect family Cicadellidae distributed in America's tropical forests. To measure the possible effects of deforestation and related activities on leafhopper biodiversity, a survey of 143 historic collecting localities was conducted to determine whether species documented in the Mexican dry tropical forests during the 1920s to 1940s were still present. Biostatistical diversity analysis was performed to compare historical to recent data on species occurrences. A data matrix of 577 geographical records was analysed. In total, 374 Athysanini data records were included representing 115 species of 41 genera. Historically, species richness and diversity were higher than found in the recent survey, despite greater collecting effort in the latter. A strong trend in species decline was observed (−53%) over 75 years in this endangered seasonally dry ecosystem. Species completeness was dissimilar between historic and present data. Endemic taxa were significantly less important and represented in the 1920s–1940s species records. All localities surveyed in the dry tropical forest are disturbed and reduced by modern anthropogenic processes. Mexico harbours highly endemic leafhopper taxa with a large proportion of these inhabiting the dry forest. These findings provide important data for conservation decision making and modelling of distribution patterns of this threatened seasonally dry tropical ecosystem.