Tropical montane cloud forest: environmental drivers of vegetation structure and ecosystem function

Cross-scale assessments of the impacts and resilience of subtropical montane cloud forests to chronic seasonal droughts and episodic typhoons

Montane cloud forests (MCFs) are ecosystems frequently immersed in fog and are vital for the terrestrial hydrological cycle and biodiversity hotspots. However, the potential impacts of climate change, particularly intensified droughts and typhoons, on the persistence of ecosystems remain unclear. Our study conducted cross-scale assessments using 6-year (2016-2021) ground litterfall and 21-year (2001-2021) satellite greenness data (the Enhanced Vegetation Index [EVI] and the EVI anomaly change [ΔEVI% ]), gross primary productivity anomaly change (ΔGPP% ), and meteorological variables (the standardized precipitation index [SPI] and wind speed). We found a positive correlation between summer EVI and ΔGPP% with the SPI-3 (3-month time scale), while winter litterfall showed a negative correlation. Maximum typhoon daily wind speed was negatively correlated with summer and the monthly ΔEVI% and ΔGPP% . These findings suggest vegetation damage and productivity loss were related to drought and typhoon intensities. Furthermore, our analysis highlighted that chronic seasonal droughts had more pronounced impacts on MCFs than severe typhoons, implying that high precipitation and frequent fog immersion do not necessarily mitigate the ramifications of water deficit on MCFs but might render MCFs more sensitive and vulnerable to drought. A significant negative correlation between the summer and winter ΔEVI% and ΔGPP% of the same year, suggesting disturbance severity during summer may facilitate vegetation regrowth and carbon accumulation in the subsequent winter. This finding may be attributed to the ecological resilience of MCFs, which enables them to recover from the previous summer. In the long-term, our results indicated an increase in vegetation resilience over two decades in MCFs, likely driven by rising temperatures and elevated carbon dioxide levels. However, the enhancement of resilience might be overshadowed by the potential intensified droughts and typhoons in the future, potentially causing severe damage and insufficient recovery times for MCFs, thus raising concerns about uncertainties regarding their sustained resilience.

Ecophysiological controls on water use of tropical cloud forest trees in response to experimental drought

Tropical montane cloud forests (TMCFs) are expected to experience more frequent and prolonged droughts over the coming century, yet understanding of TCMF tree responses to moisture stress remains weak compared with the lowland tropics. We simulated a severe drought in a throughfall reduction experiment (TFR) for 2 years in a Peruvian TCMF and evaluated the physiological responses of several dominant species (Clusia flaviflora Engl., Weinmannia bangii (Rusby) Engl., Weinmannia crassifolia Ruiz & Pav. and Prunus integrifolia (C. Presl) Walp). Measurements were taken of (i) sap flow; (ii) diurnal cycles of stem shrinkage, stem moisture variation and water-use; and (iii) intrinsic water-use efficiency (iWUE) estimated from foliar δ13C. In W. bangii, we used dendrometers and volumetric water content (VWC) sensors to quantify daily cycles of stem water storage. In 2 years of sap flow (Js) data, we found a threshold response of water use to vapor pressure deficit vapor pressure deficit (VPD) > 1.07 kPa independent of treatment, though control trees used more soil water than the treatment trees. The daily decline in water use in the TFR trees was associated with a strong reduction in both morning and afternoon Js rates at a given VPD. Soil moisture also affected the hysteresis strength between Js and VPD. Reduced hysteresis under moisture stress implies that TMCFs are strongly dependent on shallow soil water. Additionally, we suggest that hysteresis can serve as a sensitive indicator of environmental constraints on plant function. Finally, 6 months into the experiment, the TFR treatment significantly increased iWUE in all study species. Our results highlight the conservative behavior of TMCF tree water use under severe soil drought and elucidate physiological thresholds related to VPD and its interaction with soil moisture. The observed strongly isohydric response likely incurs a cost to the carbon balance of the tree and reduces overall ecosystem carbon uptake.

Impacts of anthropogenic climate change on tropical montane forests: an appraisal of the evidence

In spite of their small global area and restricted distributions, tropical montane forests (TMFs) are biodiversity hotspots and important ecosystem services providers, but are also highly vulnerable to climate change. To protect and preserve these ecosystems better, it is crucial to inform the design and implementation of conservation policies with the best available scientific evidence, and to identify knowledge gaps and future research needs. We conducted a systematic review and an appraisal of evidence quality to assess the impacts of climate change on TMFs. We identified several skews and shortcomings. Experimental study designs with controls and long-term (≥10 years) data sets provide the most reliable evidence, but were rare and gave an incomplete understanding of climate change impacts on TMFs. Most studies were based on predictive modelling approaches, short-term (<10 years) and cross-sectional study designs. Although these methods provide moderate to circumstantial evidence, they can advance our understanding on climate change effects. Current evidence suggests that increasing temperatures and rising cloud levels have caused distributional shifts (mainly upslope) of montane biota, leading to alterations in biodiversity and ecological functions. Neotropical TMFs were the best studied, thus the knowledge derived there can serve as a proxy for climate change responses in under-studied regions elsewhere. Most studies focused on vascular plants, birds, amphibians and insects, with other taxonomic groups poorly represented. Most ecological studies were conducted at species or community levels, with a marked paucity of genetic studies, limiting understanding of the adaptive capacity of TMF biota. We thus highlight the long-term need to widen the methodological, thematic and geographical scope of studies on TMFs under climate change to address these uncertainties. In the short term, however, in-depth research in well-studied regions and advances in computer modelling approaches offer the most reliable sources of information for expeditious conservation action for these threatened forests.

Assessing microhabitat, landscape features and intraguild relationships in the occupancy of the enigmatic and threatened Andean tiger cat (Leopardus tigrinus pardinoides) in the cloud forests of northwestern Colombia

Mesocarnivores play a key role in ecosystem dynamics through the regulation of prey populations and are sensitive to environmental changes; thus, they are often considered good model organisms for conservation planning. However, data regarding the factors that influence the habitat use of threatened small wild felids such as the Andean tiger cat (Leopardus tigrinus pardinoides) are scarce. We conducted a two-year survey with 58 camera trap stations to evaluate the determinants of Andean tiger cat habitat use in three protected areas in the Middle Cauca, Colombia. We developed site occupancy models and found that Andean tiger cat habitat use increased with leaf litter depth at intermediate elevations and far from human settlements. Through conditional cooccurrence models, we found that Andean tiger cat habitat use was invariant to the presence of prey or potential intraguild competitors and killers/predators, but its detectability increased when they were present and detected. This suggests that Andean tiger cats may be more likely to be detected in sites with high prey availability. We found that Andean tiger cats preferred sites with deep leaf litter, which is a particular feature of cloud forests that provides suitable conditions for ambush hunting and hiding from intraguild enemies. Our results showed that Andean tiger cats avoided human settlements, which may minimize potential mortality risks in those areas. Moreover, the restricted use of middle elevations by Andean tiger cats suggested that they could be used as a sentinel species to track the effects of climate change since their suitable habitat is likely to be projected upward in elevation. Future conservation actions must be focused on identifying and mitigating human-related threats close to the Andean tiger cat habitat while preserving microhabitat conditions and the existing networks of protected areas.

Small-fragment, high turnover: soil microenvironment fluctuation effect on tree diversity in a Neotropical montane oak forest

Background

Soil microenvironmental variables showed an important key in α and β-tree diversity in Neotropical montane oak forest. Thus, understanding the microenvironment fluctuation at small-fragment effects on tree diversity is crucial in maintaining the montane oak ecosystems. In this study, we hypothesized that within a relatively small-fragment (151.63 ha), tree α and β-diversity fluctuate and specific soil microenvironmental factors could influence tree species diversity to answer three questions: Do tree α and β-diversity differ among transects, even in a short-distance between them? Do microenvironmental variables influence tree diversity composition that occurs within a relict Neotropical montane oak forest? Is there a particular microenvironmental variable influencing tree species-specific?

Methods

We established four permanent transects during a year in a relict Neotropical montane oak forest, we assessed tree diversity and specific microenvironmental variables (soil moisture, soil temperature, pH, depth litterfall and light incidence). This allowed us to evaluate how microenvironmental variables at small-fragment influence α and β-tree diversity and tree species-specific.

Results

Our results showed that α-diversity was not different among transects; however, β-diversity of tree species was mostly explained by turnover and soil moisture, soil temperature, and light incidence were the microenvironmental variables that triggered the replacement (i.e., one species by another). Those variables also had effect on tree species-specific: Mexican beech (Fagus mexicana), Quebracho (Quercus delgadoana), Pezma (Cyathea fulva), Aguacatillo (Beilschmiedia mexicana), Pezma (Dicksonia sellowiana var. arachneosa), and Mountain magnolia (Magnolia schiedeana).

Discussion

Our results confirm our hypothesis related to β-diversity but not with α-diversity; however, the tree community structure of the diversity was similar among transects. Our study represents the first effort to evaluate and link the soil microenvironmental effect on tree α and β-diversity, finding a high replacement in a small-fragment of Neotropical montane oak forest from eastern Mexico.

Soil microbes support Janzen’s mountain passes hypothesis: The role of local-scale climate variability along a tropical montane gradient

Tropical montane ecosystems are the centers of biodiversity, and Janzen proposed that mountain climate variability plays a key role in sustaining this biodiversity. We test this hypothesis for soil bacteria and fungi along a 265–1,400 m elevational gradient on Hainan Island of tropical China, representing diverse vegetation types from deciduous monsoon forest to cloud forest. We found that bacterial and fungal diversity declined as elevation increased, and the dissimilarity of both groups increased with increasing separation in elevation, although changes in bacteria were larger than in fungi. Seasonal alterations and the range of soil moisture in the growing season were found to be the dominant drivers of fungal richness and Shannon diversity, whereas soil pH was the major driver of bacterial diversity. Dissimilarities of bacterial and fungal communities were best predicted by climate, particularly seasonal changes in soil temperature, with weaker influences of soil physicochemistry and vegetation. The dominant effect of seasonality in soil temperature was further detected in cloud forests, which harbored a higher proportion of unique bacterial species and dissimilarity of bacterial and fungal communities. Our findings suggest that local-climate variability plays a crucial role in structuring the distribution of soil microbial communities along a tropical montane gradient, which generally supports Janzen’s hypothesis. Such a sensitivity to climatic variability suggests that soil microbial communities along tropical montane gradients may shift in response to future climate scenarios.

Leading nutrient foraging strategies shaping by root system characteristics along the elevations in rubber (Hevea brasiliensis) plantations

When it comes to root and mycorrhizal associations that define resource acquisition strategy, there is a need to identify the leading dimension across root physiology, morphology, architecture and whole plant biomass allocation to better predict the plant's responses to multiple environmental constraints. Here, we developed a new framework for understanding the variation in roots and symbiotic fungi by quantifying multiple-scale characteristics, ranging from anatomy to the whole plant. We chose the rubber (Hevea brasiliensis) grown at three elevations to test our framework and to identify the key dimensions for resource acquisition. Results showed that the quantities of absorptive roots and root system architecture, rather than single root traits, played the leading role in belowground resource acquisition. As the elevation increased from the low to high elevation, root length growth, productivity and root mass fraction (RMF) increased by 2.9-, 2.3- and 13.8-fold, respectively. The contribution of RMF to the changes in total root length was 3.6-fold that of specific root length (SRL). Root architecture exhibited higher plasticity than anatomy and morphology. Further, mycorrhizal colonization was highly sensitive to rising elevations with a non-monotonic pattern. By contrast, both leaf biomass and specific leaf area (traits) co-varied with increasing elevation. In summary, rubber trees changed root system architecture by allocating more biomass and lowering the reliance on mycorrhizal fungi rather than improving single root efficiency in adapting to high elevation. Our framework is instructive for traits-based ecology; accurate assessments of forest carbon cycling in response to resource gradient should account for the leading dimension of root system architecture.

Potential distributions of pre-Columbian people in Tropical Andean landscapes

Much has yet to be learned of the spatial patterning of pre-Columbian people across the Tropical Andes. Using compiled archaeological data and a suite of environmental variables, we generate an ensemble species distribution model (SDM) that incorporates general additive models, random forest models and Maxent models to reconstruct spatial patterns of pre-Columbian people that inhabited the Tropical Andes east of the continental divide, within the modern countries of Bolivia, Peru and Ecuador. Within this region, here referred to as the eastern Andean flank, elevation, mean annual cloud frequency, distance to rivers and precipitation of the driest quarter are the environmental variables most closely related to human occupancy. Our model indicates that 11.04% of our study area (65 368 km²) was likely occupied by pre-Columbian people. Our model shows that 30 of 351 forest inventory plots, which are used to generate ecological understanding of Andean ecosystems, were likely occupied in the pre-Columbian period. In previously occupied sites, successional trajectories may still be shaping forest dynamics, and those forests may still be recovering from the ecological legacy of pre-Columbian impacts. Our ensemble SDM links palaeo- and neo-ecology and can also be used to guide both future archaeological and ecological studies. This article is part of the theme issue 'Tropical forests in the deep human past'.

Interannual variation in rainfall modulates temperature sensitivity of carbon allocation and flux in a tropical montane wet forest

Tropical forests exert a disproportionately large influence on terrestrial carbon (C) balance but projecting the effects of climate change on C cycling in tropical forests remains uncertain. Reducing this uncertainty requires improved quantification of the independent and interactive effects of variable and changing temperature and precipitation regimes on C inputs to, cycling within and loss from tropical forests. Here, we quantified aboveground litterfall and soil-surface CO₂ efflux ("soil respiration"; F_S ) in nine plots organized across a highly constrained 5.2°C mean annual temperature (MAT) gradient in tropical montane wet forest. We used five consecutive years of these measurements, during which annual rainfall (AR) steadily increased, in order to: (a) estimate total belowground C flux (TBCF); (b) examine how interannual variation in AR alters the apparent temperature dependency (Q₁₀ ) of above- and belowground C fluxes; and (c) quantify stand-level C allocation responses to MAT and AR. Averaged across all years, F_S , litterfall, and TBCF increased positively and linearly with MAT, which accounted for 49, 47, and 46% of flux rate variation, respectively. Rising AR lowered TBCF and F_S , but increased litterfall, with patterns representing interacting responses to declining light. The Q₁₀ of F_S , litterfall, and TBCF all decreased with increasing AR, with peak sensitivity to MAT in the driest year and lowest sensitivity in the wettest. These findings support the conclusion that for this tropical montane wet forest, variations in light, water, and nutrient availability interact to strongly influence productivity (litterfall+TBCF), the sensitivity of above- and belowground C fluxes to rising MAT (Q₁₀ of F_S , litterfall, and TBCF), and C allocation patterns (TBCF:[litterfall+TBCF]).

High aboveground carbon stock of African tropical montane forests

Tropical forests store 40-50 per cent of terrestrial vegetation carbon¹. However, spatial variations in aboveground live tree biomass carbon (AGC) stocks remain poorly understood, in particular in tropical montane forests². Owing to climatic and soil changes with increasing elevation³, AGC stocks are lower in tropical montane forests compared with lowland forests². Here we assemble and analyse a dataset of structurally intact old-growth forests (AfriMont) spanning 44 montane sites in 12 African countries. We find that montane sites in the AfriMont plot network have a mean AGC stock of 149.4 megagrams of carbon per hectare (95% confidence interval 137.1-164.2), which is comparable to lowland forests in the African Tropical Rainforest Observation Network⁴ and about 70 per cent and 32 per cent higher than averages from plot networks in montane^2,5,6 and lowland⁷ forests in the Neotropics, respectively. Notably, our results are two-thirds higher than the Intergovernmental Panel on Climate Change default values for these forests in Africa⁸. We find that the low stem density and high abundance of large trees of African lowland forests⁴ is mirrored in the montane forests sampled. This carbon store is endangered: we estimate that 0.8 million hectares of old-growth African montane forest have been lost since 2000. We provide country-specific montane forest AGC stock estimates modelled from our plot network to help to guide forest conservation and reforestation interventions. Our findings highlight the need for conserving these biodiverse^9,10 and carbon-rich ecosystems.

Diversity and function of soil microbes on montane gradients: the state of knowledge in a changing world

Mountains have a long history in the study of diversity. Like macroscopic taxa, soil microbes are hypothesized to be strongly structured by montane gradients, and recently there has been important progress in understanding how microbes are shaped by these conditions. Here, we summarize this literature and synthesize patterns of microbial diversity on mountains. Unlike flora and fauna that often display a mid-elevation peak in diversity, we found a decline (34% of the time) or no trend (33%) in total microbial diversity with increasing elevation. Diversity of functional groups also varied with elevation (e.g. saprotrophic fungi declined 83% of the time). Most studies (82%) found that climate and soils (especially pH) were the primary mechanisms driving shifts in composition, and drivers differed across taxa-fungi were mostly determined by climate, while bacteria (48%) and archaea (71%) were structured primarily by soils. We hypothesize that the central role of soils-which can vary independently of other abiotic and geographic gradients-in structuring microbial communities weakens diversity patterns expected on montane gradients. Moving forward, we need improved cross-study comparability of microbial diversity indices (i.e. standardizing sequencing) and more geographic replication using experiments to broaden our knowledge of microbial biogeography on global gradients.

Elevation and latitude drives structure and tree species composition in Andean forests: Results from a large-scale plot network

Our knowledge about the structure and function of Andean forests at regional scales remains limited. Current initiatives to study forests over continental or global scales still have important geographical gaps, particularly in regions such as the tropical and subtropical Andes. In this study, we assessed patterns of structure and tree species diversity along ~ 4000 km of latitude and ~ 4000 m of elevation range in Andean forests. We used the Andean Forest Network (Red de Bosques Andinos, https://redbosques.condesan.org/) database which, at present, includes 491 forest plots (totaling 156.3 ha, ranging from 0.01 to 6 ha) representing a total of 86,964 identified tree stems ≥ 10 cm diameter at breast height belonging to 2341 identified species, 584 genera and 133 botanical families. Tree stem density and basal area increases with elevation while species richness decreases. Stem density and species richness both decrease with latitude. Subtropical forests have distinct tree species composition compared to those in the tropical region. In addition, floristic similarity of subtropical plots is between 13 to 16% while similarity between tropical forest plots is between 3% to 9%. Overall, plots ~ 0.5-ha or larger may be preferred for describing patterns at regional scales in order to avoid plot size effects. We highlight the need to promote collaboration and capacity building among researchers in the Andean region (i.e., South-South cooperation) in order to generate and synthesize information at regional scale.

Distribution and Community Assembly of Trees Along an Andean Elevational Gradient

Highlighting patterns of distribution and assembly of plants involves the use of community phylogenetic analyses and complementary traditional taxonomic metrics. However, these patterns are often unknown or in dispute, particularly along elevational gradients, with studies finding different patterns based on elevation. We investigated how patterns of tree diversity and structure change along an elevation gradient using taxonomic and phylogenetic diversity metrics. We sampled 595 individuals (36 families; 53 genera; 88 species) across 15 plots along an elevational gradient (2440-3330 m) in Ecuador. Seventy species were sequenced for the rbcL and matK gene regions to generate a phylogeny. Species richness, Shannon-Weaver diversity, Simpson's Dominance, Simpson's Evenness, phylogenetic diversity (PD), mean pairwise distance (MPD), and mean nearest taxon distance (MNTD) were evaluated for each plot. Values were correlated with elevation and standardized effect sizes (SES) of MPD and MNTD were generated, including and excluding tree fern species, for comparisons across elevation. Taxonomic and phylogenetic metrics found that species diversity decreases with elevation. We also found that overall the community has a non-random phylogenetic structure, dependent on the presence of tree ferns, with stronger phylogenetic clustering at high elevations. Combined, this evidence supports the ideas that tree ferns have converged with angiosperms to occupy the same habitat and that an increased filtering of clades has led to more closely related angiosperm species at higher elevations.

Bryophyte stable isotope composition, diversity and biomass define tropical montane cloud forest extent

Liverworts and mosses are a major component of the epiphyte flora of tropical montane forest ecosystems. Canopy access was used to analyse the distribution and vertical stratification of bryophyte epiphytes within tree crowns at nine forest sites across a 3400 m elevational gradient in Peru, from the Amazonian basin to the high Andes. The stable isotope compositions of bryophyte organic material (¹³C/¹²C and ¹⁸O/¹⁶O) are associated with surface water diffusive limitations and, along with C/N content, provide a generic index for the extent of cloud immersion. From lowland to cloud forest δ¹³C increased from -33‰ to -27‰, while δ¹⁸O increased from 16.3‰ to 18.0‰. Epiphytic bryophyte and associated canopy soil biomass in the cloud immersion zone was estimated at up to 45 t dry mass ha^-1, and overall water holding capacity was equivalent to a 20 mm precipitation event. The study emphasizes the importance of diverse bryophyte communities in sequestering carbon in threatened habitats, with stable isotope analysis allowing future elevational shifts in the cloud base associated with changes in climate to be tracked.

Contrasting thermal strategies of montane Neotropical bats at high elevations

In the Neotropics, captive vespertilionid bats substantially reduce their metabolic rate at low ambient temperatures, similar to their temperate counterparts, whereas the ability of phyllostomids to lower metabolic rate seems to be more limited, even in mountain species. Nevertheless, field data on the thermal behaviour of syntopic individuals from these two families is lacking. Consequently, we aimed to test whether torpor was more common and deeper in vesper bats compared to leaf-nosed bats by studying skin temperature (Tsk) variation in individuals experiencing the same environmental conditions at a mountain area. Bats experienced ambient temperatures below 15 °C. Average Tsk was 10 °C in Myotis oxyotus gardneri (Vespertilionidae) during the day, while Sturnira burtonlimi (Phyllostomidae) regulated diurnal Tsk above 30 °C. Constant food availability may explain why diurnal Sturnira burtonlimi pay the high energetic cost required to remain normothermic and to defend a wide Ta-Tsk gap but further studies are needed to elucidate additional strategies that may be employed by these bats to reduce the energetic demands of normothermy. Our study shows that the contrasting thermal strategies and torpor use adopted by vespertilionid insectivores and phyllostomid frugivores in captive settings also occur in free-ranging conditions, thus providing a basis to develop further studies with predictions more accurately rooted in field data.

Vapor pressure deficit predicts epiphyte abundance across an elevational gradient in a tropical montane region

PREMISE OF THE STUDY

Tropical Montane Cloud Forests (TMCFs) are important ecosystems to study and preserve because of their high biodiversity and critical roles in local and regional ecosystem processes. TMCFs may be particularly affected by changes in climate because of the narrow bands of microclimate they occupy and the vulnerability of TMCF species to projected increases in cloud base heights and drought. A comprehensive understanding of the structure and function of TMCFs is lacking and difficult to attain because of variation in topography within and across TMCF sites. This causes large differences in microclimate and forest structure at both large and small scales.

METHODS

In this study, we estimated the abundance of the entire epiphyte community in the canopy (bryophytes, herbaceous vascular plants, woody epiphytes, and canopy dead organic matter) in six sites. In each of the sites we installed a complete canopy weather station to link epiphyte abundance to a number of microclimatic parameters.

KEY RESULTS

We found significant differences in epiphyte abundance across the sites; epiphyte abundance increased with elevation and leaf wetness, but decreased as vapor pressure deficit (VPD) increased. Epiphyte abundance had the strongest relationship with VPD; there were differences in VPD that could not be explained by elevation alone.

CONCLUSIONS

By measuring this proxy of canopy VPD, TMCF researchers will better understand differences in microclimate and plant community composition across TMCF sites. Incorporating such information in comparative studies will allow for more meaningful comparisons across TMCFs and will further conservation and management efforts in this ecosystem.

Towards integrated ecological research in tropical montane cloud forests

Tropical tropical montane cloud forests (TMCFs) cover a small portion of the Earth, yet they are significant biodiversity hotspots and centres of endemism, and they provide important hydrological and biogeochemical functions that affect human livelihoods. Given their fundamental sensitivity to climate, TMCFs also serve as an early warning system for climate change impacts. This paper outlines a new international initiative, CloudNet, that aims to promote integrated research across TMCFs, and introduces a special issue that reviews emerging themes and topics in the ecology of TMCFs, highlighting knowledge gaps and suggesting new directions for research. CloudNet is helping coordinate several new research projects and protocols: (1) a global repository of TMCF data and meta-analyses across multiple sites; (2) a multi-site study of plant functional traits across TMCFs; (3) a multi-site study of decomposition processes across TMCFs; (4) a protocol for standardizing climate data collection across TMCFs. These studies are intended to evaluate the extent to which general patterns emerge, accounting for biogeographic, phylogenetic and environmental differences among sites. Common data collection across TMCFs should also allow better integration across disciplines, such as linking nutrient limitation, seed production and propagule recruitment, and enable cross-site comparisons of how TMCFs respond to drivers of global change, including rising cloud bases, increasing temperatures, altered disturbance regimes, biological invasions and extinction, and changing human land use.

Seed-dispersal ecology of tropical montane forests

Seed-dispersal ecology in tropical montane forests (TMF) differs in some predictable ways from tropical lowland forests (TLF). Environmental, biogeographic and biotic factors together shape dispersal syndromes which in turn influence forest structure and community composition. Data on diaspore traits along five elevational gradients from forests in Thailand, the Philippines, Tanzania, Malawi and Nigeria showed that diaspore size decreases with increasing altitude, fleshy fruits remain the most common fruit type but the relative proportion of wind-dispersed diaspores increases with altitude. Probably corresponding to diaspore size decreasing with increasing elevation, we also provide evidence that avian body size and gape width decrease with increasing altitude. Among other notable changes in the frugivorous fauna across elevational gradients, we found quantitative evidence illustrating that the proportion of bird versus mammalian frugivores increases with altitude, while TMF primates decrease in diversity and density, and switch diets to include less fruit and more leaf proportionately. A paucity of studies on dispersal distance and seed shadows, the dispersal/predation balance and density-dependent mortality thwart much-needed conclusive comparisons of seed dispersal ecology between TMF and TLF, especially from understudied Asian forests. We examine the available evidence, reveal knowledge gaps and recommend research to enhance our understanding of seed dispersal ecology in tropical forests. This review demonstrates that seed dispersal is a more deterministic and important process in tropical montane forests than has been previously appreciated.

Natural disturbance, vegetation patterns and ecological dynamics in tropical montane forests

Disturbance is a central process in forest dynamics, yet the role of natural disturbance in tropical montane forests (TMFs) has not been systematically addressed. We posit that disturbance in TMFs has a wider role than commonly acknowledged and its effects are distinctive because: (1) TMFs often have very low rates of productivity due to low resources, and so recovery from disturbance may be slow, (2) montane forests have marked environmental heterogeneity which interacts with disturbance, (3) a large percentage of TMFs are regularly exposed to high energy windstorms and landslides, and (4) TMFs contain a biogeographically rich mixture of tree species with divergent evolutionary histories that interact differently with different disturbance types. We reviewed the literature on natural disturbance in TMFs and found 119 peer-reviewed papers which met our search criteria. Our review shows that disturbance is widespread in TMFs with pronounced effects on structure, function, composition and dynamics. Disturbance is also evident in the ecology of TMF biota with clear examples of plant life-history traits adapted to disturbance, including disturbance-triggered germination, treefall gap strategies and resprouting ability. Important aspects of TMF disturbances are stochastic and site-specific, but there are broad patterns in disturbance type, frequency and severity along latitudinal, altitudinal and environmental gradients. Compared with the lowland tropics, TMF disturbances are more spatially structured, TMFs experience more disturbance types in a given area due to environmental complexity, and TMFs are much more prone to small-scale yet severe landslides as well the large and potentially catastrophic disturbances of cyclones, forest die-back and fire. On the whole, natural disturbance should assume a larger role in models of ecosystem processes and vegetation patterns in TMFs. An improved understanding of what creates variation in disturbance severity and post-disturbance recovery rates, how composition and diversity feedback on disturbance type and likelihood, and how global change will alter these dynamics are important priorities in future TMF ecology research.

Geographic, environmental and biotic sources of variation in the nutrient relations of tropical montane forests

Tropical montane forests (TMF) are associated with a widely observed suite of characteristics encompassing forest structure, plant traits and biogeochemistry. With respect to nutrient relations, montane forests are characterized by slow decomposition of organic matter, high investment in below-ground biomass and poor litter quality, relative to tropical lowland forests. However, within TMF there is considerable variation in substrate age, parent material, disturbance and species composition. Here we emphasize that many TMFs are likely to be co-limited by multiple nutrients, and that feedback among soil properties, species traits, microbial communities and environmental conditions drive forest productivity and soil carbon storage. To date, studies of the biogeochemistry of montane forests have been restricted to a few, mostly neotropical, sites and focused mainly on trees while ignoring mycorrhizas, epiphytes and microbial community structure. Incorporating the geographic, environmental and biotic variability in TMF will lead to a greater recognition of plant–soil feedbacks that are critical to understanding constraints on productivity, both under present conditions and under future climate, nitrogen-deposition and land-use scenarios.