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Abandoned tropical pastures offer opportunities for passive and active restoration of native forest communities. Tree architecture of remnant canopy trees may be one important factor that can facilitate native plant recruitment in abandoned pastures but has largely been overlooked. Here, we evaluated patterns of native woody plant recruitment under remnant trees in abandoned pastures on Hawai’i Island and how these might be related to both tree architectural features and landscape variables. We measured native woody stems (excluding sprouts of the tree itself) in a 5 m radius around the base of each tree and modeled total basal area of native stems as a function of tree architectural characteristics. Recruitment was positively correlated with tree height as well as horizontal woody area below 1 m (tree structure that occurred below 1 m and was

Seed dispersal is an important ecological process that structures plant communities and influences ecosystem functioning. Loss of animal dispersers therefore poses a serious threat to forest ecosystems, particularly in the tropics where zoochory predominates. A prominent example is the near-total extinction of seed dispersers on the tropical island of Guam following the accidental introduction of the invasive brown tree snake (Boiga irregularis), negatively impacting seedling recruitment and forest regeneration. We investigated frugivory by a remnant population of Såli (Micronesian starling –Aplonis opaca) on Guam and two other island populations (Rota, Saipan) to evaluate their ecological role as a seed disperser in the Mariana archipelago. Using a combination of behavioural observations, nest contents and fecal samples, we documented frugivory of 37 plant species. Native plants comprised the majority (66%) of all species and 90% of all seeds identified in fecal and nest contents. Diet was highly similar across age classes and sampling years. In addition, plant species consumed by Såli comprised 88% of bird-dispersed adult trees and 54% of all adult trees in long-term forest monitoring plots, demonstrating the Såli’s broad diet and potential for restoring native forests. Overall, we provide the most comprehensive assessment to date of frugivory by the Såli and confirm its importance as a seed disperser on Guam and throughout the Marianas.

Trees can have large effects on soil nutrients in ways that alter succession, particularly in the case of nitrogen-(N)-fixing trees. In Hawai'i, forest restoration relies heavily on use of a native N-fixing tree, Acacia koa (koa), but this species increases soil-available N and likely facilitates competitive dominance of exotic pasture grasses. In contrast, Metrosideros polymorpha ('ōhi'a), the dominant native tree in Hawai'i, is less often planted because it is slow growing; yet it is typically associated with lower soil N and grass biomass, and greater native understory recruitment. We experimentally tested whether it is possible to reverse high soil N under koa by adding 'ōhi'a litter, using additions of koa litter or no litter as controls, over 2.5 yr. We then quantified natural litterfall and decomposition rates of 'ōhi'a and koa litter to place litter additions in perspective. Finally, we quantified whether litter additions altered grass biomass and if this had effects on native outplants. Adding 'ōhi'a litter increased soil carbon, but increased rather than decreased inorganic soil N pools. Contrary to expectations, koa litter decomposed more slowly than 'ōhi'a, although it released more N per unit of litter. We saw no reduction in grass biomass due to 'ōhi'a litter addition, and no change in native outplanted understory survival or growth. We conclude that the high N soil conditions under koa are difficult to reverse. However, we also found that outplanted native woody species were able to decrease exotic grass biomass over time, regardless of the litter environment, making this a better strategy for lowering exotic species impacts.

Current understanding of mutualistic networks is grounded largely in data on interaction frequency, yet mutualistic network dynamics are also shaped by interaction quality—the functional outcomes of individual interactions on reproduction and survival. The difficulty of obtaining data on functional outcomes has resulted in limited understanding of functional variation among a network's pairwise species interactions, of the study designs that are necessary to capture major sources of functional variation, and of predictors of functional variation that may allow generalization across networks. In this community‐scale study, we targeted a key functional outcome in plant–frugivore networks: the impact of frugivore gut passage on seed germination. We used captive frugivore feeding trials and germination experiments in an island ecosystem, attaining species‐level coverage across all extant native frugivores and the plants they consume to (a) assess sources of functional variation, (b) separate effects of pulp removal from those of scarification via gut passage, and (c) test trait‐based correlates of gut passage effect sizes. We found antagonistic seed predation effects of a frugivore previously assumed to be a seed disperser, highlighting the need to consider functional outcomes rather than interaction frequency alone. The other frugivores each exhibited similar impacts for individual plant species, with benefits primarily caused by pulp removal rather than scarification, supporting the use of animal functional groups in this context. In contrast, plant species varied widely in impacts of gut passage on germination. Species with smaller seeds and more frugivore partners had larger benefits of gut passage, showing promise for network metrics and functional traits to predict functional variation among plants. Synthesis. Combining network and demographic approaches, we assessed the degree and sources of variation in a key functional outcome of plant–frugivore interactions across an entire network. Using a detailed study design, our work shows how simpler study designs can capture primary sources of functional variation and that functional traits and network metrics may allow generalization across networks. Efficiently measuring and generalizing sources of functional variation within mutualistic networks will strengthen our ability to model network dynamics and predict mutualist responses to global change.

Islands experience major impacts from introduced species, especially nocturnal predators. The brown tree snake (Boiga irregularis) extirpated almost the entire native avifauna on Guam after introduction. Reintroductions from neighbouring islands can restore bird communities but will take place in heavily managed units where snake populations can be controlled. Yet, reintroductions often proceed without relevant biological information such as habitat and space use when nocturnal predators are present. To guide efforts, we studied the avian frugivore community's diurnal and nocturnal habitat and space use on the island of Saipan, where the snake is absent. Using radiotelemetry and a Bayesian framework, we compared data typically accessible to resource managers (diurnal home range [DHR] and habitat selection), which often form the basis of reintroduction plans. We contrasted this with data on nocturnal habitat use that is often not available but relevant when nocturnal predation threat is high. Diurnal home range size varied within and among species with Micronesian Starlings (Aplonis opaca) having DHRs at least 45 ha larger than other species. For all species, night roost locations were generally spatially clustered within a DHR and covered a smaller spatial extent. Two species had higher probabilities of roosting outside DHR boundaries but, when outside, roosts were often within 200 m of the DHR boundary. All species selected forested habitats during day and night, with some species choosing native forest over non‐native habitat. Synthesis and applications. If individuals roosted randomly throughout diurnal home ranges (DHRs), landscape‐level suppression of the brown tree snake on Guam might be the only viable option for reintroductions. However, we found that all species showed clustered roosting and high site fidelity. If site fidelity is common, then individuals that roost within fenced areas where snake populations are severely reduced will experience high survival even if diurnal home ranges extend into the surrounding unprotected matrix. There was a large overlap in habitat selection during day and night, indicating managed areas should be composed of native forest with non‐native forests of secondary importance. In systems with nocturnal predators, understanding variability in diurnal and nocturnal habitat use could lead to better informed management decisions.

Vertebrate frugivores enhance tropical forest regeneration by dispersing seeds into degraded areas. However, the importance of individual species as dispersers may vary within a community. Management and restoration would benefit from understanding which species are critical in moving native seeds into degraded habitats. We compared habitat composition of bird start and end locations for movement intervals based on mean gut passage times for the avian frugivore community on the island of Saipan. The proportion of movement intervals that began in intact, native forest varied among species, with Golden White-eyes having the highest proportion. However, this species tended to remain in intact forest and only rarely crossed into degraded habitats. Bridled White-eyes and Mariana Fruit Doves exhibited slightly higher rates of crossing from intact forest to degraded habitats, suggesting an ability to disperse native seeds to degraded areas. White-throated Ground Doves were never recorded crossing from intact forest to degraded habitats. Despite having a low proportion of movement intervals beginning in intact forest, Micronesian Starlings showed a higher proportion and absolute number of movements from intact forest to degraded habitats, due to their propensity to move frequently, across long distances, and across habitat types. In this species-poor frugivore network, seed dispersal into degraded habitats appears highly dependent on one species within the community. Regeneration of degraded lands may be severely hindered if this key disperser is lost.

Unlike traditional successional theory, Alternate Stable Equilibrium (ASE) theory posits that more than one community state is possible in a single environment, depending on the order that species arrive. ASE theory is often invoked in management situations where initial stressors have been removed, but native-dominated communities are not returning to degraded areas. Fundamental to this theory is the assumption that equilibria are maintained by positive feedbacks between colonizers and their environment. While ASE has been relatively well studied in aquatic ecosystems, more complex terrestrial systems offer multiple challenges, including species interactions across trophic levels that can lead to multiple feedbacks. Here, we discuss ASE theory as it applies to terrestrial, invaded ecosystems, and detail a case study from Hawai'i that exemplifies how species interactions can favour the persistence of invaders, and how an understanding of interactions and feedbacks can be used to guide management. Our system includes intact native-dominated mesic forest and areas cleared for pasture, planted with non-native grasses, and later planted with a monoculture of a native nitrogen-fixing tree in an effort to restore forests. We discuss interactions between birds, understorey fruiting native species, understorey non-native grasses, soils and bryophytes in separate feedback mechanisms, and explain our efforts to identify which of these feedbacks is most important to address in a management context. Finally, we suggest that using models can help overcome some of the challenges that terrestrial ecosystems pose when studying ASE.

Restoration of tropical forests can lead to enhanced ecosystem services and increases in native biodiversity. Bryophytes may be an integral part of the forest restoration process and can serve a critical role in forest functioning. However, the recovery of bryophytes and their ability to facilitate woody plant establishment during restoration remains poorly studied, especially in the tropics. We investigate how bryophyte abundance and community composition, as well as woody plant seedling associations with bryophyte mats and other ground cover types change from under the canopy of intact forest to under trees in restoration corridor plantings in Hawaii. Restoration corridors consisted of corridors of koa ( Acacia koa ) trees that were planted roughly 30 years ago. Some corridors were planted around remnant ʻōhiʻa trees ( Metrosideros polymorpha ) that can be several hundred years old. We sampled under ʻōhiʻa in intact forest and both koa and ʻōhiʻa trees in restoration corridors. In restoration corridors, bryophyte abundance was low relative to intact forest and species diversity was a subset of that found in intact forest despite restoration corridors being several decades old. Seedlings strongly associated with bryophytes across all habitats suggesting that bryophytes may significantly enhance forest seedling establishment when present in restoration corridors. Other ground cover types like woody litter and nurse logs also had a positive association with forest seedlings but were rare in restoration corridors. Grass remained a dominant ground cover type in restoration corridors under koa and remnant ʻōhiʻa trees and only a single seedling was ever found growing in this grass. Enhancing bryophyte growth and recovery within restoration plantings through the reduction of grass cover could facilitate native plant establishment.

Populations occurring at species' range edges can be locally adapted to unique environmental conditions. From a species' perspective, range‐edge environments generally have higher severity and frequency of extreme climatic events relative to the range core. Under future climates, extreme climatic events are predicted to become increasingly important in defining species' distributions. Therefore, range‐edge genotypes that are better adapted to extreme climates relative to core populations may be essential to species' persistence during periods of rapid climate change. We use relatively simple conceptual models to highlight the importance of locally adapted range‐edge populations (leading and trailing edges) for determining the ability of species to persist under future climates. Using trees as an example, we show how locally adapted populations at species' range edges may expand under future climate change and become more common relative to range‐core populations. We also highlight how large‐scale habitat destruction occurring in some geographic areas where many species range edge converge, such as biome boundaries and ecotones (e.g., the arc of deforestation along the rainforest‐cerrado ecotone in the southern Amazonia), can have major implications for global biodiversity. As climate changes, range‐edge populations will play key roles in helping species to maintain or expand their geographic distributions. The loss of these locally adapted range‐edge populations through anthropogenic disturbance is therefore hypothesized to reduce the ability of species to persist in the face of rapid future climate change.

The elevation of altitudinal treelines is generally believed to occur where low mean temperatures during the growing season limit growth and prevent trees from establishing at higher elevations. Accordingly, treelines should move upslope with increasing global temperatures. Contrary to this prediction, tropical treelines have remained stable over the past several decades despite increasing mean temperatures. The observed stability of tropical treelines, coupled with the drastically different temperature profiles between temperate and tropical treelines, suggests that using mean measures of temperature to predict tropical treeline movements during climate change may be overly simplistic. We hypothesize that frost events at tropical treelines may slow climate driven treeline movement by preventing tree recruitment beyond the established forest canopy. To assess this hypothesis, we measured freezing resistance of four canopy-forming treeline species (Weinmannia fagaroides, Polylepis pauta, Clethra cuneata, and Gynoxys nitida) at two life stages (juvenile and adult) and during two seasons (warm-wet and cold-dry). Freezing resistances were then compared to microclimatic data to determine if freezing events in the grassland matrix above treeline are too harsh for these forest species. Freezing resistance varied among species and life stages from -5.7 degrees C for juveniles of P. pauta to -11.1 degrees C for juveniles of W. fagaroides. Over a four-year period, the lowest temperatures recorded at 10 cm above ground level in the grasslands above treeline and at treeline itself were -8.9 degrees C and -6.8 degrees C, respectively. Juveniles maintained freezing resistances similar to adults during the coldest parts of the year and ontogenetic differences in freezing resistance were only present during the warm season when temperatures did not represent a significant threat to active plant tissue. These findings support the hypothesis that rare extreme freezing events at and above tropical treelines can prevent recruitment outside of closed canopy forest for some tree species and may significantly slow treeline advancement despite warming mean temperatures. Predictions of treeline shifts under climate change should be reevaluated to include species-specific' climatic tolerances and measures of climatic variability.

The upper elevational range edges of most tropical cloud forest tree species and hence the ‘treeline’ are thought to be determined primarily by temperatures. For this reason, the treeline ecotone between cloud forests and the overlying grasslands is generally predicted to shift upslope as species migrate to higher elevations in response to global warming. Here, we propose that other factors are preventing tropical trees from shifting or expanding their ranges to include high elevation areas currently under grassland, resulting in stationary treelines despite rising mean temperatures. The inability of cloud forest species to invade the grasslands, a phenomenon which we refer to as the ‘grass ceiling’ effect, poses a major threat to tropical biodiversity as it will greatly increase risk of extinctions and biotic attrition in diverse tropical cloud forests. In this review, we discuss some of the natural factors, as well as anthropogenic influences, that may prevent cloud forest tree species from expanding their ranges to higher elevations. In the absence of human disturbances, tropical treelines have historically shifted up- and down-slope with changes in temperature. Over time, increased human activity has limited forests to lower elevations (i.e. has depressed treelines), and often broken the equilibrium between species range limits and climate. Yet even in areas where anthropogenic influences are halted, cloud forests have not expanded to higher elevations. Despite the critical importance of understanding the distributional responses of tropical species to climate change, few studies have addressed the factors that influence treeline location and dynamics, severely hindering our ability to predict the fate of these diverse and important ecosystems.

1. The relationship between stomatal conductance (gs) and leaf water potential (Ψl) is key tothe understanding of plant function under changing climate. The variability among tree speciesgave rise to selection towards either of two contrasting water management types: isohydric oranisohydric. This study explores the variability of gs to Ψl across tree species.2. Curves of gs(Ψl) were collected from the scientific literature for 70 woody plant species.The data set is comprised of angiosperm and gymnosperm species from all major forest biomes.The hypothesis that curves from different tree species diverge between isohydric andanisohydric behaviours was tested.3. Species-specific curves formed a continuum, rather than dichotomy between isohydric andanisohydric, as confirmed by distribution models. Alternatively, the water potential at 50% ofthe maximum gs (Ψgs50) was used to quantitatively compare between species. A major differenceemerged among xylem anatomy classes whereby ring-porous species had higher absolutegs at Ψl > 2 MPa than diffuse-porous and coniferous species. A positive, linear correlationwas shown between Ψgs50 and Ψl at 50% loss of xylem conductivity.4. The results suggest that stomatal sensitivity to leaf water potential strongly relates to xylemcharacteristics. The use of Ψgs50 offers a quantitative alternative to the current, yet biased,distinction between isohydric and anisohydric species.

Montane plant species around the globe are predicted to shift their distributions upslope in response to increasing temperatures associated with climate change. In the tropical Andes, which are one of the most diverse and threatened of all biodiversity hotspots, many plant and animal species have already started to shift their distributions to higher elevations in response to warming. However, a variety of biotic and abiotic factors may stabilize Andean timberlines and halt further upslope migrations into the high elevation grasslands (referred to as “puna” in southern Peru) above the forest. One possibility is that small forest patches that occur above-timberline throughout the Andes may facilitate forest expansion into newly suitable areas in the puna, yet little is known about the ecology or function of these patches. In this study, we examined seedling recruitment patterns, seed dispersal, and microclimate at the timberline, in and around above-timberline forest patches, and in the puna. The above-timberline forest patches had similar patterns of seed dispersal as the timberline but overal

Analyses of topography show that mountains do not monotonically decrease in area with elevation as is commonly believed and that in reality land area often increases at higher elevations. This finding bodes well for the future of biodiversity since it means that in many parts of the world there are sufficient upslope areas for low- and mid-elevation species to migrate into as temperatures increase. However, more attention needs to be given to determining if migrating species can actually reach these expansive high-elevation areas. Many factors can prevent species from migrating upslope including stable ecotones. Often ecotonal boundaries are not set by mean temperatures alone and thus are not shifting upslope with warming. An example of this are tropical alpine treelines, which are not shifting upslope despite rapid warming potentially due to the stabilizing influences of climatic factors other than mean temperatures (e.g., extreme cold events) or non-climatic factors (e.g., soil or human disturbances). Stable ecotones can potentially prevent species from expanding their ranges into upland areas in which case the amount of land at higher elevations is irrelevant and species may face “mountain top extinctions” long before they reach the actual tops of the mountains.

Species migrations in response to climate change have already been observed in many taxonomic groups worldwide. However, it remains uncertain if species will be able to keep pace with future climate change. Keeping pace will be especially challenging for tropical lowland rainforests due to their high velocities of climate change combined with high rates of deforestation, which may eliminate potential climate analogs and/or increase the effective distances between analogs by blocking species movements. Here, we calculate the distances between current and future climate analogs under various climate change and deforestation scenarios. Under even the most sanguine of climate change models (IPSL_CM4, A1b emissions scenario), we find that the median distance between areas in the Amazon rainforest and their closest future (2050) climate analog as predicted based on just temperature changes alone is nearly 300 km. If we include precipitation, the median distance increases by over 50% to >475 km. Since deforestation is generally concentrated in the hottest and driest portions of the Amazon, we predict that the habitat loss will have little direct impact on distances between climate analogs. If, however, deforested areas also act as a barrier to species movements, nearly 30% or 55% of the Amazon will effectively have no climate analogs anywhere in tropical South America under projections of reduced or Business-As-Usual deforestation, respectively. These ‘disappearing climates’ will be concentrated primarily in the southeastern Amazon. Consequently, we predict that several Amazonian ecoregions will have no areas with future climate analogs, greatly increasing the vulnerability of any populations or species specialized on these conditions. These results highlight the importance of including multiple climatic factors and human land-use in predicting the effects of climate change, as well as the daunting challenges that Amazonian diversity faces in the near future.

Frugivores play differing roles in shaping dispersal patterns yet seed dispersal distance is rarely quantified across entire communities. We model seed dispersal distance using gut passage times and bird movement for the majority (39 interactions) of known bird–tree interactions on the island of Saipan to highlight differences in seed dispersal distances provided by the five avian frugivores. One bird species was found to be a seed predator rather than a disperser. The remaining four avian species dispersed seeds but differences in seed dispersal distance were largely driven by interspecific variation in bird movement rather than intraspecific variation in gut passage times. The median dispersal distance was at least 56 m for all species-specific combinations, indicating all species play a role in reducing high seed mortality under the parent tree. However, one species—the Micronesian Starling—performed 94% of dispersal events greater than 500 m, suggesting this species could be a key driver of long-distance dispersal services (e.g. linking populations, colonizing new areas). Assessing variation in dispersal patterns across this network highlights key sources of variation in seed dispersal distances and suggests which empirical approaches are sufficient for modelling how seed dispersal mutualisms affect populations and communities.

Conservation biology is often defined as a “mission driven crisis discipline”, and as such research priorities should ideally parallel the relative importance of different conservation threats. Conservation research has increased exponentially over the last 22 years, rising from 4000 articles in 2012. However, this growth has not and may not necessarily reflect changes in research needs. Consequently, it remains uncertain if growth and prioritization have been consistent between research themes, or subdisciplines. In other words, it is unknown if conservation priorities change in relation to research needs, or if instead to shifts in funding, which may or may not correspond to true research needs. Future conservation research priorities should ideally be based on conservation needs alone and must account for threats at both the immediate and long-term scales.

Interspersion is a key habitat component related to marsh bird abundance, but is not easily quantified. We used Fragstats 3.3 and aerial photos to measure interspersion within wetlands as m/ha of interface between vegetation and water (i.e., edge density). We then related edge density and other factors (marsh area, cover-to-water ratios, marsh area within 5 km) to the abundance of marsh birds on 16 emergent wetlands in New York during 2005. Abundance was assessed via call broadcast surveys for American Bittern (Botaurus lentiginosus), Least Bittern (Ixobrychus exilis), Pied-billed Grebe (Podilymbus podiceps), Sora (Porzana carolina), and Virginia Rail (Rallus limicola). Interspersion, as measured by edge density, was the best predictor of abundance for all species but Pied-billed Grebe (r2 = 0.30–0.71). Vegetation and water interspersed in a spatially complex pattern likely increases breeding diversity and density of marsh birds. Modern spatial analysis programs provide opportunities to quantify ...

The decline in populations of several species of marsh birds in North America has prompted development of a monitoring protocol that involves the broadcast of conspecific calls to enhance detection of these secretive species. However, with a standardized protocol, temporal (seasonal) and geographic variation in responses to the broadcast of calls could lead to inadequate monitoring of migratory species with large ranges. Our objective was to examine temporal variation in the response of marsh birds to playback of conspecific calls in west-central and northern New York to determine if use of the current protocol would permit effective monitoring of their populations. From 11 April to 8 July 2005, we conducted 572 surveys at 143 survey points on 16 marshes and detected 663 individuals of our target species. Our results revealed more detections of American Bitterns (Botaurus lentiginosus) and Pied-billed Grebes (Podilymbus podiceps) early in our survey period, and more detections of Virginia Rails (Rallus limicola) and Least Bitterns (Ixobrychus exilis) later in our survey period. Only 22% of Least Bitterns were detected before 28 May, whereas 76% of American Bitterns and 70% of Pied-billed Grebes were detected before 28 May. With the current recommended monitoring protocol, surveys are to be completed during a 44-day period that includes three 10-day sampling periods separated by 1 week. However, our results indicate that this protocol would lead to inadequate and inaccurate monitoring of marsh birds in New York. Given that the timing of peak detection of different species of marsh birds varies geographically, we recommend flexibility in the timing and duration of surveys so that surveys can be synchronized with location-specific peak-detection periods. SINOPSIS La reduccion numerica poblacional en los Estados Unidos de varias especies de aves de anegados ha fomentado el desarrollo de protocolos de monitoreo que envuelvan el uso de grabaciones de llamadas y otros sonidos (vocalizaciones) para mejorar la deteccion de especies de habitos secretivos. Sin embargo, con protocolos estandarizados, las variaciones estacionales y geograficas (como respuesta a las grabaciones) pueden dar origen a un monitoreo inadecuado particularmente de las especies migratorias de amplia distribucion. Nuesto objetivo fue examinar la varacion estacional o temporal de aves de anegados, en la respuesta a grabaciones de sonidos de conespecificos, para determinar si el uso del actual protocolo permite el monitoreo efectivo de poblaciones. El estudio se llevo a cabo en la parte oeste-central y la norte de New York. Del 11 de abril hasta el 8 de julio de 2005, llevamos a cabo 572 conteos en 143 puntos de conteo en 16 anegados, y detectamos 663 individuos de las especies que queriamos estudiar. Nuestro resultados revelan mayor cantidad de detecciones de individuos de Botarus lentiginosus y Podilymbus podiceps temprano en el periodo de conteo y mayor numero de detecciones para Rallus limicola e Ixobrichus exilis tarde en los periodos de conteo. Tan solo el 22% de los individuos de Botarus y el 70% de Podilymbus fueron detectados, despues del 28 de mayo. El actual protocolo de monitoreo recomienda que se termine con el mismo en un periodo de 44 dias, lo que incluye tres periodos de 10 dias de muestreos separados por una semana. Nuestros resultados indican, que dicho protocolo produce resultados inadecuados e inexactos en New York. Dado el caso de que el pico de deteccion de diferentes especies de aves de anegados varia geograficamente, recomendamos flexibilidad en el periodo y duracion de los conteos. Esto en aras de que los conteos puedan ser sincronizados con el pico de las respuestas de las aves en una localidad.

The study by Morueta-Holme et al. (1) provides unique and invaluable insight into how tropical montane ecosystems have responded to climate change and other anthropogenic disturbances over the past two-plus centuries. Clearly, one of the most important findings of their study is that many species and ecosystems have shifted their distributions upslope to higher elevations, in accord with rising global mean temperatures (1). Another extremely important finding of the study that is deserving of extra attention is that the high-elevation grasslands of Chimborazo (i.e., the Pajonal) have expanded their distributions not only upslope but also downslope by several hundreds of meters. The upslope …

The elevations at which tropical treelines occur are believed to represent the point where low mean temperatures limit the growth of upright woody trees. Consequently, tropical treelines are predicted to shift to higher elevations with global warming. However, treelines throughout the tropics have remained stationary despite increasing global mean temperatures. The goal of the study reported here was to build a more comprehensive understanding of the effects of mean temperature, low-temperature extremes, shading, and their interactions on seedling survival at tropical treelines. We conducted a seedling transplant study using three dominant canopy-forming treeline species in the southern tropical Andes. We found species-specific differences and contrasting responses in seedling survival to changes in mean temperature. The most abundant naturally occurring species at the seedling stage outside the treeline, Weinmannia fagaroides, showed a negative relationship between the survival of transplanted seedlings and mean temperature, the opposite of a priori expectations. Conversely, Clethra cuneata showed increased survival at higher mean temperatures, but survival also increased with higher absolute low temperatures and the presence of shade. Finally, the survival of Gynoxys nitida seedlings was insensitive to temperature but increased under shade. These findings show that multiple factors can determine the upper distributional limit of species forming the current tropical treeline. As such, predictions of future local and regional tropical treeline shifts may need to consider several factors beyond changes in mean temperature. If the treeline remains stationary and cloud forests are unable to expand into higher elevations, there may be severe species loss in this biodiversity hotspot.

The recent paper by Freeman and Class Freeman (1) adds to a growing body of literature documenting the upslope distributional shifts of tropical montane species in response to climate change. Armed with just a handful of studies from the tropics, the authors compare upslope shift rates between tropical and nontropical species and conclude that tropical species are, on average, shifting their distributions faster than temperate species. On the basis of this broad comparison, the authors contend that tropical montane species are more sensitive to temperature changes than are temperate species. Although this is a laudable first attempt to synthesize basic patterns of range shifts in tropical species, I believe that the authors drew spurious conclusions because they overlooked some basic ecological differences between taxa.

Temperature-change trajectories are being used to identify the geographic barriers and ther- mal 'cul-de-sacs' that will limit the ability of many species to track climate change by migrating. We ar- gue that there are many other potential barriers to species' migrations. These include stable ecotones, discordant shifts in climatic variables, human land use, and species' limited dispersal abilities. To illus- trate our argument, for each 0.5° latitude/longitude grid cell of the Earth's land surface, we mapped and tallied the number of cells for which future (2060-2080) climate represents an analog of the focal cell's current climate. We compared results when only considering temperature with those for which both temperature and total annual precipitation were considered in concert. We also compared results when accounting for only geographic barriers (no cross-continental migration) with those involving both geo- graphic and potential ecological barriers (no cross-biome migration). As expected, the number of future climate analogs available to each pixel decreased markedly with each added layer of complexity (e.g. the proportion of the Earth's land surface without any available future climate analogs increased from 3% to more than 36% with the inclusion of precipitation and ecological boundaries). While including additional variables can increase model complexity and uncertainty, we must strive to incorporate the factors that we know will limit species' ranges and migrations if we hope to predict the effects of climate change at a high-enough degree of accuracy to guide management decisions.

In the face of ongoing and future climate change, species must acclimate, adapt or shift their geographic distributions (i.e., "migrate") in order to avoid habitat loss and eventual extinction. Perhaps nowhere are the challenges posed by climate change more poignant and daunting than in tropical forests, which harbor the majority of Earth’s species and are facing especially rapid rates of climate change relative to current spatial or temporal variability. Due to the rapid changes in climate predicted for the tropics, coupled with the apparently low capacities of tropical tree species to either acclimate or adapt to sustained changes in environmental conditions, it is believed that the greatest hope for avoiding the loss of biodiversity in tropical forest is species migrations. This is supported by the fact that topical forests responded to historic changes in climate (e.g., post glacial warming) through distributional shifts. However, a great deal of uncertainty remains as to if tropical plant and tree species can migrate, and if so, if they can migrate at the rates required to keep pace with accelerating changes in multiple climatic factors in conjunction with ongoing deforestation and other anthropogenic disturbances. In order to resolve this uncertainty, as will be required to predict, and eventually mitigate, the impacts of global climate change on tropical and global biodiversity, more basic data is required on the distributions and ecologies of tens of thousands of plants species in combination with more directed studies and large-scale experimental manipulations.

In the face of ongoing and future climate change, species must acclimate, adapt or shift their geographic distributions (i.e., "migrate") in order to avoid habitat loss and eventual extinction. Perhaps nowhere are the challenges posed by climate change more poignant and daunting than in tropical forests, which harbor the majority of Earth’s species and are facing especially rapid rates of climate change relative to current spatial or temporal variability. Due to the rapid changes in climate predicted for the tropics, coupled with the apparently low capacities of tropical tree species to either acclimate or adapt to sustained changes in environmental conditions, it is believed that the greatest hope for avoiding the loss of biodiversity in tropical forest is species migrations. This is supported by the fact that topical forests responded to historic changes in climate (e.g., post glacial warming) through distributional shifts. However, a great deal of uncertainty remains as to if tropical plant and tree species can migrate, and if so, if they can migrate at the rates required to keep pace with accelerating changes in multiple climatic factors in conjunction with ongoing deforestation and other anthropogenic disturbances. In order to resolve this uncertainty, as will be required to predict, and eventually mitigate, the impacts of global climate change on tropical and global biodiversity, more basic data is required on the distributions and ecologies of tens of thousands of plants species in combination with more directed studies and large-scale experimental manipulations.