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Globally, treeline ecotones vary from abrupt lines to extended zones of increasingly small, stunted and/or dispersed trees. These spatial patterns contain information about the processes that control treeline dynamics. Describing these patterns consistently along ecologically meaningful dimensions is needed for generalizing hypotheses and knowledge about controlling processes and expected treeline shifts globally. However, existing spatial categorizations of treelines are very loosely defined, leading to ambiguities in their use and interpretation. To help better understand treeline‐forming processes, we present a new framework for describing alpine treeline ecotones, focusing on hillside‐scale patterns, using pattern dimensions with distinct indicative values: 1) the spatial pattern in the x‐y plane: a) decline in tree cover, and b) change in the level of clustering. Variation along these dimensions results in more or less 'discrete' or 'diffuse' treelines with or without islands. These patterns mainly indicate demographic processes: establishment and mortality. 2) Changes in tree stature: a) decline in tree height, and b) change in tree shape. Variation along these dimensions results in more or less 'abrupt' or 'gradual' treelines with or without the formation of environmental krummholz. These patterns mainly indicate growth and dieback processes. Additionally, tree population structure can help distinguish alternative hypotheses about pattern formation, while analysing the functional composition of the ecotonal vegetation is essential to understand community‐level processes, controlled by species‐specific demographic processes. Our graphical representation of this framework can be used to place any treeline pattern in the proposed multi‐dimensional space to guide hypotheses on underlying processes and associated dynamics. To quantify the dimensions and facilitate comparative research, we advocate a joint effort in gathering and analysing spatial patterns from treelines globally. The improved recognition of treeline patterns should allow more effective comparative research and monitoring and advance our understanding of treeline‐forming processes and vegetation dynamics in response to climate warming. [ABSTRACT FROM AUTHOR]

Background: Classifying species from the high tropical Andes into plant life forms (PLF) can provide a more functional understanding of tropical treeline dynamics. However, little is known about the potential response of PLFs to climate warming. Aims: The objective of this work was to evaluate the response of PLFs to environmental conditions above the upper montane forest in the Venezuelan Andes and forecast their potential distribution under warming scenarios using habitat suitability models. Methods: We classified species into nine PLFs, analysed their current distribution using CCA and non-linear multiple regressions and forecasted their potential distribution under warming scenarios. We used anatomical leaf traits for shrubs and caulescent rosettes to refine their potential responses to climate. Results: Tree cover sharply decreased with increasing elevation, while shrub and caulescent rosette distribution differed depending on their leaf traits. Projections suggested an upslope shift of all PLFs with warming and an increase in tree cover. Conclusions: The analysis of the distribution of PLFs, by linking plant adaptive strategies to topography/climate, allowed projecting changes in vegetation physiognomy in response to warming. Projections of a moderate increase in tree cover in the grassland páramo did not indicate the formation of closed forests above the forest line. [ABSTRACT FROM AUTHOR]

Background: Giant rosettes constitute one of the most distinctive growth-forms in tropical alpine ecosystems. However, their interactions with other plant species remain unexplored in high Andean páramos. Aims: Quantify the effect of a dominant rosette (Coespeletia timotensis) on soil and microclimatic conditions and relate the impacts to plant community structure. Methods: We analysed topsoil temperatures, soil organic matter (SOM) and plant species cover in areas adjacent to C. timotensis and paired areas outside, at three sites (4250–4360 m a.s.l.) in La Culata National Park, Venezuela. Species richness, total vegetation cover and percent cover of each species near and away from the rosettes were compared. Results: Topsoil temperature amplitudes were lower and SOM greater near C. timotensis stems, compared to areas away from them. C. timotensis had a consistent positive effect on species richness, vegetation cover and the cover of many abundant species (including cushions and herbs). Conclusions: The facilitation effects of C. timotensis on abiotic conditions and community structure indicate they play a key role as nurse plants, pointing to giant rosettes as foundation species for the maintenance of plant diversity in the alpine tropics. [ABSTRACT FROM AUTHOR]

Facilitation by nurse plants has received considerable attention, but the feedback effects of beneficiaries on the benefactor fitness remain comparatively unexplored. In particular, to our knowledge there have been no studies of feedback effects of exotic species on nurse plants. Furthermore, there have been few studies of how exotic plants interact with native beneficiaries, potentially generating indirect effects on the nurse. Here, through spatial association analyses, we estimate the reciprocal interactions between the cushion nurse-plant Arenaria musciformis, native beneficiaries, and the dominant exotic Rumex acetosella (a fast growing heliophile species) in a tropical alpine ecosystem of the Venezuelan Andes, locally known as páramos . We recorded the density of natives and the exotic within Arenaria cushions and in paired open areas at three sites and calculated Relative Interaction Indices (RII) to explore the effects of the cushion. Then, we analyzed the feedback effects of Rumex and the natives on the density of flowers produced by Arenaria . Finally, we analyzed how Rumex abundance is related to the abundance of native species inside and outside cushions. RIIs indicated a facilitative effect of cushions on the abundance of native plants, but no effect on Rumex . We found a negative relationship between the density of natives and cushion flower density, but no such relationship for Rumex . However, at high densities, Rumex was negatively correlated with the abundance of the natives both inside and outside of cushions. Our results suggest the possibility of complex reciprocal interactions between nurses, natives and exotics, and that native and exotic plants can differ in their feedback effects on nurse cushions. Native plants appeared to be facilitated by cushions and then develop a parasitic relationship with their nurses. In contrast, the exotic Rumex had a neutral interaction with the nurse, but competitive effects on the native beneficiary community, which could indirectly modulate the feedback effects of natives on the nurse. These results highlight the importance of analyzing direct and indirect reciprocal interactions and the impacts of exotic invaders for understanding alpine community assembly. [ABSTRACT FROM AUTHOR]

Questions Do local plant-plant interactions and facilitation play an important role in vegetation dynamics after disturbance in the high tropical Andes? In particular, does the dominant shrub Baccharis prunifolia have an effect on microclimate and local vegetation structure within old-fields at the upper forest line? Does distance from the border of the surrounding forests also influence community structure within fallow plots? Is there an interaction between facilitation and edge effects on vegetation regeneration? Location Gavidia Valley, Sierra Nevada de Mérida, Venezuela. Methods Plant cover, species richness and community structure were compared in circular micro-plots placed under the canopy of B. prunifolia shrubs and in open areas (12 micro-plots in each case) both near and far from the forest border within two 10-12 yr successional plots at 3400 m a.s.l. (a total of 96 micro-plots). Additionally, topsoil temperature was monitored for 1 mo in the four situations under study. Results Average and maximum temperature and temperature amplitudes were lower under the canopy of B. prunifolia shrubs than in open areas; this effect being more marked far from the forest edge. Species richness, vegetation cover and the density of dominant forest trees were higher under the shrubs' canopy than in the inter-shrub spaces. The proximity to the edge of the surrounding forest also had a positive effect on species richness, with no significant interaction with the local sampling situation (under shrub vs outside). Conclusions Our results suggest that both facilitation and edge effects influence the effectiveness of vegetation regeneration within old-fields in the high tropical Andes. Consequently, these processes should be considered for interpreting vegetation dynamics in environmental change scenarios at the tropical mountain tree line and for the design of ecological restoration strategies in these high-diversity ecotones, which have been subjected to large-scale anthropogenic transformations in recent decades. [ABSTRACT FROM AUTHOR]

Background:Studies on plant–plant interactions in alpine ecosystems show a strong bias for temperate vs. tropical latitudes and pay limited attention to the implications of biophysical heterogeneity beyond the local plant scale. Aims:To evaluate the effects of a dominant shrub (Hypericum laricifolium) on community organisation, integrating multiple scales of analysis, in a high Andean páramo in Venezuela. Methods:We compared plant cover, species richness and Shannon diversity between plots placed under the shrub and outside (plant scale); quantified the spatial relations ofHypericumwith its conspecifics, heterospecifics and rocks at different radial distances, using covariance functions (patch scale); analysed the effects ofHypericumshrubs on total species richness in the community (site scale); and evaluated the consistency of our results among four sites within the same region (between-site scale). Results:Average species richness, Shannon diversity, and the abundance of common forbs and conspecific shrubs were consistently higher inside theHypericumshrub, while the exoticRumex acetosellawas more abundant in open areas. However, the aggregated spatial distribution of conspecifics and other species extended beyond the local plant scale and there were no clear effects ofHypericumshrubs on community-level species richness. Conclusions:Shrubs can function as foundation species in tropical alpine environments, modifying local community structure and alpha diversity, but not necessarily the species richness of the overall community. Our results stress the need of analysing multiple spatial scales to interpret the role of plant–plant interactions (facilitation/competition) in heterogeneous alpine ecosystems. [ABSTRACT FROM AUTHOR]

Background:Studies in temperate mountains suggest that plant–plant interactions modulate tree establishment above the forest line. In tropical mountains worldwide this issue remains largely unexplored. Aims:To analyse the population structure and local spatial relationships of a dominant tree at a species-rich tropical Andean forest line. Methods:We determined changes in the population structure ofDiplostephium venezuelensealong an elevation gradient between continuous forest and open páramo and analysed plant community structure and superficial rock cover in the neighbourhood of saplings and adults at the upper forest line. Results:Sapling and adult densities were highest in low-altitude páramos adjacent to the forest line and at the borders of small forest islands. Saplings showed local spatial aggregation, were positively associated with small boulders and low shrubs, and negatively associated with mosses and lichens. However, no spatial association was found between scattered adults in the páramo and saplings of other forest trees. Conclusions:Complex species-specific local spatial interactions, suggesting both facilitative and antagonistic effects, seem to modulate the establishment of the dominant treeD. venezuelenseat and above the upper forest line. Nevertheless, the establishment of other tree species above continuous forests does not appear to be facilitated by the canopy cover offered by the isolatedD. venezuelenseindividuals established in open páramo environments. [ABSTRACT FROM PUBLISHER]

The aim of this study was to analyze indicators of soil restoration during secondary succession in a heterogeneous valley in the high tropical Andes. A combination of chronosequence and permanent plot methods was used to detect changes in this heterogeneous matrix. Thirty-six plots with different fallow times (1–9 years) and four noncultivated plots with natural vegetation (páramo) were sampled twice in a 3-year interval (1996 and 1999). The following soil properties were determined: total C and N, pH, exchangeable bases, cation exchange capacity, and microbial biomass N (MB-N). Using the chronosequence approach, successional increases in soil pH and Mg were detected, pointing to these variables as indicators of soil restoration during the fallow period. Comparing the noncultivated páramo with the fallow plots, a significant decrease in MB-N was found, suggesting that this is a sensitive agricultural disturbance indicator. The permanent plot analysis failed to detect successional trends in any of the study variables, probably as a result of a lack of sensitivity of the indicators used within the 3-year interval. Nevertheless, a strong acidification was detected by the permanent plot method when fallow plots were cultivated. We conclude that the size of important soil components such as total soil organic matter or microbial biomass is not a sensitive soil restoration indicator in these heterogeneous mountain systems but that other integrative variables such as pH could be more sensitive to successional changes in key soil processes (e.g., nitrification or nutrient losses). [ABSTRACT FROM AUTHOR]

Background: Leaf functional traits (LFT) influence resource acquisition and are important for understanding ecosystem processes. Climate and land use are key filters of community composition and LFT, however, how the relative importance of these filter changes with elevation has been little studied in the Andes. Aims: To gain insight into the functional response of Andean forests to climate and disturbance in naturally regenerated forest stands. Methods: We measured leaf blade thickness (LBT), leaf area (LA), specific leaf area (SLA), and leaf dry matter content (LDMC) from 13 secondary forest communities, along a 3000-m elevation transect. We derived basal area-weighted mean community (CWM) trait values to assess the effect of climate and disturbance on the functional structure of regenerating tree communities. Results: Community LFT progressively shifted along the elevation gradient driven by changes in temperature and successional stages towards communities with thicker leaves with low SLA. Reduction in LDMC with elevation suggested that both succulence and sclerophylly were important strategies in these forests. Conclusions: Our findings reinforce the validity of LFT as a powerful predictor to explore the ecological strategies of tree species in climate scenarios. Warmer conditions could result in a shift from slower to faster resource acquisition strategies at higher elevations. [ABSTRACT FROM AUTHOR]

Aim: The climate variability hypothesis (CVH) predicts that locations with reduced seasonal temperature variation select for species with narrower thermal ranges. Here we (a) test the CVH by assessing the effect of latitude and elevation on the thermal ranges of Andean vascular plant species and communities, and (b) assess tropical alpine plants vulnerability to warming based on their thermal traits. Location: High tropical Andes. Taxon: Vascular plants. Methods: Temperature data for 505 vascular plant species from alpine communities on 49 summits, were extracted from 29,627 georeferenced occurrences. Species thermal niche traits (TNTs) were estimated using bootstrapping for: minimum temperature, optimum (mean) temperature and breadth (maximum‐minimum). Plant community‐weighted scores were estimated using the TNTs of their constituent species. CVH was tested for species, biogeographical species groups and communities. Vulnerability to global warming was assessed for species, biogeographical species groups and communities. Results: Species restricted to the equator showed narrower thermal niche breadth than species whose ranges stretch far from the equator, however, no difference in niche breadth was found across summits' elevation. Biogeographical species groups distributed close to the equator and restricted to alpine regions showed narrower niche breadth than those with broader ranges. Community‐weighted scores of thermal niche breadth were positively related to distance from equator but not to elevation. Based on their TNTs, species restricted to equatorial latitudes and plant communities dominated by these species were identified as the most vulnerable to the projected 1.5°C warming, due to a potentially higher risk of losing thermal niche space. Main conclusions: Our study confirms that the CVH applies to high tropical Andean plant species and communities, where latitude has a strong effect on the thermal niche breadth. TNTs are identified as suitable indicators of species' vulnerability to warming and are suggested to be included in long‐term biodiversity monitoring in the Andes. [ABSTRACT FROM AUTHOR]

• Intensity of facilitation and life history strategy of beneficiaries interact to determine coexistence. • Nurse species can facilitate more than one parasite species, allowing for intense indirect facilitation. • A mean-field model finds that a K-species beneficiary can facilitate a nurse through competition with a more parasitic r-species beneficiary. • Spatial effects using a 2-D lattice model allows an r-strategist with weaker parasitic effects to facilitate nurses. • Indirect facilitation may create substantial conditionality in the outcomes of interactions among multiple species. When a nurse species facilitates the density of more than one species, strong indirect interactions can occur between the facilitated, or beneficiary, species, and these could lead to cascading interactive effects on community dynamics. In this context, negative effects of beneficiaries on the growth or reproduction of nurses are much more common than positive effects. This suggests beneficiaries frequently act as parasites of their nurses, and the consequences of this are largely unexplored. Our general aim is to analyze whether competition between parasitic beneficiaries can lead to indirect facilitation to nurse species and how this influences nurse-beneficiary systems. We explored potential outcomes of such reciprocal interactions in the general case of one facilitator and two facultative parasitic beneficiary species with different strategies for competing for space, one having a high carrying capacity but low maximum intrinsic growth rate (K-species), and the other having low carrying capacity but a higher intrinsic growth rate (r-species). These are defined in terms of the logistic equation, and reflect the abundances they can reach when growing alone. By considering a set of ordinary differential equations for the abundances of the nurse and the two parasitic beneficiaries in the mean-field approximation (where spatial correlations do not play a role), we first show analytically that coexistence of the three species is only possible when the r-species beneficiary is, at the same time, more harmful than the K-species and receives more benefit from the nurse. We then show that only the K-species can indirectly facilitate the nurse in such system. These are general, analytic results, independent of particular values of the parameters. We then explore these results using a 2-D lattice model informed by cushion plants in alpine ecosystems, and their interactions with beneficiaries with r and K strategies. Interesting spatial effects emerge in this case, such as a seeding effect: facilitation by the nurse increases beneficiary abundances also outside nurse patches. These in turn generate a negative feedback to the nurse, due to local competition for space near its edge. Spatial distribution effects are also crucial for relaxing the conditions for the survival of the r-species, allowing an r-strategist with weaker parasitic effects to indirectly facilitate the nurse through suppression of a more harmful K-species. Unexpectedly, this also has an indirect positive effect on the K species because of increased abundance of nurses. In the case of the r-species representing a ruderal invader, our lattice results would suggest that invaders have the potential to benefit both nurse and native beneficiary species via indirect facilitation. More generally, our results indicate that facilitation of more than one other species varying in competitive ability and which act as parasites on a nurse, can in turn promote indirect facilitation effects. This form of indirect facilitation has not been explicitly studied before, although it may create substantial conditionality in the outcomes of interactions among multiple species and the dynamics of nurse-beneficiary systems. [ABSTRACT FROM AUTHOR]

Facilitative effects of some species on others are a major driver of biodiversity. These positive effects of a benefactor on its beneficiary can result in negative feedback effects of the beneficiary on the benefactor and reduced fitness of the benefactor. However, in contrast to the wealth of studies on facilitative effects in different environments, we know little about whether the feedback effects show predictable patterns of context dependence., We reanalyzed a global data set on alpine cushion plants, previously used to assess their positive effects on biodiversity and the nature of the beneficiary feedback effects, to specifically assess the context dependence of how small- and large-scale drivers alter the feedback effects of cushion-associated (beneficiary) species on their cushion benefactors using structural equation modelling., The effect of beneficiaries on cushions became negative when beneficiary diversity increased and facilitation was more intense. Local-scale biotic and climatic conditions mediated these community-scale processes, having indirect effects on the feedback effect. High-productivity sites demonstrated weaker negative feedback effects of beneficiaries on the benefactor., Our results indicate a limited impact of the beneficiary feedback effects on benefactor cushions, but strong context dependence. This context dependence may help to explain the ecological and evolutionary persistence of this widespread facilitative system. [ABSTRACT FROM AUTHOR]