Environmental and geometric drivers of small mammal diversity along elevational gradients in Utah

The mechanisms shaping patterns of biodiversity along spatial gradients remain poorly known and controversial. Hypotheses have emphasized the importance of both environmental and spatial factors. Much of the uncertainty about the relative role of these processes can be attributed to the limited number of comparative studies that evaluate multiple potential mechanisms. This study examines the relative importance of six variables: temperature, precipitation, productivity, habitat heterogeneity, area, and the mid-domain effect on patterns of species richness for non-volant small mammals along four neighboring mountain ranges in central Utah. Along each of these elevational gradients, a hump-shaped relationship of richness with elevation is evident. This study evaluates whether the processes shaping this common pattern are also common to all gradients. Model selection indicates that no one factor or set of factors best explains patterns of species richness across all gradients, and drivers of diversity may vary seasonally. These findings suggest that commonality in the pattern of species richness, even among elevational gradients with a similar biogeographic history and fauna, cannot be attributed to a simple universal explanation.     

Patterns of plant beta‐diversity along elevational and latitudinal gradients in mountain forests of China

Biodiversity patterns and their underlying mechanisms have long been focal topics of study for ecologists and biogeographers. However, compared with spatial variation in species richness (α‐ and γ‐diversity), β‐diversity, or the dissimilarity of species composition between two or more sites has until recently received limited attention. In this study, we explored the large‐scale patterns of altitudinal turnover (β‐diversity) of plants in montane forests of China, based on systematic inventories of 1153 plots from 46 mountains distributed over ?30 degrees of latitude (21.9–51.7°N) and ?4100 m of altitude (160–4250 m). The β‐diversity of trees and shrubs declined significantly with increasing latitude. Along the altitudinal gradient, β‐diversity of both trees and shrubs showed non‐significant trends in most mountains. Differences in climate explained ?30.0% of the variation in tree β‐diversity (27.7, 36.5, and 26.2% for the Jaccard's, β_j, Sorenson's, β_s, and Simpson's dissimilarity, β_sim, respectively), with mean annual temperature being most important, and ≤ 10.0% of that in shrub β‐diversity (10.0, 8.2, and 7.0% for β_j, β_s, and β_sim, respectively), with annual actual evapotranspiration and annual precipitation as the main predictors. However, climatic controls of β‐diversity varied dramatically in different biogeograpical regions. The β‐diversity of trees exhibited stronger, whereas that of shrubs showed weaker, climatic patterns in temperate and arid than subtropical regions. These results suggest that mechanisms causing patterns of β‐diversity may differ between latitudinal and altitudinal gradients, and among biogeographical regions; as a result, caution should be exercised in drawing close parallels between patterns and causes of β‐diversity along latitudinal and altitudinal gradients and among regions.     

Anti-predator behavior along elevational and latitudinal gradients in dark-eyed juncos

Flight-initiation distance (FID),the distance between an individual and experimenter when it begins to flee,can be used to quantify risk-assessment.Among other ...     

Contrasting climate influences on Nothofagus pumilio establishment along elevational gradients

Predicted warmer temperatures and more frequent extreme climatic events in the southern Andes may affect the dynamics of the Patagonian forests. These environmental changes may differentially alter the probability of Nothofagus pumilio establishment across its altitudinal range of distribution. We monitored fruit fall, seedling emergence and survival at altitudinal distribution range of N. pumilio forests in Santa Cruz (49°22′ S—72°56′ W), Argentina. Fruit fall, seedling emergence and survival were tested in relation to drought, based on the SPEI (Standardized Precipitation Evapotranspiration Index), interacting with elevation. Fruit fall was consistently higher at low elevation and the abundance of fruits was not affected by drought intensity. Density of new-born seedlings was?~?9 to 24 times lower at low- than at high-elevation plots in our first observations (2014 and 2016), characterized by warm-dry climatic conditions in spring-early summers. In contrast, seedling abundance was?~?1.5 times larger at low elevations during relatively cold-wet growing seasons. Survival probability was explained by the interaction between SPEI and elevation. At low elevation and in dry periods, survival probability was lower (CI 54–72%) than in wet periods (CI 68–84%) but at high elevation similar survival was registered even with positive or negative SPEI values. Our results show interacting effects of elevation and drought on tree establishment at the elevation limits, with positive and negative drought effects at high and low elevations, respectively. Predicted increase of extreme drought events during the XXI century could be detrimental for N. pumilio establishment at dry, low-elevation forests.

     

Tropical metacommunities along elevational gradients: effects of forest type and other environmental factors

Elevational gradients provide a natural experiment for assessing the extent to which the structure of animal metacommunities is molded by biotic and abiotic characteristics that change gradually, or is molded by aspects of plant community composition and physiognomy that change in a more discrete fashion. We used a metacommunity framework to integrate species‐specific responses to environmental gradients as an approach to detect emergent patterns at the mesoscale in the Luquillo Mountains of Puerto Rico. Elements of metacommunity structure (coherence, species turnover and range boundary clumping) formed the basis for distinguishing among random, checkerboard, Gleasonian, Clementsian, evenly spaced and nested patterns. Paired elevational transects (300–1000 m a.s.l.) were sampled at 50 m intervals to decouple underlying environmental mechanisms: a mixed forest transect reflected changes in abiotic and biotic conditions, including forest type (i.e. tabonuco, palo colorado and elfin forests), whereas another transect reflected changes in environmental conditions but not forest type, as its constituent plots were located within palm forest. Based on distributional data (presence versus absence of species), the mixed forest transect exhibited Clementsian structure, whereas the palm forest transect exhibited quasi‐Gleasonian structure. In contrast, the distribution of modes in species abundance was random with respect to the latent environmental gradient in the mixed forest transect and clumped with respect to the latent environmental gradient in the palm forest transect. Such contrasts suggest that the environmental factors affecting abundance differed in form or type from those affecting distributional boundaries. Variation among elevational strata with respect to the first axis of correspondence from reciprocal averaging was highly correlated with elevation along each transect, even though axis scores were not correlated between mixed forest and palm forest transects. This suggests that the identity of the environmental characteristics, or the form of response by the fauna to those characteristics, differed between the two elevational transects. Despite the proximity of the transects, the patchy configuration of palm forest, and the pervasive distribution of the dominant palm species, the relative importance of abiotic variables and habitat in structuring gastropod metacommunities differed between transects, which is remarkable and attests to the sensitivity of metacommunity structure to environmental variation.     

Decomposition of Metrosideros polymorpha leaf litter along elevational gradients in Hawaii

We examined interactions between temperature, soil development, and decomposition on three elevational gradients, the upper and lower ends of each being situated on a common lava flow or ash deposit. We used the reciprocal transplant technique to estimate decomposition rates of Metrosideros polymorpha leaf litter during a three‐year period at warm and cool ends of each gradient. Litter quality was poorest early in soil development or where soils were most intensely leached and waterlogged. In situ litter decomposition was slowest on the young 1855 flow (k= 0.26 and 0.14 at low and high elevation, respectively). The more fertile Laupahoehoe gradient also supported more rapid in situ decay at the warmer low elevation site (k= 0.90) than at high elevation (k= 0.51). The gradient with the most advanced soil development showed no difference for in situ decay at low and high elevations (k= 0.88 and 0.99, respectively) probably due to low soil nutrient availability at low elevation, which counteracted the effect of warmer temperature. Comparisons of in situ, common litter, and common site experiments indicated that site factors influenced decomposition more than litter quality did. The effect of temperature, however, could be over‐ridden by soil fertility or other site factors. Field gradient studies of this sort yield variable estimates of apparent Q₁₀, even under the best conditions, due to interactions among temperature, moisture, nutrient availability, decomposer communities and litter quality. Such interactions may be as likely to occur with changing climate as they are along elevational gradients.     

Avian community structure along elevational gradients in the northeastern United States

Summary Breeding birds were censused along four elevational gradients in the Adirondack Mountains, New York, and the Green Mountains, Vermont. The bird communities of the four gradients were basically similar in species composition, richness and amplitude patterns. Three measures of species diversity decreased with increasing elevation. Low-elevation communities contained higher proportions of rare species and the relative abundances conformed to the broken-stick distribution. At higher elevations the communities showed greater dominance and the dominance-diversity curves approached geometric series. The species characteristic of high-elevation communities had the broadest altitudinal distributions.The upper and lower distributional limits of most species were independent of one another except at ecotones where marked changes in vegetation structure occurred. On each mountain, slightly more than half of the species limits coincided with ecotones. This is a significantly greater proportion than has been found in similar studies of tropical forest bird communities. In further contrast to tropical communities, we found no convincing cases of altitudinal competitive exclusion between species. Interspecific competition in the past seems to have been translated primarily into differences in habitat selection by temperate forest birds.Many of the differences between temperate forest breeding bird communities and tropical ones can be understood in terms of the migratory nature of most of the temperate species and the lower species richness in temperature forests.     

Terrestrial insects along elevation gradients: species and community responses to altitude

The literature on the response of insect species to the changing environments experienced along altitudinal gradients is diverse and widely dispersed. There is a growing awareness that such responses may serve as analogues for climate warming effects occurring at a particular fixed altitude or latitude over time. This review seeks, therefore, to synthesise information on the responses of insects and allied groups to increasing altitude and provide a platform for future research. It focuses on those functional aspects of insect biology that show positive or negative reaction to altitudinal changes but avoids emphasising adaptation to high altitude per se. Reactions can be direct, with insect characteristics or performance responding to changing environmental parameters, or they can be indirect and mediated through the insect's interaction with other organisms. These organisms include the host plant in the case of herbivorous insects, and also competitor species, specific parasitoids, predators and pathogens. The manner in which these various factors individually and collectively influence the morphology, behaviour, ecophysiology, growth and development, survival, reproduction, and spatial distribution of insect species is considered in detail. Resultant patterns in the abundance of individual species populations and of community species richness are examined. Attempts are made throughout to provide mechanistic explanations of trends and to place each topic, where appropriate, into the broader theoretical context by appropriate reference to key literature. The paper concludes by considering how montane insect species will respond to climate warming.     

Competition and habitat filtering jointly explain phylogenetic structure of soil bacterial communities across elevational gradients

The importance of assembly processes in shaping biological communities is poorly understood, especially for microbes. Here, we report on the forces that structure soil bacterial communities along a 2000 m elevational gradient. We characterized the relative importance of habitat filtering and competition on phylogenetic structure and turnover in bacterial communities. Bacterial communities exhibited a phylogenetically clustered pattern and were more clustered with increasing elevation. Biotic factors (i.e., relative abundance of dominant bacterial lineages) appeared to be most important to the degree of clustering, evidencing the role of the competitive ability of entire clades in shaping the communities. Phylogenetic turnover showed the greatest correlation to elevation. After controlling the elevation, biotic factors showed greater correlation to phylogenetic turnover than all the habitat variables (i.e., climate, soil and vegetation). Structural equation modelling also identified that elevation and soil organic matter exerted indirect effects on phylogenetic diversity and turnover by determining the dominance of microbial competitors. Our results suggest that competition among bacterial taxa induced by soil carbon contributes to the phylogenetic pattern across elevational gradient in the Tibetan Plateau. This highlights the importance of considering not only abiotic filtering but also biotic interactions in soil bacterial communities across stressful elevational gradients.     

Growth trends and climate responses of Norway spruce along elevational gradients in East-Central Europe

Key message

Decadal growth variability of Norway spruce increases with elevation. Recent temperature sensitivity and growth enhancement are limited to trees growing in the zone adjacent to timberline.

Abstract

Growth trends and climate responses of forest trees along elevational gradients are not fully understood. A deeper insight is, however, fundamental for predicting ecosystem functioning and productivity under future climate change. Supplementary to the effects of elevation and regional provenance on tree growth are sample depth, uneven representation of sample age and varying site conditions. Furthermore, there is only a limited number of studies addressing growth changes along elevational gradients, while at the same time applying tree-ring standardization methods that are sensitive to trend preservation. Here, we introduce 12 novel tree-ring width chronologies of Norway spruce (Picea abies[L.] Karst.) from four elevational belts encompassing montane forests and the local timberline in three regions in East-Central Europe between 15° and 19°E. Each chronology is characterized by sufficient sample replication and a comparable age structure between 1906 and 2010. Tree growth near timberline revealed substantial medium-frequency variability and sharply increasing ring widths since the 1980s. Medium-frequency growth variability of lower elevation trees was, however, relatively small, and growth rates over the last decade were either stable or even decreased. During the last four decades, Norway spruce from higher elevations exhibited a reduced response to autumn temperatures preceding ring formation. In contrast, trees from the lower-montane zone increased their sensitivity to drought during the same time. Our results emphasize not only different but also instable growth trends and climate responses of forest trees along altitudinal gradients, which should be considered in future forest management strategies.

     

Phylogenetic constraints to soil properties determine elevational diversity gradients of forest understory vegetation

Elevational diversity gradients (EDGs) of vegetation are shaped by the evolutionary histories of plants as well as by ecological factors. However, few studies of EDGs have focused on the roles of phylogenetic constraints and the effects of complicated interactions among environmental factors. Here, we examine the direct and indirect effects of environmental factors in forming EDGs of forest understory vegetation. The study plots were selected along elevational gradients in cool-temperate and sub-alpine forests in the University of Tokyo Chichibu Forest of central Japan. Tree seedlings and herbs were identified, and environmental factors (elevation, soil temperature, soil pH, soil CN ratio, forest type, basal area, canopy openness, and slope) were measured in these plots. Structural equation modeling (SEM) including taxonomic and phylogenetic diversity was used to consider the causal relationships between environmental conditions and the diversity of understory vegetation. In addition, phylogenetic signals in habitat requirements were tested. The taxonomic and phylogenetic diversities of tree seedlings increased monotonically with elevation, and the same pattern was found for the taxonomic diversity of herbs. The SEM indicated that both the taxonomic and phylogenetic diversity of tree seedlings were most affected by soil properties, although the phylogenetic diversity of herbs was determined by light conditions. These results highlight the importance of environmental filtering by soil properties in shaping EDGs of tree seedlings. This study implies that phylogenetic constraints in the adaptation to soil properties should be considered when predicting changes in EDGs under environmental fluctuations.     

Phylogenetically balanced evidence for structural and carbon isotope responses in plants along elevational gradients

We tested three hypotheses related to the functioning of mountain plants, namely their reproductive effort, leaf surface structure and effectiveness of CO₂ assimilation, using archive material from contrasting elevations. Analysis of elevational trends is at risk of suffering from two major biases: a phylogenetic bias (i.e. an elevational change in the abundance of taxonomic groups), and covariation of different environmental drivers (e.g. water, temperature, atmospheric pressure), which do not permit a mechanistic interpretation. We solved both problems in a subcontinental survey of elevational trends in key plant traits in the European Alps and the high Arctic (northern Sweden, Svalbard), using herbarium samples of 147 species belonging to the genera Carex, Saxifraga and Potentilla. We used both species and phylogenetically independent contrasts as data points. The analysis revealed enhanced reproductive efforts at higher elevation in insect-pollinated taxa (not in wind-pollinated taxa), no increase in leaf pubescence at high elevation (as is often assumed), and a strong correlation between ¹³C discrimination and elevation. Alpine taxa operate at a smaller mesophyll resistance to CO₂ uptake relative to diffusive resistance (stomata). By comparison with congeneric low altitude polar taxa (low temperature, but high atmospheric pressure), the response could be attributed to the elevational decline in atmospheric pressure rather than temperature (a mean increase in δ¹³C by 1.4‰ km^?1). The signal is consistent within and across genera and within species, suggesting rapid adjustment of leaf physiology to reduced partial pressure of CO₂. These results offer answers to long-debated issues of plant responses to high elevation life conditions.     

Comparative study of additive basal area of conifers in forest ecosystems along elevational gradients

We examined the basal area of two life forms (conifers vs. broadleaf trees) along elevational gradients on Yakushima Island, Japan and on two series of geological substrate on Mount Kinabalu, Borneo. On Yakushima, total stand basal area abruptly increased from 700 to 1,050 m in accordance with the high dominance of conifers, indicating the presence of additive basal area of conifers in conifer–broadleaf mixed forests at higher elevations (1,050–1,300 m). Along two substrate series on Kinabalu, some forests at higher elevations (1,860–3,080 m) showed relatively high dominance of conifers, but conifer basal area did not appear to be additive. Conifers were emergents above the canopy of broadleaf trees in mixed forests on Yakushima, but two life forms usually coexisted in the single-story canopy in mixed forests on Kinabalu. Litterfall rate as a surrogate of productivity decreased with decreasing temperature along elevation on both the sites, but the rate of decrease was slower on Yakushima, where mixed forests at higher elevations showed relatively high rates. Thus, we suggest that additive basal area of conifers was linked to their emergent status, and that it enhanced productivity by complementary use of light by two life forms that occupy different stories. On Yakushima, typhoons are a major disturbance, but do not severely limit the height growth of conifers, allowing the development of two-story mixed forests. On Kinabalu, a major disturbance is El Niño-driven drought, and hydraulic limitation to tree height may explain the non-additive and non-emergent nature of conifers.     

Historical processes enhance patterns of diversity along latitudinal gradients

One of the more vexing issues in ecology is how historical processes affect contemporary patterns of biodiversity. Accordingly, few models have been presented. Two corollary models (centre of origin, time-for-speciation) can be used to make quantitative predictions characterizing the tropical niche conservatism hypothesis and describe diversification as diffusion and subsequent cladogenesis of species away from the place of origin of a higher taxon in the tropics. Predictions derived from such models are: (i) species richness declines toward the periphery of the range of a higher taxon; (ii) taxa are more derived toward the periphery than the centre; (iii) ages of taxa are lower toward the periphery than the centre; and (iv) ages and measures of derivedness are less variable toward the periphery of the range of a higher taxon. I tested these predictions to better understand the formation of one of the most ubiquitous patterns of biodiversity-the latitudinal gradient in species richness. Results indicate well-supported predictions for New World leaf-nosed bats and that diversification has had strong influences on latitudinal gradients of species richness. A better understanding of how evolutionary diversification of taxa contributes to formation of patterns of species richness along environmental gradients is necessary to fully understand spatial variation in biodiversity.     

The influence of habitat boundaries on evolutionary branching along environmental gradients

It is well known that habitat boundaries affect ecological dynamics, but their influence on evolutionary dynamics is less well understood. Here, we study the effects of different kinds of boundaries on evolutionary branching in clonal populations along environmental gradients by systematically analyzing individual-based stochastic models in small- and large-range systems, as well as their large-population-size limits through deterministic approximations. Specifically, we examine four prototypical kinds of boundaries: impermeable boundaries at which individuals stop (“stopping”), or from which they continue back into the interior as if bouncing back mechanically (“reflecting”), or that let them abort the dispersal attempt, return to their original position and try a different direction (“reprising”), and semipermeable boundaries that can be crossed without hindrance, but do not allow the crossing individual to return (“absorbing”).We find that boundary conditions shape branching patterns only in small-range systems, where stopping boundaries generate disruptive selection for a wide range of parameters, whereas absorbing boundaries always generate stabilizing selection. Reflecting and reprising boundaries generate disruptive selection at low individual mobilities, and stabilizing selection at high mobilities. To further analyze these findings, we introduce a simple approximation of the invasion fitness in a mobile population, which predicts the observed outcome. The effect of stochasticity on evolutionary outcomes is small even in small populations: stochasticity causes random branch extinctions at steeper slopes and higher mobilities. In large-range systems, frequency-dependent interactions alone induce evolutionary branching for almost all parameters and independent of boundary conditions.