Elevation-related Differences Research Articles

Adaptation of Abies fargesii var. faxoniana (Rehder et E.H. Wilson) Tang S Liu seedlings to high altitude in a subalpine forest in southwestern China with special reference to phloem and xylem traits

Key messageAbies fargesii var. faxoniana (Rehder et E.H. Wilson) Tang S Liu seedlings at high elevations compensate for the low efficiency of their water conducting system and high phloem hydraulic resistance by the enhancement in xylem:leaf area, phloem:leaf area, and phloem:xylem area.ContextMaintenance of xylem and phloem transport is particularly important for the survival and growth of trees at the treeline. How plants modify the allocation to leaf, xylem, and phloem structures to adapt to the treeline environment is an important issue.AimsThe purpose of this study was to estimate how xylem and phloem anatomy and volume as well as leaf functional traits of A. fargesii seedlings vary with elevation.MethodsWe examined elevation-related differences in a variety of phloem and xylem functional areas and hydraulic conduit diameters of A. fargesii seedlings growing at elevations between 2600 and 3200 m in the subalpine conifer forest of southwest China.ResultsXylem area, last xylem ring area, and leaf:sapwood area significantly decreased, while xylem:leaf area, phloem:leaf area, and non-collapsed phloem:xylem area significantly increased with elevation. Principal components analysis showed that xylem area, non-collapsed phloem area, and xylem:phloem area were positively correlated with growth rates.ConclusionOur results showed that A. fargesii tree seedlings at the treeline tend to facilitate growth and maintain functional water and sugar balance between stem and leaves by the enhancement in xylem:leaf area, phloem:leaf area, and phloem:xylem area, but not through differences in vessel lumen diameter.

Elevation-related differences in annual survival of adult food-caching mountain chickadees are consistent with natural selection on spatial cognition

Animals inhabiting montane gradients experience varying winter climates that may result in differential selection on survival-related traits. Higher elevations in temperate climates are characterized by harsher winters with greater and longer-lasting snow cover compared to lower elevations, potentially leading to stronger selection for traits that improve fitness under these harsher conditions. For food-caching mountain chickadees, Poecile gambeli, inhabiting harsh high elevation environments, individual variation in spatial cognitive abilities related to cache retrieval is associated with significant differences in overwinter survival. Compared to lower elevations, stronger predicted selection on traits needed for overwinter survival at higher elevations can be expected to result in higher adult annual survival despite harsher environmental conditions, indicating that individuals that survive their first winter are better suited to survive similar subsequent selection events. Here, we used a Bayesian hierarchical Cormack-Jolly-Seber (CJS) model to estimate and compare survival of adult mountain chickadees at higher and lower elevations over 3 years. We showed that adult survival was consistently higher at higher elevations despite much harsher environmental conditions, supporting our hypothesis that selection on overwinter survival-related traits (such as spatial cognition) is stronger at our high elevation study area than at lower elevations. Understanding how environmental conditions are associated with different selection strengths on survival-related traits is an important question in behavioral ecology. Directly estimating differences in strength of selection is daunting, but comparing survival between environments may provide an alternative method. We tested for differences in adult survival in a resident food-caching species at higher and lower elevations varying in winter climate severity. These birds rely on food caches for winter survival, and juvenile birds with better spatial cognition (needed for cache retrieval) have higher survival during their first year at higher harsher elevations. Here, we report higher adult survival at higher elevations compared to lower elevations, despite much harsher winter environment. Such findings support our hypothesis for stronger selection in harsher winter conditions because individuals that survive their first year under stronger selection are better suited to survive subsequent selection events.

Variation in song structure along an elevation gradient in a resident songbird

Heterogeneous environments can create differential selection pressures among populations, which may result in the evolution of local adaptations. Premating isolation mechanisms may emerge due to limited movement among locally adapted individuals, thus further enhancing such adaptations. The song of male songbirds is one potential premating isolation mechanism that has long been of interest because of its implications for mate choice, local adaptation, and speciation associated with environmental heterogeneity. We tested whether locally adapted mountain chickadees exhibit variation in song structure along an elevation gradient and whether song structure shifts with the distinct change in winter climate severity around the snowline. These birds exhibit large elevation-related differences in numerous behavioral and neurological traits, including variation in spatial cognition associated with fitness consequences. We recorded song along two continuous elevation gradients and detected significant differences among the sampling sites along an elevation gradient and between “high” and “low” elevation songs. Our current results, coupled with previously reported winter climate-related differences are generally consistent with the local adaptation hypothesis explaining geographic variation in song, which suggests that variation in song structure could serve as an indicator of local adaptation. Our study shows that song differences can evolve on a small spatial scale along a continuous species distribution when there is a rapid change in environmental conditions also favoring the evolution of local adaptations. Local adaptations may evolve when heterogeneous environments result in differential selection on fitness-related traits. If locally adapted populations experience reduced movement, additional traits may also change over time and serve as indicators of origin. The ability to identify and mate with local males, which are likely adapted to the prevailing environment, should procure a fitness advantage such that offspring produced with a local individual will inherit the genetic material that made that individual successful. Temperate male bird song is initially learned near the nest, varies geographically, and is commonly used by females to assess male quality. Using mountain chickadees that inhabit an elevation gradient associated with large phenotypic differences, we found significant variation in song structure along an elevation gradient and that male song also changes significantly at an important ecological threshold, consistent with high- and low-elevation songs that may be used by females to identify and mate with local males.

Elevation-related differences in the age structure of breeding birds suggest stronger selection at harsher elevations

How environmental variation affects the strength of selection is important for understanding phenotypic variation within a population. An indirect method to evaluate differences in selection pressures is to compare the age composition of a breeding population. A larger adult/first-year breeding ratio, due to higher juvenile mortality, may indicate stronger selection. We investigated the adult/first-year breeding ratios of food-caching mountain chickadees inhabiting different elevations. Chickadees at high elevations experience harsher winter conditions and cache more food items and have better spatial cognition to recover caches compared with their low-elevation counterparts. Cache recovery and spatial cognition are critical for overwinter survival and individuals with better spatial cognitive abilities are more likely to survive their first winter; therefore, these differences are likely a result of stronger selection pressures on spatial cognition and caching behavior at higher elevations. In support of this hypothesis, we found a significantly larger adult/first-year breeding ratio at high elevations compared with low elevations. This may suggest higher overwinter juvenile mortality and hence stronger selection pressures on survival-related traits at high elevations, consistent with elevation-related phenotypic differences. The strength of selection on survival-related traits varies across heterogenous environments. One way to assess the strength of selection is to measure the traits of individuals that are able to survive a particular selection event. Here we assessed the adult/first-year breeding ratios of resident, food-caching birds experiencing differentially harsh winter climates. These birds rely on spatial cognition to retrieve food caches and survive the winter; the largest annual selection event. At the individual level, those with better cognitive ability have demonstrated higher overwinter survival. However, at the population level, it remains unclear whether or not the strength of selection differs between harsh and mild winter climates. By assessing the age of breeders, we found a significantly larger adult/first-year breeding ratio in birds inhabiting harsher, high elevations compared with milder, low elevations. This suggests higher overwinter juvenile mortality and hence stronger selection pressures on survival-related traits at high elevations.

Mountain chickadees from different elevations sing different songs: acoustic adaptation, temporal drift or signal of local adaptation?

Song in songbirds is widely thought to function in mate choice and male–male competition. Song is also phenotypically plastic and typically learned from local adults; therefore, it varies across geographical space and can serve as a cue for an individual's location of origin, with females commonly preferring males from their respective location. Geographical variation in song dialect may reflect acoustic adaptation to different environments and/or serve as a signal of local adaptation. In montane environments, environmental differences can occur over an elevation gradient, favouring local adaptations across small spatial scales. We tested whether food caching mountain chickadees, known to exhibit elevation-related differences in food caching intensity, spatial memory and the hippocampus, also sing different dialects despite continuous distribution and close proximity. Male songs were collected from high and low elevations at two different mountains (separated by 35 km) to test whether song differs between elevations and/or between adjacent populations at each mountain. Song structure varied significantly between high and low elevation adjacent populations from the same mountain and between populations from different mountains at the same elevations, despite a continuous distribution across each mountain slope. These results suggest that elevation-related differences in song structure in chickadees might serve as a signal for local adaptation.

Problem-solving ability and response to novelty in mountain chickadees (Poecile gambeli) from different elevations

Animals inhabiting challenging and harsh environments are expected to benefit from certain phenotypic traits including cognitive abilities. In particular, innovation and habituation are traits thought to benefit animals in challenging environments and increase individual’s probability of survival via increased foraging success. Here, we tested whether mountain chickadees from two elevations varying in winter climate severity differed in two traits involving innovation and habituation–problem-solving ability and response to novelty. Higher montane elevations are associated with a significantly longer winter period characterized by lower temperatures and more snow, making winter survival more challenging due to a probable increase in foraging demands. Mountain chickadees inhabiting the harsher high elevation were significantly faster at solving a novel problem than their low elevation counterparts. Birds from both elevations responded with increased latency to approach the novel object; however, there were no elevation-related differences. Male and female chickadees responded similarly on both tasks, suggesting no sex-related differences in problem solving ability or neophobia. The problem solving results are consistent with the results for closely related black-capped chickadees along a longitudinal gradient of winter climatic harshness on a larger geographic scale, but the response to novelty data is not. Overall, our data support the hypothesis that enhanced problem solving ability might be associated with living in harsher environments either via natural selection or by plastic response to different environments and suggest that differences in problem solving ability do not have to be associated with differences in neophobia.

Elevation related variation in aggressive response to mirror image in mountain chickadees

Behavioural syndrome literature suggests that behavioural traits may be coupled to form behavioural strategies that are consistent and repeatable across contexts. Typically these behavioural types are composed of bold, aggressive, dominant individuals and shy, less aggressive, subordinate individuals. Mountain chickadees living in varying climatic conditions across a montane environment appear to exhibit consistent behavioural types, such that high elevation birds, which experience harsher winter conditions and rely more heavily on spatial memory to retrieve food caches to survive winter, explore a novel environment at a slower rate (i.e., shy) and are socially subordinate to low elevation birds. Dominance is typically expressed in individual pairwise interactions, but it is not always clear which traits predispose an individual to become a dominant in such interactions. Aggression is frequently suggested to be associated with higher dominance status, yet the data are equivocal. Here we aimed to test whether previously described elevation related differences in social dominance might be associated with elevation related differences in aggression levels in mountain chickadees by using the mirror test. The mirror test can address potential differences in reactive and proactive aggression, although caution should be taken when using the mirror test as some species are able to self-recognize. Low elevation birds responded more aggressively to their mirror image than high elevation birds, suggesting that elevation related differences in aggression may be partially responsible for previously found differences in social dominance status.

Basin Elevation-Related Differences in the Impact of Climate Change on Snowmelt Runoff

Basin Elevation-Related Differences in the Impact of Climate Change on Snowmelt Runoff

Elevation-related variation in leaf stomatal traits as a function of plant functional type: evidence from Changbai Mountain, China.

Understanding the variation in stomatal characteristics in relation to climatic gradients can reveal the adaptation strategies of plants, and help us to predict their responses to future climate changes. In this study, we investigated stomatal density (SD) and stomatal length (SL) in 150 plant species along an elevation gradient (540–2357 m) in Changbai Mountain, China, and explored the patterns and drivers of stomatal characteristics across species and plant functional types (PFTs: trees, shrubs, and herbs). The average values of SD and SL for all species combined were 156 mm–2 and 35 µm, respectively. SD was higher in trees (224 mm–2) than in shrubs (156 mm–2) or herbs (124 mm–2), and SL was largest in herbs (37 µm). SD was negatively correlated with SL in all species and PFTs (P<0.01). The relationship between stomatal characteristics and elevation differed among PFTs. In trees, SD decreased and SL increased with elevation; in shrubs and herbs, SD initially increased and then decreased. Elevation-related differences in SL were not significant. PFT explained 7.20–17.6% of the total variation in SD and SL; the contributions of CO2 partial pressure (), precipitation, and soil water content (SWC) were weak (0.02–2.28%). Our findings suggest that elevation-related patterns of stomatal characteristics in leaves are primarily a function of PFT, and highlight the importance of differences among PFTs in modeling gas exchange in terrestrial ecosystems under global climate change.

Elevation-related differences in female mate preference in mountain chickadees: are smart chickadees choosier?

Heterogeneous environments are often associated with differential selection pressures favouring the evolution of local adaptations, and assortative mating is one of the mechanisms that might enhance such local adaptations. Montane environments present an example in which environment changes rapidly and predictably along an elevation gradient, and such variation may be expected to lead to the evolution of local adaptations. In food-caching mountain chickadees, Poecile gambeli, reliance on food stores is likely to increase with elevation, and previous research has shown that individuals living at high elevations cache more food and have superior spatial memory, needed to recover food caches, while also being socially subordinate to low-elevation birds. Here, we asked whether such differences might be associated with assortative mating. Considering that superior spatial memory ability for recovering food caches may be more critical for survival at high elevations because of more severe winter conditions, it should benefit females from high elevations to mate assortatively with males from the same elevation. If spatial memory is costly but not critical at low elevations, females from low elevation should mate assortatively with males from low elevation, especially given their socially dominant status to high-elevation birds. We assessed female preference using a pairwise choice of high- and low-elevation males. We used the amount of time spent in proximity to males from the same versus different elevation to determine female preference. High-elevation females showed significant preference for high-elevation males, however, low-elevation females showed no elevation-related preference. These results suggest that high-elevation females are choosier than low-elevation females, and prefer males from their same elevation.

Elevation-related differences in novel environment exploration and social dominance in food-caching mountain chickadees

Harsh and unpredictable environments have been assumed to favor the evolution of better learning abilities in animals. At the same time, individual variation in learning abilities might be associated with variation in other correlated traits potentially forming a behavioral syndrome. We have previously reported significant elevation-related differences in spatial memory and the hippocampus in food-caching mountain chickadees. Here, we tested for elevation-related differences in novel environment exploration, neophobia, and social dominance—behavioral traits previously thought to correlate with individual variation in cognition, using different birds from the same elevations. Compared to low-elevation birds, high-elevation chickadees were slower at novel environment exploration, but there were no detectable differences in neophobia. High-elevation chickadees were also socially subordinate to low-elevation chickadees in pairwise interactions. Considering previously reported elevation-related differences in cognition and the brain, our results suggest, however indirectly, that elevation-related variation in spatial memory might be associated with differences in novel environment exploration and in ability to obtain a high social rank in winter social groups. Whether these behavioral traits represent a behavioral syndrome or whether climate might affect these traits independently, our results suggest that multiple differences between elevations might assist with elevation-related separation. High-elevation chickadees would likely experience higher mortality if they move to lower elevation due to their low social dominance status and low-elevation chickadees might experience higher mortality if they move to higher elevation due to reduced memory ability and lack of behavioral adaptations to colder climate.

Untangling Elevation-Related Differences in the Hippocampus in Food-Caching Mountain Chickadees: The Effect of a Uniform Captive Environment

Variation in environmental conditions associated with differential selection on spatial memory has been hypothesized to result in evolutionary changes in the morphology of the hippocampus, a brain region involved in spatial memory. At the same time, it is well known that the morphology of the hippocampus might also be directly affected by environmental conditions. Understanding the role of environment-based plasticity is therefore critical when investigating potential adaptive evolutionary changes in the hippocampus associated with environmental variation. We previously demonstrated large elevation-related variation in hippocampus morphology in mountain chickadees over an extremely small spatial scale. We hypothesized that this variation is related to differential selection pressures associated with differences in winter climate severity along an elevation gradient, which make different demands on spatial memory used for food cache retrieval. Here, we tested whether such variation is experience based, generated by potential differences in the environment, by comparing the hippocampus morphology of chickadees from different elevations maintained in a uniform captive environment in a laboratory with those sampled directly from the wild. In addition, we compared hippocampal neuron soma size in chickadees sampled directly from the wild with those maintained in laboratory conditions with restricted and unrestricted spatial memory use via manipulation of food-caching experiences to test whether memory use can affect neuron soma size. There were significant elevation-related differences in hippocampus volume and the total number of hippocampal neurons, but not in neuron soma size, in captive birds. Captive environmental conditions were associated with a large reduction in hippocampus volume and neuron soma size, but not in the total number of neurons or in neuron soma size in other telencephalic regions. Restriction of memory use while in laboratory conditions produced no significant effects on hippocampal neuron soma size. Overall our results showed that captivity has a strong effect on hippocampus volume, which could be due, at least partly, to a reduction in neuron soma size specifically in the hippocampus, but it did not override elevation-related differences in hippocampus volume or in the total number of hippocampal neurons. These data are consistent with the idea of the adaptive nature of the elevation-related differences associated with selection on spatial memory, while at the same time demonstrating additional environment-based plasticity in hippocampus volume, but not in neuron numbers. Our results, however, cannot rule out that the differences between elevations might still be driven by some developmental or early posthatching conditions/experiences.

Effect of warming on extracted soil carbon pools of Abies faxoniana forest at two elevations

Carbon (C) fluxes are mainly controlled by small labile pools, and the C storage is determined by soil recalcitrant chemical compounds, which are directly associated with litter decomposition. Hence, surveys of soil C fractions, litter decomposition and CO2 efflux can be used to predict the impact of future global warming on subalpine forest ecosystems. Elevation-related differences of Abies faxoniana forest soil can be used as a proxy for future changes of C accumulation in soil. A four-year field experiment was conducted in an A. faxoniana forest using an open top chamber method at two elevations. Two-step sulfuric acid hydrolysis was used to quantify labile and recalcitrant C fractions in soil. Soil warming accelerated labile carbon decomposition and increased recalcitrant C pools, but there were altitudinal differences. Soil warming enhanced soil microbial biomass C (MB-C), accelerated litter decomposition and stimulated the release of carbon dioxide. However, the respiration quotients were not significantly affected by soil warming at either elevation. The proportions of the soil C fractions differed significantly between the two elevations. The responses of the high altitude site to soil warming were larger than those of the low altitude site. Our results suggested that decreases in soil labile C fractions and increases in soil respiration probably resulted from slight increases in MB-C under soil warming, which may enhance C loss, especially at a higher elevation where labile C is more abundant.

Elevation-related differences in memory and the hippocampus in mountain chickadees, Poecile gambeli

Harsh environments may lead to increased demands on memory in animals that rely on memory for survival. We previously showed that winter severity is associated with non-experience-based differences in memory and the hippocampus over a large continental scale in food-caching black-capped chickadees, Poecile atricapillus. However, large climatic differences also occur along steep elevational gradients in montane environments over a small geographical scale. Here, we demonstrate for the first time that large differences in memory and the hippocampus exist over extremely short distances (10 km) along the elevation gradient. We found that food-caching mountain chickadees (P. gambeli) from the highest elevations in the Sierra Nevada Mountains had significantly better spatial memory associated with larger hippocampi, with almost twice the number of hippocampal neurons, than individuals only 600 m lower in elevation. We found similarly large differences in hippocampal neurogenesis rates as indicated by the total number of immature neurons. Our study therefore suggests that climate-related environmental differences can produce dramatic differences in memory and the hippocampus in animals within close proximity on small spatial scales and that currently observed trends in global climate may have significant effects on cognition and the brain.

Forest Productivity, Leaf Area, and Terrain in Southern Appalachian Deciduous Forests

Abstract Leaf area index (LAI) is an important structural characteristic of forest ecosystems which has been shown to be strongly related to forest mass and energy cycles and forest productivity. LAI is more easily measured than forest productivity, and so a strong relationship between LAI and productivity would be a valuable tool in forest management. While a linear relationship has been observed between LAI and forest productivity, most of these data have been collected in needle-leaved evergreen stands. The generality and consistency of the relationship between LAI and productivity has not been as well established for deciduous forests. Leaf area index (LAI) and aboveground net primary production (ANPP) were measured on 16 forest stands in the southern Appalachian Mountains. These stands span a range of elevation, slope position, temperature, and moisture regimes. LAI averaged 5.8 m2 m-2 and ranged from 2.7 to 8.2. ANPP averaged 9.2 Mg ha-1 yr-1 and ranged from 5.2 to ll.8 Mg ha-1 yr-1. LAI and ANPP decreased significantly from cove to ridge sites, and ANPP decreases significantly from low to high elevation (P &amp;lt; 0.05, linear regression slope). Elevation-related differences in ANPP do not appear to be due to changes in precipitation, leaf nitrogen content, or site N mineralization rates. Linear ANPP-LAI equations fit to the data measured in this study were significant (P &amp;lt; 0.05). These relationships were not significantly different (P &amp;gt; 0.1) from linear relationships based on data reported in most other studies of ANPP and LAI in eastern deciduous forests of North America. However, the slope of a linear regression model based on North American eastern deciduous forests was significantly different (P &amp;lt; 0.05) from one based on data collected in temperate deciduous forests for the rest of the globe. The differences were slight over the range of observed data, however, and the difference may be due to a narrower range of data for North American deciduous forests. For. Sci. 47(3):419–427.