Lower Elevational Limits Research Articles

Strong differences in egg heat tolerance between closely related lowland and alpine butterfly species

Abstract Temperature variation is one of the best‐known and studied factor constraining life history along the altitudinal gradients. Ectotherms such as insects are sensitive to temperature across all life stages, resulting in a wide range of adaptations in populations and species living at different elevations. In butterflies, the egg stage is particularly sensitive to temperature variation as eggs cannot move and deploy behavioural responses to overheating. Using a gradient polymerase chain reaction thermocycler to finely control temperature treatments, we tested for differences in egg heat tolerance between five closely related Coenonympha (Nymphalidae: Satyrinae) species living at different elevations. We found strong differences in egg heat tolerance ranges between lowland species (C. arcania (Linnaeus) and C. pamphilus (Linnaeus)) and two high elevation alpine species (C. gardetta (Prunner) and C. cephalidarwiniana Verity). Surprisingly, the third alpine species (C. darwiniana Staudinger) showed a heat tolerance range closer to that of the lowland species than that of alpine species. The sensitivity of eggs to overheating matched the temperature regimes recorded in the microhabitats utilised for oviposition and may explain the oviposition site selection of females observed in the wild. The narrower egg heat tolerance of some of alpine species may partly explain the position of their lower elevational limit but also raises concern about the fate of these species in the face of current climate change. Our results highlight the need to study all life stages, as well as microclimate choice for oviposition, to understand species distribution and the threats to their persistence.

Traits shaping urban tolerance in birds differ around the world

As human density increases, biodiversity must increasingly co-exist with urbanization or face local extinction. Tolerance of urban areas has been linked to numerous functional traits, yet few globally consistent patterns have emerged to explain variation in urban tolerance, which stymies attempts at a generalizable predictive framework. Here, we calculate an Urban Association Index (UAI) for 3,768 bird species in 137 cities across all permanently inhabited continents. We then assess how this UAI varies as a function of ten species-specific traits and further test whether the strength of trait relationships vary as a function of three city-specific variables. Of the ten species traits, nine were significantly associated with urban tolerance. Urban-associated species tend to be smaller, less territorial, have greater dispersal ability, broader dietary and habitat niches, larger clutch sizes, greater longevity, and lower elevational limits. Only bill shape showed no global association with urban tolerance. Additionally, the strength of several trait relationships varied across cities as a function of latitude and/or human population density. For example, the associations of body mass and diet breadth were more pronounced at higher latitudes, while the associations of territoriality and longevity were reduced in cities with higher population density. Thus, the importance of trait filters in birds varies predictably across cities, indicating biogeographic variation in selection for urban tolerance that could explain prior challenges in the search for global patterns. A globally informed framework that predicts urban tolerance will be integral to conservation as increasing proportions of the world's biodiversity are impacted by urbanization.

On the predictability of phenotypic divergence in geographic isolation.

Do related populations that are separated by barriers predictably evolve differences from one another over time, or is such divergence idiosyncratic and unpredictable? We test these alternatives by investigating patterns of trait evolution for 54 sister pairs of Andean forest birds that live in similar environments on either side of the arid Marañón Gap, a strong dispersal barrier for humid montane species. We measured divergence in both sexual (song and plumage) and ecological (beak size and beak shape) traits. Sexual traits evolve in a clock-like fashion, with trait divergence positively correlated with genetic distance (r = 0.6-0.7). In contrast, divergence in ecological traits is uncorrelated or only loosely correlated with genetic distance (r = 0.0-0.3). Thus, for geographically isolated Andean montane forest birds that live in similar environments, divergence is predictable in sexual traits, but not for ecological traits. This means that sexual trait divergence occurs independently of adaptive ecological divergence within the mega-diverse tropical Andean avifauna. Last, we show that variation in genetic divergence across a biogeographic barrier is associated with traits that are proxies for species' opportunities for dispersal (low elevation limit and elevational niche breadth), but not with traits that are proxies for species' dispersal abilities (hand-wing index and foraging strata).

Ranges of woody plant species and ferns on forested elevational gradients on Aotea-Great Barrier Island, New Zealand: the role of zones of permanent and temporary establishment

The arrangement of plant species along elevational gradients is prominent in the debate between individualistic versus community-unit concepts in plant ecology. We obtained elevational ranges (upper and lower elevation limits) for woody species and ferns on the three highest mountains on Aotea (Great Barrier Island), Aotearoa-New Zealand. These data allowed potential species composition to be obtained for any elevation and were analysed using regression, ordination, and classification. Both woody plants and ferns demonstrated linear declines in richness with increasing elevation.

Reduced forest vulnerability due to management on the Hualapai Nation

Tribal nations in the US have worked to uphold their long history of managing forests in ways that reduce fuels, support ecosystem functioning, and enhance Indigenous livelihoods. Forests on the Hualapai Nation at the western end of the Grand Canyon have been actively managed for decades using fire and other treatments. We collected data on tree size and age structure, forest understory characteristics, and surface fuels to explore how the legacy of forest management, historical surface fire, and recent prescribed fire, have influenced the contemporary structure of the ponderosa pine-Gambel oak forest on the Hualapai tribal lands (hereafter Hualapai forest). Current overstory tree density (range: 361.1 to 1664.0 trees ha−1) and basal area (16.0 to 29.8 m2 ha−1) place the Hualapai forest in an intermediate state among more open vs. denser southwestern forests, but additional characteristics set the Hualapai forest apart from others in the region. In particular, diameter distributions of live ponderosa pine in the Hualapai forest are dominated by mid-diameter trees (trees ∼18-33 cm dbh), while diameter distributions of live ponderosa pine on forest lands across Arizona indicate a higher number of small-diameter trees (trees <23 cm dbh) and a lower number of large-diameter trees. This finding of a relatively lower number of small-diameter trees, which serve as ladder fuels, in the Hualapai forest may indicate better protection against severe wildfire, although some individual sites might be at greater risk due to higher numbers of small live trees and snags, seedlings and saplings, along with higher levels of coarse woody debris. Beyond fire risk, the Hualapai forest is lacking in large live trees and snags that support cultural needs and sustain its ecological functioning. Current management approaches are helping support the recovery of large trees and snags. While the Hualapai forest is at risk due to its location at the lower elevational limit of ponderosa pine in the Southwest, the legacy of past management and prescribed fire has differentiated it from similar forests in the region. Continued management informed by Indigenous perspectives can best position the Hualapai forest to sustain its structure and function as climate warms as well as achieve socio-cultural and ecological outcomes most important to the people of the Hualapai Tribe.

Drought years promote bark beetle outbreaks in Mexican forests of Abies religiosa and Pinus pseudostrobus

Climate change is an important risk factor for forest ecosystems through alteration of forest disturbance regimes such as bark beetle outbreaks, which in some places now are more successfully attacking host trees weakened by hotter drought events. In Mexico, ties between climate and amplified outbreaks of bark beetles have begun to be documented, although these relationships are not entirely clear. This project aims to identify the geographic patterning and relations between climate and bark beetle outbreaks in Abies religiosa and Pinus pseudostrobus populations located in the Trans-Mexican Volcanic Belt region. We used: 1) a database of phytosanitary logging authorizations and locations issued to enable salvage logging of trees infested or killed by bark beetle outbreaks from 2009 to 2018; 2) a previously developed gridded database of contemporary suitable climatic habitat of these tree species that will be lost by 2060 to determine if outbreaks occurred in sites with climatic habitat lost; and 3) elevation values to determine if outbreaks occur at xeric limits (drought limits) of tree hosts. Climate analysis was conducted with TerraClimate data using PDSI (Palmer’s Drought Severity Index) values. We find that bark beetle outbreaks do not necessarily occur at sites where models project that both species will lose suitable climatic habitat. For A. religiosa (Sacred Fir), of the 4091 ha treated by sanitary logging, 40% occurs between 3000 and 3100 m of elevation, where the pure and relatively better-preserved Sacred Fir stands are found. For P. pseudostrobus, more than 54% of the sanitary logging area (4664 ha) ranges from 2200 to 2400 m a.s.l., which coincides with the lower elevation limit (xeric limit) of the natural distribution of the species. Regarding relationships with PDSI, for A. religiosa there was a one-year lag between the year of mortality (expressed by the year of the phytosanitary logging authorization) and the year with the most negative (driest) PDSI anomaly (one year before), while for Pinus pseudostrobus, phytosanitary logging authorizations were concentrated in the driest year. An investigation that considers more variables, especially anthropogenic factors, is necessary to be able to better understand the dynamics of forest pests and prevent unusual outbreaks induced by expected climate-change-amplified droughts.

Investigating factors that set the lower elevational limit of Canada Jays (Perisoreus canadensis) on Vancouver Island, British Columbia, Canada

The biotic and abiotic factors responsible for determining ranges of most species are poorly understood. The Canada Jay (Perisoreus canadensis (Linnaeus, 1766)) relies on perishable cached food for over-winter survival and late-winter breeding and the persistence of cached food could be a driver of range limits. We confirmed that the Canada Jay’s lower elevational limit on Vancouver Island, British Columbia, Canada, matches that of the subalpine zone (900 m) and then conducted simulated caching experiments to examine the influence of antimicrobial properties of subalpine tree species (biotic) and of temperature (abiotic) on the preservation of cached food. We found that two high-elevation species, yellow cedar (Callitropsis nootkatensis (D. Don) D.P. Little) and Amabilis fir (Abies amabilis Douglas ex J. Forbes), preserved cached blueberries and chicken flesh better than other trees, but they also occurred well below the lower limit of Canada Jays. The effect of temperature was similarly unclear; while food cached at 1150 m retained 17% more mass than food cached at 550 m, there was no difference in percent mass remaining of food placed 70 m above versus 120 m below the Canada Jay’s lower elevational limit. Thus, we were unable to provide definitive evidence that either of the proposed abiotic or biotic factors was responsible for setting thelower elevational limit of resident Canada Jays.

Pinus pseudostrobus assisted migration trial with rain exclusion: maintaining Monarch Butterfly Biosphere Reserve forest cover in an environment affected by climate change

In the Trans-Mexican Volcanic Belt in central-western Mexico, drought stress causes the decline of two of the most ecologically and economically important conifers: Abies religiosa and Pinus pseudostrobus. Since P. pseudostrobus is distributed immediately below the lower elevation limit of A. religiosa, it would be a logical choice of species to fill gaps that may be created by the decline of A. religiosa. We assessed the feasibility of upward assisted migration and explored the effect of rain reduction using four P. pseudostrobus provenances in a rain exclusion field test at the border of the Monarch Butterfly Biosphere Reserve (MBBR) in Mexico. Our results show that, although populations were transferred between − 1.4 and − 3.3 °C mean annual temperature, and 30% of the precipitation (− 200 mm) was excluded, no statistically significant differences were found among treatments or populations for height increment (overall average 1.52 m) or survival (overall average 87%). These findings suggest that it is feasible to migrate P. pseudostrobus upward, towards the MBBR border, which also coincides with the sites where A. religiosa is in decline. This may present the additional benefit of discouraging change in land use from forestry to potato production by maintaining a healthy barrier of trees. If assisted migration is not included in management and reforestation programs, climate change could induce significant mortality in the present A. religiosa forest, facilitating expansion of the agricultural frontier toward the MBBR.

Sensitivity to water stress drives differential decline and mortality dynamics of three co-occurring conifers with different drought tolerance

Drought-induced tree decline and accelerated mortality under global change threaten forests worldwide. Identifying environmental and structural factors driving tree mortality of species with different ecology is essential to understand forest dynamics. We compared three coniferous species and trees with different health status (i.e. healthy, declining and dead) on a xeric Mediterranean ecotone where Pinus pinaster shows widespread decline (defoliation, mistletoe, high mortality) while more drought-tolerant Pinus pinea and Juniperus oxycedrus do not. We analysed individual tree and stand characteristics and regeneration of all species to understand how different abiotic factors at different time scales affected stand species dynamics in relation to tree decline and mortality. Only dead and declining pine trees showed recent multiannual negative radial growth trends. The growth decline started after two extreme droughts, being 10 years longer in the declining species, P. pinaster, than in non-declining P. pinea. We observed no signs of decline or mortality in the most drought-tolerant species, J. oxycedrus. Variables related to moisture availability, like stand dominant height and the tree response to precipitation, increased survival probability in P. pinaster but not in P. pinea. Despite P. pinaster dominating the canopy, its regeneration was very scarce and less abundant than that of co-occurring more drought-tolerant species. Our results suggest that drought-induced mortality was related to species-specific sensitivity to water stress and may be inducing compositional changes towards dominance of more drought-tolerant species. Consequently, at its lower elevation limit P. pinaster might be able to persist only in microsites with higher moisture availability.

A disconnect between upslope shifts and climate change in an Afrotropical bird community

AbstractClimate change threatens to push species to higher elevations and eventual extinction. Birds, in particular, are shown to be shifting upslope in the Neotropics and Southeast Asia. Yet previous studies have lacked the temporal resolution to investigate distributional dynamics over time in relation to climatic fluctuations, especially in the understudied Afrotropics. Here, we used 15 years of point‐count data from across an elevational gradient (1,767–2,940 m) in Rwanda, to assess elevational shift rates and dynamics in a community of Afrotropical birds. In general, species shifted their elevations upslope by 1.9 m/year, especially at their lower elevational limits which shifted by 4.4 m/year. Importantly, these shifts occurred despite the fact that local temperature and precipitation showed little trend over the study period. Moreover, the interannual distributions of few species were associated with temperature, suggesting that temperature played little direct role in determining elevational distributions of birds. Instead, upslope shifts may be more related to incremental shifts in habitat and resources which lag behind decades of increased temperature in the region. Precipitation appeared to have more of an effect than temperature in determining interannual elevational changes, allowing species to expand their ranges in years of higher rainfall. Our results highlight the need to understand the mechanisms driving upslope shifts as they occur throughout the tropics. It will be critical for montane regions of the tropics to preserve contiguous blocks of forest across elevational gradients to allow wildlife to shift unimpeded.

Risk factors for the conservation of saltmarsh vegetation and blue carbon revealed by earthquake-induced sea-level rise

Vegetated coastal ecosystems (VCEs) are in global decline and sensitive to climate change; yet may also assist its mitigation through high rates of ‘blue’ carbon sequestration and storage. Alterations of relative sea-level (RSL) are pervasive drivers of change that reflect the interaction between tidal inundation regimes and ground surface elevation. Although many studies have investigated sediment accretion within VCEs, relatively few have addressed spatiotemporal patterns of resilience in response to RSL change. In this study, we used high resolution elevation models and field surveys to identify RSL changes and socio-ecological responses in a tidal lagoon system following earthquakes in New Zealand. We expected that vegetation changes would result from RSL effects caused by surface-elevation changes in intertidal zones. Elevation measurements showed a sequence of vertical displacements resulting from major earthquakes in 2011 and 2012, and additional surface-elevation loss since. VCE losses were recorded over an 8 year period post-2011 in response to high rates of RSL rise (up to 41 mm yr−1). Anthropogenic factors influenced the pattern of losses and illustrate opportunities for managing risks to other VCEs facing RSL rise. Four key principles for building VCE resilience were identified: i) anthropogenic encroachment results in resilience loss due to the need for landward migration when changes exceed the tolerance thresholds of VCEs at their lower elevational limits; ii) connectivity losses exacerbate encroachment effects, and conversely, are a practical focus for management; iii) landscape-scale risk exposure is disproportionately influenced by the largest wetland remnants illustrating the importance of site-specific vulnerabilities and their assessment; and iv) establishing new protected areas to accommodate the movement of VCEs is needed, and requires a combination of land tenure rearrangements and connectivity conservation. Embracing these concepts offers promise for improving whole-system resilience to help address the challenge of global climate change.

Large variation in branch and branch-tip hydraulic functional traits in Douglas-fir (Pseudotsuga menziesii) approaching lower treeline.

Few studies have quantified intraspecific variation of hydraulic functional traits in conifers across elevation gradients that include range boundaries. In the Intermountain West, USA, the lower elevational limit of forests (lower treeline) is generally assumed to be caused by water limitations to growth and water relations, yet few studies directly show this. To test this assumption, we measured changes in a suite of traits that characterize drought tolerance such as drought-induced hydraulic vulnerability, hydraulic transport capacity and morphological traits in branch tips and branches of Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) along a 400-m elevation gradient in southeastern Idaho that included lower treeline. As elevation decreased, vulnerability to hydraulic dysfunction and maximum conductivity both decreased in branches; some hydraulic safety-efficiency trade-offs were evident. In branch tips, the water potential at the turgor loss point decreased, while maximum conductance increased with decreasing elevation, highlighting that branch-tip-level responses to less moisture availability accompanied by warmer temperatures might not be coordinated with branch responses. As the range boundary was approached, we did not observe non-linear changes in parameters among sites or increased variance within sites, which current ecological hypotheses on range limits suggest. Our results indicate that there is substantial plasticity in hydraulic functional traits in branch tips and branches of Douglas-fir, although the direction of the trends along the elevation gradient sometimes differed between organs. Such plasticity may mitigate the negative impacts of future drought on Douglas-fir productivity, slowing shifts in its range that are expected to occur with climate change.

Dietary analysis of an uncharacteristic population of the Mountain Pygmy-possum (Burramys parvus) in the Kosciuszko National Park, New South Wales, Australia.

BackgroundThe Mountain Pygmy-possum (Burramys parvus) is a critically endangered marsupial, endemic to alpine regions of southern Australia. We investigated the diet of a recently discovered population of the possum in northern Kosciuszko National Park, NSW, Australia. This new population occurs at elevations well below the once-presumed lower elevation limit of 1,600 m.Goals and MethodsFaecal material was analysed to determine if dietary composition differed between individuals in the newly discovered northern population and those in the higher elevation southern population, and to examine how diet was influenced by rainfall in the southern population and seasonal changes in resource availability in the northern population.Results and DiscussionThe diet of B. parvus in the northern population comprised of arthropods, fruits and seeds. Results indicate the diet of both populations shares most of the same invertebrate orders and plant species. However, in the absence of preferred food types available to the southern population, individuals of the northern population opportunistically consumed different species that were similar to those preferred by individuals in higher altitude populations. Differing rainfall amounts had a significant effect on diet, with years of below average rainfall having a greater percentage composition and diversity of invertebrates. Seasonal variation was also recorded, with the northern population increasing the diversity of invertebrates in their diet during the Autumn months when Bogong Moths (Agrotis infusa) were absent from those sites, raising questions about the possum’s dependence on the speciesConclusionsMeasurable effects of rainfall amount and seasonal variation on the dietary composition suggest that predicted climatic variability will have a significant impact on its diet, potentially impacting its future survival. Findings suggest that it is likely that B. parvus is not restricted by dietary requirements to its current pattern of distribution. This new understanding needs to be considered when formulating future conservation strategies for this critically endangered species.

Influence of temperature on historic and future population fitness of the western spruce budworm, Choristoneura occidentalis

Climate change affects the geographic range and outbreak behavior of forest insects. Both the range and dynamics of insect populations are linked to physiological responses to abiotic conditions and trophic interactions via their effects on individual fitness. We develop a process-based simulation model of population fitness for the western spruce budworm, Choristoneura occidentalis (Freeman) to examine the effects of temperature on historical range and observed outbreak behavior in the complex topography western North America. Model predictions are consistent with observed changes in the distribution and frequency of this insect’s outbreaks over the past 100 years. These changing patterns are the result of several direct and indirect responses of the insect to temperature. Overwintering survival is negatively affected by warming and determines its southern and lower-elevation limit. Ability to complete its life cycle before killing frosts limits its range to the north and at higher elevations. Interactions affecting synchrony between the insect’s feeding life stages and host foliage development also determines fitness and will increase the area favoring fitness of western spruce budworm in the future, especially in western Canada where host trees extend much farther north than the insect currently does.

Leaf‐level trade‐offs between drought avoidance and desiccation recovery drive elevation stratification in arid oaks

AbstractUnderstanding the extent to which climate limitations drive elevation stratification among species is integral to predicting the impacts of climate change. Zonation patterns of species within mountains have been well documented, and shifts in these patterns have been correlated with recent warming. However, the physiological mechanisms that explain these zonation patterns are not well understood. We used a system of broadly sympatric oak species within semi‐arid mountains to (1) investigate the extent to which species elevation ranges correlate with climate, (2) test for associations of cold and drought resistances with upper and lower elevation limits, and for trade‐offs between resistance mechanisms with elevation, and (3) examine the extent to which species‐wide climatic ranges predict traits that drive local community assembly along an elevation gradient. We found that aridity gradients but not winter minimum temperatures predict oak stratification. Species differed in drought resistance, demonstrating a trade‐off between drought avoidance and drought recovery. At lower elevations, species avoided drought stress during the dry season through leaf abscission; at upper elevations, they maintained transpiration but recovered from daily desiccation via higher leaf water storage capacity, rather than tolerating desiccation via lower turgor loss points. Freezing resistance, measured as stem electrolyte leakage, was not correlated with elevation differences. Taken together, these results indicate that elevation stratification is linked to drought resistance rather than freezing resistance. We also found evidence of niche partitioning among closely related oaks linked to contrasting leaf phenology. The functional, phenological, and physiological traits important for elevation stratification were correlated with species’ range‐wide mean annual precipitation and precipitation seasonality, but not aridity. Our findings indicate that drought resistance along a leaf avoidance–recovery trade‐off is integral to species stratification within this semi‐arid montane system. Additionally, the mechanism of stratification acts upon traits and strategies conserved at the species level. Species within this system are likely vulnerable to range retraction under increased drought as a consequence of this phenological avoidance–physiological tolerance trade‐off.

Linking species thermal tolerance to elevational range shifts in upland dung beetles

Climate warming has been proposed as the main cause of the recent range shifts seen in many species. Although species' thermal tolerances are thought to play a key role in determining responses to climate change, especially in ectotherms, empirical evidence is still limited. We investigate the connection between species' thermal tolerances, elevational range and shifts in the lower elevational limit of dung beetle species (Coleoptera, Aphodiidea) in an upland region in the northwest of England. We measured thermal tolerances in the laboratory, and used current and historical distribution data to test specific hypotheses about the area's three dominant species, particularly the species most likely to suffer from warming: Agollinus lapponum. We found marked differences between species in their minimum and maximum thermal tolerance and in their elevational range and patterns of abundance. Overall, differences in thermal limits among species matched the abundance patterns along the elevation gradient expected if distributions were constrained by climate. Agollinus lapponum abundance increased with elevation and this species showed lower maximum and minimum thermal limits than Acrossus depressus, for which abundance declined with elevation. Consistent with lower tolerance to high temperature, we recorded an uphill retreat of the low elevation limit of A. lapponum (177 m over 57 yr) in line with the increase in summer temperature observed in the region over the same period. Moreover, this species has been replaced at low and mid‐elevations by the other two warm‐tolerant species (A. depressus and Agrilinus ater). Our results provide empirical evidence that species' thermal tolerance constrains elevational ranges and contributes to explain the observed responses to climate warming. A mechanistic understanding of how climate change directly affects species, such as the one presented here, will provide a robust base to inform predictions of how individual species and whole assemblages may change in the future.

Toward ecologically realistic predictions of species distributions: A cross-time example from tropical montane cloud forests.

There is an urgent need for more ecologically realistic models for better predicting the effects of climate change on species' potential geographic distributions. Here we build ecological niche models using MAXENT and test whether selecting predictor variables based on biological knowledge and selecting ecologically realistic response curves can improve cross-time distributional predictions. We also evaluate how the method chosen for extrapolation into nonanalog conditions affects the prediction. We do so by estimating the potential distribution of a montane shrew (Mammalia, Soricidae, Cryptotis mexicanus) at present and the Last Glacial Maximum (LGM). Because it is tightly associated with cloud forests (with climatically determined upper and lower limits) whose distributional shifts are well characterized, this species provides clear expectations of plausible vs. implausible results. Response curves for the MAXENT model made using variables selected via biological justification were ecologically more realistic compared with those of the model made using many potential predictors. This strategy also led to much more plausible geographic predictions for upper and lower elevational limits of the species both for the present and during the LGM. By inspecting the modeled response curves, we also determined the most appropriate way to extrapolate into nonanalog environments, a previously overlooked factor in studies involving model transfer. This study provides intuitive context for recommendations that should promote more realistic ecological niche models for transfer across space and time.

Growth strategies and threshold responses to water deficit modulate effects of warming on tree seedlings from forest to alpine

Abstract Predictions of upslope range shifts for tree species with warming are based on assumptions of moisture stress at lower elevation limits and low‐temperature stress at high‐elevation limits. However, recent studies have shown that warming can reduce tree seedling establishment across the entire gradient from subalpine forest to alpine via moisture limitation. Warming effects also vary with species, potentially resulting in community shifts in high‐elevation forests. We examined the growth and physiology underlying effects of warming on seedling demographic patterns. We evaluated dry mass (DM), root length, allocation above‐ and below‐ground, and relative growth rate (RGR) of whole seedlings, and their ability to avoid or endure water stress via water‐use efficiency and resisting turgor loss, for Pinus flexilis, Picea engelmannii and Pinus contorta seeded below, at and above tree line in experimentally warmed, watered and control plots in the Rocky Mountains, USA. We expected that growth and allocation responses to warming would relate to moisture status and that variation in drought tolerance traits would explain species differences in survival rates. Across treatments and elevations, seedlings of all species had weak turgor‐loss resistance, and growth was marginal with negative RGR in the first growth phase (−0.01 to −0.04 g g−1 day−1). Growth was correlated with soil moisture, particularly in the relatively small‐seeded P. contorta and P. engelmannii. Pinus flexilis, known to have the highest survivorship, attained the greatest DM and longest root but was also the slowest growing and most water‐use efficient. This was likely due to its greater reliance on seed reserves. Seedlings developed 15% less total DM, 25% less root DM and 11% shorter roots in heated compared with unheated plots. Higher temperatures slightly increased DM, root length and RGR where soils were wettest, but more strongly decreased these variables under drier conditions. Synthesis. The surprising heat inhibition of tree seedling establishment at the cold edge of forests appears to have a physiological basis: newly germinated seedlings have poor moisture stress tolerance, which appears related to marginal initial growth and heavy reliance on seed reserves. Variation in these attributes among tree species at tree line helps explain their different climate responses.

Environment and host identity structure communities of green algal symbionts in lichens.

An understanding of how biotic interactions shape species' distributions is central to predicting host-symbiont responses under climate change. Switches to locally adapted algae have been proposed to be an adaptive strategy of lichen-forming fungi to cope with environmental change. However, it is unclear how lichen photobionts respond to environmental gradients, and whether they play a role in determining the fungal host's upper and lower elevational limits. Deep-coverage Illumina DNA metabarcoding was used to track changes in the community composition of Trebouxia algae associated with two phylogenetically closely related, but ecologically divergent fungal hosts along a steep altitudinal gradient in the Mediterranean region. We detected the presence of multiple Trebouxia species in the majority of thalli. Both altitude and host genetic identity were strong predictors of photobiont community assembly in these two species. The predominantly clonally dispersing fungus showed stronger altitudinal structuring of photobiont communities than the sexually reproducing host. Elevation ranges of the host were not limited by the lack of compatible photobionts. Our study sheds light on the processes guiding the formation and distribution of specific fungal-algal combinations in the lichen symbiosis. The effect of environmental filtering acting on both symbiotic partners appears to shape the distribution of lichens.

An upward elevation shift of native and non‐native vascular plants over 40 years on the island of Hawai'i

AbstractQuestionsHave vascular plants shifted in elevation along a tropical mountain gradient on Mauna Loa, Hawai'i over the past 40 yr? If so, do native and non‐native species differ in their shifts of mean elevation range, upper elevation limit or lower elevation limit?LocationHawai'i Volcanoes National Park, southeast slope of Mauna Loa, Hawai'i.MethodsWe resurveyed an elevation belt transect of vegetation plots that was first surveyed in the early 1970s. Differences between 1970 and 2010 were calculated for 69 plant species’ mean elevation, upper elevation limit, lower elevation limit and elevation range. Statistical significance of differences was assessed by Monte Carlo permutation tests.ResultsAveraging across all 69 species, a statistically significant upward shift of 65.2 m was detected in mean elevation. Both upper and lower elevation limits shifted significantly upward by 31.6 and 90.5 m, respectively. Elevation range did not change significantly. For the 49 native species, the upper elevation limit did not change, but the lower elevation limit shifted significantly upward by 94.1 m. In contrast, the 20 non‐native species displayed a different pattern of a significant upward shift in both their upper and lower elevation limits, by 126.4 and 81.6 m, respectively. Native species seemed to display a contracting elevation range of 101.2 m, but this was not significant (P = 0.053), while non‐native species maintained a stable elevation range (P = 0.365).ConclusionsVascular plants have shifted upward in elevation on the tropical island of Hawai'i, but the shifting trend differs between native and non‐native species. Non‐native species, which were often found at lower elevations, are in an upward niche expansion phase. Native species show trends toward range contraction due to lack of expansion in upper elevation limit, probably due to drought stress correlating with a trade‐wind inversion (TWI) that severely limits rainfall at higher elevations. As more non‐native species reach theTWIelevation, their upward expansion may also become more limited.