Population divergence of developmental thermal optima in Swedish common frogs,Rana temporaria

Growth rate, like any other trait, should be under balancing selection in natural populations, with selection adjusting mean growth rate in a population in relation to its site‐specific costs and benefits. In the present study, we tested for differences in thermal growth optima between a northern and a southern region of Rana temporaria by rearing tadpoles in three different temperatures in the laboratory (10, 15 and 20 °C). Because of the rapid increase in post‐melt temperature at high latitudes, spawn and tadpoles from the northern region experience significantly higher minimum, mean and maximum temperature throughout the period of pre‐metamorph development. Frogs up north also enjoys a shorter breeding season and activity season overall. This suggests that growth and development should be maximized at a relatively higher temperature in the north as a result of directional selection. In accordance with this prediction we found that tadpoles from the northern region grew faster at relatively higher temperature than frogs in the south, whereas the opposite was true at relatively lower temperatures. North tadpoles also had a higher mortality and poorer physiological performance than south tadpoles at low temperatures. In summary, our results conclusively support the hypothesis that frogs in the north are adapted to relatively warmer developmental conditions than frogs in the south.     

Plasticity in age and size at metamorphosis in Rana temporaria - comparison of high and low latitude populations

Effects of different combinations of stressors (viz. temperature, food level) on growth, developmental and survival rates of Rana temporaria tadpoles from two geographically widely (∼ 1500 km) separated populations were studied in a common garden experiment. In both populations, low temperature and low food level lead to towered growth rates and delayed metamorphosis, whereas high temperature and high food level had the opposite effect. Tadpoles from north metamorphosed earlier and exhibited higher growth rates than tadpoles from south, suggesting local adaptation to shorter growth period and cooler ambient temperature in north. Size at metamorphosis did not differ between the two populations, but when the differences in metamorphic age were accounted for, then the tadpoles from north were larger than those from south. These results suggest considerable adaptive genetic differentiation in growth rates, size and timing of metamorphosis between northern and southern R. temporaria populations. In both populations, high food levels tended to reduce tadpole survival rates and there was a negative correlation between growth and survival rates across different treatments in both populations. In general, tadpoles from north experienced high mortality rates in high food level - low temperature treatments, whereas southern tadpoles experienced high mortality in high food level-high temperature treatments. This suggest that there may be genetic differences among different populations as how they would be influenced by high nutrient loads, such as brought along for example by fertilization of forest or agricultural soils.     

Growth-ring variations of dwarf shrubs reflect regional climate signals in alpine environments rather than topoclimatic differences

Aim Our main aim is to determine if ring‐width variations in Empetrum hermaphroditum reflect regional or local topoclimate signals in an alpine environment. In the case that topoclimate provides the dominant signal, a secondary aim is to link these to spatial distribution patterns of different vegetation types. Location The study area is situated in the middle alpine belt in the Vågåmo region, Central Norwegian Scandes. Sampling sites cover different topoclimates: ridges, north‐facing slopes and south‐facing slopes. Methods We constructed ring‐width chronologies of E. hermaphroditum for each type of microsite for the common period 1951–2004. Climate data were prepared on an hourly, daily and growing‐season time scale. Climate–growth relationships were evaluated using bivariate correlations and regression tree methods for continuous time‐series analyses. In addition, extreme growth anomalies (pointer years) were compared with the climate conditions in those years. The impact of water supply on wood anatomy was determined by correlating the conductive area (percentage of vessel per growth ring) with a running mean (sum) of 10‐day intervals for temperature and precipitation. Results This study indicates that mean summer (June–August) temperatures determine the width of the growth rings of E. hermaphroditum irrespective of topoclimate. The length of the growing season, which is the most differentiating climatic factor between microsites, does not substantially alter the anatomical ring structure. Microsite differences in mean growth rates are attributed to the higher frequency of warm days. Extremely warm days limit ring‐width development at south‐facing slopes, while plants at ridges and north‐facing slopes still benefit from higher temperatures. As a consequence, pointer years are not developed synchronously at all microsites. Vessel formation is affected by available moisture, especially in the later part of the growing season. Main conclusions Topoclimate induces slight modifications of annual growth‐ring increments of E. hermaphroditum at different microsites. In contrast to the distribution patterns of vegetation types that are determined by snow cover, growth‐ring variations are related to summer temperature conditions, and the prominent regional climate signal is still reflected at all microsites. This offers the opportunity to reconstruct climatic change in alpine regions from dwarf shrub ring‐width chronologies.     

Widespread triazole pesticide use affects infection dynamics of a global amphibian pathogen

The sixth mass extinction is a consequence of complex interplay between multiple stressors with negative impact on biodiversity. We here examine the interaction between two globally widespread anthropogenic drivers of amphibian declines: the fungal disease chytridiomycosis and antifungal use in agriculture. Field monitoring of 26 amphibian ponds in an agricultural landscape shows widespread occurrence of triazole fungicides in the water column throughout the amphibian breeding season, together with a negative correlation between early season application of epoxiconazole and the prevalence of chytrid infections in aquatic newts. While triazole concentrations in the ponds remained below those that inhibit growth of Batrachochytrium dendrobatidis, they bioaccumulated in the newts' skin up to tenfold, resulting in cutaneous growth-suppressing concentrations. As such, a concentration of epoxiconazole, 10 times below that needed to inhibit fungal growth, prevented chytrid infection in anuran tadpoles. The widespread presence of triazoles may thus alter chytrid dynamics in agricultural landscapes.     

Stem tilting in the inter‐tropical cactus Echinocactus platyacanthus: an adaptive solution to the trade‐off between radiation acquisition and temperature control

While plants require radiation for photosynthesis, radiation in warm deserts can have detrimental effects from high temperatures. This dilemma may be solved through plant morphological attributes. In cold deserts, stem tilting keeps reproductive organs warm by increasing radiation interception at the cost of decreased annual light interception. Conversely, little is known about stem tilting in warm deserts. We hypothesised that stem tilting in Echinocactus platyacanthus prevents high temperatures near the apex, where reproduction occurs. The study was conducted in the warm, inter‐tropical portion of the Chihuahuan Desert, Mexico. We found that cacti preferentially tilted towards the south, which reduced temperatures of reproductive organs during the hot season, but increased total annual near‐apex PAR interception. Tilting also maximised reproduction, a likely consequence of temperature control but perhaps also of the difficulty in translocating photosynthates in cacti; therefore, annual energy acquisition near floral meristems may be largely allocated to reproduction. Unlike plants of higher latitudes, in inter‐tropical deserts sunlight at noon comes either from the north or the south, depending on the season, and thus stem tilting may more strongly affect total annual radiation received in different portions of the stem. Inter‐tropical cacti can synchronise reproduction with irradiance peaks if flowering occurs in a specific (north or south) portion of the stem; also, they effectively solve the conflict between maximising annual PAR interception and minimising temperature at the hottest time of day. Notably, the two inter‐tropical cacti in which stem tilting has been studied successfully solve this conflict.     

Photoperiod cues and patterns of genetic variation limit phenological responses to climate change in warm parts of species’ range: Modeling diameter‐growth cessation in coast Douglas‐fir

The phenology of diameter‐growth cessation in trees will likely play a key role in mediating species and ecosystem responses to climate change. A common expectation is that warming will delay cessation, but the environmental and genetic influences on this process are poorly understood. We modeled the effects of temperature, photoperiod, and seed‐source climate on diameter‐growth‐cessation timing in coast Douglas‐fir (an ecologically and economically vital tree) using high‐frequency growth measurements across broad environmental gradients for a range of genotypes from different seed sources. Our model suggests that cool temperatures or short photoperiods can induce cessation in autumn. At cool locations (high latitude and elevation), cessation seems to be induced primarily by low temperatures in early autumn (under relatively long photoperiods), so warming will likely delay cessation and extend the growing season. But at warm locations (low latitude or elevation), cessation seems to be induced primarily by short photoperiods later in autumn, so warming will likely lead to only slight extensions of the growing season, reflecting photoperiod limitations on phenological shifts. Trees from seed sources experiencing frequent frosts in autumn or early winter tended to cease growth earlier in the autumn, potentially as an adaptation to avoid frost. Thus, gene flow into populations in warm locations with little frost will likely have limited potential to delay mean cessation dates because these populations already cease growth relatively late. In addition, data from an abnormal heat wave suggested that very high temperatures during long photoperiods in early summer might also induce cessation. Climate change could make these conditions more common in warm locations, leading to much earlier cessation. Thus, photoperiod cues, patterns of genetic variation, and summer heat waves could limit the capacity of coast Douglas‐fir to extend its growing season in response to climate change in the warm parts of its range.     

The effects of acute and developmental temperature on burst swimming speed and myofibrillar ATPase activity in tadpoles of the Pacific tree frog, Hyla regilla

The effects of acute and developmental temperature on maximum burst swimming speed, body size, and myofibrillar ATPase activity were assessed in tadpoles of the Pacific tree frog, Hyla regilla. Tadpoles from field-collected egg masses were reared in the laboratory at 15 degrees (cool) and 25 degrees C (warm). Body size, maximum burst swimming speed from 5 degrees to 35 degrees C, and tail myofibrillar ATPase activity at 15 degrees and 25 degrees C were measured at a single developmental stage. Burst speed of both groups of tadpoles was strongly affected by test temperature (P<0. 001). Performance maxima spanned test temperatures of 15 degrees -25 degrees C for the cool group and 15 degrees -30 degrees C for the warm group. Burst speed also depended on developmental temperature (P<0.001), even after accounting for variation in body size. At most test temperatures, the cool-reared tadpoles swam faster than the warm-reared tadpoles. Myofibrillar ATPase activity was affected by test temperature (P<0.001). Like swimming speed, enzyme activity was greater in the cool-reared tadpoles than in the warm-reared tadpoles, a difference that was significant when assayed at 15 degrees C (P<0. 01). These results suggest a mechanism for developmental temperature effects on locomotor performance observed in other taxa.     

Population‐specific effects of developmental temperature on body condition and jumping performance of a widespread European frog

All physiological processes of ectotherms depend on environmental temperature. Thus, adaptation of physiological mechanisms to the thermal environments is important for achieving optimal performance and fitness. The European Common Frog, Rana temporaria, is widely distributed across different thermal habitats. This makes it an exceptional model for studying the adaptations to different thermal conditions. We raised tadpoles from Germany and Croatia at two constant temperature treatments (15°C, 20°C), and under natural temperature fluctuations (in outdoor treatments), and tested how different developmental temperatures affected developmental traits, that is, length of larval development, morphometrics, and body condition, as well as jumping performance of metamorphs. Our results revealed population‐specific differences in developmental time, body condition, and jumping performance. Croatian frogs developed faster in all treatments, were heavier, in better body condition, and had longer hind limbs and better jumping abilities than German metamorphs. The populations further differed in thermal sensitivity of jumping performance. While metamorphs from Croatia increased their jumping performance with higher temperatures, German metamorphs reached their performance maximum at lower temperatures. These population‐specific differences in common environments indicate local genetic adaptation, with southern populations being better adapted to higher temperatures than those from north of the Alps.     

Identification of candidate loci for adaptive phenotypic plasticity in natural populations of spadefoot toads

Phenotypic plasticity allows organisms to alter their phenotype in direct response to changes in the environment. Despite growing recognition of plasticity's role in ecology and evolution, few studies have probed plasticity's molecular bases—especially using natural populations. We investigated the genetic basis of phenotypic plasticity in natural populations of spadefoot toads (Spea multiplicata). Spea tadpoles normally develop into an “omnivore” morph that is favored in long‐lasting, low‐density ponds. However, if tadpoles consume freshwater shrimp or other tadpoles, they can alternatively develop (via plasticity) into a “carnivore” morph that is favored in ephemeral, high‐density ponds. By combining natural variation in pond ecology and morph production with population genetic approaches, we identified candidate loci associated with each morph (carnivores vs. omnivores) and loci associated with adaptive phenotypic plasticity (adaptive vs. maladaptive morph choice). Our candidate morph loci mapped to two genes, whereas our candidate plasticity loci mapped to 14 genes. In both cases, the identified genes tended to have functions related to their putative role in spadefoot tadpole biology. Our results thereby form the basis for future studies into the molecular mechanisms that mediate plasticity in spadefoots. More generally, these results illustrate how diverse loci might mediate adaptive plasticity.     

Effects of density and predation on Scaphiopus couchi tadpoles in desert ponds

Summary The effects of density on growth and development of Scaphiopus couchi tadpoles in desert ponds were investigated, and sources of mortality over a three-year period were documented. In 16 of the 82 ponds monitored, predation was the principal cause of death, demonstrating that tadpoles in desert ponds may be exposed to high levels of predation, although the overall importance of predation is less here than in more mesic areas. Desiccation was the primary cause of mortality in 49 ponds. Growth and development were extremely slow in most high density ponds and as a result most tadpoles were unable to metamorphose before ponds dried. Only 8 ponds produced metamorphs, and mortality was high even in these. Food-supplementation resulted in some metamorphosis in high density ponds, although the effect was diminished by the extreme crowding in most ponds. In low density ponds, S. couchi tadpoles can develop very quickly and metamorphose. High mortality due to desiccation is largely a consequence of high density: tadpoles rarely completed development in high density ponds, regardless of pond duration.     

ADAPTIVE PLASTICITY IN DEVELOPMENT OF SCAPHIOPUS COUCHII TADPOLES IN DESERT PONDS

Couch's spadefoot toads (Scaphiopus couchii) breed in ephemeral desert ponds that are highly variable in duration. Rapid development is expected to be advantageous in short-duration ponds, but slower development, allowing more time for growth, may be advantageous in ponds of longer duration. Previous experiments have revealed both genetic variation in development time and phenotypic plasticity in response to pond drying. In this paper, I examine the norms of reaction of five sibships of tadpoles to see whether there is genetic variation in the effect of pond duration, i.e., in phenotypic plasticity. Several important results emerged. 1) Differences among sibships in development time that were seen in the lab were also seen in the field. 2) There was no evidence for genetic variation in plasticity of development; all sibships exhibited faster development and decreased larval period in ponds of short duration. Plasticity in development appears to be adaptive, as size at metamorphosis was correlated with duration of larval period. The slowest developing sibship, however, suffered higher mortality compared to other sibships in short duration ponds. 3) Sibships did not differ in growth or size at metamorphosis in short-duration ponds, but the slowest developing sibship metamorphosed at the largest size in long duration ponds, resulting in a significant genotype x environment interaction for size at metamorphosis. Thus, although only one of the five sibships responded differently, there appears to be genetic variation for plasticity in growth, and a genetically determined trade-off between fitness in short-duration ponds (via rapid development) and fitness in long duration ponds (via large size at metamorphosis). This may explain the existence of both phenotypic plasticity and genetic variation in development. A single genotype, although capable of adaptive plasticity, is not sufficiently flexible to have equally high fitness in both long- and short-duration ponds.     

PERFORMANCE OF TADPOLES FROM THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX: INTERACTIONS WITH POND DRYING AND INTERSPECIFIC COMPETITION

The performance of three genotypes (LL, LR, RR) of tadpoles resulting from the hybrid mating system of Rana lessonae (phenotype L, genotype LL) and Rana esculenta (phenotype E, genotype LR) was determined in artificial ponds. The effects of interspecific competition and pond drying on growth, development, and survival of tadpoles were used to measure the performance of genotypes and the relative fitness of offspring. Among the three genotypes, tadpoles from the homogametic mating RR had the lowest survival, growth, and development under all environmental conditions. Body size of the LL and LR genotype tadpoles at metamorphosis was reduced by competition and pond drying. Days to metamorphosis were also higher for the LL and LR genotype tadpoles in competition ponds. The proportion of individuals metamorphosing of each genotype was differentially lowered by competition and pond drying. The LL genotype produced more metamorphs than the LR genotype in the constant water level ponds, but the LR genotype produced more in drying ponds. In competition ponds, the LR genotype produced more metamorphs than the LL genotype, but the LL genotype produced more metamorphs in ponds without competition. The RR genotype produced no metamorphs in any of the experimental environments. Increased performance of LR offspring from the heterogametic mating, in harsh conditions, and reduced performance of RR offspring from the homogametic mating, even under favorable conditions, relative to the parental genotype (LL) suggests that the population dynamics of this hybridogenetic system is strongly dependent on mate choice in mixed populations and the subsequent pond environment females select for oviposition and larval development.     

Microgeographical variation in thermal preference by an amphibian

Ectotherms use behaviour to buffer effects of temperature on growth, development and survival. While behavioural thermoregulation is widely reported, localized adaptation of thermal preference is poorly documented. Larval amphibians live in wetlands ranging from entirely open to heavily shaded by vegetation. We hypothesized that populations undergo localized selection leading to countergradient patterns of thermal preference behaviour. Specifically, we predicted that wood frog (Rana sylvatica) larvae from closed canopy ponds would be more strongly temperature selective and would prefer higher temperatures than conspecifics from populations found in open canopy ponds. In a study of six breeding ponds in north‐eastern Connecticut, USA, these predictions were upheld. The countergradient, microgeographical variation in thermal preference documented here implies that wood frog populations may have diverged rapidly in the face of contrasting selection pressures. Rapid, behaviourally mediated responses to changing thermal environments have important implications for understanding population responses to climate change.     

Factors affecting the distribution and abundance of an unpigmented heterotrophic alga Prototheca richardsi

1. Numbers of the contramensal alga Prototheca richardsi were high in spring in two ponds used for breeding by anuran amphibians, but lower at other times of year and undetectable in two ponds not used by anurans. 2. Prototheca richardsi became abundant in the silt of eight experimental ponds which contained tadpoles, but remained undetectable in four otherwise identical ponds lacking tadpoles. 3. Prototheca richardsi numbers in laboratory microcosms remained stable for many days in sterile tap water, but declined with a half-life of about 6 days in pond water at 20°C. 4. Further studies with microcosms using antibiotics and electron microscopy indicated that mortality of P. richardsi was caused primarily by pathogenic bacteria.     

The shape of things to come: linking developmental plasticity to post‐metamorphic morphology in anurans

Development consists of growth and differentiation, which can be partially decoupled and can be affected by environmental factors to different extents. In amphibians, variation in the larval environment influences development and causes changes in post‐metamorphic shape. We examined post‐metamorphic consequences, both morphological and locomotory, of alterations in growth and development. We reared tadpoles of two phylogenetically and ecologically distant frog species (the red‐eyed treefrog Agalychnis callidryas and the African clawed frog Xenopus laevis) under different temperatures with ad libitum food supply and under different food levels at a constant temperature. Low temperature and low food levels both resulted in similarly extended larval periods. However, low temperature yielded relatively long‐legged frogs with a lower degree of ossification than warm temperature, whereas low food yielded relatively short‐legged frogs with a higher degree of ossification than high food levels. Such allometric differences had no effect on locomotor performance of juveniles. Our results provide a basis for understanding the relationship between growth, differentiation and post‐metamorphic shape in anurans and help explain many of the discrepancies reported in previous studies.     

Predator-induced Swarms in the Tadpoles of an African Savanna Frog,Phrynomantis microps

Aggregations in tadpoles of the West African savanna frog Phrynomantis microps were often observed in their breeding ponds in our study area, situated in Comoé National Park, Ivory Coast. Experiments under seminatural conditions demonstrated that this behaviour was only shown while predators were present. The tadpoles reacted differently to different predators. Factors inducing swarm behaviour were optical (the predator) and/or chemical (liquid from injured tadpoles). Alarm substances are not species-specific. Kinship seems not to play a role in swarm formation.     

Effects of hatching age on development and hatchling morphology in the red-eyed tree frog, Agalychnis callidryas

The red-eyed treefrog, Agalychnis callidryas , lays eggs on leaves overhanging ponds. Tadpoles hatch and enter the water at different ages, and late-hatched tadpoles survive aquatic predators better than do early-hatched tadpoles. Here I assess developmental consequences of hatching age through: (1) a morphological study of embryos and tadpoles through the plastic hatching period; (2) a behavioural assay for an effect of hatching age on feeding; and (3) a field experiment testing the effect of hatching age on growth to metamorphosis. Substantial development of feeding, digestive, respiratory and locomotor structures occurs in embryos over the plastic hatching period. Hatchling morphology thus varies with age, with consequences for behaviour and predation risk. Hatched tadpoles develop faster than embryos, and early-hatched tadpoles feed before late-hatched tadpoles. After all tadpoles have hatched, the effect of hatching age on size decreases. I found no evidence for an effect of hatching age on size at metamorphosis and only weak evidence for an effect on larval period. Hatching age affects the sequence of developmental change: early-hatched tadpoles lose external gills while otherwise more developed embryos maintain them. Plasticity in external gill resorption may be adaptive given differences in the respiratory environments of embryos and tadpoles. Early-hatched tadpoles also diverge from embryos in shape, growing relatively smaller tails. The study of functional morphology and developmental plasticity will contribute to understanding hatching as an ontogenetic niche shift.     

Magnetic Compass Orientation in Larval Iberian Green Frogs, Pelophylax Perezi

Experiments were carried out to investigate whether Iberian green frog tadpoles Pelophylax perezi (formerly Rana perezi) are able of using the geomagnetic field for y‐axis orientation (i.e. orientation toward and away from shore). Tadpoles were trained outdoor for 5 d, in two different training configurations: (i) a training tank aligned along the magnetic north–south axis, with shore facing south, and (ii) a training tank aligned along the magnetic east–west axis, with shore located east, and similar to the shore–deep water axis (‘y‐axis’) found in their home stream, which flows from south to north. After training, tadpoles were individually tested for magnetic orientation in a water‐filled circular outdoor arena surrounded by a pair of orthogonally aligned cube‐surface‐coils used to alter the alignment of the earth's magnetic field. Tadpoles held in the east–west training tank oriented towards shore, indicating that they were able to distinguish between the shoreward and waterward direction along the y‐axis. Tadpoles trained in the tank that was aligned along the north–south axis showed bimodal magnetic compass orientation along the shore–deep water magnetic axis. These findings provide evidence for the use of magnetic compass cues for y‐axis orientation by P. perezi tadpoles.     

COSTS AND BENEFITS OF A PREDATOR-INDUCED POLYPHENISM IN THE GRAY TREEFROG HYLA CHRYSOSCELIS

The phenotypes of gray treefrog (Hyla chrysoscelis) tadpoles vary depending on whether predators are present in the pond. Tadpoles reared in ponds with predatory dragonfly larvae are relatively inactive compared with tadpoles in predator-free ponds, and have relatively large, brightly colored tailfins with dark spots along the margins. Models for the evolution of plasticity predict that induced phenotypes such as this should confer high fitness relative to the typical phenotype when in the presence of predators, but should be costly when the predator is absent. Our study tested for the predicted fitness trade-off in H. chrysoscelis by first rearing tadpoles in mesocosms under conditions that induce the alternate phenotypes, and then comparing the performance of both phenotypes in both environments. We generated the two phenotypes by rearing tadpoles in 600-liter outdoor artificial ponds that contained either two caged dragonflies (Anax junius) or an empty cage. Tadpoles from the two environments showed significantly different behavior, tail shape, and tail color within two weeks of exposure. We compared the growth and survival of both phenotypes over four weeks in ponds where there was no actual risk of predation. Under these conditions, both phenotypes grew at the same rate, but the predator-induced phenotype had significantly lower survival than the typical phenotype, indicating that induced tadpoles suffered greater mortality from causes other than odonate predation. We tested the susceptibility of both phenotypes to predation by exposing them to dragonflies in 24-h predation trials. The predator-induced phenotype showed a significant survival advantage in these trials. These results confirm that the predator-induced phenotype in H. chrysoscelis larvae is associated with fitness costs and benefits that explain why the defensive phenotype is induced rather than constitutive.     

Responses of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline

From 2001 to 2004 we experimentally warmed 40 large, naturally established, white spruce [Picea glauca (Moench) Voss] seedlings at alpine treeline in southwest Yukon, Canada, using passive open‐top chambers (OTCs) distributed equally between opposing north and south‐facing slopes. Our goal was to test the hypothesis that an increase in temperature consistent with global climate warming would elicit a positive growth response. OTCs increased growing season air temperatures by 1.8°C and annual growing degree‐days by one‐third. In response, warmed seedlings grew significantly taller and had higher photosynthetic rates compared with control seedlings. On the south aspect, soil temperatures averaged 1.0°C warmer and the snow‐free period was nearly 1 month longer. These seedlings grew longer branches and wider annual rings than seedlings on the north aspect, but had reduced Photosystem‐II efficiency and experienced higher winter needle mortality. The presence of OTCs tended to reduce winter dieback over the course of the experiment. These results indicate that climate warming will enhance vertical growth rates of young conifers, with implications for future changes to the structure and elevation of treeline contingent upon exposure‐related differences. Our results suggest that the growth of seedlings on north‐facing slopes is limited by low soil temperature in the presence of permafrost, while growth on south‐facing slopes appears limited by winter desiccation and cold‐induced photoinhibition.