Subalpine ecosystems are experiencing rapid changes in snow pack, temperature, and precipitation regime as a result of anthropogenic climate forcing. These changes in climate can have a profound effect on subalpine ecosystem structure and functioning, which may ultimately feed back to climate change. In this study, I examined the response of the subalpine meadow plant communities at the Rocky Mountain Biological Laboratory to natural and simulated climate change. First, I looked at whether changes in growing season precipitation or temperature regime would have the larger effect on subalpine ecosystem carbon flux. In a simulated warming experiment, changes in growing season precipitation had a tenfold larger effect on cumulative carbon flux than did the warming treatment. Along a natural climatic and elevational gradient, precipitation stimulates carbon uptake, particularly at higher elevations. Given projected decreases in summer precipitation in the high elevation Rockies, we predict a 20% decrease in carbon uptake from subalpine meadows. Second, I compared the taxonomic, phylogenetic and functional structure of plant communities along an elevational gradient to infer which climatic and biotic factors influence community assembly at each elevation. Floral and phenology traits become overdispersed at high elevation, mirroring phylogenetic relatedness, and suggesting pressure to diversify to attract pollinators during the abbreviated growing season. At the same time, leaf functional traits become clustered at high elevation, indicating multiple opposing assembly mechanisms in subalpine communities. Finally, I studied the natural history of sagebrush, Artemisia tridentate ssp. vaseyana, at its elevational range limit in subalpine meadows. In particular, I focused on the importance of warming and species interactions in elevational advance of the species. I found that facilitation by neighboring forbs was critical for sagebrush seedling survival, decreasing mortality by 75%. Seedling mortality was overwhelmingly due to desiccation of seedlings; therefore, neighboring forbs moderate temperature and water stress for seedlings. Despite the extremely limited growing season at high elevation caused by subfreezing temperatures, subalpine ecosystem structure and function are closely tied to water availability during the growing season. Therefore, improved predictions of future precipitation regimes over the Rocky Mountains will be our best tool for conservation of these fragile habitats.

Com o objetivo de investigar a influência da elevação topográfica na distribuição da fauna mirmecológica da serapilheira e de verificar se há um padrão de riqueza para o gradiente investigado, estudei a distribuição das formigas ao longo de um gradiente elevacional no Núcleo Picinguaba do Parque Estadual da Serra do Mar, Ubatuba, SP. Para isso, adotei seis faixas elevacionais (50, 200, 400, 600, 800 e 1000 metros) na costa leste dos Morros do Corisco e do Cuscuzeiro, coletando 20 amostras de 1 m2 de serapilheira em cada uma delas ao longo de dois transectos de 225 m cada e medindo a espessura de cada amostra. As 120 amostras foram submetidas ao extrator mini-Winkler para retirada das formigas. A riqueza observada em todo o gradiente foi de 176 espécies e a estimada (N= 206, dp= 5,17) foi calculada utilizando os índices mais indicados pela literatura, relacionando a riqueza observada à variação elevacional e à espessura da serapilheira através de modelos regressivos. O padrão de riqueza observado apresentou um pico na faixa intermediária do gradiente (cota de 400 metros), com 103 espécies, sendo testada a hipótese do domínio médio para explicar sua distribuição. Através deste teste pude constatar que a distribuição é influenciada por processos estocásticos e fatores bióticos e/ou abióticos. Para verificar se houve variação da fauna ao longo do gradiente apliquei índices de dissimilaridade adequados para o tipo de dados, correlacionando os resultados obtidos com a variação elevacional através do teste de Mantel. Além disso, utilizei a análise de correspondência destendenciada (DCA) para entender como a comunidade mirmecológica em questão está estruturada. As análises de dissimilaridade e o teste de Mantel mostraram que a fauna variou significativamente ao longo do gradiente. Por sua vez, a análise de DCA indicou que a substituição acontece de forma gradual ao longo do gradiente elevacional com a distinção de duas assembléias, uma nas áreas baixas e a outra nas áreas altas, ligadas por um continuum entre elas. Além disso, verifiquei o comportamento da riqueza de espécies exclusivas de cada faixa elevacional para entender a dinâmica da comunidade, utilizando um modelo de regressão. A riqueza dessas espécies foi maior nas extremidades do gradiente, principalmente, na cota de 1000 metros, onde a área é menor, e menor nas faixas intermediárias (200 e 400 metros), onde as estimativas de riqueza foram maiores. Considerando os resultados obtidos, sugiro que os fatores bióticos têm papel preponderante na distribuição das espécies e na definição do padrão de riqueza. Entretanto, a influência de tais fatores só poderá ser avaliada com o estudo da estrutura de guildas de formigas ao longo do gradiente elevacional. / Aiming to investigate the influence of the elevation in topography on the myrmecological leaf litter fauna and to verify if there is a richness pattern for the investigated gradient, I studied the distribution of these ants along an elevational gradient in the Núcleo Picinguaba of the Serra do Mar State Park, Ubatuba, SP. For this, I adopted six elevational bands (50, 200, 400, 600, 800 and 1000 meters) at the Eastern coast of the Corisco and Cuzcuzeiro mountains, where I collected 20 1 m2 leaf litter samples at each band along two transect of 225 m each and measured the leaf litter depth of each sample. The 120 samples were submitted to mini-Winkler extractors to remove of the ants. The observed richness in the whole gradient was 176 species and the estimated richness (N = 206; sd = 5,17) was calculated with the best index according the literature, relating observed richness to the elevational variation and to the leaf litter depth using regressive models. The observed richness pattern revealed a peak at the mid-elevational band of the gradient (400 meters quota) with 103 species; the mid-domain effect theory being tested to explain the species distribution. The results indicate that the distribution is influenced by stochastic processes and/or biotic or abiotic factors. To verify if there is variation on the ant fauna along the gradient, I applied the appropriate dissimilarity index for the data types, correlating the obtained results with the elevational variation using the Mantel test. Moreover, I used a detrended correspondence analysis (DCA) to understand how the myrmecological community in question is structured. The dissimilarity analysis and the Mantel test showed that the fauna changed significantly along the gradient. However, the DCA indicated a gradual substitution along the elevational gradient with two distinct assemblages, one in the lower zone and other in the upper zone, with a continuum between them. Furthermore, I verified the presence of exclusive species at each elevational band to understand the dynamics of the community, using a regressive model. The richness of these exclusive species was greatest at the extremities of the gradient, mainly at the 1000 meters quota, where the area is smaller; and it was smallest at the mid-elevations (200 and 400 meters), where the richness estimatives were greater. Considering the obtained results, I suggest that the biotic factors have a preponderant role in the species distribution and in the definition of the richness pattern. However, the influence of these factors may be only evaluated with the study of the ant guilds structure along the elevational gradient.

Com o objetivo de investigar a influência da elevação topográfica na distribuição da fauna mirmecológica da serapilheira e de verificar se há um padrão de riqueza para o gradiente investigado, estudei a distribuição das formigas ao longo de um gradiente elevacional no Núcleo Picinguaba do Parque Estadual da Serra do Mar, Ubatuba, SP. Para isso, adotei seis faixas elevacionais (50, 200, 400, 600, 800 e 1000 metros) na costa leste dos Morros do Corisco e do Cuscuzeiro, coletando 20 amostras de 1 m2 de serapilheira em cada uma delas ao longo de dois transectos de 225 m cada e medindo a espessura de cada amostra. As 120 amostras foram submetidas ao extrator mini-Winkler para retirada das formigas. A riqueza observada em todo o gradiente foi de 176 espécies e a estimada (N= 206, dp= 5,17) foi calculada utilizando os índices mais indicados pela literatura, relacionando a riqueza observada à variação elevacional e à espessura da serapilheira através de modelos regressivos. O padrão de riqueza observado apresentou um pico na faixa intermediária do gradiente (cota de 400 metros), com 103 espécies, sendo testada a hipótese do domínio médio para explicar sua distribuição. Através deste teste pude constatar que a distribuição é influenciada por processos estocásticos e fatores bióticos e/ou abióticos. Para verificar se houve variação da fauna ao longo do gradiente apliquei índices de dissimilaridade adequados para o tipo de dados, correlacionando os resultados obtidos com a variação elevacional através do teste de Mantel. Além disso, utilizei a análise de correspondência destendenciada (DCA) para entender como a comunidade mirmecológica em questão está estruturada. As análises de dissimilaridade e o teste de Mantel mostraram que a fauna variou significativamente ao longo do gradiente. Por sua vez, a análise de DCA indicou que a substituição acontece de forma gradual ao longo do gradiente elevacional com a distinção de duas assembléias, uma nas áreas baixas e a outra nas áreas altas, ligadas por um continuum entre elas. Além disso, verifiquei o comportamento da riqueza de espécies exclusivas de cada faixa elevacional para entender a dinâmica da comunidade, utilizando um modelo de regressão. A riqueza dessas espécies foi maior nas extremidades do gradiente, principalmente, na cota de 1000 metros, onde a área é menor, e menor nas faixas intermediárias (200 e 400 metros), onde as estimativas de riqueza foram maiores. Considerando os resultados obtidos, sugiro que os fatores bióticos têm papel preponderante na distribuição das espécies e na definição do padrão de riqueza. Entretanto, a influência de tais fatores só poderá ser avaliada com o estudo da estrutura de guildas de formigas ao longo do gradiente elevacional. / Aiming to investigate the influence of the elevation in topography on the myrmecological leaf litter fauna and to verify if there is a richness pattern for the investigated gradient, I studied the distribution of these ants along an elevational gradient in the Núcleo Picinguaba of the Serra do Mar State Park, Ubatuba, SP. For this, I adopted six elevational bands (50, 200, 400, 600, 800 and 1000 meters) at the Eastern coast of the Corisco and Cuzcuzeiro mountains, where I collected 20 1 m2 leaf litter samples at each band along two transect of 225 m each and measured the leaf litter depth of each sample. The 120 samples were submitted to mini-Winkler extractors to remove of the ants. The observed richness in the whole gradient was 176 species and the estimated richness (N = 206; sd = 5,17) was calculated with the best index according the literature, relating observed richness to the elevational variation and to the leaf litter depth using regressive models. The observed richness pattern revealed a peak at the mid-elevational band of the gradient (400 meters quota) with 103 species; the mid-domain effect theory being tested to explain the species distribution. The results indicate that the distribution is influenced by stochastic processes and/or biotic or abiotic factors. To verify if there is variation on the ant fauna along the gradient, I applied the appropriate dissimilarity index for the data types, correlating the obtained results with the elevational variation using the Mantel test. Moreover, I used a detrended correspondence analysis (DCA) to understand how the myrmecological community in question is structured. The dissimilarity analysis and the Mantel test showed that the fauna changed significantly along the gradient. However, the DCA indicated a gradual substitution along the elevational gradient with two distinct assemblages, one in the lower zone and other in the upper zone, with a continuum between them. Furthermore, I verified the presence of exclusive species at each elevational band to understand the dynamics of the community, using a regressive model. The richness of these exclusive species was greatest at the extremities of the gradient, mainly at the 1000 meters quota, where the area is smaller; and it was smallest at the mid-elevations (200 and 400 meters), where the richness estimatives were greater. Considering the obtained results, I suggest that the biotic factors have a preponderant role in the species distribution and in the definition of the richness pattern. However, the influence of these factors may be only evaluated with the study of the ant guilds structure along the elevational gradient.

Andean montane forests are one of the most diverse ecosystems on Earth, but are also highly vulnerable to climate change. Therefore, the link between plant distribution and ecosystem productivity is a critical point to investigate in these ecosystems. Are the patterns in productivity observed in montane forest due to species turnover along the elevational gradients? Methodological constraints keep this question unanswered. Also, despite their importance, belowground biomass remains poorly quantified and understood. I measured two plant functional traits in seedlings, root:shoot ratio and specific leaf area, to identify different strategies in growth and biomass allocation across elevations. A tradeoff in specific leaf area with elevation was found in only one species, and no generalized directional change was detected with elevations for root:shoot ratio. Lack of information for the ontogeny of the measured plant traits could confounding the analysis.

Moderate livestock grazing is often identified as a useful management tool for sustaining biodiverse grassland communities and has long been practiced in Scandinavian mountains. Despite of a growing number of studies focusing on grazing effects along elevational gradients worldwide, there is still limited knowledge of how grazing intensity varies over time and how it affects plant communities on species and functional group level. Moreover, the majority of existing studies are conducted in enclosures with controlled stocking rate, while free-range grazing with a highly varied livestock number is the common practise in Scandinavia. Therefore, the aim of this study was to quantify free-range grazing intensity over the growing season along an elevational gradient and to evaluate how grazing affects plant communities on species, functional group and community level. In order to quantify grazing intensity, we conducted a grazer exclusion experiment along an elevational gradient in Norway in the growing season of 2019. We quantified grazing intensity as the difference in dry above-ground biomass between grazed and caged plots and analyzed plant composition on community and functional group level as well as recorded species-specific grazing damage and potentially influential abiotic factors. To capture the effects of temporal variation in grazing we also relocated caged and grazed plots in four-week intervals during the growing season. We demonstrated that grazing intensity gradually decreased with elevation and elevation explained 77% of the variation in grazing among sites. Lower grazing intensity at high elevation is likely related to decreasing plant productivity and differences in plant community composition along the gradient. Grazers reduced the biomass of graminoids and forbs in the intensively grazed lower sites. Grazing damage varied among sites for five of the eight most commonly grazed plant species, which was mainly driven by a higher grazing damage in the intermediate site (900m a.s.l.). Selective grazing was observed in three sheep-dominated study sites. Grazing intensity did not significantly vary during the growing season at any site, possibly due to weak temporal effects of an overall short growing season. In accordance with other studies, we demonstrated that grazing homogenized plant community composition in actively grazed sites that were relatively species rich. To conclude, livestock grazing can help maintain biodiverse semi-natural grasslands, yet when grazing is applied as a management tool in the mountainous landscapes, we suggest considering that grazing intensity, and therefore its effects, will vary with elevation.

Island biogeography theory predicts that the patterns and distributions of spatially isolated populations are governed by large scale processes. The high elevations forests in the Southern Appalachians represent a series of naturally fragmented islands that harbor many isolated populations of species at the southern limits of their range. Understanding the governing forces of population dynamics in this region will enhance the probability of species persistence in the face of threats such as global warming and human development. We surveyed bird populations across multiple elevations in Virginia and combined this with a multi-scale habitat analysis to determine influences of species presence and species richness. We detected 101 species across the elevation gradient, including 12 species with special conservation status and ten species whose presence increased with increasing elevation. These ten elevation sensitive species responded to habitat variables at both the microhabitat and landscape scale, with species-specific patterns of habitat variable correlation emerging. Habitat type was least effective in predicting species presence for any elevation sensitive species. Species richness declined over the elevation gradient until the highest elevations, where this trend reversed and richness began to increase. This pattern was driven by an increase in short-distance migrants beginning at mid-elevations, which ultimately overpowered a corresponding decrease in long-distance migrants beginning at similar elevations. Habitat analysis linked these patterns to a preference of short-distance migrants for smaller, more isolated non-forested patches, and a historical lack of persistence for long-distance migrants. Conservation and management decisions for the region should focus on a multi-scale approach that preserves all habitat types for continued species presence and high species richness, although the persistence of particular elevation sensitive species is compounded by unique species-habitat relationships and the perception of islands as species-specific. Continued monitoring of these fragmented populations in light of both short- and long-term threats which span multiple scales of influence will maintain high species richness and ensure the persistence of crucial breeding habitat. / Master of Science

Living (biotic) and non-living (abiotic) factors drive the function of ecosystems across a variety of scales from the root-soil interface to the watershed. Biotic and abiotic global change pressures such as increasing temperature and invasive species are shifting how ecosystems function. Thus, exploring and understanding how these factors shape function across the landscape is an important research area. For example, climate change both directly and indirectly affects soil microbial functions – such as carbon mineralization and nitrogen transformations – through increasing activity under warming and altering inputs to the soil through species composition changes. Mountains provide a useful tool for studying relationships among biotic and abiotic factors because climate and species diversity shift along gradients. Here, I measured carbon and nitrogen soil processes as well as microbial extracellular enzyme activity along an elevational gradient to explore how changes in climate, edaphic properties, and biotic composition affects ecosystem function. As expected, climate and species composition varied in predictable ways along the gradient – actual evapotranspiration declined, and conifer dominance increased. Soil functions also shifted along the gradient. Potential carbon mineralization increased with elevation and with conifer dominance. Potential nitrogen mineralization rates increased with elevation and with conifer dominance. Surprisingly, there were few predictors for potential soil nitrification, which increased only with soil functional diversity. While temperature and moisture availability drive ecosystem function at broad scales and biotic factors typically drive function at the regional scale, we saw that function of soils at the mountain watershed scale was best explained by a combination of both abiotic and biotic factors.

Orientadores: Kikyo Yamamoto, George John Shepherd / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Biologia / Made available in DSpace on 2018-08-15T02:40:57Z (GMT). No. of bitstreams: 1 Bertoncello_Ricardo_M.pdf: 8895754 bytes, checksum: 5b50a52fbb7e96dfd8126d57935601cf (MD5) Previous issue date: 2009 / Resumo: A classificação e a delimitação das diferentes formações fitogeográficas que ocorrem no domínio da Mata Atlântica nas regiões Sul e Sudeste do Brasil constituem um desafio para botânicos, ecólogos e fitogeógrafos. A distribuição atual destas formações está relacionada ao histórico de ocupação das áreas e a complexos gradientes ambientais, que ocorrem nos sentidos horizontal (latitudinal, longitudinal e edáfico) e vertical (altitudinal). Visando a contribuir para o conhecimento das formações florestais que integram a Mata Atlântica, este trabalho foi realizado no Morro do Cuscuzeiro (Ubatuba-SP) com os objetivos de: (1) descrever as variações florísticas e estruturais nas comunidades arbóreas em função da altitude, e verificar se existe uma Floresta Nebular que possa ser discriminada por parâmetros florísticos e estruturais; e (2) verificar a situação fitogeográfica das comunidades encontradas neste local no contexto da Mata Atlântica nas regiões Sul e Sudeste do Brasil. O Morro do Cuscuzeiro se situa no Núcleo Picinguaba do Parque Estadual da Serra do Mar (23° 18' 14" S e 44° 47' 16" W) e possui 1277m de altitude. O gradiente altitudinal foi representado por amostras de quatro cotas altimétricas, 820m, 970m, 1120m e 1270m, obtidas em 10 parcelas de 10 x 10 m em cada cota, exceto na mais alta que foi dividida em duas 'sub-cotas' com 5 parcelas cada. O critério de inclusão amostral da flora arbórea foi de 15 cm de CAP (exceto nas duas 'sub-cotas' superiores, 10 cm de CAP). Em sentido base-topo, foi verificada diminuição na riqueza, na diversidade, na altura, no diâmetro e no volume das árvores, e aumento na densidade. Uma mudança abrupta na composição da comunidade arbórea foi detectada a 1120m, acima da qual aparece uma formação que identificamos como Floresta Nebular, em substituição à floresta de encosta da Serra do Mar, que ocorre abaixo daquela altitude. As espécies destas duas formações florestais foram inseridas em uma matriz de 1546 espécies registradas em 112 levantamentos de 78 localidades do domínio da Mata Atlântica nas regiões Sul e Sudeste do Brasil. As análises multivariadas resultaram em cinco grupos de levantamentos: 1-Florestas Nebulares; 2- Florestas da província costeira (posteriormente sub-dividido em (a) Florestas de encosta e (b) Florestas de topo de morro e florestas da planície costeira); 3-Florestas de Araucária; 4-Florestas Semi-decíduas; e 5- Florestas Semi-decíduas Montanas. As formações encontradas no Morro do Cuscuzeiro foram incluídas nos grupos das Florestas Nebulares (de 1120m e 1270m) e das florestas de encosta da província costeira (820m e 970m), o que reforçou os resultados da análise dos dados locais, mostrando que pequenas diferenças em altitude podem resultar em mudança abrupta na composição das comunidades, evidenciada pela presença de espécies de distribuição disjunta que são compartilhadas com outras formações de Florestas Nebulares das regiões Sul e Sudeste do país / Abstract: The classification and delimitation of different vegetational formations into a phytogeographic system in Brazil has been a challenge to ecologists, botanists, and phytogeographers. The current distribution of these formations is related to historical process of land use and complex environment gradients, occurring in horizontal (latitude, longitude and edafic) and vertical (altitude) ways. The aims of this study were (a) to describe the changes in floristic composition and structure of tree species along an elevational gradient on Morro do Cuscuzeiro, Ubatuba (SP), and to verify the occurrence of a Cloud Forest that can be discriminated by floristic and structural paremeters; and (b) to analyze the phytogeographic position of the communities found in this mountain in relation to other surveys of the Atlantic Rain Forest domain in Southern and Southeastern Brazil. Mountain Cuscuzeiro is located in the Serra do Mar State Park (at 23° 18' 14" S, 44° 47' 16" W it is 1277m in height). A survey of tree species was made in four elevational levels, 820m, 970m, 1120m, and 1270m, using 10 samples of 10x10m on each level, except on the higher one, which was sub-divided in two sub-levels whith five samples on each. The sampling criterion was 4,8cm of DBH (except in the two higher sub-levels, where 3,2cm of DBH was used). The analysis resulted in consistent groups at the different levels, indicating a strong altitude influence on the floristic composition. An abrupt change of the vegetation was identified at 1120m, from where a typical Ombrophilous Dense Forest (slope forest from 820m to 970m) shifted into a Cloud Forest formation (1120m to 1270m). These two formations were inserted in a matrix made by a total of 1546 species of 112 surveys from 78 locations of Atlantic Rain Forest domain in southern and southeastern Brazil. The multivariate analysis resulted in five groups of samples: 1- The Cloud Forest; 2- The coastal province (further subdivided into (a) the slope forests and (b) a subgroup composeded by the mountaintop and the coastal plain forests); 3- The Araucaria Forest; 4-The Semi-Deciduous Forest; and 5- The Montane Semi-Deciduous Forest. The formations found on Mt. Cuscuzeiro were included in the groups of the Cloud Forest (from 1120m to 1270m) and of the Coastal Province (from 820m to 970m), which reinforced the results of the local elevational gradient analysis, showing that minor changes on altitude can lead to abrupt changes in community composition due to the occurrence of species with disjunct distribution that are shared with other Cloud Forest formations of southern and southeastern Brazil / Mestrado / Biologia Vegetal / Mestre em Biologia Vegetal

No description available.

Le changement climatique rapide auquel nous assistons actuellement est déjà en train de modifier le cycle de vie d’un grand nombre d’organismes. Des études basées sur des modèles d’enveloppe bioclimatique apportent des réponses mais ces prédictions de nouvelles aires de répartition ne tiennent pas compte d’une part de l’adaptation rapide des espèces (plasticité phénotypique et diversité génétique non neutre), et d’autre part des interactions interspécifiques ou de la dynamique des populations. Ce travail de recherche est centré sur les mécanismes permettant la persistance des individus dans un environnement changeant.Nous avons travaillé dans les Pyrénées françaises sur deux espèces à large répartition européenne (chêne sessile et hêtre commun) sur un gradient altitudinal de 1500 m de dénivelé correspondant en moyenne à 8°C d’amplitude thermique. Ce gradient a été répété dans deux vallées parallèles, distantes de 30 km. Basée sur l’étude de traits fonctionnels, nous avons caractérisé les variations altitudinales de traits morphologiques et physiologiques de ces caractères dans des conditions naturelles. Les capacités écophysiologiques apparaissent plus élevée pour le chêne et pour les populations de haute altitude, suggérant une adaptation locale à un environnement stressant. Dans une étude préliminaire, nous avons établi qu’il était nécessaire d’intégrer les variations de pression partielle de dioxyde de carbone et non se concentration dans les mesures d’échanges gazeux. Tous les traits fonctionnels mesurés en populations naturelles varient avec l’altitude. Ainsi la taille des feuilles diminue avec l’altitude tandis que les autres traits augmentent, avec des valeurs de 1.3 à 3.9 fois supérieures entre le haut et le bas du gradient tous traits confondus. Nous avons ensuite cherché à déterminer l’origine de ces variations phénotypiques. Une expérimentation en test de provenance a ainsi permis de déterminer les traits dont la variation in situ était en partie sous contrôle génétique. Les résultats de notre étude montrent que le gradient altitudinal a induit une différenciation génétique au niveau de la croissance, de la phénologie et de traits fonctionnels foliaires pour ces espèces, malgré la proximité des populations étudiées dans leur milieu naturel. Néanmoins, les capacités photosynthétiques pour le hêtre et la surface spécifique foliaire pour le chêne ne montre aucune différentiation génétique, ainsi que la densité stomatique pour les deux espèces. D’autre part, l’expérimentation de transplantations réciproques le long du gradient, a mis en évidence une forte plasticité phénotypique pour les deux espèces, ce qui suggère que les populations peuvent dans une certaine mesure répondre immédiatement aux variations climatiques rencontrées le long de ce gradient. Cependant, tandis que la température optimale pour la longueur de saison de végétation ne semble pas encore atteinte, les populations pourraient réagir négativement à une élévation de la température en terme de croissance. En conclusion, les mécanismes adaptatifs mis en évidence le long d’un gradient climatique naturel, pourraient permettre aux populations de faire face au changement climatique actuel. / The rapid climate change, which we are currently witnessing, is already modifying the physiology and distribution of species. Predictions of changes in species distributions do not take evolutionary mechanisms and biotic interactions into account. Our main objective was to assess the inherent adaptive capacities of tree populations by i) quantifying the phenotypic variations of functional traits with altitude and ii) studying the extent to which these variations are environmentally driven (phenotypic plasticity) and/or genetically fixed (adaptation). The study took place in the French Pyrenees along an altitudinal gradient range of 1500 m corresponding on average to 8°C of thermal amplitude. We focused on two broadleaved species with a wide European distribution (sessile oak and common beech). This gradient was repeated in two parallel valleys, distant from 30 km. Altitudinal trends were investigated for several morphological, physiological and phenological traits in natural conditions (in situ), in a common garden experiment and in reciprocal transplant experiments (RTEs). The phenotypic variability observed in situ showed significant altitudinal trends for all the studied traits and followed similar patterns for both species. We established that together with temperature, it was also necessary to integrate the variations of atmospheric gas partial pressure along the altitudinal gradient. In the common garden experiment, our results showed that the altitudinal gradient induced genetic differentiations for growth, leaf phenology and several morphological and physiological traits. This experiment made it possible to demonstrate, for both species, a weaker effect of genetic variations than in situ observed variations, suggesting a strong effect of the environment on leaf functional traits. A higher intrapopulation than interpopulation genetic variability was also observed for all traits. Finally, the reciprocal transplant experiments highlighted a high magnitude of phenotypic plasticity whatever the trait and the species.