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The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature-trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.

Numerous factors contribute to the variability in treeline change; however, the potential role of insect predation in limiting seed productivity is not well documented. Conditions for seed germination, establishment, and survival are not limiting at the Mealy Mountains treeline (Labrador, Canada), but seedlings are rarely encountered, suggesting a seed-related bottleneck to recruitment. Mature cones were collected in 2008 from four tree species (black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.), balsam fir (Abies balsamea (L.) Mill.), eastern larch (Larix laricina (Du Roi) K. Koch), and white spruce (Picea glauca (Moench) Voss)) across three treeline zones (forest, transition, and krummholz) to assess potential seed limitation. During that year, an unexpectedly high diversity of insect larvae caused extensive reproductive loss and damaged the cones of ~85% of trees sampled, confirming that treeline change models should include seed predation. Seed germination was low and variable in all treeline zones, although significantly higher in black spruce and eastern larch. Most reproductive quality measures decreased significantly with elevation, although no differences among zones or tree species in the percentages of seeds damaged by insects were found (mean ± standard deviation: 31% [+ or -] 23%). Based on tree density and seed production, black spruce is predicted to lead the treeline expansion in the Mealy Mountains. Although climate warming may create conditions conducive for increased seed production, predispersal seed predation may limit future treeline expansion. Key words: climate change, forest-tundra ecotone, insect herbivory, multispecies, predispersal seed predation, reproductive bottleneck. Plusieurs facteurs contribuent a la variabilite de la modification de la limite des arbres mais le role potentiel des epidemies d'insecte dans la reduction de la productivite des graines a ete peu etudie. A la limite des arbres dans les montagnes Mealy (Labrador, Canada), les conditions sont adequates pour la germination, l'etablissement et la survie mais on trouve rarement des semis. Cela indique qu'il y a un probleme de recrutement relie aux graines. Des cones matures ont ete collectes en 2008 chez quatre especes (epicea noir (Picea mariana (Mill.) Britton, Sterns & Poggenb.), sapin baumier (Abies balsamea (L.) Mill.), meleze laricin (Larix laricina (Du Roi) K. Koch) et epicea blanc (Picea glauca (Moench) Voss)) dans trois zones de la limite des arbres (foret, transition et krummholz) pour evaluer les problemes potentiels relies aux graines. Durant cette annee, une diversite etonnamment elevee de larves d'insecte ont cause des pertes importantes en endommageant les cones d'environ 85 % des arbres echantillonnes. Cela a confirme que les modeles qui simulent les variations de la limite des arbres devraient inclure la predation par les insectes. La germination des graines etait faible et variable dans toutes les zones a la limite des arbres quoique significativement meilleure chez l'epicea noir et le meleze. La plupart des mesures de qualite de la reproduction diminuaient significativement avec l'altitude bien qu'aucune difference dans le pourcentage de graines endommagees par les insectes entre les zones ou les especes d'arbre n'ait ete observee (moyenne : 31% ± 23%ET). Sur la base de la densite des arbres et de la production de cones ou de graines, on prevoit que l'epicea noir dominera l'expansion de la limite des arbres dans les montagnes Mealy. Bien que le rechauffement du climat puisse creer des conditions favorables a l'augmentation de la production de graines, la predation des graines avant qu'elles n'aient ete dispersees pourrait limiter l'expansion future de la limite des arbres. [Traduit par la Redaction] Mots-cles : changements climatiques, ecotone entre la foret et la toundra, insectes herbivores, multiespeces, predation des graines avant leur dispersion, problemes de reproduction., Introduction At the global scale, upslope and northward advance of the thermally constrained forest-tundra ecotone (treeline; sensu Scott (1995)) is expected given projected warming scenarios associated with climate change (Grace [...]

Although many studies have focused on factors influencing treeline advance with climate change, less consideration has been given to potential changes in tree spatial pattern across the forest-tundra ecotone. We investigated trends in spatial pattern across the forest-tundra ecotone and geographical variation in the Yukon, Manitoba, and Labrador, Canada. Tree cover was measured in contiguous quadrats along transects up to 100 m long located in Forest, Ecotone, and Tundra sections across the forest-tundra transition. Spatial patterns were analyzed using new local variance to estimate patch size and wavelet analysis to determine the scale and amount of aggregation. Compared with the Forest, tree cover in the Ecotone was less aggregated at most sites, with fewer smaller patches of trees. We found evidence that shorter trees may be clumped at some sites, perhaps due to shelter from the wind, and we found little support for regular spacing that would indicate competition. With climate change, trees in the Ecotone will likely become more aggregated as patches enlarge and new patches establish. However, results were site-specific, varying with aspect and the presence of krummholz (stunted trees); therefore, strategies for adaptation of communities to climate change in Canada's subarctic forest would need to reflect these differences. Resume: Alors que plusieurs etudes ont mis l'accent sur les facteurs qui influencent la progression de la limite des arbres due aux changements climatiques, peu d'attention a ete accordee aux changements potentiels du patron de repartition spatiale des arbres dans l'ecotone entre la foret et la toundra. Nous avons etudie les changements de la repartition spatiale dans l'ecotone entre la foret et la toundra de meme que sa variation geographique au Yukon, au Manitoba et au Labrador, au Canada. Le couvert forestier a ete mesure dans des quadrats contigus etablis le long de transects pouvant atteindre 100 m de longueur et situes dans les sections de foret, d'ecotone et de toundra dans la zone de transition entre la foret et la toundra. Les patrons spatiaux ont ete analyses avec les methodes de la nouvelle variance locale pour estimer la taille des ilots de foret et avec l'analyse par ondelettes pour determiner l'echelle et le degre d'agregation. Comparativement a la foret, le couvert forestier de l'ecotone etait moins groupe dans la plupart des stations, ce qui s'est traduit par des ilots de foret plus petits et moins nombreux. Nous avons trouve des preuves que les petits arbres peuvent etre regroupes dans certaines stations, probablement pour se proteger du vent, et nous avons trouve peu de signes d'espacement regulier qui auraient indique la presence de competition. Avec les changements climatiques, les arbres etablis dans l'ecotone se regrouperont probablement davantage par l'agrandissement des ilots et par l'etablissement de nouveaux ilots. Cependant, ces resultats sont specifiques aux stations puisqu'ils varient selon leur orientation et en fonction de la presence de krummholz (arbres prostres). Ainsi, les strategies d'adaptation des communautes aux changements climatiques dans la foret subarctique canadienne devraient refleter ces differences. [Traduit par la Redaction], Introduction Increases in global temperature associated with climate change have caused range shifts in a wide array of taxa (Parmesan 2006; Woodall et al. 2009; Harsch et al. 2009). At [...]

The fundamental niche of many species is shifting with climate change, especially in sub‐arctic ecosystems with pronounced recent warming. Ongoing warming in sub‐arctic regions should lessen environmental constraints on tree growth and reproduction, leading to increased success of trees colonising tundra. Nevertheless, variable responses of treeline ecotones have been documented in association with warming temperatures. One explanation for time lags between increasingly favourable environmental conditions and treeline ecotone movement is reproductive limitations caused by low seed availability. Our objective was to assess the reproductive constraints of the dominant tree species at the treeline ecotone in the circumpolar north. We sampled reproductive structures of trees (cones and catkins) and stand attributes across circumarctic treeline ecotones. We used generalized linear mixed models to estimate the sensitivity of seed production and the availability of viable seed to regional climate, stand structure, and species‐specific characteristics. Both seed production and viability of available seed were strongly driven by specific, sequential seasonal climatic conditions, but in different ways. Seed production was greatest when growing seasons with more growing degree days coincided with years with high precipitation. Two consecutive years with more growing degree days and low precipitation resulted in low seed production. Seasonal climate effects on the viability of available seed depended on the physical characteristics of the reproductive structures. Large‐coned and ‐seeded species take more time to develop mature embryos and were therefore more sensitive to increases in growing degree days in the year of flowering and embryo development. Our findings suggest that both moisture stress and abbreviated growing seasons can have a notable negative influence on the production and viability of available seed at treeline. Our synthesis revealed that constraints on predispersal reproduction within the treeline ecotone might create a considerable time lag for range expansion of tree populations into tundra ecosystems. [ABSTRACT FROM AUTHOR]

Aim: Recent and rapid warming is reorganizing terrestrial vegetation, creating novel species assemblages, and shifting range limits. Relative to the evergreen species that currently dominate much of the boreal forest landscape, Larix (larch) distributions may be particularly responsive to climatic change due to their deciduous habit, and quick growth and reproduction. Here, we amassed data from 83 studies to describe and explain observed patterns of Larix range shifts under contemporary climate change. Location: Northern hemisphere. Taxon: Species of the genus Larix, deciduous gymnosperms. Methods: With 181 observations of Larix range limit dynamics, we used five distribution parameters (tree line advance, stand infilling, tree line recession, stand thinning, no response) and their determinants (climate, land use change, natural disturbance) to describe and explain observed patterns of Larix range shifts under contemporary climate change. We ran a redundancy analysis on the matrix of five distribution parameters considered with other climatic and nonclimatic parameters as explanatory variables. We also characterized the climatic niche of Larix species (temperature and precipitation) and how the niche has changed during the 20th century. Results: Of 173 sites studied over the full distribution of Larix, 63% experienced Larix population increases, 18% had population decreases, and no response was detected at 19% of sites. Latitudinal Larix tree lines in Siberia and North America appear to be infilling and shifting their distributions northward, whereas Larix recession and thinning was more common in southern regions, suggesting southern populations may be experiencing greater drought stress than their northern counterparts. Climatic niches of most Larix species shifted towards warmer and wetter conditions, with tree line advance/forest infilling in cool/dry climate space, and recession/thinning in warm/dry space. Main conclusions: Northern expansion is underway or seems imminent for boreal Larix species, primarily L. laricina in North America. Retraction in southern regions and disappearance of some mountainous populations may be inevitable due to their narrow ecological niches. Species restricted to mountainous habitats may expand locally, though will likely not contribute to broad scale range expansion. These changes will depend on suitable climate, disturbance, and dispersal mechanisms. [ABSTRACT FROM AUTHOR]