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Climate change is causing significant shifts in tree species distributions to higher elevations and latitudes. Seed germination and seedling establishment are particularly important steps in tree range expansion under warmer conditions, yet seedling establishment is influenced by a range of factors beyond temperature, including herbivory, microenvironment, and the timing and amount of precipitation. We conducted an experiment to assess how augmented precipitation regimes, wildlife herbivory, and microclimate influence germination and first-season survival of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) near the upper elevational limit of its range in the southern Rocky Mountains. Germination was strongly influenced by moisture, with over three times higher germination in watered treatments. Seedling survival was similar across watered treatments but was negatively associated with microenvironments with higher maximum temperatures. These results indicate that soil moisture effects on germination and the negative impact of hot growing-season temperatures on seedling survival limit initial seedling establishment in Douglas-fir, even at the cooler and wetter end of its range, suggesting that the planting of this species will be most successful in cooler and wetter microsites. Taken together, this study suggests that continued warming and projected increases in droughts may strongly limit Douglas-fir regeneration and thus its ability to shift upwards with climate change. Key words: seedling establishment, microclimate, species distribution, soil moisture, Douglas-fir, climate change. Le changement climatique provoque des modifications importantes dans l'aire de repartition des especes arborescentes aux latitudes et altitudes elevees. La germination des graines et l'etablissement des semis sont des etapes particulierement importantes dans l'expansion de l'aire de repartition des arbres sous des conditions plus chaudes. Par contre l'etablissement des semis est influence par une foule de facteurs au-dela de la temperature, incluant le broutage, le microenvironnement ainsi que le moment et la quantite de precipitation. Nous avons realise une experience pour evaluer de quelle facon la pluviosite accrue, le broutage de la faune et le microclimat influencent la germination et la survie lors de la premiere saison de croissance du douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) pres de la limite altitudinale superieure de son aire de repartition dans la partie sud des Montagnes Rocheuses. La germination etait fortement influencee par l'humidite et plus de trois fois plus elevee dans les traitements irrigues. La survie des semis etait semblable parmi les traitements irrigues, mais elle etait negativement associee aux microenvironnements avec des temperatures maximales plus elevees. Ces resultats indiquent que les effets de l'humidite du sol sur la germination et l'impact negatif des temperatures chaudes durant la saison de croissance sur la survie des semis limitent l'etablissement initial des semis de douglas de Menzies, meme aux limites plus fraiches et plus humides de son aire de repartition, indiquant qu'on aura davantage de succes en plantant cette espece sur des microsites plus frais et plus humides. Cette etude indique qu'ensemble, un rechauffement progressif et l'augmentation prevue des secheresses pourrait fortement reduire la regeneration du douglas de Menzies et par consequent sa capacite a migrer vers le haut en altitude pour s'adapter au changement climatique. Mots-cles : etablissement des semis, microclimat, repartition des especes, humidite du sol, douglas de Menzies, changement climatique., Introduction Changing climatic conditions have begun to exceed thresholds for tree species regeneration across significant portions of their ranges, causing spatial shifts in the regenerative niche (Kelly and Goulden 2008; [...]

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Although closed‐canopy forests are characterized by low‐light availability and slow population dynamics, many are under threat from non‐native, invasive woody species that combine high colonization ability and fast growth potential with high low‐light survival. This 'superinvader' phenotype contravenes expected trade‐offs predicted by successional niche theory, posing a challenge to both invasion and forest succession theory.We propose a parsimonious conceptual model based on the whole‐plant light compensation point (WPLCP) that, across a variety of plant strategies and growth forms, can explain greater competitive abilities of forest invaders in the context of both high‐light growth rate and shade tolerance. The model requires only that non‐native species experience relatively fewer carbon costs than native species, enabling resource‐acquisitive species to establish in low‐light conditions.We review evidence for lower carbon costs in invasive species resulting from (1) enemy release, (2) recent environmental changes that favour less stress‐tolerant phenotypes and (3) phylogenetically constrained native floras. We also discuss implications of invader shade tolerance in the context of other life‐history strategies that, combined with canopy disturbances, facilitate their rapid numerical dominance.Synthesis. An invasion framework driven by carbon dynamics suggests renewed focus on whole‐plant carbon costs, including below‐ground respiration and tissue turnover, which are rarely measured in functional studies of forest invaders. [ABSTRACT FROM AUTHOR]

Copyright of Biotropica is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Functional Ecology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The ability of tree species to acclimate and tolerate projected increases in drought frequency and intensity has fundamental implications for future forest dynamics with climate change. Inquiries to date on the drought tolerance capacities of tree species, however, have focused almost exclusively on mature trees with scant in situ work on seedlings, despite the central role that regeneration dynamics play in forest responses to changing conditions. We subjected naturally established seedlings of co-dominant subalpine conifer species (Abies lasiocarpa and Picea engelmannii) in the southern Rocky Mountains to 2 years of in situ summer precipitation exclusion, simulating summer drought conditions similar to a failure of the North American monsoon. We compared the morphological and physiological responses of seedlings growing in drought vs. ambient conditions to assess the relative changes in drought tolerance traits as a function of seedling size. Drought treatments had a marked impact on soil moisture: volumetric water content averaged ≈5–8 % in drought treatments and ≈8–12 % in ambient controls. We detected no significant shifts in morphology (e.g. root biomass, leaf:stem area ratio) in response to drought for either species, but net photosynthesis in drought treatments was 78 % lower for spruce and 37 % lower for fir. Greater stomatal control associated with increasing stem diameter conferred greater water use efficiencies in larger seedlings in both species but was not significantly different between drought and ambient conditions, suggesting an overall lack of responsivity to water stress and a prioritization of carbon gain over investment in drought mitigation traits. These results indicate a canonization of traits that, while useful for early seedling establishment, may portend substantial vulnerability of subalpine seedling populations to prolonged or recurrent droughts, especially for spruce. [ABSTRACT FROM AUTHOR]