Background/Question/Methods   Theory and experimental data concerning habitat-related ecophysiological traits suggest carbon allocation strategy depends on resources availability. Species tend to shift from a conservation strategy in a resource-limited environment to a production strategy in rich environment. In high latitudes, global warming tends to increase soil nutrient availability, removing important nutrient limitation. The objective of this study was to test if the related increase in productivity will be coupled with a decrease of allocation to reproduction and storage. This change in allocation may cause the dominant species to become sensitive to extreme weather events, which may hinder their long-term dominance. In the arctic tundra, ecosystem-responses to global change include a progressive encroachment of shrub species. This response has been shown to vary between sites and functional groups. In a long-term fertilization/warming/shading experiment, we quantified carbon allocation pattern of growth, storage and reproduction for a dominant deciduous schrub (Betula nana, B.n.) and a dominant evergreen species (Ledum palustre, L.p.) in Alaskan tundra. The goals of this study were to (1) assess the changes in allocation strategy in a changing environment, (2) determine the limitation factors of development and (3) identify the carbon sources and/or sinks under limitation, in species from contrasting functional groups.

Results/Conclusions   Both species presented similar growth efficiency (GE ~ 1.2 mgC/leaf cm2) whereas L.p. presented higher reproduction efficiency (RE ~ 2.1 mgC/leaf cm2) than B.n. (RE ~ 0.2 mgC/leaf cm2). Under warming (W) and fertilization (F) treatments, B.n. greatly dominated over L.p. whereas under shading (Sh), L.p. was dominant. For both species, W induced an increase of leaf area (LA) without modifying GE, and F induced an increase of both LA and GE. W induced a decrease of RE for L.p. and not for B.n., whereas F induced an increase of RE for B.n. and not for L.p. Under Sh, L.p. presented an increase of RE but stable LA and GE, whereas B.n. presented a decrease of LA, GE and RE. This study shown that (1) the environmental determinism of growth and reproduction is different and change between species, (2) as hypothesized, the allocation strategy depends on the environmental resources in association with species and/or functional groups considered, (3) the enhancement of environmental conditions may not systematically favor productivity, depending on the functional group. Continuing analyses of carbohydrates and lipids pools will give greater insights into the processes of developmental limitations.