C. Marty

Leaf life span optimizes annual biomass production rather than plant photosynthetic capacity in an evergreen shrub.

SUMMARY *Owing to nitrogen (N) translocation towards new leaves, the shedding of old leaves can increase the whole-plant carbon gain. It occurs when their photosynthetic nitrogen use efficiency (PNUE) declines below a given threshold. *Here, we investigated variations in net photosynthetic capacity (A(max)), N resorption and PNUE in populations of Rhododendron ferrugineum presenting different mean leaf life spans (LLS). *Both populations had comparable annual leaf surface area production and A(max) across leaf-age cohorts. Branch photosynthetic capacity was up to 95% higher in the population with the longer LLS mainly because of the high contribution of old leaves to the total leaf area. Despite lower N concentrations, old leaves maintained relatively high A(max) and consequently PNUE that were higher than or similar to the values found in current-year leaves. *As the ratio of PNUE in old to PNUE in new leaves was always higher than the fraction of leaf N resorbed during leaf shedding, we concluded that leaf shedding did not improve plant photosynthetic capacity. We suggest that in R. ferrugineum, leaf shedding is mainly controlled by the leaf storage function and, therefore, that models aiming to explain LLS should not only consider the leaf carbon assimilation function, particularly in nutrient-poor habitats.

Complex interactions between a legume and two grasses in a subalpine meadow

Interactions between plants are a complex combination of positive and negative interactions, with the net outcome depending on environmental contexts. The more frequent association of Trifolium alpinum (legume) with Festuca eskia than with Nardus stricta (grasses) in many Pyrenean subalpine meadows suggests a differential ability to use nitrogen (N) derived from N(2) fixation. In the field, we investigated the interactions between the legume and grasses and, in the glasshouse, the transfer of (15)N from the legume to the grasses. In one grass-Trifolium mixture, the legume had a strong positive effect on the biomass and N content of the grass as compared to pure grass stands. When both grasses grew together with the legume, only Festuca benefited from the presence of Trifolium but, surprisingly, the benefit decreased with increasing Trifolium abundance. Leaf labeling experiments with (15)N-NH(4)(+) revealed a higher transfer of (15)N from Trifolium to Festuca than to Nardus, suggesting a more direct N pathway between the two species. This more direct pathway could prevent Nardus from benefiting from the legume N in the three-species mixtures. Thus, the positive interactions between N-fixers and nonfixers appear to be largely species-specific and to depend strongly on the species in the plant assemblage.

Effect of the Relative Abundance of Conifers Versus Hardwoods on Soil δ13C Enrichment with Soil Depth in Eastern Canadian forests

Soils are a major component of the global C cycle, and considerable effort has been dedicated to improve our understanding of factors controlling soil organic C (SOC) turnover and stabilization in the last decades. Carbon stable isotopes are useful in this respect as they represent an integrative indicator of SOC biogeochemical processing. In the present study, C concentration and δ13C were measured in soil horizons of 21 forest sites located at the transition zone between the hardwood and the conifer forest in Québec, Canada, and related to 13 biophysical variables to identify the main drivers of SOC storage and turnover. Carbon concentrations in the forest floor (FF) and the B- and C-horizons were, respectively, strongly correlated with percentage of clay (Pclay), the mean annual precipitation: potential evapotranspiration ratio (MAP:PET), and percentage of hardwoods (Phwd). In FF, δ13C was poorly correlated with the studied variables, whereas in mineral horizons, it was significantly correlated with mean annual air temperature (MAAT) and the percentage of conifers (Pc) and Pclay. Across the studied area, δ13C increased on average by 2.0‰ from the FF to the C-horizon. The isotopic enrichment with soil depth (β) was strongly negatively correlated with Pc, which explained 55% of its variability among sites. This suggests that the vegetation type is an important driver of soil C long-term turnover rate in forest ecosystems. Overall, our data suggest that hardwood forest expansion in response to climate change might reduce the stability and the storage of SOC in the future.

Canopy interaction with precipitation and sulphur deposition in two boreal forests of Quebec, Canada

The interaction of atmospheric sulphur (S) was investigated within the canopies of two boreal forests in Québec, Canada. The net canopy exchange approach, i.e. the difference between S-SO(4) in throughfall and precipitation, suggests high proportion of dry deposition in winter (up to 53%) as compared to summer (1-9%). However, a 3.5‰ decrease in δ(18)O-SO(4) throughfall in summer compared to incident precipitation points towards a much larger proportion of dry deposition during the warm season. We suggest that a significant fraction of dry deposition (about 1.2 kg ha(-1) yr(-1), representing 30-40% of annual wet S deposition) which contributed to the decreased δ(18)O-SO(4) in throughfall was taken up by the canopy. Overall, these results showed that, contrary to what is commonly considered, S interchanges in the canopy could be important in boreal forests with low absolute atmospheric S depositions.