Co-variation between plant above-ground biomass and phenology in sub-alpine grasslands

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Abstract

Question

Resources quality and quantity are both important determinants of habitat use for large herbivores. We aim to understand how these two variables vary throughout the growing season in sub-alpine grasslands. How do productivity and phenology (quality) of different plant communities within a landscape vary over time? Do productivity and phenology co-vary? Which environmental constraints or growth form composition best explain differences in productivity and phenology among plant communities?

Location

Calcareous sub-alpine grasslands, the external Alps, France.

Methods

We assessed how productivity (above-ground biomass) and phenology varied with date, slope, radiation and altitude among seven plant communities. Then we explored (1) co-variation among maximum biomass, the date of maximum biomass and average community flowering date for each plant community, and (2) whether these variables were related to snow regime or growth form composition.

Results

Temporal dynamics of biomass and phenology varied markedly among plant communities. More productive communities reached their maximum biomass later. Flowering occurred after the biomass peak, except for one plant community. However, the later the biomass peak, the shorter the lag to flowering peak. The timing of flowering and date of maximum biomass were best explained by differences in snow regimes among communities. The impact of growth form composition was marginal.

Conclusions

Plant communities are under strong time constraints to grow and flower. Snowmelt regime plays a crucial role at one end of the growing season, while time left for fruiting and maturing is likely to influence the other end. Sub-alpine grassland communities are highly heterogeneous in productivity, timing of maximum productivity and flowering phenology, creating small-scale variability in the forage quality and quantity available to large herbivores.

Covariation between phenology and temporal biomass dynamics at plant community level may be important criteria of resource selection for upper trophic levels. In subalpine grasslands, flowering, followed maximum productivity but with a shorter lag the later the peak of productivity. Snow regime explained when biomass peaked but the end of the growing season imposed a constraint on flowering dates. Picture by Marc Cornillon.

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