Abstract

Mechanisms controlling transient responses to elevated CO2 concentration and climate change in an unfertilized grassland on the Swiss Plateau were examined in light of simulations with PROGRASS, a process-based model of grass-clover interactions. Daily weather for a series of tran- sient climate scenarios spanning the 21st century were developed for the study site with the help of the LARS-WG weather generator. Changes in the length of dry and wet spells, temperature, precip- itation and solar radiation defining the scenarios were obtained from regional climate simulations carried out in the framework of the PRUDENCE project. Compared to 1961-1990, the latter indicated that for 2071-2100 there would be a noticeable increase in temperature (roughly 3°C in winter and 5°C in summer), a significant drop in summer precipitation (of the order of -30%) and a nearly 2-fold increase in the length of dry spells. Assuming that clover is less drought-tolerant than grass, we examined whether clover abundance would decrease as a consequence of increasing heat and water stress, or be promoted on account of CO2 stimulation. Results indicated that: (1) at our site, climate change alone did not curtail biological N2 fixation to the point as to alter the composition of the sward but nevertheless entailed a decrease in grassland productivity; (2) increasing CO2 clearly promoted clover growth (via feedbacks on nitrogen acquisition) and grassland productivity; and (3) even with CO2 stimulation, the additional N input from symbiotic N2 fixation was not sufficient to substantially improve the mineral N status of the system and promote grass competitiveness.

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