Root production and turnover in an upland grassland subjected to artificial soil warming respond to radiation flux and nutrients, not temperature.

Root demographic processes (birth and death) were measured using minirhizotrons in the soil warming experiments at the summit of Great Dun Fell, United Kingdom (845 m). The soil warming treatment raised soil temperature at 2 cm depth by nearly 3°C. The first experiment ran for 6 months (1994), the second for 18 (1995-1996). In both experiments, heating increased death rates for roots, but birth rates were not significantly increased in the first experiment. The lack of stimulation of death rate in 1996 is probably an artefact, caused by completion of measurements in late summer of 1996, before the seasonal demography was concluded: root death continued over the winter of 1995-1996. Measurements of instantaneous death rates confirmed this: they were accelerated by warming in the second experiment. In the one complete year (1995-1996) in which measurements were taken, net root numbers by the end of the year were not affected by soil warming. The best explanatory environmental variable for root birth rate in both experiments was photosynthetically active radiation (PAR) flux, averaged over the previous 5 (first experiment) or 10 days (second experiment). In the second experiment, the relationship between birth rate and PAR flux was steeper and stronger in heated than in unheated plots. Death rate was best explained by vegetation temperature. These results provide further evidence that root production acclimates to temperature and is driven by the availability of photosynthate. The stimulation of root growth due to soil warming was almost certainly the result of changes in nutrient availability following enhanced decomposition.