Effects of increased temperature, drought and nitrogen supply on two upland perennials of contrasting functional type: Calluna vulgaris and Pteridiumaquilinum

Mature heather (Calluna vulgaris) and bracken (Pteridium aquilinum) turfs, transplanted from the field, were subjected to factorially combined experim ental treatments for three consecutive years. Summer drought had the greate st effect, decreasing photosynthesis, growth and reproductive output in bot h species, and opening the bracken canopy. The timing of the drought relati ve to plant development was critical to which species was worst affected; b racken was worst affected by an early drought, heather by a later drought. Both species showed physiological damage during drought but, as predicted o n the basis of their functional types, heather showed greater acclimation o f water-use efficiency to drought stress. Contrary to expectations based on functional types, heather responded more rapidly than bracken to increased nitrogen supply (50 kg N ha(-1) yr(-1)). Added nitrogen caused both specie s to start above-ground growth earlier in the spring. For bracken this stim ulation was short-lived; added nitrogen might be preferentially allocated t o the rhizome and the longer-term consequences of this are unknown. For hea ther, nitrogen promoted growth and flowering throughout the season. There w as no positive effect on the photosynthetic physiology of either species; c hanges in resource partitioning, and thus photosynthate production at the c anopy level, are the most likely mechanism for the increase in heather shoo t growth. Warmer temperatures increased heather shoot growth from early spr ing onwards but did not advance bracken crozier emergence, although frond h eight and the proportion of fertile fronds were subsequently increased. No significant effects of warming on the photosynthetic physiology of either s pecies were found. Predictions of responses of heather and bracken to envir onmental change are complicated by the strong interactive effects of unpred ictable climatic events such as drought and extreme winter temperatures. Wh en drought was imposed, damage to heather was much greater in plants receiv ing increased nitrogen supply. Stimulation of growth by nitrogen resulted i n a water demand that was unsustainable in drought conditions, leading to w ilting, reduced shoot growth and some acclimation of water-use efficiency. Additionally, a very cold winter spell proved most damaging to heather that had been droughted in the previous summer. For bracken, winter damage occu rred in plants that had been warmed, with significantly fewer fronds emergi ng in the next spring and thus canopy photosynthetic potential being reduce d. We predict that positively managed heather has the potential to limit th e bracken problem in conditions of environmental change, provided that high levels of nitrogen deposition do not coincide with increased drought frequ ency.