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Summary • This study investigates effects of nitrogen and phosphorus on high Arctic heath vegetation, particularly bryophytes. • Heath communities received factorial combinations of nitrogen (0, 10 and 50 kg ha−1 yr−1) and phosphorus (0 and 5 kg ha−1 yr−1) in five applications per growing season, for 8 yr. • Nitrogen decreased lichen cover but did not affect cover of any other functional type. However, just 10 kg ha−1 yr−1 increased the proportion of physiologically active bryophte shoots, and decreased their nitrate assimilation capacity. Phosphorus had greater effects, and the combination of both nutrients altered species composition. Individual bryophyte species displayed contrasting responses to fertilization, suggesting that they should not be grouped as a single functional type. • The ‘critical load’ of nitrogen for Arctic heath lies below 10 kg ha−1 yr−1. Nitrogen and phosphorus are colimiting in this sytem, so the critical load of nitrogen will be lower where phosphorus availability is greater. Responses of vegetation to any increase in net mineralisation due to soil warming will depend on the ratio in which nitrogen and phosphorus availabilities increase. The effects of nutrient enhancement are very persistent.

Mature heather (Calluna vulgaris) and bracken (Pteridium aquilinum) turfs, transplanted from the field, were subjected to factorially combined experimental treatments for three consecutive years. Summer drought had the greatest effect, decreasing photosynthesis, growth and reproductive output in both species, and opening the bracken canopy. The timing of the drought relative to plant development was critical to which species was worst affected ; bracken was worst affected by an early drought, heather by a later drought. Both species showed physiological damage during drought but, as predicted on the basis of their functional types, heather showed greater acclimation of 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 −" yr −" ). Added nitrogen caused both species to start above-ground growth earlier in the spring. For bracken this stimulation was shortlived ; added nitrogen might be preferentially allocated to the rhizome and the longer-term consequences of this are unknown. For heather, nitrogen promoted growth and flowering throughout the season. There was no positive effect on the photosynthetic physiology of either species ; changes in resource partitioning, and thus photosynthate production at the canopy level, are the most likely mechanism for the increase in heather shoot growth. Warmer temperatures increased heather shoot growth from early spring onwards but did not advance bracken crozier emergence, although frond height and the proportion of fertile fronds were subsequently increased. No significant effects of warming on the photosynthetic physiology of either species were found. Predictions of responses of heather and bracken to environmental change are complicated by the strong interactive effects of unpredictable climatic events such as drought and extreme winter temperatures. When drought was imposed, damage to heather was much greater in plants receiving increased nitrogen supply. Stimulation of growth by nitrogen resulted in a water demand that was unsustainable in drought conditions, leading to wilting, 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 occurred in plants that had been warmed, with significantly fewer fronds emerging in the next spring and thus canopy photosynthetic potential being reduced. We predict that positively managed heather has the potential to limit the bracken problem in conditions of environmental change, provided that high levels of nitrogen deposition do not coincide with increased drought frequency.

Summary Competition for water between Calluna vulgaris (heather) and Pteridium aquilinum (bracken) was studied in conditions of increased temperature, drought and increased nitrogen supply. All these factors increased the intensity of competition for water, with the combination of drought and increased nitrogen having the greatest effect on water use. 2. Both species increased water-use efficiency in response to increased nitrogen and drought. The effects of temperature were however, equivocal. Calluna had a greater water demand than Pteridium but acclimated to water stress more readily. 3. Calluna was the superior competitor for water; its water-use efficiency was reduced as a consequence of its roots depleting water from the Pteridium rooting zone. Pteridium, the poorer competitor, increased water-use efficiency to cope with reduced water availability owing to competition. 4. There was a strong relationship between carbon isotope discrimination (∆) and instantaneous water-use efficiency for both species, but discrimination provided a more sensitive measure of seasonal water-use efficiency. Reconstruction of the plant's history of water-use efficiency by retrospective measurement of ∆ proved a useful technique for Calluna leaves but was inappropriate for Pteridium rhizome.