The ecology of severe moorland fire on the North York Moors: chemical and physical constraints on moss establishment from spores

1. Bryophytes played a dominating role in recolonizing bare surfaces left by severe fires which destroyed the vegetation and variable amounts of peat on parts of the North York Moors in the dry summer of 1976. Ashed peat (AH) was normally first colonised by Ceratodon purpureus, followed by Polytrichum piliferum with dominance later tending to pass to P. commune. Charred surfaces and progressively granulating peat (GP) remained largely bare except for patches of Dicranella heteromalla, Pohlia nutans and Polytrichum spp. 2. On charred surfaces recreated in this study and GP bryophyte cover was < 10% after one year, compared with 22% on ash. Shelter and lime treatments resulted in nearly 100% cover on some charred peat and ash, but superficially charred peat and GP still had < 50% cover. 3. Glasshouse trials confirmed that charred surfaces and GP had some residual chemical deficiency or toxicity even when physical conditions were improved. 4. In hanging-drop cultures, Pohlia nutans germinated well on extracts of all surface-types and Funaria hygrometrica germinated well on extracts from AH and GP; Polytrichum commune gave about 10% germination on AH, and D. heteromalla about 10% germination on AH and GP. Germination of C. purpureus on all the burnt surfaces, and of F. hygrometrica on the charred surfaces appeared to be nutrient or pH-limited. Germination of F. hygrometrica, D. heteromalla and the Polytrichum species was apparently inhibited to varying degrees by water-soluble extracts from the charred surfaces. 5. Protonema of Pohlia nutans grew well on extracts from all surfaces. D. heteromalla grew well on AH and to some extent on GP; F hygrometrica showed some growth on extracts from AH and GP. Growth of C. purpureus and D. heteromalla appeared to be nutrient- or pH-limited on some or all of the burnt surfaces. Extracts from the burnt surfaces reduced growth of F. hygrometrica and almost or completely suppressed growth of Campylopus pyriformis and the Polytrichum species. 6. The pH appeared to be an important factor affecting germination and growth for most species; there was a significant positive correlation between germination and pH of the culture solution (range 3.5-6.4) for all species but Ceratodon purpureus and Pohlia nutans. 7. In glasshouse trials, toxicity from lichen/alga crusts on intact peat surfaces reduced germination of D. heteromalla by c. 50%: the similar effect on germination of Polytrichum piliferum was not statistically significant. 8. Wind-tunnel experiments showed that the surface of charred peats and GP dried quicker than AH under high-radiation conditions. Except for Campylopus pyriformis, which survived to 390 MPa, the protonema of all the mosses was killed by 24h exposure at -23 MPa: protonema of Polytrichum commune was the most sensitive to slight desiccation. 9. Ash is chemically and physically favourable for moss establishment. Freshly charred surfaces are chemically unfavourable. Establishment on these surfaces can only occur if there is an adequate interval between amelioration of chemical conditions and the onset of physical instability and granulation of the peat surface.