The ecology of severe moorland fire on the North York Moors: seed distribution and seedling establishment of Calluna vulgaris

1. Regeneration of Calluna vulgaris following normal heath fires is from surviving stem bases or, where survival is poor, from a seed bank. Severe fires causing ignition of surface organic horizons kill stem bases and cause a serious depletion of the seed bank, most of which (up to 10(6) seeds m-2) lies in the litter and upper 2-3 cm of soil or organic material. Germinable seeds were detected in appreciable numbers down to 6 cm beneath the boundary of the F/O soil horizons in soil cores from mature heath vegetation on a blanket peat and peaty stagnopodzol. However, severe fires in which more than 10 cm of peat is ignited will destroy all of the viable seed bank. 2. At the site studied, the seed-rain density for Calluna was approximately inversely proportional to the dispersal distance, regression equations predicted an annual seed rain of only c. 25 seeds m-2 at 10 m from source plants. Small numbers of wind-dispersed flowers retaining some seeds may be important in long-distance dispersal. 3. The crustose lichens Lecidea granulosa and L. uliginosa and several species of algae form a near-ubiquitous film over the surface of exposed organic soil following severe moorland fires. This creates a crust overlying unconsolidated sub-surface peat. The lichen-alga film significantly reduced germination of Calluna seeds by about 40%. 4. No evidence was found to link poor germination or growth to low pH, low nutrient levels or soil toxicity. Fertilizer experiments show that addition of potassium increases the root:shoot ratio of Calluna seedlings while nitrogen increases shoot growth at the expense of root development. It is suggested that the seedling root:shoot ratio may be important in establishment and in surviving the effects of needle ice formation, surface instability and desiccation. 5. Transplant experiments and observations on naturally occurring seedlings show that summer desiccation of seedlings, winter browning, surface instability due to needle ice formation, and trampling by sheep are all major factors preventing seedling establishment. The relative importance of these factors varies from site to site.