Disturbance studies on open Juncus trifidus heath and other Cairngorms vegetation
The studies were set against a background of concern over the effects of high levels of recreation in the Gairn Gorm area on vegetation and fauna. The aims were to compare the effects of disturbance by trampling on different mountain vegetation types, to examine these effects in detail on one locally widespread vegetation type, Open Juncus trifidus heath, and to make detailed studies of the structure and dynamics of this vegetation type. Comparison of the effects of experimental trampling on six vegetation types showed Alpine Nardus grassland to be the least damaged, followed by Open Juncus trifidus heath. Dwarf-shrub heaths were more heavily damaged. Open Juncus trifidus heath also showed moderate recovery. Vegetation responses were largely determined by the responses of the most abundant species. Such responses supported groupings of species as showing high, moderate or low susceptibility to trampling. Density (4- 4-1 m-** excluding seedlings) and size-class structure were determined for a J.trifidus population at Cairn Lochan, and J.trifidus tussocks were found to be randomly dispersed. Numbers of seedlings seemed to vary from year to year and their spatial distribution was related to the type of surface. Most seedlings occurred on bare soil and on surfaces of fine and medium gravel. A reduction in seedling density from in June, 1980 to 1 . 5m22 in June, 1981 was recorded. The quantitative morphology of J.trifidus tussocks was documented. Tussock size was related to age, based on node counts and the annual production of tillers. The annual cycle of growth and the changes in tussock morphology with age were described. The annual rate of radial growth of tussocks was indicated to be 2 - 3mm per year. An age-state classification for J.trifidus was described. The effects of different surfaces on the germination and establishment of J.trifidus were examined experimentally using a range of gravel sizes and depths at four sites. The benefits of a gravel covering at high altitude were clear. Differences between treatments in numbers of seeds germinating, germination rate, survival, growth and development were sometimes difficult to interpret, but clear altitudinal trends existed. In general, the results supported observations on seedling occurrence in terms of differential effects of surfaces on germination and establishment of J.trifidus? Six Open Juncus trifidus heath sites were used to study the effects of disturbance level on the vegetation and the structure of the J.trifidus population. Significantly lower total plant and J.trifidus cover were only found where the level of disturbance was high. However, moderate disturbance was associated with an increase in smaller J.trifidus individuals, and the number of these and other sized individuals decreased with increasing disturbance. These results were supported by the findings of two studies of footpath disturbance. Wind tunnel experiments showed that surfaces modify microenvironmental conditions with consequent effects on the germination and early growth of J.trifidus. Changes in rates of water loss were important, and a thin layer of gravel benefited J.trifidus establishment. In general, disturbance was thought to influence the recruitment of individuals into the J.trifidus population and thereby modify population structure. The overall dynamics of Open Juncus trifidus heath were discussed and suggestions made for future work.