The ecology of poor fen & willow carr on Goss Moor NNR, Cornwall
Goss Moor NNR is a unique, rare and nationally-important wetland habitat in Mid- Cornwall. The majority of the habitats were created as a result of a long history of tin-stream mining, which ceased in the early 1900s. Phytosociological surveys of poor-fen and willow carr communities provide the first formal descriptions of the vegetation at this site. The poor-fen survey revealed twelve poor-fen vegetation types, which were distributed along a primary environmental gradient of organic matter depth, surface water height and bare substrate. Separation of the poor-fen communities by a moisture gradient was considered as spatial evidence for hydroseral succession, which begins with the colonisation of open-water pools created by tin excavations. The Salix cinerea ssp. oleifolia willow carr was divided by eight understorey communities, according to age, defined by reference to five sets of aerial photographs of Goss Moor taken over the last six decades. The average number of poor-fen species per unit area in the understorey generally decreased with age. This relationship was related to the increase in canopy cover and, therefore, shade. Willow was found to invade areas with the greatest amounts of accumulated organic material and a low water table. In the oldest and driest willow, oak saplings were found, indicating the beginning of secondary woodland. An architectural analysis of willow showed that useful age descriptors were the height of the first fork, the number of live secondary shoots, tree height and dbh, all of which generally increased with age. Spatial successional patterns were characterised using the lattice-wombling technique in three large rectangles or 'tranomes'. Plant communities were associated with either abrupt or diffuse boundary types. Abrupt boundaries or ecotones were found between heath communities and densely vegetated tall-herb fen and species-poor willow carr wetland vegetation. Diffuse or ecocline transitions occurred between communities with subtle differences in their composition. Spatial relationships between swamp and poor-fen communities were taken as evidence for space-for- time successions, these patterns varied according to location and microtopography. Investigations into the water regime showed water depth was governed by substrate heterogeneity. Homogeneous microtopography was associated with deep inundations and greatest amplitude in water depth, and most closely resembled rainfall fluctuations. The most complex microtopography resulted from the most intense tin-streaming activity. Therefore the anthropogenic history of Goss Moor plays an important role in governing the contemporary water regime and vegetation distribution. Of the wetland communities, rush pasture was the driest and poor-fen the wettest. The communities of open habitats were wetter than the willow communities. The youngest willow community was drier in the summer than the other five vegetation types studied, which was indicative of the conditions necessary for willow scrub colonisation to take place. The N:P ratio revealed that nitrogen was the limiting nutrient in all of the wetland vegetation types suggesting an early stage of successional development. High water levels were thought to be responsible for the prevalence of N-limitation on Goss Moor, creating deoxygenated substrates and leading to the demise of nitrifying bacteria and thus a reduction in the rate of soil N mineralisation. Plant strategies were used to classify the species from a number of wetland communities ranging from open-water pools to willow carr, in order to apply them to Grime's triangular model. The ten communities were ordered into a logical successional sequence. However, the model needs to be modified to account for succession in the aquatic environment. Based on the findings of this thesis, a number of suggestions were made for the effective management of the wetland habitats on Goss Moor. These include: evaluation of willow scrub before removal so those areas of vegetation subsequently opened-up can be monitored; and the creation of new ponds to encourage the growth of certain poor-fen communities, which are species-rich, but only account for a small area of the whole resource.