THE MORPHOLOGY OF BRACKEN (PTERIDIUM-AQUILINUM (L) KUHN) IN THE NORTHYORK MOORS - A COMPARISON OF THE MATURE STAND AND THE INTERFACE WITH HEATHER (CALLUNA-VULGARIS (L) HULL) .2. THE RHIZOME

The rhizome system of mature bracken (Pteridium aquilinum (L.) Kuhn) c ontains large reserves of both biomass (mean 8.63 kg m(-2) fr. wt) and buds (mean 565 m(-2)) which are largely responsible for both its pers istence and its often rapid rates of vegetative encroachment. Within a reas such as the North York Moors the spread of bracken into areas pre viously dominated by heather and grass is considered undesirable becau se of reduced land value in terms of both agriculture and ecological d iversity. In this paper we describe the morphology of bracken rhizome within a mature bracken stand, and at advancing and stationary stand m argins where bracken-heather interfaces occur. Stationary margins, i.e . those where bracken is not encroaching into heather at a significant rate, often have morphological characteristics intermediate to those of a mature stand and an advancing margin. In the mature stand rhizome biomass is dominated by carbohydrate-storing long shoots which compri se 63% of the total fresh weight, whilst the majority of rhizome buds (89% of all active and 86% of all dormant buds) are found on frond-bea ring short shoots. At the margins of a bracken stand the proportion of rhizome which is composed of long shoots is even greater, and that of short shoots small relative to that iri the mature stand. More transi tional shoots are also found at the stand margins. Hence close to the margin a greater proportion of fronds is found on transitional rhizome than is the case in more mature parts of the stand. The majority of b uds on all types of rhizome are in a dormant state. The proportion of buds which are active is, however, greater on long and transitional sh oots than on shea shoots. Hence, a larger proportion of buds are activ e close to the margin where the rhizome is composed less of short shoo ts than is the case further into a mature stand. The differences in th e morphology of bracken in a mature stand and at the stand margins whi ch are identified here support the idea of controlling bracken at stan d margins in preference to the spraying of large areas of dense, matur e bracken. Morphological differences include an increased proportion o f active buds, greater frequency of fronds per unit rhizome biomass, r educed biomass reserves. Improved conditions for the re-invasion and r e-establishment of alternative vegetation are also available at stand margins in comparison with the centre of a dense bracken stand.