The morphology of bracken (Pteridium aquilinum (L.) Kuhn) in the North York Moors—a comparison of the mature stand and the interface with heather (Calluna vulgaris (L.) Hull) 2. The rhizome

Abstract

SummaryThe rhizome system of mature bracken (Pteridium aquilinum (L.) Kuhn) contains 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 persistence and its often rapid rates of vegetative encroachment. Within areas such as the North York Moors the spread of bracken into areas previously dominated by heather and grass is considered undesirable because of reduced land value in terms of both agriculture and ecological diversity.In this paper we describe the morphology of bracken rhizome within a mature bracken stand, and at advancing and stationary stand margins 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 comprise 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‐bearing 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 in the mature stand. More transitional 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 buds on all types of rhizome are in a dormant state. The proportion of buds which are active is, however, greater on long and transitional shoots than on short shoots. Hence, a larger proportion of buds are active close to the margin where the rhizome is composed less of short shoots than is the case further into a mature stand.The differences in the morphology of bracken in a mature stand and at the stand margins which are identified here support the idea of controlling bracken at stand margins in preference to the spraying of large areas of dense, mature bracken. Morphological differences include an increased proportion of active buds, greater frequency of fronds per unit rhizome biomass, reduced biomass reserves. Improved conditions for the re‐invasion and re‐establishment of alternative vegetation are also available at stand margins in comparison with the centre of a dense bracken stand.