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) 1. The fronds

The encroachment of bracken (Pteridium aquilinum (L.) Kuhn) into areas previously dominated by heather represents a threat to the ecology, agricultural economy and landscape value of many UK upland areas, including the moorland of the North York Moors National Park. The morphology of bracken, within a mature stand and at several bracken-heather interfaces, has been studied at a number of sites within the National Park. Differences have been found in the frond growth of bracken in a mature stand, at stationary stand margins, and at advancing stand margins where bracken is encroaching into heather. Frequency of fronds present on bracken rhizome growing at a stationary stand margin close to the interface with heather (1–2 m behind the boundary) are approximately the same as those found within a mature stand. At advancing margins (again 1–2 m behind the boundary), maximum frond densities were often found to exceed those present in either a mature stand or at a stationary margin. Frond numbers decline rapidly at the stand margins as distance from the stand increases. This is especially true where the front is stationary and bracken is not encroaching into heather at a significant rate. Maximum frond heights in a mature stand consistently exceed those at stand margins (even 1–2 m into the stand) and are greater at stationary margins than at advancing margins. Outlying fronds at the edges of bracken stands are generally present in greater numbers, and further into the area dominated by heather, where the margin is advancing. Heights of outlying fronds fall as distance from the bracken stand increases, as does stipe length. Fronds at the edges of bracken stands emerge each spring before those further into the stand and are therefore particularly vulnerable to frost damage. Outlying fronds are not, however, the first to emerge. Early emerging fronds reach their maximum height and eventually become senescent before later emerging fronds. Whilst most fronds emerge before the end of June a few fronds continue to emerge throughout the summer. Frond densities close to the edges of bracken stands (1–2 m into the stand) are comparable to those in a mature stand. At advancing stand margins frond densities generally exceed those in a mature stand, suggesting that a large number of potential entry points for foliage-applied herbicides are available for bracken control at the stand margins. The ratio of potential uptake points to biomass of rhizome is also greatest at the edges of the stand, and the canopy 1–2 m into the stand is usually almost completely closed. It is possible therefore, the efficacy of herbicides could be improved by the use of small scale applications, using tractors or hand-held sprayers, close to the margins of bracken stands.