Surveys conducted a third of a century apart reveal changes to in-stream large wood, riparian vegetation and stream planform in response to management within a UK national park

Abstract

Despite their importance for the structure and biogeomorphological function of river landscapes, riparian trees and wood have long been heavily managed. In 1991, Gregory et al. (1993) undertook a walkover survey of ~60 km of streams within the Lymington River catchment, New Forest, England, including mapping the characteristics of in-stream large wood, riparian vegetation and stream planform. In 2024, the survey was repeated. Before 1991, wood and fallen trees were frequently removed from stream channels to improve drainage and support growth of non-native conifers in plantations. Arterial drains were also cut and linked to realigned, resectioned streams within planted areas. From ~1990, major stream and land management changes were progressively introduced, including a reduction in wood removal from streams, restoration of realigned streams to more natural planforms and smaller channel sizes and clearance of conifers from floodplains to permit recolonisation by native deciduous species.

Comparative analysis of the 1991 and 2024 survey data reveals >100% increase in the number of wood jams and fallen trees, with fallen trees and the most hydraulically-influential wood jam types increasing by >200%. These increases are particularly associated with sinuous streams draining deciduous woodland. Streams draining coniferous woodland and heath-scrub-lawn-mire show less fallen trees and wood jams and smaller increases between surveys, with particularly low abundances along streams disturbed by conifer clearance and stream restoration. This analysis has quantified the transformation of extended lengths of New Forest streams, revealed the likely degree to which the transformation is attributable to different management actions, and provided insights into possible future trajectories of change.

Following brief training, the survey is suitable for application by river volunteers and professionals. Repeat surveys in future decades and on other sites can quantify changes to in-stream wood, riparian vegetation and stream planform, providing invaluable insights into the consequences of management actions.