Tall herb sites as a guide for planning, maintenance and engineering of riparian continuous forest cover

Abstract

Continuous cover riparian forests host significant plant and animal species richness, a range of habitats, and natural processes of importance for both terrestrial and aquatic ecosystems. Riparian forest is thus a green infrastructure for biodiversity conservation. However, a long history of landscape alteration now calls for maintenance and restoration by ecological engineering. This study evaluates management guidelines advocating constant vs. variable width of riparian forest protected zones in managed landscapes. In naturally dynamic forests, stands with gap-phase dynamic along streams often provide a network of habitats with a high degree of continuity in tree canopy cover and dead wood for biodiversity conservation and delivery of ecosystem services including water purification. Based on the observation that tall herb sites indicate a potential for temporally continuous forest cover, we tested three null hypotheses. Tall herb sites (1) are equally common in the riparian zone and in the surrounding forest landscape; (2) have the same width on both sides of a stream; and (3) their widths are independent of the width of the adjacent stream. We described the ground vegetation in transects along and perpendicular to streams, and in the surrounding landscape, in six 3rd stream order catchment located in Sweden, Lithuania and the Komi Republic of Russia. The results showed that tall herb sites were 21–27 times more common along streams compared to in the rest of the landscape, the width of tall herb sites varied considerably along streams, and it was independent of the width of the adjacent stream. This study suggests that rather than fixed-width guidelines for riparian set-asides, to support cost-efficient maintenance of riparian forest, local site conditions should be used as guide for planning, maintenance and engineering of riparian ecotones. Because tall herb forest sites were historically cleared for agricultural purposes, the potential natural amount of riparian forest is severely underestimated.