Historical harvests reduce neighboring old‐growth basal area across a forest landscape

Abstract

While advances in remote sensing have made stand, landscape, and regional assessments of the direct impacts of disturbance on forests quite common, the edge influence of timber harvesting on the structure of neighboring unharvested forests has not been examined extensively. In this study, we examine the impact of historical timber harvests on basal area patterns of neighboring old-growth forests to assess the magnitude and scale of harvest edge influence in a forest landscape of western Oregon, USA. We used lidar data and forest plot measurements to construct 30-m resolution live tree basal area maps in lower and middle elevation mature and old-growth forests. We assessed how edge influence on total, upper canopy, and lower canopy basal area varied across this forest landscape as a function of harvest characteristics (i.e., harvest size and age) and topographic conditions in the unharvested area. Upper canopy, lower canopy, and total basal area increased with distance from harvest edge and elevation. Forests within 75 m of harvest edges (20% of unharvested forests) had 4% to 6% less live tree basal area compared with forest interiors. An interaction between distance from harvest edge and elevation indicated that elevation altered edge influence in this landscape. We observed a positive edge influence at low elevations (<800 m) and a negative edge influence at moderate to high elevations (>800 m). Surprisingly, we found no or weak effects of harvest age (13-60 yr) and harvest area (0.2-110 ha) on surrounding unharvested forest basal area, implying that edge influence was relatively insensitive to the scale of disturbance and multi-decadal recovery processes. Our study indicates that the edge influence of past clearcutting on the structure of neighboring uncut old-growth forests is widespread and persistent. These indirect and diffuse legacies of historical timber harvests complicate forest management decision-making in old-growth forest landscapes by broadening the traditional view of stand boundaries. Furthermore, the consequences of forest harvesting may reach across ownership boundaries, highlighting complex governance issues surrounding landscape management of old-growth forests.

Piecing together the fragments: elucidating edge effects on forest carbon dynamics

“…In old‐growth temperate coniferous forests in Oregon, increases and decreases in basal area with proximity to edges were observed in forests at lower elevations (<800 m) and at higher elevations (>800 m), respectively (Bell et al . ). In the broadleaf forests of Quebec, Canada, no changes in aboveground C stocks were observed within 100 m of an edge, although stem density increased (Ziter et al .…”

“…; Bell et al . ), whereas at higher elevations and along clear‐cut edges, where winds are likely to be stronger, mortality and basal area declines of large trees increase with proximity to the edge (Chen et al . ; Bell et al .…”

“…In old‐growth temperate coniferous forests, no effect of harvest age was found on basal area in edges between 13 and 60 years old created by clear‐cutting (Bell et al . ), and edge growth enhancement of Douglas‐fir and western hemlock were sustained for at least 10–15 years following edge creation (Chen et al . ).…”

“…Edge effects on aboveground C pools vary considerably across forest types, with aboveground biomass reported to increase (Bowering et al . 2006 ;Bell et al 2017 ; Reinmann and Hutyra 2017 ), decrease (Laurance et al . 1997 ;Chaplin-Kramer et al .…”

“…1997 ;Chaplin-Kramer et al . 2015 ;Bell et al 2017 ), or remain essentially unchanged (Ziter et al . 2014 ) with proximity to forest edges.…”