Measuring woody debris in the small streams of New Zealand's pine plantations

Abstract

To assess the impact of harvesting on woody debris volumes in streams, a method was required with sufficient precision to provide meaningful evaluation and comparison of pre‐ and post‐harvest levels of woody debris. Before harvest, woody debris volumes were measured in 24 first‐ to third‐order stream sites in New Zealand's mature pine plantations (22–34 years of age). An adaptation of the Van Wagner line intersect method was used to measure the small woody debris 1–9 cm in diameter (SWD). All large woody debris ≥10 cm in diameter (LWD) was measured for diameter and length. Woody debris volumes in the stream channel ranged from 2 to 345 m3ha‐1, averaging 112 m3 ha‐1 (±34, 95% confidence interval (CI)). Woody debris surface areas averaged 2883 m2 ha‐1 (±688), range 220–6769 m2 ha‐1. Most of the woody debris volume (87%) was composed of LWD. Sixty‐seven percent of the woody debris volume was located above the stream, the remainder was lying in‐stream or on the floodplain. Woody debris volumes in streams of mature pine plantations in New Zealand were similar to woody debris volumes in streams of temperate native forests in New Zealand and North America. These sites will be remeasured after harvest to identify any changes in woody debris characteristics.