Abstract

There is a demand for more progressive restoration directives to regenerate forest ecosystems impacted by harvesting, wildfire, insect outbreaks, and mineral resource extraction. Forest restoration may take many decades and even centuries without active silvicultural intervention to grow large trees that provide suitable habitat for various wildlife species. We tested the hypotheses (H) that, compared with unmanaged (unthinned and old-growth) stands, large-scale precommercial thinning (heavy thinning to <500 stems/ha) of young lodgepole pine (Pinus contorta var. latifolia), at 20–25 years post-treatment, would enhance: (H1) the architecture of large overstory trees (e.g., diameter, height, and crown dimensions); (H2) mean (i) total abundance and species diversity of forest-floor small mammals, (ii) abundance of tree squirrels; and (H3) relative habitat use by mule deer (Odocoileus hemionus). There were three levels of thinning with mean densities of crop trees/ha: 353 (low), 712 (medium) and 1288 (high), an unthinned, and old-growth stand replicated at three areas in south-central British Columbia, Canada. Mammal abundance and habitat use were measured during the period 2013 to 2015. Mean diameter of crop trees was significantly different among stands with the low-density, medium-density, and old-growth stands having diameters larger than the high-density and unthinned stands. Mean height of crop trees was highest in the old-growth stands. Mean crown volume of crop trees was significantly different among stands with the low-density stands 2.1 to 5.8 times higher than the high-density, unthinned, and old-growth stands, and hence partial support for H1. Mean total abundance of forest-floor small mammals was significantly different among stands with the low-density and old-growth stands 1.9 to 2.4 times higher than the other three treatment stands. Mean abundances per stand of the red squirrel (Tamiasciurus hudsonicus) (range of 4.8 to 12.0) and the northern flying squirrel (Glaucomys sabrinus) (range of 3.2 to 4.3) were similar among stands. Mean relative habitat use by mule deer was similar among stands, but variable with counts of pellet-groups/ha in the thinned stands were 3.8 to 4.2 and 2.1 to 2.3 times higher than the unthinned and old-growth stands, respectively. Thus, mean total abundance of forest-floor small mammals of H2 was supported, but species diversity and abundance of tree squirrels was not. Enhanced relative habitat use by mule deer (H3) was not supported. To our knowledge, this is the first concurrent measurement of several mammal species in heavily thinned, unthinned, and old-growth forest across three replicate study areas at 20–25 years post-treatment. Although not all mammal responses were significant, there was a strong indication that restored forests via heavy thinning (<500 trees/ha) produced large overstory trees (at least for diameter and crown dimensions) in stands 33 to 42 years old. Comparable old-growth stands, albeit with crop trees of greater height and merchantable volume, ranged from 120 to 167 years of age. Restored forests with large trees capable of supporting at least these mammal species may be achieved in decades rather than centuries.

Highlights

  • Forest restoration is heralded as a crucial endeavour for future economic and environmental sustainability in temperate and boreal coniferous forests of North America and Eurasia in the 21st century [1,2]

  • Mean merchantable volume of crop trees seemed to be higher in the old-growth than other stands owing to greater height growth, but the medium-density, high-density and unthinned stands were comparable in this economic metric

  • We evaluated the responses of forest-floor small mammals, tree squirrels, and mule deer concurrently in a range of thinned, unthinned, and old-growth stands of lodgepole pine at 20–25 years post-treatment

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Summary

Introduction

Forest restoration is heralded as a crucial endeavour for future economic and environmental sustainability in temperate and boreal coniferous forests of North America and Eurasia in the 21st century [1,2]. Despite the increased production imperative, there are new conservation strategies that seek to maintain intact habitat as well as increase the size of protected areas to conserve forest biodiversity and species at risk [9,10] Concurrent with these conservation efforts is the concern for loss of old-growth forests, in some jurisdictions in the Pacific Northwest (PNW) of North America where as little as 14–30% or less remain or is of poor quality [11,12]. All of these factors suggest strongly that a timber supply crisis is imminent or, has been affecting the forest industry for some time, at least in many parts of North America and Europe [7,13]

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