Abstract
Variable retention harvest promotes biodiversity conservation in managed boreal forests by ensuring forest continuity and structural complexity. However, do post-harvest and post-fire patches maintain the same structural complexity? This study compares post-harvest and post-fire residual patches and proposes retention modalities that can maintain the same structural complexity as in natural forests, here considering both continuous forest stands and post-fire residual patches. In boreal black spruce forests, 41 post-fire residual patches, and 45 post-harvest retention patches of varying size and ages (exposure time to disturbed matrix) and 37 continuous forest stands were classified into six diameter structure types. Types 1 (inverted-J) and 2 (trunked-unimodal) characterized stands dominated by small trees. The abundance of small trees decreased and the abundance of large trees increased from Type 1 to Type 6. Type 6 had the most irregular structure with a wide range of diameters. This study indicates that: (1) old post-harvest residual retentions maintained the range of structural complexity found in natural stands; (2) Types 1 and 2 were generally associated with young post-fire patches and post-harvest retention clumps; (3) the structure of residual patches containing only small trees was usually younger (in terms of the age of the original forest from which residual patches were formed) than those with larger trees. To avoid the risk of simplifying the structure, retention patches should be intentionally oriented towards Types 3–6, dominated by intermediate and large trees.
Highlights
In boreal forests, landscape mosaics are as likely to have been shaped by harvest as by fire [1]
In order to examine the local environmental factors that may explain the differences among structural types, we considered four factors: Time since last fire, which corresponds to the age of the original forest from which the patch was formed; tree anchoring substrate, estimated by the thickness of the organic layer; and site area and shape
It is increasingly recognized that conservation of biodiversity requires the preservation of the structural attributes of natural forest stands in the context of forest management [11]
Summary
In boreal forests, landscape mosaics are as likely to have been shaped by harvest as by fire [1]. Fire severity is spatially heterogeneous, with partially or entirely intact tree patches in the burned matrix, here called post-fire residual patches. These residual patches are believed to preserve pre-fire continuous forest structure, including old growth structure. In managed landscapes, harvesting, including large-scale clearcuts practiced in the last half century, progressively homogenizes the forest mosaic and simultaneously reduces the proportion of old-growth forests [7,8].
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