Biodiversity conservation
In the absence of human intervention, Fennoscandian boreal forests would be shaped by a mixture of large-scale disturbance-succession processes (following e.g. fires) and processes operating at a smaller grain (e.g. gap dynamics; Angelstam and Kuuluvainen
2004). As a result, highly heterogeneous forest environments historically characterized the forest landscapes, which included the regular occurrence of old trees and the presence of areas where forest cover was maintained more or less continuously over time (Angelstam and Kuuluvainen
2004). Systematically shortening rotations throughout the landscape is inconsistent with aims to better emulate natural disturbance regimes, as it curtails forest stand development at an age younger than would be found in substantial parts of naturally dynamic forest landscapes. In contrast, extending rotations on some proportion of the land base can be expected to alleviate some of the differences between managed and naturally dynamic forest landscapes (Burton et al.
1999; Lindenmayer and Franklin
2002).
Old forest supports microhabitats that are rare or absent in younger forest (e.g. Lassauce et al.
2013) and critical to large numbers of red-listed forest species (Bernes
2011). Important microhabitats on old or large trees include cavities, thick creviced bark, exposed wood, fungal fruiting bodies and mycelium, large branches and dead roots (Siitonen
2012). In older stands, dead wood accumulates more quickly as a result of increased tree mortality and the diameter of the dead wood is larger (Jonsson et al.
2006). Therefore, landscape-scale habitat availability for species specialized on old-tree microhabitats and larger-diameter dead wood may increase if rotations are extended (Dettki and Esseen
2003; Jonsson et al.
2006). However, some structures (e.g. very large tree hollows created by wood decay) develop at such a late age that they do not occur in trees grown for timber production (Lindenmayer and Franklin
2002). Shorter rotations imply more frequent clearfelling and soil scarification events, which typically result in the destruction of a large proportion of the dead wood pool (Hautala et al.
2004). In addition, shorter rotations allow less time for the gradual development of an understory of shade-tolerant trees (e.g. Hansen et al.
1995), with possible negative impacts on species dependent on a vertically complex vegetation structure. However, potential changes to thinning regimes associated with rotation-length modifications also need to be considered. Indeed, thinning may influence habitat quality via reduced tree density and mortality (and hence reduced dead wood accumulation), increased growth of residual trees, reduced vertical heterogeneity of the main tree layer, changes to understory structure, and altered micro-climatic conditions. Moreover, salvage logging of trees damaged by wind, insects or pathogens within stands may reduce some of the conservation benefits of extended rotations by preventing dead wood accumulation (Żmihorski
2010). Generally, extending rotations without additional thinning or salvage logging seems to be the most beneficial rotation scenario for species dependent on old-forest attributes such as dead wood (Table
2).
Some of the negative impacts of shortened rotations on biodiversity can be partly alleviated through specific conservation measures, such as retention of green trees and dead wood at clearfelling (Lindenmayer and Franklin
2002). The absolute contribution of tree retention to the provision of large and old trees may be larger under shorter rotations, because more frequent opportunities are provided to replace blowndown retention trees with new ones (Nilsson et al. unpubl.).
The different stand-scale effects of altered rotations translate to the landscape scale. At a given point in time, the proportion of forest containing older-forest attributes will decrease if rotation lengths are systematically shortened, thereby potentially reducing the connectivity of such habitat. Moreover, in each stand subjected to shortened rotations, the suitable temporal window for old-forest associated species becomes narrower, which decreases the time available for colonization, reproduction and dispersal (Dettki and Esseen
2003). Reduced spatial connectivity and shorter temporal persistence of old-forest habitats may increase the extinction risk of old-forest specialists. In addition, shortening rotations results in increased amounts of edges between open habitat and forest, with potentially detrimental effects on edge-sensitive species (Harris
1984).
Shortened rotations may, however, improve the situation for some species by increasing the availability of open habitat (Hansen et al.
1995). Provided that sufficient amounts of dead wood are retained at harvesting, threatened beetle (Coleoptera) species dependent on sun-exposed wood could benefit from increases in the amounts of open areas and edges (Rubene et al.
2014). Whether shortened rotations would increase, retain or decrease the amount of sun-exposed dead wood is however unclear, because the availability of that substrate is the product of the expanse of open areas and the amount of dead wood, two factors which may be influenced in opposite directions by a given change in rotation length (Table
2). Acknowledging potential positive effects of shortened rotations on open-habitat species, the provision of old-forest structures and habitats is generally considered a higher conservation priority in today’s Fennoscandian landscapes (Bernes
2011). Hence, extending rotations is expected to be more widely consistent with current biodiversity conservation objectives in Fennoscandia, at least if the extension is substantial enough to allow the development of old-forest attributes.