Level and pattern of overstory retention interact to shape long‐term responses of understories to timber harvest

Abstract

In many regions of the world, variable retention has replaced clear-cutlogging as the principal method of regeneration harvest. Partial retention of the overstory is thought to ensure greater continuity of the species and ecological processes that characterize older forests. Level (amount) and spatial pattern of overstory retention are two basic elements of forest structure that can be manipulated to achieve specific ecological or silvicultural objectives. However, experiments that elucidate the relative importance of retention level and pattern (or their interaction) are rare. Here we assess long-term (> 10 yr) responses of forest understories to experimental harvests of mature coniferous forests replicated at five sites in the Pacific Northwest (PNW). Treatments contrast both the level of retention (40% vs. 15% of original basal area) and its spatial distribution (dispersed vs. aggregated in 1-ha patches). For most vascular plant groups (early seral, forest generalist, and late seral), postharvest changes in cover and richness were reduced at higher levels of retention and in dispersed relative to aggregated treatments. Although retained forest patches were stable, changes in adjacent harvested (cleared) areas were significantly greater than in dispersed treatments. Late-seral herbs were highly sensitive to level and pattern of retention, with extirpations most frequent in the cleared areas of aggregated treatments and at low levels of dispersed retention. In contrast, early-seral species were most abundant in these environments. Forest-floor bryophytes exhibited large and persistent declines regardless of treatment, suggesting that threshold levels of disturbance or stress were exceeded. Our results indicate that 15% retention (the minimum standard on federal forestlands in the PNW) is insufficient to retain the abundance or diversity of species characteristic of late-seral forests. Although 1-ha aggregates provide refugia, they are susceptible to edge effects or stochastic processes; thus, smaller aggregates are unlikely to serve this function. The ability to achieve multiple ecological or silvicultural objectives with variable retention will require the spatial partitioning of habitats to include dispersed retention and larger undisturbed aggregates along with cleared areas.

Short- and long-term benefits for forest biodiversity of retaining unlogged patches in harvested areas

“…Biotic responses to retention forestry have rarely been investigated in the long-term (e.g., Lõhmus and Lõhmus, 2010;Halpern et al, 2012). However, it is likely that habitat changes and ecological succession alter the importance of retained structure over time (Halpern et al, 2012).…”

“…However, it is likely that habitat changes and ecological succession alter the importance of retained structure over time (Halpern et al, 2012). For example, the structural contrast between harvested and retained forest, and the strength of edge influence on microclimate in the harvested area, change with regrowth of the regenerating forest )-particularly the pace at which the canopy closes.…”

Influence of Mature Overstory Trees on Adjacent 12-Year Regeneration and the Woody Understory: Aggregated Retention versus Intact Forest

“…While aggregated retention achieves some ecological goals regardless of retention patch size [42], retaining larger areas (>1 ha) of mature forest is generally recommended [10]. Given the similarities in influence on regeneration between intact forest and small aggregates observed here, managers choosing to retain larger aggregates may be able to do so without increasing the extent of reduced productivity into harvested areas.…”

“…The survivorship of mature trees also affects seed availability, seed dispersal (as species differ in their response to edge conditions [46]), and the microclimate. Thus, seedling germination and establishment are subject to forest influence and differ according to the arrangement and proximity of mature overstory [48] at short-to medium-terms after harvest [10]. Moreover, while mostly similar between aggregates and intact forest, some evidence suggests a greater depth of microclimatic influence into harvested areas associated with intact forest [43].…”

Structural Complexity and Ecosystem Functions in a Natural Mixed Forest under a Single-Tree Selection Silviculture

“…Recently, the use of partial harvesting systems has been raising attention on managing forests to harmonize timber production and other goals, because it engenders stands with complex structures and compositions [3][4][5]. Retaining a desired quantity of trees and biological legacies preserves biodiversity and the ecosystem functions of the original ecosystem, and reduces the negative impacts of logging [6][7][8].…”