Hemiboreal forest: natural disturbances and the importance of ecosystem legacies to management

Abstract

Abstract. The condition of forest ecosystems depends on the temporal and spatial pattern of management interventions and natural disturbances. Remnants of previous conditions persisting after disturbances, or ecosystem legacies, collectively comprise ecosystem memory. Ecosystem memory in turn contributes to resilience and possibilities of ecosystem reorganization following further disturbance. Understanding the role of disturbance and legacies is a prerequisite for maintaining resilience in the face of global change. Several legacy concepts discussed in the peer-reviewed literature, including disturbance, biological, soil, land-use, and silvicultural legacies, overlap in complex ways. Here, we review these established legacy concepts and propose that the new terms "material legacy" (individuals or matter, e.g., survivors, coarse woody debris, nutrients left after disturbance) and "information legacy" (adaptations to historical disturbance regimes) cut across these previous concepts and lead to a new classification of legacies. This includes six categories: material legacies with above-and belowground, and biotic and abiotic categories, and information legacies with above-and belowground categories. These six legacies are influenced by differential patterns of editing and conditioning by "legacy syndromes" that result from natural or human-manipulated disturbance regimes that can be arranged along a gradient of naturalness. This scheme is applied to a case study of hemiboreal forests in the Baltic States of Estonia, Latvia, and Lithuania, where natural disturbance, traditional clearcut silviculture, and afforestation of abandoned agricultural lands constitute the three main legacy syndromes. These legacy syndromes in turn influence forest response to management actions and constrain resilience, leading to a mosaic of natural, manipulated, and artificial (novel) ecosystems across the landscape, depending on how the legacies in each syndrome affect ecological memory.

The forest fire regime in Latvia during 1922–2014

“…Forest fires are a part of the natural disturbance regime in boreal and hemiboreal and boreal forests, shaping landscape patterns, vegetation dynamics, and biodiversity (Granström 2001;Niklasson and Drakenberg 2001;Kalnina et al 2015;Jogiste et al 2017). In addition to their ecological importance, forest fires have strong socio-economic effects, including economic costs and displacement of people from their homes (Tedim et al 2015).…”

Major disturbances test resilience at a long‐term boreal forest monitoring site

“…In conjunction with other results outlined above, this shows that the hypothesized refuges are indeed functioning as such, with a substantially preserved predisturbance vegetation community despite their obvious susceptibility to edge effects in this heterogeneously disturbed habitat. This surprising persistence can be conceived of as a form of conservative ecological memory of the previous ecosystem state enhancing the ecological resilience of the forest (Allen et al, ; Jõgiste et al, ; Johnstone et al, ). At the same time, the nonrefuges have moved in a direction which is more typical of postdisturbance community composition.…”

Incorporating Intermediate-Severity Disturbances in Oak Stand Development

“…This phase may be short-lived however as the disturbance may provide for the establishment of advanced reproduction. As an understory layer develops and canopy openings close, the stand would shift to the understory reinitiation stage, but material and information legacies would remain in the system [24,25]. In this example, the duration of the mixed stage may be influenced by a variety of factors including the sprouting potential of killed trees, seed production of residual trees, and the suitability of the seedbed for germination and stem establishment.…”

“…Thus, the range of disturbance events that may be classed as intermediate severity is considerably greater than the range of gap-scale and stand replacing events. This inherent variability in biological legacies, which represent the effects of prior disturbances [24,25] Figure 2 with uniform, aggregated, and linear patterns of disturbance). A species-specific insect outbreak may remove trees singly or in small groups beyond the background mortality rate from throughout a stand (canopy gaps formed beyond the background rate would be an intermediate-severity event), whereas a low intensity tornado may remove most all trees in a linear swath, but leave the majority of the stand undisturbed [23,26,27].…”