Hemiboreal Tree Dynamics: Comparison
Please note this is a comparison between Version 3 by Raimundas Petrokas and Version 2 by Camila Xu.

Climate change and the associated disturbances have disrupted the relative stability of tree species composition in hemiboreal forests. The natural ecology of forest communities, including species occurrence and composition, forest structure, and food webs, have been affected. Yet, the hemiboreal forest zone of Lithuania is the least studied in the country for climate change risks and possible management adaption techniques. Moreover, the adverse effects of the phenomenon of global climate change can undermine the resilience of forest ecology in terms of its capacity for natural regeneration to occur successfully on the scale of the expected time. Especially since forests are multi-scale, multi-species networks that constantly evolve toward the successional processes and patterns of natural regeneration which cannot be reached at an individual tree species level. Therefore, in the context of hemiboreal forests, assisted natural regeneration lays the groundwork necessary to consider the life-cycle features of trees that affect the organic relationships between individual species and ecological communities indirectly via their effects on growth, reproduction, and survival, such as tree regeneration strategies that correspond to the various trade-offs in the adaptations to competition, stress, and forest disturbances. With the concept of assisted natural regeneration, embedded in the principle of deep ecology, the natural powers of forest ecology can be revitalized and fast-tracked to keep up with the pace of global climate change or even overtake it ahead its disruptions.

  • hemiboreal trees
  • assisted natural regeneration
  • deep ecology

1. Introduction

Natural recurring forest processes are often self-organised and implicate sustainability processes in environmental changes. Niche construction, ecological engineering, and biosemiotics processes are different forms of indirect, background interaction and communication of organisms in the environment [1]. Self-organisation of an ecosystem includes all the diversity that cannot be reduced to the properties of an individual system’s components, such as molecules, genes, populations, and species in both time and space [2]. Meaning is generated across all the organisational levels [3]. The strategy of life expansion is realized through the spread of life in space—the proliferation and collaborative construction of ecosystems and the biosphere by organisms. Forests are multi-scale, multi-species networks that constantly evolve toward the successional processes and patterns of natural regeneration which cannot be reached at an individual tree species level. In this direction, this research focuses on the super-organism approach of forest communities that considers succession as a comprehensive ecological process of multiple events where the forest vegetation communities are directly related to environmental condition with regard to climate change [4].
The life history traits of species are controlled by natural patterns and processes recurring over time and space at multiple scales [5][6]. Natural selection has matched trees to site and environmental conditions for millennia [7] and is considered a key evolutionary process that can increase the adaptation rate of species to environmental change [8]. Natural selection can be confirmed through field observations of ecological communities and their development towards self-organisation. Tree species’ life histories, reproductive character, regeneration times, mode of dispersion, and other evolutionary phenomena are interconnected in the immense and complex system of self-sustaining interactions of forest communities [9]. The ecology of a forest never ceases to evolve. The probability of seed germination, tree growth, development and recruitment is dependent on a species’ genetic profiles and life history traits to cope with the changes in environmental conditions [10][11][12]. Dynamics in forest communities are driven by a wide range of factors, including species’ invariable life history strategies [4].
However, traditional forest management, climate change and increased disturbances have disrupted the relative stability of tree species composition in combination with the edaphic site conditions in European hemiboreal forests [13][14]. This is a key problem. The natural ecology of forest communities, including species occurrence and composition, forest structure, and food webs have been affected [15]. Developing knowledge about natural forest disturbance dynamics and their relationship to anthropogenic impacts and management practices is essential towards the mitigation of impacts on forest ecosystems in the light of climate change [16]. This warrants the basis for proposing the assisted natural regeneration (ANR) strategy as an alternative adaptive model for forest management in Europe.
The self-organisation of an ecological community can play a major role in providing resilience to future climate change [56,57,58] but must be accelerated with ANR strategies. In forest management, the most fundamental methods of ANR are the protection and promotion of parent tree growth and regeneration specific to site conditions and natural disturbance regimes [60]. This is in line with the philosophy of deep ecology which emphasizes diversity as the bastion of sustainability.

2. European Hemiboreal Tree Species: The Case of Lithuania

Focusing on the European hemiboreal forests of Lithuania because it is one of only two countries (Latvia and Lithuania) that falls completely within the hemiboreal forest zone in Europe [17]. The hemiboreal forest zone is the flux zone between the temperate forest zone to the south and boreal zone to the north. Unfortunately, the forests of the hemiboreal zone are often overlooked in climate impact and adaption studies, while attention is focused on the other two zones [14]. Lithuania’s hemiboreal forest site types (as well as the 13 Natura 2000 forest habitat types of European Community) can be classified into three main forest habitat types based on the concept of potential vegetation and soils [14][18][19]: (1) mixed broadleaved forests on rich sites; (2) mixed species forests on mesic sites dominated by Norway spruce; and (3) Scots pine (Pinus sylvestris) forests on poor sites. Soil moisture and fertility of Lithuania’s forests are considered the main drivers of forest disturbances and succession [17]. As such, Lithuania’s forests have been classified by the Food and Agriculture Organisation (FAO, Rome, Italy) soil classification system [20][21] based on soil typological groups (Figure 1).
Figure 1. Lithuania’s forest site type scheme. The bold codes refer to Lithuanian’s forest site types based on soil fertility and moisture, and the small non-bold letters refers to the forest vegetation type series [18][20][22]: N—normally moist, L—temporarily over moist, U—over moist, P—peatland, and f—very eutrophic soils, d—eutrophic soils, c—mesotrophic soils, b—oligotrophic soils, a—very oligotrophic soils; aeg—Aegopodiosa, c—Caricosa, cal—Calamagrostidosa, cir—Carico-iridosa, cl—Cladoniosa, cmh—Carico-mixtoherbosa, csp—Carico-sphagnosa, fil—Filipendulo-mixtoherbosa, hox—Hepatico-oxalidosa, lsp—Ledo-sphagnosa, m—Myrtillosa, mox—Myrtillo-oxalidosa, msp—Myrtillo-sphagnosa, ox—Oxalidosa, oxn—Oxalido-nemorosa, ur—Urticosa, v—Vacciniosa, vm—Vaccinio-myrtillosa.

3. Hemiboreal Tree Dynamics

3.1. Tree Regeneration Strategy

Forests are characterized by the development of contiguous communities of trees that are relatively uniform in composition, structure, age, size, class, distribution, spatial arrangement, site quality, condition, and location to distinguish them from adjacent communities created by human intervention [23][24][25]. The absence of structural legacies at multiple scales is one of the most distinguishing features of modified forests subjected to intense and frequent anthropogenic disturbances [26][27]. Species’ life history traits are interrelated with natural disturbances and associated site conditions, and these account for the interactions (patterns and processes) in species distribution [14][28]. There is also increasing evidence that the intrinsic influences of disturbance susceptibility are phylogenetically inherited, implying that species-level traits are constrained by developmental, genetic, or other correlated limitations [29]. Being the primary species of forest ecosystems, long-lived trees are pivotal in providing associated organisms with a combination of resources and habitats that range from beneficial to detrimental [30]. Therefore, the forest development and growth dynamics of tree species follow relatively fixed patterns and can be difficult to modify in the light of the interactions of both biological and physical processes. This is also the case with hemiboreal trees’ natural regeneration. There are four tree natural regeneration strategies, i.e., the establishment and growth of trees in forest gaps [31][32][33][34][35]: (i) colonization; (ii) occupation; (iii) invasion; and (iv) expansion (Table 1). These are inter-intuitive with Clark and Clark’s [36] tree species regeneration groups (A–D), Whitmore’s [37] tree species groups, having an increasing “pioneer index” (1–4), and Grime’s [38] four types of secondary ecological strategies in trees that are derived from the theoretical triangular scheme of competitor (C), stress-tolerant (S) and ruderal (R) primary plant ecological strategies—stress-tolerant ruderals (S-R), competitive stress-tolerant ruderals (C-S-R), competitive ruderals (C-R), and competitive stress-tolerators (C-S). Colonization (D, 4, S-R) implies that even-aged seedlings are being established after gap formation and grow only in gaps. This relates to stress-tolerant species that possess a ruderal strategy without advanced regeneration. Juveniles have the highest growth potential. A ruderal strategy is a characteristic of many species that never become established in ruderal habitats. Ruderal species are plants that grow only in habitats that have been completely disturbed and damaged by human activity [39]. Occupation (C, 3, C-S-R) relates to the competitive stress-tolerant ruderal strategy species occurring as gap makers. Their seeds germinate better in gaps with intermediate canopy openness than in the understorey or large gaps, saplings can survive in closed forests. Invasion (B, 2, C-R) implies that trees regenerate from saplings recruited before gap or stand formation. This type involves competitive species with a ruderal strategy of advance regeneration, allowing already established juveniles to survive in newly created gaps. Expansion (A, 1, C-S) implies that trees in the forest regenerate as advanced regeneration under shade. This usually involve competitive stress-tolerant species. Juveniles have average growth rates.
Table 1. Conceptual framework of hemiboreal tree dynamics in Lithuania [31][40][41]. Capital letters indicate the main tree species that form forest stands in gap dynamics (G), successional development (S), or multi-cohort succession (M), whereas small letters (g, s, and m) indicate secondary ones which are a valuable admixture in these stands.


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