Ecology of mixedwood forest and successional pathways

In natural forest landscapes, stand diversity occurs mainly as a result of three components: abiotic conditions (physical, environment, and climate), succession (dispersion, pre-disturbance composition), and disturbance regimes (fire, insect outbreak, harvesting, etc.) (Gauthier, Leduc, and Bergeron 1996). In Quebec’s territory, the diversity and distribution of ecosystems are classified through a hierarchical ecological classification (ecological districts) as a function of relief, geology, and geomorphology. This classification helps to identify the susceptibility of stands to natural disturbance occurrences as a function of soil characteristics and vegetal composition. Related to forest disturbances, in Abitibi the mixedwood forest landscape may be characterized by a mixed influence of fire and forest management (Reyes et al. 2010, Wang and Cumming 2010). Stand and landscape composition in the mixedwood boreal forests are particular to each ecological region as an effect of abiotic condition.

For instance, in the northern border of the mixedwood boreal forests, the spruce-feathermoss bioclimatic domain composed by the ecological regions 6a and 6c have a landscape dominated by black spruce stands and occasional balsam fir; while in the center of the mixedwood boreal forests the balsam fir-white birch bioclimatic domain, composed of the ecological regions 5a and 5b, have a landscape dominated by hardwood or mixed stands with intolerant hardwood as trembling aspen, white birch and jack pine. In the south of the mixedwood boreal forest the balsam fir-yellow birch bioclimatic domain composed of the ecological regions 4a and 4c, have a landscape dominated by mixed stands of yellow birch (Betula alleghaniensis Britt.) and softwoods (Bergeron 2000, Saucier et al. 2011, Seedre and Chen 2010). In relationship to the effect of the succession on stand and landscape composition, the distribution of species depends on the strength of the natural succession process and the degree to which disturbances affect a stand. In particular, succession in the boreal forest is driven by the interaction between factors such as the type and means of regeneration, species shade tolerance, species longevity and growth rate, and the species adaptation to thrive from disturbances (Bergeron 2000, Seedre and Chen 2010). Although the course of the succession process tends to converge towards dominance by conifers in an almost predictable manner, the succession pathways may give rise to a wide variety of stand dynamics (stand structure and composition), because the development of the succession process is a function of species life history, biotic interactions and abiotic conditions, the aforementioned regeneration process, and even insects outbreaks or another secondary natural disturbances (Bergeron et al. 2014).

Regarding the influence of disturbances regimes on species composition, disturbances affect the stand structure and forest succession dynamics, causing a variety of pathways. Disturbance typically initiates succession by creating suitable conditions for the predominance of shade-intolerant species. In the boreal forest, trembling aspen and white birch are the first species to be established (Ilisson and Chen 2009) due to their high light demand to germinate (Scheller and Mladenoff 2005). However, conifers shade-intolerants such as jack pine can be established with the early successional hardwoods when a seed source is nearby (Bergeron and Charron 1994). In mid-succession stages and given long enough time since fire, the canopy transition allows to shade mid-tolerant conifer such as black spruce and white spruce to grow from the understory and replace the hardwood shade-tolerant pioneers (Bergeron and Dubuc 1989, Brassard and Chen 2010). Finally, in the late succession stage, tree recruitment occurs mostly in small openings created by gap dynamics (MacDonald and Weingartner 1995). In this stage black spruce, white cedar (Thuya occidentalis L.) or balsam fir are the typical dominant species. The simple replacement by the understory coniferous trees of the overstory cohort dominated by deciduous trees is not the most likely scenario. The presence of mixed sub-canopy layers could lead to the development of mixed stands (Bergeron and Charron 1994). Additionally, in mixedwood forests, stands rarely endure as climax stands, because fire, harvesting or other disturbances return these forests to early or mid-successional stages before they reach mature states (Bergeron 2000). Therefore, the equilibrium of these forests can be considered only at landscape level where a relatively stable stand age distribution can be observed (Bergeron and Dubuc 1989).

Forest management on mixedwood boreal forests Harvesting over the last century has been a key factor influencing the landscape in mixedwood forests in eastern Canada. Since the 1960s, modern forest industry began with the mechanization of forest operations (Vincent 1995). During this period, clearcutting was the most commonly used treatment. It contributed to change the stand age distribution producing a large-scale shift from the initial predominance of coniferous to a mixed and deciduous forests (Fenton et al. 2009), and also to increase the landscape heterogeneity (Boucher et al. 2015b, Brassard et al. 2008, Molina, Valeria, and De Grandpre 2018, Pickell, Andison, and Coops 2013). Generally, the practice of large-scale harvesting like clearcutting has a significant impact that favors the increase of deciduous species rather than conifers, since in addition to extracting conifer species (balsam fir, spruce, pine, and larch), it eliminates seed-bearing trees and destroy potential and pre-stablished regeneration (Boucher et al. 2009). Because of the negative effects of traditional harvesting practices on forest composition and structure, and also on the landscape configuration, in recent decades an Forest Ecosystem Management (FEM) have been proposed as the new forest management approach (Simard et al. 2009). FEM try to emulate natural disturbances and then, maintain biodiversity, resilience, and adaptive capacity of forest ecosystems by reducing the gap between managed and natural landscapes to ensure long-term maintenance of ecosystem functions and thereby retain the social and economic benefits they provide to society (Gauthier et al. 2009). Currently, the most widely used FEM harvesting strategies in eastern Canada include partial cutting (it comprises commercial thinning, shelterwood, and selection cutting).

In theory, by adjusting the forest management to resemble the patterns observed in natural disturbances, most of the natural habitat structures, processes, and species of the ecosystem could be safeguarded. However, none of these new approaches has been able to resemble natural disturbances, and currently there are differences in structure and composition during the initiation stage of stand development between post-fire and post-harvesting stands (Brassard and Chen 2010). For instance, harvesting has not yet been able to reproduce the distribution of forest age classes observed after natural disturbances, because harvesting continues being undertaken systematically in mature and overmature stands, and it is occurring much faster than natural disturbances would (Bergeron, Richard, et al. 1998, Boucher et al. 2009, Cyr et al. 2009). Therefore, mature and overmature stands continue decreasing considerably in forest landscapes, and a simplification of stand structure has been observed (Brassard and Chen 2010). Furthermore, conifer cover is declining and making way for hardwood and mixedwood forests (Bouchard and Pothier 2011). In terms of spatial configuration, the mosaic formed by harvesting is more fragmented than the one created by natural disturbances (Wang and Cumming 2010). This pattern is associated with the variability in size, form, and time of the harvested areas. These patterns of fragmentation modify the amount of interior habitats and connectivity between mature stands, which in turn influence the movement and dispersion of species (Bergeron and Charron 1994, Bergeron and Dubuc 1989, Fleming and Candau 1998, NCASI 2006). The main objective of this thesis was to characterize the mid-term changes of mixedwood boreal forest landscape in north-west Quebec and improving our knowledge regarding the relationships between such changes and the expected variations on fires as a consequence of the future climate change and forest management.