Importance of understories in natural forests
Understory vegetation contributes greatly to overall forest structure and function. It helps facilitate energy flow, cycles nutrients, and affects canopy succession (Hong Huo, Qi Feng & Yong-hong Su, 2014). Understories often contain the largest portion of floristic diversity, although they contribute relatively little to total plant biomass. Vegetation at this level is also crucial to forest regeneration as it can have an effect on germination, survival, and growth of tree seedlings by competing for light, water, and nutrients (Hong Huo, Qi Feng & Yong-hong Su, 2014). It is also habitat for many macro- and micro-soil organisms (Bokhorts et al. 2014), as well as amphibians, reptiles, and mammals (Fontúrbel et al., 2021). Plants at this layer are extremely important to bird populations as they provide movement pathways, nesting sites, and shelter. Birds are key to forest ecosystems as they play roles as pollinators, seed dispersers, and help control herbivore insect populations (Fontúrbel et al., 2021). Furthermore, much of the vegetation is culturally significant to humankind (Rosero-Toro et al., 2018), suggesting that forests provide cultural and social functions in addition to ecological.
Understory ecology
Because the understory environment is so heterogeneous in resources like light, moisture, and nutrients, plants here have physiological, morphological, and phenological adaptations to survive and thrive in their unique environments. Currently, there is limited knowledge pertaining to the autecology of many (Boreal) understory species, however, plants at this level can be coarsely grouped, largely based on their ability to tolerate shade. For example, understory obligates (i.e., shade-tolerants) are typically species with resource conservative traits and are non-competitive and slow growing (Grime 1977; Blondeel et al. 2020). These species are also slow colonizers and usually rely on clonal reproduction as they produce small volumes of seeds that are generally not dispersed over long distances (Grime 1977). As a result, obligates are typically constrained to older forests with closed canopies which inhibit more competitive species due to resource limitation. Understory obligates tend to allocate more resources to leaves, creating specific morphological leaf traits that can be associated with efficacy of light capture in this environment (Mestre et al. 2017). In contrast, less shade-tolerant species are often fast colonizers with resource acquisitive traits, are tall in stature, and produce significant leaf area (Grime 1977; Blondeel et al. 2019). These species are more commonly found in newly disturbed or early successional forests or in forests that allow for significant light penetration to the forest floor (Blondeel et al 2019). In these higher light environments, there is a competitive advantage in having greater height growth to overtop competitors (Valladares & Niinemets, 2008; Rundel et al. 2020) often producing multiple vertical sub-canopy layers depending on size and shade tolerance. Understory vegetation can also be grouped based on phenological adaptations (i.e., how organisms adapt to seasonal and interannual variations in climate), as vegetation here matures, reproduces, and sensesces at different times throughout the growing season.
Current forest restoration practices
Ecological restoration can be divided into two broad categories: passive (i.e., natural regeneration) and active restoration. Passive restoration occurs when the source of the disturbance is removed (i.e., ending the prior anthropogenic land-use type) from the ecosystem and colonization by shrubs and trees and secondary natural succession takes over (Vaughn et al., 2010; Morrison & Lindell, 2011). Passively restored forests have the potential to regain considerable functionality and many ecosystem services (Lamb, 2018) and are attractive from a cost perspective (Holl & Aide, 2011), as little human intervention such as planting, seeding, or weeding is required. Active restoration is where human intervention occurs through planting vegetation, weeding, burning, and/or thinning to achieve a desired structure (Morrison & Lindell, 2011). Active restoration is used in various applications worldwide, especially in areas that are highly degraded and/or have no residual patches of forest nearby to supply propagation material (Lamb, 2014), such as open pit mining sites and temporary drilling pads (Dhar et al., 2018; Jones et al., 2018). Planting of nursery-grown seedlings and direct seeding are the most frequently used active restoration methods employed in forest restoration (Brancalion et al., 2016; Grossnickle & MacDonald 2018; Palma & Laurance, 2015).
Understory vegetation contributes greatly to overall forest structure and function. It helps facilitate energy flow, cycles nutrients, and affects canopy succession (Hong Huo, Qi Feng & Yong-hong Su, 2014). Understories often contain the largest portion of floristic diversity, although they contribute relatively little to total plant biomass. Vegetation at this level is also crucial to forest regeneration as it can have an effect on germination, survival, and growth of tree seedlings by competing for light, water, and nutrients (Hong Huo, Qi Feng & Yong-hong Su, 2014). It is also habitat for many macro- and micro-soil organisms (Bokhorts et al. 2014), as well as amphibians, reptiles, and mammals (Fontúrbel et al., 2021). Plants at this layer are extremely important to bird populations as they provide movement pathways, nesting sites, and shelter. Birds are key to forest ecosystems as they play roles as pollinators, seed dispersers, and help control herbivore insect populations (Fontúrbel et al., 2021). Furthermore, much of the vegetation is culturally significant to humankind (Rosero-Toro et al., 2018), suggesting that forests provide cultural and social functions in addition to ecological.
Understory ecology
Because the understory environment is so heterogeneous in resources like light, moisture, and nutrients, plants here have physiological, morphological, and phenological adaptations to survive and thrive in their unique environments. Currently, there is limited knowledge pertaining to the autecology of many (Boreal) understory species, however, plants at this level can be coarsely grouped, largely based on their ability to tolerate shade. For example, understory obligates (i.e., shade-tolerants) are typically species with resource conservative traits and are non-competitive and slow growing (Grime 1977; Blondeel et al. 2020). These species are also slow colonizers and usually rely on clonal reproduction as they produce small volumes of seeds that are generally not dispersed over long distances (Grime 1977). As a result, obligates are typically constrained to older forests with closed canopies which inhibit more competitive species due to resource limitation. Understory obligates tend to allocate more resources to leaves, creating specific morphological leaf traits that can be associated with efficacy of light capture in this environment (Mestre et al. 2017). In contrast, less shade-tolerant species are often fast colonizers with resource acquisitive traits, are tall in stature, and produce significant leaf area (Grime 1977; Blondeel et al. 2019). These species are more commonly found in newly disturbed or early successional forests or in forests that allow for significant light penetration to the forest floor (Blondeel et al 2019). In these higher light environments, there is a competitive advantage in having greater height growth to overtop competitors (Valladares & Niinemets, 2008; Rundel et al. 2020) often producing multiple vertical sub-canopy layers depending on size and shade tolerance. Understory vegetation can also be grouped based on phenological adaptations (i.e., how organisms adapt to seasonal and interannual variations in climate), as vegetation here matures, reproduces, and sensesces at different times throughout the growing season.
Current forest restoration practices
Ecological restoration can be divided into two broad categories: passive (i.e., natural regeneration) and active restoration. Passive restoration occurs when the source of the disturbance is removed (i.e., ending the prior anthropogenic land-use type) from the ecosystem and colonization by shrubs and trees and secondary natural succession takes over (Vaughn et al., 2010; Morrison & Lindell, 2011). Passively restored forests have the potential to regain considerable functionality and many ecosystem services (Lamb, 2018) and are attractive from a cost perspective (Holl & Aide, 2011), as little human intervention such as planting, seeding, or weeding is required. Active restoration is where human intervention occurs through planting vegetation, weeding, burning, and/or thinning to achieve a desired structure (Morrison & Lindell, 2011). Active restoration is used in various applications worldwide, especially in areas that are highly degraded and/or have no residual patches of forest nearby to supply propagation material (Lamb, 2014), such as open pit mining sites and temporary drilling pads (Dhar et al., 2018; Jones et al., 2018). Planting of nursery-grown seedlings and direct seeding are the most frequently used active restoration methods employed in forest restoration (Brancalion et al., 2016; Grossnickle & MacDonald 2018; Palma & Laurance, 2015).
Establishing understories in restored forests
As previously stated, understory redevelopment is often left to passive processes (MacDonald et al. 2012). There is a growing body of evidence suggesting that this is not a feasible option for highly severe disturbances such as open-pit mining and temporary drilling pads. In their 2015 paper, Errington and Pinno detail the conditions needed for passive understory restoration it to be successful. First, clonal encroachment is mostly important for small disturbances that are within short distances to intact vegetation (Rydgren et al. 1998). Second, depth of soil disturbance most likely also has an impact on the regenerating plant community, as different overall abundance and species composition represented in the seed bank varies at different soil depths (Moore and Wein 1977; Qi and Scarratt 1998). Lastly, colonization from seed/spore rain often relies on exposed soil for germination (Halpern 1988; Whittle et al. 1997; Rydgren et al. 1998; Hautala et al. 2001; Wolken et al. 2010), so this may also depend on the level of soil disturbance and distance from a seed source (Hughes and Fahey 1991). Severe disturbances, whether anthropogenic or natural (i.e., severe fire) can remove the important criteria passive understory restoration relies on to be successful (i.e., small scale, minimal soil disturbance, short distance from intact vegetation). Actively restoring the understory could be an option, though it has yet to be explored in any depth in the field of restoration research. Some efforts have been made, such as on reclamation sites in the Athabasca Oil Sands Region of Alberta, for example, where during tree planting, several shrub species are being planted simultaneously (Dhar et al., 2018; Schoonmaker et al. 2014), though there are no long-term studies evaluating outplanting success of these seedlings. Research objectives
The main objectives of this research project are to determine which environmental variables (i.e., soil physical and chemical parameters, percent cover of vegetation functional types, presence of a canopy) have an effect on the survival of four nursery-grown Boreal shrub species, and gain insight into which site conditions and temporal stages of reclamation are optimal for including shrubs in post-disturbance revegetation efforts. Expected results Due to the limited knowledge in this area of inquiry, we have employed an inductive approach where we will be studying seedling outplanting performance and environmental conditions at each site to determine if any relationships are present. Ideally, the outcomes of this project will aid reclamation practitioners in deciding when and where to plant shrub seedlings while actively restoring the understories of reclaimed forests. |