Modeling tolerant hardwood sapling density and occurrence probability in the Acadian forests of New Brunswick, Canada: Results 14 years after harvesting

Natural forest regeneration after natural or anthropogenic disturbance is difficult to predict given its high variability. The process is poorly documented for commercial northern hardwood species in the Acadian forest of eastern Canada. Our objective was to identify the silvicultural, environmental, and ecological factors that best explain the variability in sapling density and occurrence of two commercial tolerant hardwood species in New Brunswick: American beech (Fagus grandifolia Ehrh.) and sugar maple (Acer saccharum Marsh.). Forty-three permanent sample plots were established in 2002 and measured before harvesting in 2004. Sapling density and occurrence were measured 14 years after harvesting. The results showed that the interactions between the species and the residual merchantable basal area and between the species and the percent of hardwoods in the original stand best explained the sapling density and occurrence variation of tolerant hardwoods. The sapling density of sugar maple increased with increasing merchantable residual basal area. However, the effect of this variable was not significant for the density of American beech saplings. The density and occurrence of tolerant hardwood saplings both increased along with the percent of hardwoods in the original stand. These results provide an improved understanding about tolerant hardwood regeneration dynamics in New Brunswick forests.

Beyond oak regeneration: modelling mesophytic sapling density drivers along topographic, edaphic, and stand-structural gradients in mature oak-dominated forests of Appalachian Ohio

Shade-tolerant mesophytic tree species tend to dominate the understories of present-day oak–hickory and mixed-hardwood forests in the eastern United States. We quantified the sapling density associations with abiotic and biotic variables for three important mesophytic species: red maple (Acer rubrum L.), sugar maple (Acer saccharum Marsh.), and American beech (Fagus grandifolia Ehrh.) in southeastern Ohio. In this study, we sampled 165 permanent plots in oak-dominated, topographically diverse, mature (>90 years old), second-growth forests following a time span of 21–25 years (1993–1995 and 2016–2018) between samples on the Athens and Marietta units of the Wayne National Forest. Our models showed that sugar maple was strongly associated with high pH soils and red maple was strongly associated with low pH soils. Additionally, red maple was associated with upper slope positions and older stands, while American beech was associated with lower slopes, northeasterly aspects, and northeast-facing upper slopes. Basal area of competing species, solum depth, and management unit were not significantly related to sapling density for our focal species. American beech sapling density doubled between the two sampling periods, while densities of both maple species declined by half. Our results will help scientists and managers by providing insight into potential future composition of currently oak-dominated forests in areas without active management intervention.

Moose Browsing Tends Spruce Plantations More Efficiently Than a Single Mechanical Release

Forest vegetation management can improve planted seedling survival and growth and is thus widely used in plantation silviculture. In some jurisdictions, mechanical release using brushsaws has replaced the traditional use of chemical herbicides for forest vegetation management purposes. However, its associated costs and the increasing difficulty of finding qualified labor represent a challenge. The browsing of competition by large herbivores may represent an alternative to mechanical release when planted seedlings are resistant to browsing. Here, we compare the efficacy of moose browsing relative to mechanical release in controlling competing vegetation and in promoting white spruce growth in plantations. In a high moose density region, we used an experimental design consisting of four pairs of moose exclosures and unfenced plots; fifty percent of both the access-restricted and unrestricted study areas received a mechanical release treatment. Moose browsing was more efficient than mechanical release in diminishing the sapling density and basal area of competing species. Mechanical release only reduced the sapling density of taller competitors (height > 201 cm), whereas browsing reduced the sapling densities of competitors across a greater size range (height > 130 cm). These effects of moose browsing on competition translated into a greater positive effect of moose browsing on the basal area of planted spruces. We attribute the higher effectiveness of moose browsing relative to mechanical release to its chronic nature. Moose browsed continuously throughout the year and for multiple years, whereas mechanical release was applied only one time between the second and fourth years after planting. Our results suggest that pairing wildlife management and silviculture decisions could be in the best interest of both the hunting and forestry industries in regions where plantations are frequent and use browse-resistant crop trees. Favouring browsers in controlling the density of competing species could increase the hunting experience and income, while providing an effective, cost-free, and socially acceptable forest vegetation management service.

The Best of Both Worlds? Integrating Sentinel-2 Images and airborne LiDAR to Characterize Forest Regeneration

Sustainable forest management relies on practices ensuring vigorous post-harvest regeneration. Data on regeneration structure and composition are often collected through intensive field surveys. Remote sensing technologies (e.g., Light Detection and Ranging (LiDAR), satellite imagery) can cover a much larger spatial extent, but their ability to estimate regeneration characteristics is often challenged by the obstruction associated with canopy foliage. Here, we determined whether the integration of LiDAR and Sentinel-2 images can increase the accuracy of sapling density estimates and whether this accuracy decreased with canopy cover in the Acadian forest of New Brunswick, Canada. Using random forest regression, we compared the accuracy of three models (LiDAR and Sentinel-2 images alone or combined) to estimate sapling density for two species groups: saplings of all species or commercial species only. The integration of both sensors did not increase the accuracy of sapling density estimates, nor did it reduce the negative influence of canopy cover for either species group compared to LiDAR, but it increased the accuracy by approximately 15% relative to Sentinel-2 images. Under very high canopy cover, the accuracy of density estimates for all species combined was significantly lower with Sentinel-2 images only. We recommend using LiDAR and high-resolution satellite images acquired in the fall to obtain more accurate estimates of sapling density.

Regeneration Dynamics Following the Formation of Understory Gaps in a Slovakian Beech Virgin Forest

The frequency and size of canopy gaps largely determine light transmission to lower canopy strata, controlling structuring processes in the understory. However, quantitative data from temperate virgin forests on the structure of regeneration in gaps and its dynamics over time are scarce. We studied the structure and height growth of tree regeneration by means of sapling density, shoot length growth and cumulative biomass in 17 understory gaps (29 to 931 m2 in size) in a Slovakian beech (Fagus sylvatica L.) virgin forest, and compared the gaps with the regeneration under closed-canopy conditions. Spatial differences in regeneration structure and growth rate within a gap and in the gap periphery were analyzed for their dependence on the relative intensities of direct and diffuse radiation (high vs. low). We tested the hypotheses that (i) the density and cumulative biomass of saplings are higher in gaps than in closed-canopy patches, (ii) the position in a gap influences the density and height growth of saplings, and (iii) height growth of saplings increases with gap size. Sapling density and biomass were significantly higher in understory gaps than under closed canopy. Density of saplings was positively affected by comparatively high direct, but low diffuse radiation, resulting in pronounced spatial differences. In contrast, sapling shoot length growth was positively affected by higher levels of diffuse radiation and also depended on sapling size, while direct radiation intensity was not influential. Conclusively, in this forest, regeneration likely becomes suppressed after a short period by lateral canopy expansion in small gaps (<100 m2), resulting in a heterogeneous understory structure. In larger gaps (≥100 m2) saplings may be capable even at low plant densities to fill the gap, often forming a cohort-like regeneration layer. Thus, gaps of different sizes imprint on the resulting canopy structure in different ways, enhancing spatial heterogeneity.

Strategies for Modeling Regeneration Density in Relation to Distance from Adult Trees

Research Highlights: We proposed new methodologies for the spatial analysis of regeneration processes and compared with existing approaches. Background and Objectives: Identifying the spatial relationship between adult trees and new cohorts is fundamental to understanding the dynamics of regeneration and therefore helps us to optimize the stand density and natural regeneration when undertaking regeneration fellings. Most of the statistical approaches analyzing the spatial dependence between adult trees and new individuals (seedlings or saplings) require a complete census and mapping of all individuals. However, approaches considering individuals grouped into sampling points or subplots (i.e., density data) are limited. In this study, we reviewed and compared approaches (intertype point pattern analyses and a generalized additive model) to describe the spatial relationship between adult trees and density regeneration in a Pinus sylvestris L. monospecific stand in Spain. We also proposed a new approach (intertype mark variance function) to disentangle the effect of the tree-size on sapling density and the effect of the spatial pattern. Materials and Methods: To this end, we used a half-hectare plot in which all the individuals of P. sylvestris have been mapped and measured. Results: Our results indicated that sapling distribution was related to distance from the adult trees, thus displaying distance-dependence patterns, but it was not related to the size of the adult trees. The intertype mark correlation function was an useful tool to distinguish the effect of the marks (sapling density and tree size) from the effect of the spatial pattern of the classes (trees cohorts in our case). Conclusions: The largest number of saplings was found with increased distance between adult trees (>11 m), and the generalized additive model may be useful to explain spatial relationships between adult trees and regenerating cohorts when other measured biotic variables (e.g., soil stoniness, etc.) and repeated measurements are available.

High-severity fire reduces early successional boreal larch forest aboveground productivity by shifting stand density in north-eastern China

Climate warming is predicted to increase fire activity across the Eurasian boreal larch forest in the 21st century, which could have serious consequences on carbon storage. Quantifying the effects of fire disturbance on forest structure and aboveground net primary productivity (ANPP) could aid in our ability to predict future carbon storage on a regional and biome level. In this study, we examined the spatial heterogeneity of forest structure and ANPP on sites of varying fire severity and topographic position in a recently burned landscape in the Great Xing’an Mountains, China. Results indicated that after 11 years of vegetation regrowth, fire severity significantly affected forest regeneration ANPP. Spatial heterogeneities in forest regeneration ANPP were explained by both tree sapling density and understorey vegetation abundance. Although understorey vegetation productivity on average contributed 50% of total ANPP after fire, the increase in understorey productivity with fire severity could not offset the decrease in tree productivity in severely burned stands where tree sapling density was limited. Our results suggest that high-severity fire can decrease forest regeneration ANPP by altering forest structure in the early post-fire successional stage and that this shift in forest structure may influence future forest productivity trajectories over an extended period.

Demography and biomass change in monodominant and mixed old-growth forest of the Congo

Mbau forest covers much of the Congo, and shifts in its composition could have a large impact on the African tropics. The Ituri forest in east Congo is near a boundary between the monodominant mbau type and non-mbau mixed forest, and two 20-ha censuses of trees ≥ 1 cm diameter were carried out over 12 y to monitor forest change. Based on published diameter allometry, mbau forest had 535 Mg ha−1 biomass above ground and gained 1.1 Mg ha−1 y−1. Mixed forest had 399 Mg ha−1 and gained 3 Mg ha−1 y−1. The mbau tree (Gilbertiodendron dewevrei) increased its share of biomass from 4.1% to 4.4% in mixed forest; other common species also increased. Sapling density declined at both sites, likely because increased biomass meant shadier understorey, but the mbau tree increased in sapling density, suggesting it will become more important in the future. Tree mortality and growth rates were low relative to other tropical forests, especially in the mbau plots. Shifting toward G. dewevrei would represent a large gain in carbon in the mixed forest, but mbau is presently more important as a high-carbon stock: biomass lost during forest harvest could not recuperate for centuries due to slow community dynamics.