Mitigating post-fire regeneration failure in boreal landscapes with reforestation and variable retention harvesting: At what cost?

Successive disturbances such as fire can affect post-disturbance regeneration density, with documented adverse effects on subsequent stand productivity. We conducted a simulation study to assess the potential of reactive (reforestation) and proactive (variable retention harvesting) post-fire regeneration failure mitigation strategies in a 1.37-Mha fire-prone boreal landscape dominated by black spruce and jack pine. We quantified their respective capacity to maintain landscape productivity and post-fire resilience, as well as their associated financial returns under current and projected (RCP 8.5) fire regimes. While post-fire reforestation with jack pine revealed to be the most effective strategy to maintain potential production, associated costs quickly became prohibitive when applied over extensive areas. Proactive strategies such as an extensive use of variable retention harvesting, combined with replanting of fire-adapted jack pine only in easily accessible areas, appeared as a more promising approach. Despite this, our results suggest an inevitable erosion of forest productivity due to post-fire regeneration failure events, highlighting the importance to integrate fire a priori in strategic forest management planning as well as its effects on long-term regeneration dynamics.

The Impact of Variable Retention Harvesting on Growth and Carbon Sequestration of a Red Pine (Pinus resinosa Ait.) Plantation Forest in Southern Ontario, Canada

As atmospheric carbon dioxide concentrations continue to rise and global temperatures increase, there is growing concern about the sustainability, health, and carbon sequestration potential of forest ecosystems. Variable retention harvesting (VRH) has been suggested to be a potential method to increase forest biodiversity, growth, and carbon (C) sequestration. A field trial was established in an 88-year-old red pine (Pinus resinosa Ait.) plantation in southern Ontario, Canada, using a completely randomized design to examine the response of tree productivity and other forest values to five harvesting treatments: 33% aggregate retention (33A), 55% aggregate retention (55A), 33% dispersed retention (33D), and 55% dispersed retention (55D) in comparison to an unharvested control (CN). In this study, we explored the impacts of VRH on aboveground stem radial growth and annual C increment. Standard dendrochronological methods and allometric equations were used to quantify tree- and stand-level treatment effects during a five-year pre-harvest (2009–2013) and post-harvest (2014–2018) period. Tree-level growth and C increment were increased by the dispersed retention pattern regardless of retention level. At the stand level, the total C increment was highest at greater retention levels and did not vary with retention pattern. These results suggest that the choice of retention level and pattern can have a large influence on management objectives as they relate to timber production, climate change adaptation, and/or climate change mitigation.

Water Dynamics in the Understory of a Pine Plantation Forest After Variable Retention Harvesting

Background: Variable Retention Harvesting (VRH) is a silvicultural technique applied to enhance forest growth, and restore forest stands to closely resemble their natural compositions. This study used sapflow and understory eddy covariance flux measurements to examine the impacts of four different VRH treatments on the dominant components of evapotranspiration including canopy transpiration and water flux from understory vegetation and soil. These VRH treatments were applied to an 83-year-old red pine (Pinus resinosa) plantation forest in the Great Lakes region in Canada and included 55% aggregated crown retention (55A), 55% dispersed crown retention (55D), 33% aggregated crown retention (33A), 33% dispersed crown retention (33D) and unharvested control (CN) plot. Results: Study results showed a positive relationship between thinning intensity and the growth of understory vegetation, and hence enhanced evapotranspiration. The contribution to evapotranspiration from understory vegetation and soil was more pronounced in the dispersed thinning treatments, as compared to the aggregated. Overall, canopy transpiration contributed to 83% of total evapotranspiration in the un-thinned control plot and 55, 58, 30, and 23% for the 55A, 55D, 33A and 33D plots, respectively. The thinning or retention harvesting enhanced the water use efficiency in all treatments.Conclusion: Our results suggest VRH treatments that follow a dispersed harvesting pattern may provide the optimal balance between forest productivity and evapotranspiration or stand water use. Furthermore, a balance of contributions from both the canopy and successional understory vegetation and soil, as observed in the 55% retention harvesting treatment, may increase the resiliency of forest to climate change. These findings will help researchers, forest managers and decision-makers to improve their understanding of thinning impacts on water and carbon exchanges in forest ecosystems and adopt appropriate forest management practices to enhance their carbon sequestration capabilities, water use efficiency and resilience to climate change.

Dispersed Variable-Retention Harvesting Mitigates N Losses on Harvested Sites in Conjunction With Changes in Soil Microbial Community Structure

As an alternative to clear-cutting, variable-retention harvesting is now standard forest management practice on the coast of British Columbia and in temperate forests globally, due to the benefits associated with maintaining mature forest species and forest structural diversity. Although there is some evidence that variable-retention harvesting, particularly single-tree (dispersed) retention will mitigate the impacts of clear-cutting on soil microbial communities and nutrient cycling, findings have been inconsistent. We examined microbial community structure (phospholipid-fatty acid), and nutrient availability (PRSTM probes) in a large (aggregated) retention patch and over three harvesting treatments: dispersed retention, clear-cut and clear-cut edge 2 years after harvest. Unlike previous studies, we did not observe elevated nitrate in the harvested areas, instead ammonium was elevated. Availability of N and other nutrients were surprisingly similar between the dispersed-retention treatment and the retention patch. The microbial community, however, was different in the clear-cut and dispersed-retention treatments, mostly due to significantly lower abundance of fungi combined with an increase in bacteria, specifically Gram-negative bacteria. This was accompanied by lower δ13CPDB value of the Gram-negative PLFA's in these treatments, suggesting the decline in mycorrhizal fungal abundance may have allowed the dominant Gram-negative bacteria to access more of the recently photosynthesized C. This shift in the microbial community composition in the dispersed-retention treatment did not appear to have a major impact on microbial functioning and nutrient availability, indicating that this harvesting practice is more effective at maintaining generic microbial functions/processes. However, as Mn levels were twice as high in the retention patch compared to the harvested treatments, indicating the other “narrow” processes (i.e., those performed by a small number of specialized microorganisms), such as lignin degradation, catalyzed by Mn peroxidase, which concomitantly removes Mn from solution, may be more sensitive to harvesting regimes. The effect of harvesting on such narrow nutrient cycling processes requires further investigation.

Variable retention harvesting: conceptual analysis according to different environmental ethics and forest valuation

Background Conceptual clarity is important to attain precise communication of scientific knowledge and to implement appropriate technological and policy actions. Many concepts referring to forest management are widely used by decision-makers, regardless of their complexity. Although the scientific and methodological issues of forestry practices are frequently discussed in the literature, their normative dimensions are rarely treated. Thus, linguistic uncertainty increases when different environmentally ethical perspectives and ways of valuing forests are considered. The objective was to compare different conceptualizations on the silvicultural systems suggested for forest management and the implications they have for conservation. We have conceptually contrasted high-intensity forestry practices with variable retention harvesting, considering different environmentally ethical perspectives and forest valuation alternatives. Results Clear boundaries between clear-cutting, selective logging, and variable retention harvesting can be evidenced when different ethical points of view and alternatives in the human-nature relationships are considered. We have found a variety of definitions of variable retention harvesting that can be analyzed under different ethical positions. Sharply contrasting views on variable retention harvesting can be evidenced if nature is considered to be purely at human’s service or if it is conceptualized as humans co-inhabiting with nature. The latter position implies that the maintenance of ecological, evolutionary, and historical processes supported by unmanaged forest stands is a crucial step for forest management proposals based on variable retention harvesting. Conclusions Forestry practices that are focused on forest yields and that misinterpret functional uncertainty of forest functioning would be risky. Moreover, forestry with variable retention harvesting could imply good yields with reasonable conservation management in some contexts, while it could be unacceptable in other socio-ecological contexts. The improvement of conceptual clarity on the different meanings of variable retention harvesting and the development of indicators for forest management based on the variations of this concept can reduce controversies.