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.

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.

Substrates mediate responses of forest bryophytes to a gradient in overstory retention

Forest bryophytes are sensitive to the disturbances and environmental changes associated with forest management. We asked whether the substrates on which bryophytes grow mediate responses to exposure following canopy removal. We measured bryophyte cover and richness in 0.1 m2 quadrats on the forest floor, decayed logs, and tree bases along a gradient of dispersed overstory retention (100%, 40%, and 15% of initial basal area) 7 to 8 years after harvest of mature Pseudotsuga forests. Cover, local richness, and, to a lesser degree, species evenness declined steeply across the retention gradient on decayed logs and tree bases but not on the forest floor. Liverworts were more sensitive than mosses, particularly on decayed logs and on the southwestern aspects of trees (>97% declines in cover under 15% retention). Richness and evenness at the treatment scale also declined sharply on decayed logs and on the southwestern aspects of trees but changed little or increased under 40% retention on the forest floor. Our results indicate that even moderate levels of dispersed retention cannot sustain the abundance and overall diversity of wood-associated bryophytes in these forests. During regeneration harvests, conservation of these species may require retention of intact forest aggregates in which substrate quality and microclimatic stability can be maintained.

Twelve-year responses of planted and naturally regenerating conifers to variable-retention harvest in the Pacific Northwest, USA

We studied patterns of conifer regeneration over 12 years as part of a regional-scale experiment in variable-retention harvest in the Pacific Northwest, the DEMO Study. We compared survival and height growth of planted conifers and density and seral composition of natural regeneration among treatments with differing retention levels (15% versus 40%) and patterns (dispersed versus aggregated) replicated across a range of latitudes and forest zones. We also assessed plot-scale relationships of natural regeneration with overstory density and basal area, competing vegetation, and slash accumulations. Early (1- to 2-year) survival of planted seedlings was greater in dispersed treatments (Pinus monticola Douglas ex D. Don, Abies spp.) or unaffected by retention level or pattern (Pseudotsuga menziesii (Mirb.) Franco). Later (5- to 12-year) survival did not differ (all species), but growth was distinctly reduced in dispersed treatments and (or) at higher levels of retention. Density of natural regeneration was 1.5–2.5 times greater in dispersed treatments than in the cleared areas of aggregated treatments. Low-level dispersed retention promoted Pseudotsuga, the early-seral dominant, presumably by enhancing seed rain within a relatively high-light environment. Dispersed retention favored late-seral conifers. The ability to manipulate retention pattern and level to influence regeneration density and composition provides managers with flexibility in developing structurally complex and compositionally diverse forests.

Early responses of bats to alternative silvicultural treatments in wet eucalypt forests of Tasmania.

Bats are sensitive to forest management and different silvicultural treatments are likely to impact on species in different ways. We compared the early responses of ultrasonic bat activity to alternative silvicultural treatments in the Warra Long-term Ecological Research Site, in the tall wet eucalypt forests of Tasmania. We sampled 45 sites and recorded 2424 bat passes early in the maternity season of 2008, which provides a base-line for future comparisons. Total bat activity differed little between the silvicultural treatments, but locations within treatments did influence activity. Pre-planned contrasts revealed significantly lower activity at the centre of clear-fell and dispersed retention coupes than control coupes. However, there was no detectable difference between the centre of aggregate retention and control coupes, indicating that this silvicultural treatment appears effective at ameliorating the impacts of logging on bat activity. However, the retained aggregates themselves, both in their centre and along the edge, were seldom used by bats. Activity on the coupe edge was similar to control treatments, and greater than the centre of clear-fell coupes, but not the other treatments. Individual bat taxa responded to treatments consistent with predictions from ecomorphology. We also compared species level activity at paired bat detectors on the ground and in the sub-canopy (20-37 m) along the nearby Tahune airwalk. Activity was 4.5 times greater in the sub-canopy compared to ground detectors set in openings within the understorey. Adjusting the ground activity by this factor in the tall forests of the control coupes suggests that the logging impacts on bat activity are considerably greater than those described above. However, this conclusion makes the assumption that all bats flying in the range of ground-based detectors within logged gaps and adjacent to retained trees are recorded and require no adjustment.

Early impacts of harvesting and burning disturbances on vegetation communities in the Warra silvicultural systems trial, Tasmania, Australia

Impacts on the understorey vegetation of a range of silvicultural alternatives to clearfelling in lowland Eucalyptus obliqua wet forest were studied over a decade in the Warra silvicultural systems trial in southern Tasmania. The treatments were clearfell with understorey islands, patchfell, stripfell, dispersed retention, aggregated retention, and single-tree/small-group selection. High intensity burning, low intensity burning and no burning were variously applied as part of these treatments. Three understorey types were studied, including one wet sclerophyll community and two rainforest communities. Wherever burning occurred across the research trial, the regenerating vegetation was floristically wet sclerophyll with an incipient composition consistent with that of the pre-harvest wet sclerophyll community. Sites previously occupied by rainforest understoreys retained occasional rainforest elements, but the regeneration was overwhelmingly sclerophyll in nature. There were no consistent differences in the floristic composition of the regenerating vegetation, after burning or harvesting disturbance, that could be attributed to the silvicultural system. However, field observations and the results of a related, subsequent study suggest that, in designing silvicultural trials similar to the present one, close attention should be paid to the size of quadrats in relation to the level at which disturbance impacts are operating. The response of the vegetation at edges created by the treatments, and in the undisturbed forest beyond, supports the finding that edge effects on the vascular flora extend for less than 10 m into the undisturbed forest.