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.

Assessing the short-term impacts of variable retention harvesting (VRH) and climatic drivers on carbon sequestration and growth of a red pine (Pinus resinosa) plantation in southern Ontario, Canada

<p>Variable retention harvesting (VRH) is a silvicultural management practice that has been utilized to promote biodiversity, growth, and carbon sequestration in managed forests. VRH has been implemented as a climate mitigation strategy in response to increased climate warming and more erratic precipitation patterns which are occurring due to anthropogenic production of greenhouse gases. The aim of this study is to determine the impact of VRH and climate change on the inter-annual growth and carbon sequestration in a 20 ha red pine (Pinus resinosa) plantation forest located in southern Ontario, Canada over a 5-year period (2014-2018). The VRH treatment was implemented in 2014 within twenty, 1-hectare plots, which were subjected to one of four experimental thinning treatment types; 33% and 55% crown retention in an aggregated pattern (33A and 55A, respectively); and 33% and 55% crown retention in a dispersed pattern (33D and 55D, respectively), while four plots were maintained as unharvested control (CN) plots. In each plot, approximately sixty 5 mm increment cores were collected from residual trees during the spring and summer of 2019. These cores were processed for annual incremental growth according to standard dendrochronological methods to identify the climatic drivers on growth. Biomass and carbon sequestration were estimated using a species-specific allometric growth equation to quantify the impact of the different VRH treatments on red pine growth. Results of climate-growth assessments indicate that red pine growth in this region is dependent on maximum temperatures and total precipitation during the summer growing season. These relations were best captured in the May-July standard precipitation evapotranspiration index with a 3-month memory (SPEI3). May to August average maximum temperature and May to June total precipitation are also important drivers of red pine growth in all plots. We found that the dispersed crown retention patterns are the only VRH treatments that result in an increase in post-harvest growth; 8.12 ± 9.83% increase for 33D and 7.52 ± 5.71% increase for 55D. This suggests that dispersed retention may spatially optimize growth for the climatic conditions and be best suited for managing these forests under changing climatic conditions in the future. Aggregated treatments are found to have significantly less growth post-harvest; 33A had an average of 0.34 kg less biomass post-harvest (2.50 ± 1.94% decrease) and 55A had an average of 0.44 kg less biomass post-harvest (6.36 ± 3.82 % decrease). Our control sites showed that post-harvest growth showed an average of 0.56 kg less biomass post-harvest (6.01 ± 3.39% decrease). Within the aggregated treatments, exterior trees demonstrated increased growth, annual biomass accumulation, and carbon sequestration compared to trees growing in the interior of the aggregated plots. Our results suggest that dispersed crown retentions are most optimal when the goal of VRH is to increase growth or carbon sequestration. This research is ultimately important in informing future forest management practices in similar plantation forests across southern Ontario and elsewhere in the Great Lakes region and northeastern North America. </p>

Managing plantation density through initial spacing and commercial thinning: Yield results from a 60-year-old red pine spacing trial experiment

We report on a 60-year-old Pinus resinosa spacing trial experiment located in Ontario (Canada) that included the combinations between six initial spacings (from 1.2–3.0 m) and the presence/absence of a commercial thinning (CT) regime, and their impacts on quadratic mean diameter (QMD) and stand volume yield. The CT regime, initiated at age 30, targeted a residual basal area (BA) of 38 m2 ha-1 after each of four entries. Without thinning, as initial spacing increased, QMD increased; gross and net volume production peaked in the 2.1–2.4 m spacings. With thinning, similar trends with spacing were evident for QMD, although piece sizes were larger and differences between spacing were lower. The immediate increase of average tree size caused by tree selection explained most of the differences in QMD between thinned and unthinned plots. Thinning to a common target BA resulted in similar standing volume across spacings. Cumulative gross yield was similar between spacings <2.1 m for both unthinned/thinned stands and decreased for thinned plots for wider spacings. Greater net volume production in thinned stands with the narrower spacings confirmed that mortality was captured. Lower gross and net production for wider spacings suggested that thinning resulted in under-utilized growing space.

Long-term pine regeneration, shrub layer dynamics, and understory community composition responses to repeated prescribed fire in Pinus resinosa forests

Prescribed fire is increasingly viewed as a valuable tool for reversing ecological consequences of fire suppression within fire-adapted forests. While the use of burning treatments in northern temperate conifer forests has received considerable attention, the long-term (>10 year) effects on understory composition and dynamics have not been quantified. We describe the persistence of prescribed fire effects on the woody and herbaceous understory in a mature red pine (Pinus resinosa Ait.) forest in northern Minnesota, USA, over a ∼50-year period, as well as the relative roles of fire season and frequency in affecting vegetation responses. Burning treatments were applied from 1960 to 1970 on 0.4 ha experimental units and crossed fire season and frequency in a randomized block design. Burning altered shrub layer dynamics and composition in both the short and long terms and was influenced by both fire season and frequency, with frequent summer season burns having the largest impact, including greatest control of hazel (Corylus spp.). The application of fire facilitated regeneration of pine; however, recruitment into the overstory was limited. Additionally, community composition of the herbaceous understory diverged 40+ years following burning. This study highlights the importance of continued burning in affecting vegetation responses and the potential of fire as a long-lasting vegetation management tool in these forests.