The effect of boreal jack pine harvest residue retention on soil environment and processes
Introduction
Approaches for managing non-commercial tree residues (“slash”) remaining after forest harvest operations have varied across regions and over time. In the Canadian boreal forest context, prior to the 2000 s the common harvesting approaches included whole tree (WT) harvesting and de-limbing at road side and stem only (SO) harvest where de-limbing was done at the stump. In the former, slash was then either piled and/or burned at the roadside, and in the latter slash was left in situ, sometimes with or without piling and/or burning. In the last 20 years there has been movement towards greater use of tree biomass beyond just the bole, resulting in whole tree (WT) harvesting with larger portions of the formerly uncommercial slash being taken for biomass uses (e.g., bioenergy production, pellets, etc.) (Abbas et al., 2011). While there is a benefit to more fully using tree biomass in terms of carbon sequestration in harvested wood products and off-sets of fossil fuel use, there are also negative impacts of leaving less slash on site (Klockow et al., 2013, Egnell et al., 2016).
Slash that remains on-site plays an important function in terms of soil nutrition for the growth of the next generation forest (Ring et al., 2016, Ranius et al., 2018, Lim et al., 2020). Leaving slash on-site after harvesting influences the physical and biogeochemical processes within the soil (see recent summary by Page-Dumroese et al., 2021). Removing the slash removes the pool of woody debris available to decompose over time that would provide carbon (C) and nutrients to the soil and exposes the soil surface to greater temperature fluctuations (Harvey et al., 1976, Covington, 1981, Sinclair, 1992). However, leaving too much slash may result in increased risk of fires due to high fuel loads, as well as difficulties in planting and establishing trees (Jurgensen et al., 1997, Page-Dumroese et al., 2010, Harrington et al., 2013). Finding the optimal amount of slash to leave on-site is challenging (Abbas et al., 2011). Many agencies recommend leaving one-third of harvest residues on site (Hendrickson et al., 1989, Mann et al., 1988, Hazlett et al., 2014, Thiffault et al., 2014), but a “one size fits all” approach may not be suitable for poor and productive sites alike (Klockow et al., 2013, Egnell et al., 2016).
Identifying approaches that optimize wood use while maintaining soil fertility and developing and validating indicators of site suitability for forest harvest residue are essential for ensuring sustainable forestry (Thiffault et al., 2015). Considerable work has focused on understanding impacts of slash removal on soil nutrient pools and tree regeneration and growth (e.g., Worrell and Hampson, 1997, Thiffault et al., 2011, Binkley et al., 2020), but there has been very little focus on understanding interacting physical, chemical and biological processes within soils. To better understand and accurately model the impacts of slash management, a better understanding of these soil processes occurring over short and long time scales is required. This is particularly of concern on nutrient poor soils that may be particularly sensitive to nutrient losses (Worrell and Hampson, 1997, Thiffault et al., 2011).
At the Island Lake Biomass Harvest Experiment (ILBHE, Kwiaton et al., 2014) the impacts of different levels of biomass removal (Stem Only [SO], Full Tree Biomass [FTbio] where effort was made to remove as much slash as possible, Stumping [ST] and Blading [BL]) have been studied on various ecosystem components (e.g., soil microbes [Smenderovac et al., 2017], vegetation, macro invertebrates [Rousseau et al., 2019], food web structure [Laigle et al., 2021]) over a period of ten years in a boreal, jack pine dominated forest on sandy soil. One previous study (Webster et al., 2016) showed that soil respiration (surface CO2 efflux) normalized to 15 °C (R15) was lower in biomass harvest treatments than in the uncut stand and a mature 80-yr-old fire-origin natural stand. Among harvest treatments, R15 was positively related to the amount of C retained, with the general pattern of FTbio plus blading <FTbio plus stumps removed <FTbio ~ SO harvest. Given the area constraints in the experimental design, no additional slash loading levels (i.e., greater than ~40 Mg ha−1 of the tree length treatment) could be tested at the plot scale. Furthermore, slash on large plots could not be evenly distributed to examine impacts at finer scale processes. To investigate physical, chemical and biological impacts on finer-scale soil processes over a wider range of slash loads (blading [forest floor removed], 0 [forest floor remains], 15, 30 and 60 Mg ha−1) at a relatively nutrient poor (site index at 50 years = 19 m), a slash manipulation study was established within the FTbio matrix of the ILBHE.
The goal of the manipulative study is to provide a better process-based understanding of the short-term impacts of a range of slash loadings on key physical, chemical and biological processes, including: soil temperature and soil moisture (soil environment), soil pore water chemistry (soil nutrient status), soil respiration (Rs, decomposition and nutrient cycling) and Net Ecosystem Exchange (NEE, understory productivity). The hypothesis is that intermediate amounts of forest residue left on a conifer-dominated, nutrient poor site will moderate soil environmental conditions, yet still provide sufficient inputs of C, nitrogen (N) and other nutrients to maintain soil processes. The expectation is that higher slash loadings, compared to lower slash loadings will result in: (1) cooler temperatures due to shading, (2) drier soil due to interception losses, (3) higher soil water concentrations of nutrients due to presence of residual biomass, (4) higher rates of soil respiration due to larger pools of decomposable substrate, although cooler and drier conditions may offset that increase, and; (5) lower NEE due to shading and reduced establishment. Having a more fulsome understanding of these interacting mechanisms will help to answer questions about the optimal levels of residue to leave after harvesting at nutrient poor sites, which will, in turn inform guidelines for slash management in the future.
Section snippets
Study area and experimental design
The ILBHE (Fig. 1) is situated on a 50 ha site near Chapleau, Ontario (N 47.7° W 83.6°). The mean annual temperature for the area is 2.0 °C, with 1444°days (>5 °C) and 92 frost free days, normally from early June to early September. The highest daily maximum temperatures are in July and the coldest are in January. Mean annual precipitation is 827 mm (545 mm in rainfall, 282 cm in snowfall) with September being the wettest month and February being the driest (Environment Canada, 2014). The soils
Physical environment
Soil temperature varied over the 4 years of the experiment with 2016 the warmest and 2019 the coldest (Table 6; Fig. 4). Among the treatments (Fig. 5A) the bladed treatment was always the warmest, except in October when it was the coldest (data not shown). Average soil temperatures (across all years and months) decreased in a linear or concave up manner with increasing slash loads (Table 6; Fig. 5A).
Soil moisture varied over the 4 years of the experiment with 2018 and 2019 drier than 2016 and
Physical environment
It was clear that the amount of slash left on the ground had a dramatic effect on the physical environment. The blading treatment was exposed to more extremes in temperature, having no insulated barrier from a forest floor or from slash. The forest floor only treatment (0 Mg ha−1 slash) showed warm and dry conditions likely due to drying and evapotranspiration in the forest floor. Intermediate levels of slash (15 and 30 Mg ha−1) moderated both temperature and moisture conditions. The highest
Application and limitations
Many slash retention guidelines recommend leaving one-third of the biomass from pre-harvest live crown trees >10 cm as residue on the ground (Thiffault et al. 2015). For the ILBHE, the average pre-harvest live crown trees >10 cm (foliar + branches, i.e., the potential residue, so does not including stem wood or stem bark) was 26 Mg ha−1 (Kwiaton et al., 2014), thus the one-third recommendation would be ~9 Mg ha−1 for this site. This is slightly less than what was left on the FTbio treatment
Conclusion
Retention of slash on site following harvesting performs many different ecosystem functions. One of the most important functions is ensuring a source of nutrients, which upon decomposition, can promote growth of regenerating forest. Over the short term (5 years), it is clear that the presence of forest floor is the dominant structural attribute that promotes ideal physical conditions (soil moisture and temperature) for decomposition (as measured by soil respiration). Although additional slash
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: ‘Kara Webster reports financial support was provided by Natural Resources Canada – Canadian Forest Service. Kara Webster reports a relationship with Natural Resources Canada – Canadian Forest Service that includes: employment’.
Acknowledgements
We gratefully acknowledge the laboratory assistance of Linda Vogel, Kristi Broad, Sharon Gibbs, Linda Buchan and Laura Hawdon. We also acknowledge Rob Fleming for his assistance with quantitative analysis. We thank the anonymous reviewers for constructive comments on the original manuscript. Financial support for this work was provided by NRCan‐CFS A-base to Webster and Hazlett.
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