The effect of boreal jack pine harvest residue retention on soil environment and processes

https://doi.org/10.1016/j.foreco.2021.119517Get rights and content

Highlights

  • Forest residue (“slash”) left after harvest affects soil environment for regenerating forest.

  • Retention of forest floor most important component in moderating soil environmental conditions.

  • One-third residue retention is sufficient over short term.

Abstract

Forest residue left on the ground following harvesting (i.e., “slash”) plays an important role in moderating the physical and chemical environment of the soil for future forest growth. Leaving too little slash can leave the soil exposed to extreme fluctuations in soil conditions and fewer nutrients that may hinder future forest growth, while leaving too much slash makes it difficult for new trees to establish or increases fire hazard. This study investigated the impacts of blading and different loadings of slash (0, 15, 30 and 60 Mg ha−1 dry mass) on soil physical (temperature and moisture), chemical (soil solution) and biological (soil respiration and net ecosystem exchange) processes over 4 summers at a harvested jack pine stand within the Island Lake Biomass Harvest Experiment in northeastern Ontario. Soil temperature and moisture were highest in the bladed and lowest in the 60 Mg ha−1 slash loading. Soil solution chemistry was generally similar among the 0, 15 and 30 Mg ha−1 slash loadings. However, total organic carbon and potassium had higher concentrations at 60 Mg ha−1 treatment and lower concentrations in the bladed treatment, the opposite trend occurred for pH and nitrate. Over three years the concentrations of cations decreased and nitrogen species increased for bladed to 30 Mg ha−1 treatments. The 60 Mg ha−1 treatment had increases in some solutes over time suggesting there is a lag effect as needles and bark are incorporated into the soil. The soil respiration data showed that lowest rates of CO2 production occurred in the bladed treatment, but increased over time as the forest floor developed. CO2 production was highest in the 60 Mg ha−1 slash loading, with high rates of soil respiration in the first year, as fine debris from slash deposited onto the soil, however little photosynthesis occurred in these treatments. Thus retention of small to moderate amounts of slash seem to be sufficient for maintaining a suitable balance of soil conditions for a regenerating forest over the short term.

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.

References (64)

  • S.D. Roberts et al.

    Harvest residue and competing vegetation affect soil moisture, soil temperature, N availability, and Douglas-fir seedling growth

    For. Ecol. Manage.

    (2005)
  • L. Rousseau et al.

    Woody biomass removal in harvested boreal forest leads to a partial functional homogenization of soil mesofaunal communities relative to unharvested forest

    Soil Biol. Biochem.

    (2019)
  • E. Thiffault et al.

    Developing and validating indicators of site suitability for forest harvesting residue removal

    Ecol. Ind.

    (2014)
  • B.D. Titus et al.

    Soil solution concentrations on three white birch sites in Central Newfoundland following different harvesting intensities

    Biomass Bioenergy

    (1997)
  • A. Trottier-Picard et al.

    Amounts of logging residues affect planting microsites: a manipulative study across northern forest ecosystems

    For. Ecol. Manage.

    (2014)
  • K.L. Webster et al.

    Soil CO2 efflux and net ecosystem exchange following biomass harvesting: Impacts of harvest intensity, residue retention and vegetation control

    For. Ecol. Manage.

    (2016)
  • D. Zabowski et al.

    Timber harvesting residue treatment: Part 1. Responses of conifer seedlings, soils and microclimate

    For. Ecol. Manage.

    (2000)
  • N. Bélanger et al.

    The soil acid-base status of boreal black spruce stands after whole-tree and stem-only harvesting

    Can. J. For. Res.

    (2003)
  • A. Belleau et al.

    Soil nutrient dynamics after harvesting and slash treatments in boreal aspen stands

    Soil Sci. Soc. Am. J.

    (2006)
  • Binkley, D., Richter, D.D., Pouyat, R.V., Geiser, L.H., 2020. State of forest and rangeland soils research in the...
  • K. Cassman et al.

    Nitrogen mineralization as affected by soil moisture, temperature, and depth

    Soil Sci. Soc. Am. J.

    (1980)
  • X.Q. Cheng et al.

    Short-term effects of thinning on soil respiration in a pine (Pinus tabulaeformis) plantation

    Biol. Fertil. Soils

    (2014)
  • N. Clarke et al.

    Effects of forest residue harvesting on short-term changes in soil solution chemistry

    Scand. J. For. Res.

    (2018)
  • W. Covington

    Changes in forest floor organic-matter and nutrient content following clear cutting in northern hardwoods

    Ecology

    (1981)
  • Environment Canada, 2014. National Climate Data and Information Archive....
  • B.R. Frey et al.

    The influence of partial harvesting and forest floor disturbance on nutrient availability and understory vegetation in boreal mixedwoods

    Can. J. For. Res.

    (2003)
  • Harvey, A.E., Jurgensen, M.F., Larsen, M.J., 1976. Intensive fiber utilization and prescribed fire: effects on the...
  • P.W. Hazlett et al.

    Effects of biomass removals on site carbon and nutrients and jack pine growth in boreal forests

    Soil Sci. Soc. Am. J.

    (2014)
  • P.-O. Hedwall et al.

    Effects of clear-cutting and slash removal on soil water chemistry and forest-floor vegetation in a nutrient optimised Norway spruce stand

    Silva Fenn.

    (2013)
  • O.Q. Hendrickson et al.

    Nutrient cycling following whole-tree and conventional harvest in northern mixed forest

    Can. J. For. Res.

    (1989)
  • R. Hyvönen et al.

    Dynamics of soil C, N and Ca in four Swedish forests after removal of tops, branches and stumps as predicted by the Q model

    Scand. J. For. Res.

    (2012)
  • M.F. Jurgensen et al.

    Impacts of timber harvesting on soil organic matter, nitrogen, productivity and health of inland northwest forests

    For. Sci.

    (1997)
  • View full text