Coarse Woody Debris Dynamics Following Biomass Harvesting: Tracking Carbon and Nitrogen Patterns During Early Stand Development in Upland Black Spruce Ecosystems

Abstract

Coarse woody debris (CWD) in the boreal ecosystem has been hypothesized to play an important nutritional role following stand-replacing disturbances such as fire or harvest. Sites with shallow soil over bedrock, or those with coarse-textured soils, can be especially susceptible to overstory removal because low carbon and nutrient pools may limit stand productivity in subsequent rotations. On these site types, CWD can provide essential nutrition to the developing second growth stand, prior to internal cycling processes stabilizing at crown closure (15 years to 20 years after stand initiation) through slow and steady decomposition. The current study sites were established in 1994 and in 2008 (14 years following harvesting) and were approaching crown closure. The experimental harvest areas were designed to document carbon loss and nutrient fluxes after the application of four levels of biomass removal from mature black spruce forested stands in northwestern Ontario, Canada. Two soil types (fresh, loamy : dry, sandy), with stand replicates (blocks), were selected to test whether residual CWD represents a source or sink for nutrients, and if the decay pattern varied depending on soil type. Measurement/sampling of CWD was done immediately after the harvest treatments were applied, and again in year 4 and year 14. The biomass removal treatment with the greatest carbon loss and fastest CWD decay rate had the highest initial mass of CWD, indicating possible synergistic decay dynamics. Nitrogen concentration in the CWD continued to increase from the initial measurements to year 14 (from 900 ppm to 2400 ppm), but was largely a function of increasing carbon loss. When converted to N content in CWD (kg ha-1), however, nitrogen exhibited an initial upward trend (i.e., immobilization) through years 1to 4 (from 50 kg ha-1 up to 80 kg ha-1) and a subsequent release in years 5 to 14 (from 80 kg ha-1 down to 27 kg ha-1). This trend was more apparent on the dry, sandy sites where N content peaked at almost 100 kg ha-1 at year 4, but then reduced to 26 kg ha-1 by year 14. We compared the average loss of N from CWD in years 4 to 14 (5.3 kg ha-1 yr-1) to the total soil inorganic N pool (based on a fresh K 2 SO 4 extraction), and found that the N loss from CWD represented a substantive portion (80%) of the available N pool, particularly on the dry, sandy sites. After an initial peak in year 4, black spruce foliar N decreased significantly (p<.0001) through to year 10 but began to rebound by year 15. This increase, presumably, was in part the result of the documented release of N from CWD. These results suggest that CWD, although a small contributor to the total N pool, makes a substantial contribution to the relatively small available N pool, especially on dry, sandy soils. The trend of initial N immobilization and subsequent release shows CWD may also serve to buffer the initial leaching of nutrients from the site following harvesting and provide an available source of N to the regenerating stand prior to crown closure.

Black spruce growth response to varying levels of biomass harvest intensity across a range of soil types: 15-year results

“…Nutrient loss, particularly for N, P, and K, was even greater during the first 6 months (approximately 60% loss). Spruce foliar nutrient concentrations increased significantly through to year 4, responding to the "assart" flush (i.e., increased soil mineralization and decomposition of the fine slash); however, there were no significant differences reported between the SO and FT treatments (Wiebe et al 2012).…”

“…On the peatland sites, there were significant height declines (Ht 15 , p = 0.016) for both the CH and FT treatments. It is possible that without the ameliorating (compensating) effect of the fine litter (i.e., needles) decomposition and accompanying nutrient release associated with the SO treatment (Symonds et al 2013), N immobilization by the coarse logging debris (i.e., chips or solid wood), and their associated high C:N ratios (chips, 200:1; solid wood, 300:1; unpublished data), may have lowered N availability, in turn, contributing to these growth declines on the CH and FT treatments (Wiebe et al 2012). The slight improvement in height growth exhibited on the FTB treatment on the peatland sites is in all likelihood due to the elimination of the dense ericaceous shrub layer, shown to inhibit black spruce growth (Fenton et al 2005;Lavoie et al 2007;Morris et al 2009), and the exposure of richer (Om/Oh horizons) peat layers (Lafleur et al 2011).…”

Community‐level natural selection modes: A quadratic framework to link multiple functional traits with competitive ability

“…Trait data were not available for all species used in Keddy et al. experiment, and the resulting dataset was limited to 37 species characterised in terms of plant height, LDMC, LS, and SLA (see list of species in Appendix ; references for these data include Aubin, Beaudet, & Messier, ; Aubin et al., ; Aubin, Messier, & Kneeshaw, ; Aubin & Ricard, ; Masse, Prescott, Müller, & Grayston, ; Morris, ; Wiebe, Morris, Luckai, & Reid, ). The TRY database provided several trait values for most of these species.…”

Modeling persistence of coarse woody debris residuals in boreal forests as an ecological property

“…Coarse woody debris also plays a crucial role in nutrient cycles (Wiebe et al 2014), and acts as a nutrient buffer maintaining long-term soil fertility (Herrmann and Bauhus 2018). In particular, CWD can act as a nitrogen (N) sink during the decomposition process (Palviainen et al 2010), promoting initial N immobilization followed by a slow-release increasing the N efficiency of the stand (Wiebe et al 2012), and increasing total N stocks (Wiebe et al 2014). Additionally, CWD improves soil characteristics such as pH and physical properties (Moghimian et al 2020).…”