Contrasting conifer species productivity in relation to soil carbon, nitrogen and phosphorus stoichiometry of British Columbia perhumid rainforests

Abstract

Abstract. Temperate rainforest soils of the Pacific Northwest are often carbon (C) rich and encompass a wide range of fertility, reflecting varying nitrogen (N) and phosphorus (P) availability. Soil resource stoichiometry (C : N : P) may provide an effective measure of site nutrient status and help refine species-dependent patterns in forest productivity across edaphic gradients. We determined mineral soil and forest floor nutrient concentrations across very wet (perhumid) rainforest sites of southwestern Vancouver Island (Canada) and employed soil element ratios as covariates in a long-term planting density trial to test their utility in defining basal area growth response of four conifer species. There were strong positive correlations in mineral soil C, N, and organic P (Po) concentrations and close alignment in C : N and C : Po both among and between substrates. Stand basal area after 5 decades was best reflected by mineral soil and forest floor C : N, but in either case included a significant species–soil interaction. The conifers with ectomycorrhizal fungi had diverging growth responses displaying either competitive (Picea sitchensis) or stress-tolerant (Tsuga heterophylla, Pseudotsuga menziesii) attributes, in contrast to a more generalist response by an arbuscular mycorrhizal tree (Thuja plicata). Despite the consistent patterns in organic matter quality, we found no evidence for increased foliar P concentrations with declining element ratios (C : Po or C : Ptotal) as we did for N. The often high C : Po ratios (as much as 3000) of these soils may reflect a stronger immobilization sink for P than N, which, along with ongoing sorption of PO4-, could limit the utility of C : Po or N : Po to adequately reflect P supply. The dynamics and availability of soil P to trees, particularly as Po, deserves greater attention, as many perhumid rainforests were co-limited by N and P, or, in some stands, possibly P alone.

Post‐disturbance conifer tree‐ring δ15N reflects openness of the nitrogen cycle across temperate coastal rainforests

“…Other temperate forests of eastern North America, in contrast, often have lower C:N ratios (<20 in mineral soils), possibly due to the influence of ARB angiosperms on N cycling (Zhu, McCormack, Lankau, Egan, & Wurzburger, 2018). On a methodological note, C:N ratios of mineral and organic substrates in these coastal ecosystems are offset and not entirely parallel (Kranabetter, Sholinder, et al., 2020), which, along with distinct relationships in N mineralization (Figures 2a and 3a), made it impractical to derive a single C:N value for the upper profile (forest floor plus mineral soil). For this reason, we used gross N mineralization rates rather than C:N ratio as a covariate to test responses in tree‐ring δ 15 N.…”

“…Soil C and N concentrations for the 12 plots were measured in 2018 and reported previously (see Kranabetter, Sholinder, et al., 2020 for further details). As a relative index of N availability, we measured gross N mineralization rates of the forest floor and mineral soils over an 8‐week period using in situ buried bags (Hart, Stark, Davidson, & Firestone, 1994).…”

“…The study sites are part of the planting density trial (EP571) described by Kranabetter, Sholinder, et al. (2020). Briefly, we selected six locations along southwestern Vancouver Island (between Port Renfrew [48°33′00″N, 124°25′00″W] and Bamfield [48°50′00″N, 125°08’00″W], B.C.…”

“…The relationship between forest floor N mineralization and C:N ratio was nonlinear and consequently log transformed before testing. The soil C and N data was originally portrayed (along with phosphorus) as molar ratios (Kranabetter, Sholinder, et al., 2020) but here we present the more commonly used mass‐based C:N ratio to facilitate comparisons with related literature.…”

“…A portion of the landscape is very productive, with a strong legacy of N 2 ‐fixation by red alder Alnus rubra that contributes to high soil N capital and substantial rates of N mineralization and nitrification (Cromack Jr., Miller, Helgerson, Smith, & Anderson, 1999; Perakis et al., 2006; Prescott, Chappell, & Vesterdal, 2000). Favourable parent materials and soil moisture regimes can further contribute to the development of relatively high soil N concentrations and correspondingly low C:N ratios (Kabzems & Klinka, 1987; Kranabetter, Sholinder, & de Montigny, 2020; Littke, Harrison, Briggs, & Grider, 2011), while very infrequent disturbance intervals allows N capital and forest biomass to accumulate for centuries (Giesen, Perakis, & Cromack Jr.,, 2008; Perakis, Tepley, & Compton, 2015). Consequently, temperate rainforest stands approaching natural N‐saturation may have substantial post‐disturbance losses in through accelerated nitrification and leaching, more so than might be found in less humid, N‐poor ecosystems (Perakis, Sinkhorn, & Compton, 2011).…”