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.
Highlights
One of the largest global expanses of temperate rainforest is located along the Pacific coast of northwestern North America (DellaSala et al, 2011), an area renowned for towering, conifer-dominated forests of extraordinary productivity and longevity (Waring and Franklin, 1979; Smithwick et al, 2002; Keith et al, 2009). Carpenter et al (2014) highlighted the high diversity of soil types across this region and some key attributes related to soil carbon (C), nitrogen (N) and phosphorus (P) status of these forests
We found a considerable range in nutrient concentrations (e.g., 0.15–0.60 % N; Table 1) and strong, positive correlations among C, N and Po for mineral soils (Pearson r>0.7) across these temperate rainforest sites (Table 2, Fig. 1)
C : N of forest floors widened with increasing % C (Fig. 2b, Table 2) but C : N of both substrates were well aligned across sites (r = 0.85, p< 0.001; Fig. 3a)
Summary
One of the largest global expanses of temperate rainforest (approximately 25 million ha) is located along the Pacific coast of northwestern North America (DellaSala et al, 2011), an area renowned for towering, conifer-dominated forests of extraordinary productivity and longevity (Waring and Franklin, 1979; Smithwick et al, 2002; Keith et al, 2009). Carpenter et al (2014) highlighted the high diversity of soil types across this region and some key attributes related to soil carbon (C), nitrogen (N) and phosphorus (P) status of these forests. One of the largest global expanses of temperate rainforest (approximately 25 million ha) is located along the Pacific coast of northwestern North America (DellaSala et al, 2011), an area renowned for towering, conifer-dominated forests of extraordinary productivity and longevity (Waring and Franklin, 1979; Smithwick et al, 2002; Keith et al, 2009). Carpenter et al (2014) highlighted the high diversity of soil types across this region and some key attributes related to soil carbon (C), nitrogen (N) and phosphorus (P) status of these forests. Temperate rainforests often have substantial accumulations of organic matter at the soil surface (forest floors) and in mineral horizons that rank among the highest in global soil C stocks (Sun et al, 2004; Homann et al, 2005; McNicol et al, 2019). Kranabetter et al.: Contrasting conifer species productivity in relation to soil carbon
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