Conifer establishment and root architectural responses to forest floor heterogeneity in an old-growth subalpine forest in central Japan

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We examined niche differentiation in the root foraging strategies of four late-successional species, namely, Abies mariesii, Abies veitchii, Picea jezoensis var. hondoensis, and Tsuga diversifolia, in an old-growth subalpine forest on Mt. Ontake in central Japan. Substrate type and substrate height were assessed for all the trees (H ≥ 2 m) in a 1.0 ha plot. Root architectural characteristics were assessed based on the total length and vertical depth of taproot and horizontal root extensity for the seedlings on logs with three different degrees of decay (and soil for the two Abies species). The two Abies species established on the lowest substrates, including soil, and woody substrates with higher decay classes. These seedlings effectively extended their taproots deep into the soil or well-decayed logs, where the substrate height was low. P. jezoensis var. hondoensis mainly established on substrates, including less-decayed woody substrates, that were taller than those on which the two Abies species established. Their seedlings were expected to acquire belowground resources efficiently by means of greater horizontal root extensity on less-decayed logs, where nutrients are mostly concentrated in the thinner humus layer. Of the four species, T. diversifolia established on the tallest substrates, which covered small projection areas and were difficult for the other three species to establish on. Their seedlings appear to endure poor nutrient conditions by having small root architecture both vertically and horizontally. Thus, we observed species-specific root foraging strategies closely associated with regeneration of the four conifer species.