Architecture and neighbourhood competition of understorey saplings in a subalpine forest in central Japan

Abstract

We investigated the effects of both overstorey shading and understorey sapling competition on the sapling architecture of three conifers, Abies mariesii, A. veitchii, and Picea jezoensis var. hondoensis, in a subalpine forest in central Japan. With increased overstorey shading, the two Abies species tended to have flatter crowns than P. jezoensis var. hondoensis. Because a flattened crown enhances the efficiency of light interception under low light availability, both Abies species appear more adapted to shading. Conversely, P. jezoensis var. hondoensis seems better adapted to canopy gaps, because its conical crown provides an advantage for faster height growth. Unlike A. veitchii, A. mariesii increased its stem volume to support the greater snow-loading resulting from its flattened crown. Therefore, A. mariesii seems better designed for enduring shading under snowy conditions. However, A. veitchii showed faster height growth and thus presents regeneration traits intermediate between those of A. mariesii and P. jezoensis var. hondoensis. Because sapling competition is intense in sapling-crowded microsites conditions under less crowded overstorey conditions, saplings need faster height increase to escape from other sapling competition. Conversely, because sapling competition is weaker and more space is available under a closed understorey, saplings need to have flatter crowns to compensate for overstorey shading. Species-specific architecture in response to such living conditions is closely associated with the regeneration characteristics of the three conifers studied.