Hardwood mixture enhances soil water infiltration in a conifer plantation

Soil hydraulic properties affect soil infiltration capacity. Limited information is available about how hardwood mixture influences water infiltration in conifer plantations through its effects on soil hydraulic properties. We evaluated how, and to what extent, infiltration rates were affected by abiotic and biotic conditions along gradients of abundance and diversity of hardwoods and abundance of understory plants in nine stands representing three replicates of three different thinning intensities (unthinned [Control], 33% thinning [Weak], and 67% thinning [Intensive]) in a Cryptomeria japonica (Thunb. ex L. f.) D. Don plantation. Correlation analyses and a structural equation model (SEM) indicated that infiltration rates were enhanced by hardwood mixture (i.e., abundance and species richness of hardwoods) and abundance of understory plants, via increases in the fine root density and accumulation of broadleaf litter, because the fine root density decreased soil bulk density and accumulation of litter increased the porosity and both of which greatly improved the soil hydraulic conditions. In the Intensive treatment, complementarity effects stemming from vertical partitioning of resource acquiring organs (i.e., leaves) among species might have increased accumulation of broadleaf litter. If increased volumes of broadleaf litter promote increased earthworm abundance, earthworms may in turn cause higher porosity, because the burrowing activity of earthworms increases the number of macropores. Consequently, hardwood mixture with abundant understory plants promotes increased infiltration rates. Our results demonstrate that intensively thinned stands with an abundance and diversity of hardwoods, including broadleaf understory plants, can absorb and store more water compared to lightly thinned stands with only understory plants, owing to increased soil infiltration capacity. Our results point to the importance of hardwood mixture in enhancing ecosystem functioning in conifer plantations.