The Right-Skewed Distribution of Fine-Root Size in Three Temperate Forests in Northeastern China

Trees can build fine-root systems with high variation in root size (e.g., fine-root diameter) and root number (e.g., branching pattern) to optimize belowground resource acquisition in forest ecosystems. Compared with leaves, which are visible above ground, information about the distribution and inequality of fine-root size and about key associations between fine-root size and number is still limited. We collected 27,573 first-order fine-roots growing out of 3,848 second-order fine-roots, covering 51 tree species in three temperate forests (Changbai Mountain, CBS; Xianrendong, XRD; and Maoershan, MES) in Northeastern China. We investigated the distribution and inequality of fine-root length, diameter and area (fine-root size), and their trade-off with fine-root branching intensity and ratio (fine-root number). Our results showed a strong right-skewed distribution in first-order fine-root size across various tree species. Unimodal frequency distributions were observed in all three of the sampled forests for first-order fine-root length and area and in CBS and XRD for first-order fine-root diameter, whereas a marked bimodal frequency distribution of first-order fine-root diameter appeared in MES. Moreover, XRD had the highest and MES had the lowest inequality values (Gini coefficients) in first-order fine-root diameter. First-order fine-root size showed a consistently linear decline with increasing root number. Our findings suggest a common right-skewed distribution with unimodality or bimodality of fine-root size and a generalized trade-off between fine-root size and number across the temperate tree species. Our results will greatly improve our thorough understanding of the belowground resource acquisition strategies of temperate trees and forests.

Root Biomass Distribution and Soil Physical Properties of Short-Rotation Coppice American Sycamore (Platanus occidentalis L.) Grown at Different Planting Densities

Short rotation woody crops (SRWCs) provide sustainable, renewable biomass energy and offer potential ecosystem services, including increased carbon storage, reduced greenhouse gas emissions, and improved soil health. Establishing SRWCs on degraded lands has potential to enhance soil properties through root and organic matter turnover. A better understanding of SRWC planting density and its associated root turnover impacts on soil–air–water relations can improve management. In this study, we investigate the effects of planting density for a low-input American sycamore SRWC (no fertilization/irrigation) on soil physical properties for a degraded agricultural site in the North Carolina piedmont. The objectives were (1) to estimate the distributions of coarse and fine root biomass in three planting densities (10,000, 5000, and 2500 trees per hectare (tph)) and (2) to assess the effects of planting density on soil hydraulic properties and pore size distribution. Our results show that planting at 10,000 tph produced significantly higher amounts of fine root biomass than at lower planting densities (p < 0.01). In the 25,000 tph plots, there was significantly higher amounts of coarse root biomass than for higher planting densities (p < 0.05). The 10,000 tph plots had lower plant available water capacity but larger drainable porosity and saturated hydraulic conductivity compared with lower planting densities (<0.05). The 10,000 tph plots total porosity was more dominated by larger pore size fractions compared with the 5000 and 2500 tph. Generally, our findings show similar patterns of soil hydraulic properties and pore size distributions for lower planting densities. The results from 10,000 tph indicate a higher air-filled pore space at field capacity and more rapid drainage compared with lower planting densities. Both characteristics observed in the 10,000 tph are favorable for aeration and oxygen uptake, which are especially important at wet sites. Overall, the results suggest that improved soil health can be achieved from the establishment of American sycamore SRCs on marginal lands, thereby providing a green pathway to achieving environmental sustainability with woody renewable energy.