Keitaro Yamase

Abstract Background and Aims Estimating the time required for tree root systems to reach deep soils (> 1 m) and elucidating the characteristics of soil properties and fungi associated with deep roots are crucial for understanding tree establishment in harsh coastal environments. This study aimed (1) to determine the downward elongation rate of Pinus thunbergii taproots using tree-ring analysis and (2) to explore the relationships among deep roots, deep-soil properties, and fungal associations. Methods Annual growth rings of taproots in a coastal forest in Japan were analyzed using a cross-dating procedure. Soil physicochemical properties were assessed, and root-associated fungi in deep soils were identified by DNA sequencing. Results Taproot growth followed a sigmoid function; trees with a larger stem diameter at breast height (DBH) had greater total taproot depths. Taproot elongation of the tree with the largest DBH, located at a site with the coarsest gravel, did not slow even within deep gravel horizons. Fine soils in deep horizons had C and N concentrations comparable to or higher than surface horizons, and soil C:N ratios correlated positively with taproot elongation rates. DNA sequencing of fine roots revealed the presence of endophytic and ectomycorrhizal fungi with potential saprotrophic functions. Conclusion Taproot downward elongation dynamics appear to depend on aboveground tree characteristics and soil properties. Root detritus generated through friction with gravel likely supplies C and N to deep soils, supporting sparse saprotrophic fungi. These findings underscore the importance of deep root–soil–fungi interactions in supporting tree establishment in nutrient-poor coastal environments.

Tree resistance to uprooting can be estimated as the critical turning moment in tree-pulling experiments. The depth at the center point of rotation (Dcp) in tree-pulling experiments is measured as an indicator of below-ground traits and is related to this critical turning moment. However, few researchers have investigated the relationship between the Dcp and maximum root depth. Our objective in this study was to clarify whether the Dcp in tree-pulling experiments can be estimated as the maximum root depth of Pinus thunbergii Parl. in sandy soils. We also estimated which position of displacement of the center of rotation (Cp) can be applied as the Dcp. We conducted tree-pulling experiments, and compared the Dcp obtained from images with the measured maximum root depth. We found significant positive correlations between the Dcp and maximum root depth. The Cp displacement concentrated immediately below the stem when the maximum critical turning moment was reached. This position should be measured as the Dcp, which is related to the maximum root depth. We found that the Dcp can serve as a parameter, preventing the need for uprooting, when tree-pulling experiments are performed to obtain an important below-ground trait for understanding the critical turning moment.

Three-dimensional (3D) root system architecture (RSA) is a predominant factor in anchorage failure in trees. Only a few studies have used 3D laser scanners to evaluate RSA, but they do not check the accuracy of measurements. 3D laser scanners can quickly obtain RSA data, but the data are collected as a point cloud with a large number of points representing surfaces. The point cloud data must be converted into a set of interconnected axes and segments to compute the root system traits. The purposes of this study were: (i) to propose a new method for easily obtaining root point data as 3D coordinates and root diameters from point cloud data acquired by 3D laser scanner measurement; and (ii) to compare the accuracy of the data from main roots with intensive manual measurement. We scanned the excavated root systems of two Pinus thunbergii Parl. trees using a 3D laser scanner and neuTube software, which was developed for reconstructing the neuronal structure, to convert the point cloud data into root point data for reconstructing RSA. The reconstruction and traits of the RSA calculated from point cloud data were similar in accuracy to intensive manual measurements. Roots larger than 7 mm in diameter were accurately measured by the 3D laser scanner measurement. In the proposed method, the root point data were connected as a frustum of cones, so the reconstructed RSAs were simpler than the 3D root surfaces. However, the frustum of cones still showed the main coarse root segments correctly. We concluded that the proposed method could be applied to reconstruct the RSA and calculate traits using point cloud data of the root system, on the condition that it was possible to model both the stump and ovality of root sections.

Rainwater interacts with tree canopies in forest ecosystems, which greatly influence its quality. However, little information is available regarding how tree canopies influence dissolved organic matter (DOM) in rainwater. To examine this, we collected bulk deposition (rainfall) and throughfall in a conifer (Chamaecyparis obtusa) plantation, western Japan, during a rain event, and analyzed their DOM molecular compositions using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. The dissolved organic carbon flux and the number of DOM molecular species detected were approximately seven times and three times higher in throughfall than in rainfall, respectively. We found that the average proportion of molecular species shared between five sample replicates was larger in throughfall (69%) than in rainfall (50%). Nonmetric multidimensional scaling revealed that the molecular species were significantly differentiated between throughfall and rainfall, and the dissimilarity among the replicates was much smaller in throughfall. This indicates that the quality of DOM in rainwater became spatially homogeneous due to contact with tree canopies. The number of lignin-like molecules was larger than those of any other biomolecular compounds in throughfall and seven times larger than in rainfall, suggesting that many of plant-derived DOM molecules were dissolved into rainwater.

Trees can be uprooted by typhoons which have intensive rainfall and strong winds. The ability of the tree to resist uprooting could depend on tree root systems in the soil. In this study, to clarify the difference in pull-out resistance between tree species (Cryptomeria japonica (L. fil.) D. Don) and unpalatable shrub species (Pieris japonica (Thunb.) D. Don and Edgeworthia chrysantha Lindley) , we carried out pull-out tests of root segments. As a result, the pull-out resistances of the roots in 10 mm basal diameter in C. japonica (1012.3 N) and P. japonica (844.3 N) were stronger than that in E. chrysantha (200.1 N). On the other hand, the soil reinforcement (i.e. sum of the pull-out resistance per soil profile 1 m²) was largest in C. japonica (26.8 kN) , and followed by P. japonica (15.1 kN) ,and was smallest in E. chrysantha (1.0 kN). Our results suggest that taking advantage of the characteristics of each unpalatable shrub species improve the soil reinforcement in a site heavily used by sika deer (Cervus nippon Temminck) .

[in Japanese]

In order to use for revegetation, we investigated the composition of soil seed bank both in 2005 and 2012. We collected 255 soil samples (20 cm × 20 cm × 5 cm in depth) from three districts in Hyogo prefecture, Japan. We counted emergent seedlings in the soil samples placed in an open space, and estimated the composition of soil seed bank. The environmental factors affecting the seed density and the number of taxa were analyzed using a generalized linear mixed model. The sampling year and the slope type were selected as variables for predicting the seed density. The seed density in 2005 was more on the valley slope> the straight slope> the ridge slope, but the seed density in 2012 was lower regardless of slope types. The composition of soil seed bank in 2012 against 2005 has significantly reduced in the ruderal species and the grassland species, but has almost unchanged in the coppice / summergreen species or the evergreen species. Changes of soil seed bank indicate that reestablishment of revegetation goal is necessary, assessment of soil seed bank potential is essential, and that control of surface erosion is required.

Sprouting that occurs after clear cutting of seven tree species commonly observed in broad-leaved secondary forests in the warm-temperate zone was investigated in Hyogo prefecture, Japan. The environmental factors affecting the sprouting ratio after one growing season and the survival rate of the sprout after three growing seasons were analyzed using a generalized linear mixed model. In Quercus serrata, the sprouting ratio was lower on drought-prone slopes, in short stumps and in old trees. On the other hand, the survival rate of stumps was higher on tall stumps. The older the tree, the lower the sprouting ratio in many tree species examined. This trend was most prominent in Quercus serrata. These results suggest that sprouting of Quercus serrata is sensitive to aging and that the survival rate of sprouts may be increased by controlling stump height.

This research reveals that the static resistance of trees varies according to whether trees depend on soil water conditions in the case of Japanese larch, whether they originated in seeding or transplanting in the case of Japanese oak, and whether they grow close together or not in the case of Japanese larch. This is verified by experiments of pulling down trees that grow in different conditions. It has been said that forests play important roles in preventing sediment disasters. However, it has been unknown to what extent which kinds of trees contribute to preventing debris flow in mountain areas. Therefore, we compared differences of the maximum tree-pulling moment of several kinds of trees that have different soil water conditions, different origins in growing, and different grove density by conducting experiments of pulling down the trees. In the experiments, we calculated the maximum tree-pulling moment whenever a heavy machine or TIR FOR pulled down a tree by drawing a wire passed around the tree at a height of one meter. The experiments show three results. First, there are no differences of the maximum tree-pulling moment even among trees that depend even on different soil water conditions. Second, the maximum tree-pulling moment of trees originated in seeding is the same as for those in transplanted. Finally, there are negative correlations between the maximum tree-pulling moment and the grove density

To quantify the amount of surface erosion from slopes with and without horizontal step works using thinning logs, a soil trap was installed at base of a steep slope site in Cedar and Cypress artificial forests, Hyogo Prefecture. The amount of surface erosion at heavy rains was 0.128∼1.023 m³ • ha^-1 • yr^-1 from slopes with the horizontal step works, whereas it was 0.318∼4.743 m³ • ha^-1 • yr^-1 from slopes without the horizontal step works. The support practice factor of USLE was 0.03∼0.63 with horizontal step works, and effect was seen clearly in place, it was about the same as hillside covering works.