Hideyuki Kawaguchi

Topographic variations of soil properties and stand structure in a Pinus thunbergii plantation

Soil properties and above-and belowground forest structure were studied across various topographies in a 20-year-oldPinus thunbergii Parl. plantation on Mt Tanakami, Japan. The soil properties and stand structure varied greatly with slope position fromridge top to valley floor. Soil thickness, fine soil content and soil moisture content were greater in lower slope positions. The amount of organic carbon in the forest floor was greater in upper slope positions. The organic carbon content in the mineral soil was slightly greater in lower slope positions. These changes in soil properties suggested an upslope decrease in decomposition rate and water and/or nutrient availability. The aboveground structure ofP. thunbergii was more developed at lower slope positions. The mean stem diameter, height and volume ofP. thunbergii increased downslope with decreasing tree density. However, fine root biomass increased greatly upslope. This inverse relationship between tree height and fine root biomass indicated morphological plasticity ofP. thunbergii in exploiting environmental heterogeneity. Variations in soil-plant interactions in the stand along various topographies caused spatial heterogeneity in the accumulation pattern of organic matter in plants and the soil.

Nutrient-uptake and nutrient-use efficiency ofPinus thunbergii Parl. along a topographical gradient of soil nutrient availability

To examine responses of a plant species to nutrient availability, we investigated changes in soil nutrient availability, litterfall production and nutrient content in litterfall along a topographic gradient in aPinus thunbergii Parl. plantation. Responses were evaluated in terms of three efficiency indices: (i) nutrient-uptake efficiency (the ratio of nutrient return in litterfall to soil nutrient availability); (ii) nutrient-use efficiency (the ratio of litterfall mass to nutrient return in litterfall); and (iii) nutrient-response efficiency (the ratio of litterfall mass to soil nutrient availability). These indices can distinguish the ability of a species to acquire nutrients and its ability to use them in litterfall production. Nitrogen and phosphorus availabilities in soil were lower in upper slope positions. The three efficiencies were higher in upper slope positions and negatively correlated with soil nutrient availability for both nitrogen and phosphorus. An increase in nutrient-response efficiency was achieved by both increases in nutrient-uptake and nutrient-use efficiencies.

Sex ratio and gender-dependent neighboring effects in Podocarpus nagi, a dioecious tree

We analyzed sex ratio, growth rates, and spacing among individuals of Podocarpus nagi, a dioecious tree, on Mt. Mikasa, Nara City, Japan. The sex ratio of reproductive trees ≥ 5 cm in stem diameter at breast height (dbh, 130 cm above ground level) was significantly male-biased. The sex ratio was male-biased in the < 20 cm and ≥ 50 cm size classes, while it did not depart from 1:1 in the 20 ≤ dbh < 50 cm class. Growth rate varied with tree size in males but not in females. The precocity and vigor of males suggests that differences in reproductive costs between sexes induce the biased sex ratio. Random labeling tests on the positions of reproductive trees showed that in the < 30 cm class, males and females were distributed randomly and independently from each other. In the ≥ 30 cm class, males were significantly clumped, whereas females were randomly distributed. Males and females showed significant repulsion, i.e., a spatial segregation of sexes. Both intra- and intersexual effects on the growth rate of crowding by neighbors were significant for females, but not for males. Maximum competitive interference was observed at a distance of 5 m, which corresponded approximately to the radius of clumps of large males and to the significant repulsive distance between large males and females. These results suggest that sexual differences in sensitivity to local crowding are related to the formation of gender-dependent spatial patterns. Formation of female-repulsive male clumps and a male-biased sex ratio may intensify the decreased probability of regeneration near males, as suggested by the limited seed-dispersal range of this species, thereby promoting coexistence with other species.

Nitrogen resorption from needles of Pinus thunbergii Parl. growing along a topographic gradient of soil nutrient availability

To examine the relative importance of nutrient resorption in increasing the nutrient-use efficiency of Pinus thunbergii Parl., we investigated the nitrogen contents of green and senescent needles of P. thunbergii trees growing at five positions along a slope (LS, lower slope; TR, transitional site; MS, middle slope; US, upper slope; RG, ridge) and found that soil nitrogen availability tended to decrease upslope. Nitrogen concentration in green and senescent needles decreased upslope. Nitrogen resorption efficiencies (percentage change in nitrogen content between green and senescent needles) increased upslope from 43 to 77% with decreasing soil nitrogen availability. Nitrogen resorption efficiency was related to green needle dry mass per unit length, but there was no clear correlation between nitrogen resorption efficiency and nitrogen content in green needles. We concluded that the increase in nitrogen resorption efficiency of P. thunbergii enhanced the nitrogen-use efficiency as a response to the low nitrogen availability.

Sprouting traits and population structure of co-occurring Castanopsis species in an evergreen broad-leaved forest in southern China

The role of sprouting in regeneration was compared betweenfour co-occurring Castanopsis species in an evergreen broad-leavedforest in southern China. We considered the largest stem of an individualto be the main stem, while all of the other stems were consideredto be sprouts. To evaluate the degree of dependence on sproutingin the regeneration of each species, we measured the ratio of thenumber of all sprouts to the number of main stems in a population. The ratio was 4.3 in Castanopsis fordii and > 1in Castanopsis fabri and Castanopsis carlesii, althoughit was < 1 in Castanopsis fargesii. The ratioof the number of all sprouts to the number of main stems in a populationwas represented by the product of two factors of sprouting ability:the proportion of sprouting individuals among all individuals andthe number of sprouts per sprouting individual. The proportion ofsprouting individuals among all individuals differed significantlyamong the four species, while the number of sprouts per sproutingindividual did not. Specific variations in sprouting traits werein agreement with the population structure of each species. Specieswith fewer small individuals in the size distribution had a higherratio than species with L-shaped distribution. Furthermore, specieswith a clumped distribution had a larger ratio than species thatwere randomly distributed. These results suggested that specieswhose recruitment by seedlings was temporally and spatially restrictedwere relatively more dependent on sprouting in the regeneration. The specific gradient of sprouting ability and the relative dependencyon sprouts to maintain a population can enhance the coexistenceof these four congeneric species.

Spatial pattern formation and relative importance of intra- and interspecific competition in codominant tree species, Podocarpus nagi and Neolitsea aciculata

Spatial patterns, their changes due to mortality, and intra- and interspecific competition of two codominant tree species, Podocarpus nagi and Neolitsea aciculata, were analyzed at Mt. Mikasa, Nara City, Japan. Podocarpus nagi has a higher shade tolerance but a narrower seed dispersal range than N. aciculata. We inferred the mechanisms of spatial pattern formation and coexistence of the two species. Podocarpus nagi and N. aciculata trees were clumped and showed a spatial repulsion from each other. Patches dominated by either P. nagi or N. aciculata were formed. Podocarpus nagi trees were less clumped with increasing tree size, although no significant change in spatial patterns due to mortality was detected. A patch formation of the P. nagi population seemed to be induced by the narrow seed dispersal range. On the other hand, N. aciculata trees were more clumped and more repulsive from P. nagi trees with increasing tree size. The distribution of N. aciculata trees shifted to more clumped than expected from the random mortality over the research period. Post-dispersal mortality due to competitive exclusion by P. nagi affected the patch formation of the N. aciculata population. The relative importance of intraspecific competition to interspecific competition on the relative growth rate increased with increasing tree size. The shift corresponded to an increasing spatial repulsion between the two species. The patch formation of P. nagi population may create the spatial refuge of N. aciculata from P. nagi and may enable avoidance of interspecific competition and the coexistence of the two species.

Differences in nitrogen use efficiency between leaves from canopy and subcanopy trees

We examined the effects of increasing light availability along a vertical gradient within a forest community on the efficiency of leaf nitrogen (N) use in individual trees. The N contents of green and senescent leaves in canopy and subcanopy trees of an evergreen coniferous species, Podocarpus nagi, and an evergreen hardwood species, Neolitsea aciculata, were analyzed in a mixed forest community at Mt Mikasa, Nara City, Japan. The inverse of N concentration (NC) in senescent leaves was used as an index of N use efficiency (NUE) at the leaf-level. The leaf-level NUE was higher in canopy trees than in subcanopy trees in both P. nagi and N. aciculata, although soil N mineralization rates around canopy and subcanopy trees did not differ significantly. The NC in green leaves was lower in canopy trees than in subcanopy trees. The ratio of resorbed N in senescent leaves to the N content in green leaves was higher in canopy trees than in subcanopy trees. The higher leaf-level NUE of canopy trees was partly a result of lower NC in living tissues and partly because of greater N resorption during senescence. The present study suggested that the leaf-level NUE could be increased in response to an imbalance between soil N and light availability caused by spatial community structure.

Initial nitrogen content and topographic moisture effects on the decomposition of pine needles

The effects of litter quality and site characteristics on the decomposition process were investigated using a litterbag method. Pine needle litters with differing nitrogen concentrations (0.8, 0.6 and 0.4%) were placed on the upper and lower slopes of a Pinus thunbergii Parl. plantation. After both 3 and 6 months, the mass of decomposing litter with the lower nitrogen concentration was larger than the litter with higher nitrogen concentrations. After 9 months, there were no significant differences in the litter mass remaining, regardless of the initial nitrogen concentration. Moisture content in the litter was always higher on the lower slope, although the mass of litter was smaller. Nitrogen concentration of the decomposing litter increased linearly with accumulated mass loss. The increase in nitrogen concentration of decomposing litter was greater on the lower slope, but this increase did not differ between initial nitrogen concentrations. The nitrogen release from the decomposing litter with higher initial nitrogen concentration was larger than the release from litter bags with lower nitrogen concentrations. This result suggests that there may be positive feedback between soil nutrient availability, litter quality and nutrient release from decomposing litter at the intraspecific level.

Production-dependent reproductive allocation of a tall tree speciesQuercus serrata

The reproductive allocation (allocation of net production to acorns) of a tall tree speciesQuercus serrata was estimated by a method combining branch diameter distribution and the sampling of acorns per branch. Acorn production per branch of 0.5-cm diameter significantly varied among individuals and years, but not significantly across tree size. Leaf production per branch of 0.5-cm diameter had little annual fluctuation, while that of acorns fluctuated about 6 times between the maximum and minimum. The reproductive allocation was regressed against vegetative allocation, following the general model of klinkhameret al. (1992); the reproduction had a beginning threshold and reproductive allocation increased drastically at relatively young stage after the tree began reproduction. With its small threshold production, coppicing traits and large maximum size,Q. serrata seemed adaptive to infrequent but large-scale disturbances like fires.

Use of foliar 15N and 13C abundance to evaluate effects of microbiotic crust on nitrogen and water utilization in Pinus massoniana in deteriorated pine stands of south China

We compared the foliar δ15N and δ13C values of Pinus massoniana growing on soils with and without microbiotic crust to examine the influence of the microbiotic crust on N and water use in plants in deteriorated watersheds in southern China. At our study site, litterfall and undergrowth had been intensively removed for fuel and soil N concentration was extremely low. Microbiotic crust covered the lower slope within the watersheds and pine trees were taller here than on the middle and upper slopes, although the crust reduced the amount of rainfall that could penetrate the soil. The foliar δ15N values were greater (closer to zero) in pine trees growing on soil covered with microbiotic crust on the lower slope than on the middle and upper slopes, which lacked the microbiotic crust. These data suggest that P. massoniana may depend on N fixed by the microbiotic crust on the lower slope, and on N carried by precipitation on the middle and upper slopes. The microbiotic crust did not influence foliar δ13C, an index for water use efficiency, in P. massoniana. The fact that P. massoniana biomass was greater on the lower slope, which is less permeable to rainfall, suggests that P. massoniana growth may be limited by the amount of available N rather than by water. The microbiotic crust may improve plant productivity by increasing N availability, despite its negative effect on water availability.