Natural<Superscript>15</Superscript> N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Measurement of nitrogen isotopic composition (¹⁵N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of ¹⁵N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of ¹⁵N between soil N pools and plants, and by using an existing model. ¹⁵N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO₃^– pool at two sites with different nitrification rates in a temperature forest in Japan. ¹⁵N of plants was similar to that of soil NO₃^– in the high-nitrification site. Because of high foliar NRA and the large foliar NO₃^– pool at this site, we concluded that plant ¹⁵N indicated a great reliance of plants on soil NO₃^– there. However, many ¹⁵N of soil N overlapped each other at the other site, and ¹⁵N could not provide definitive evidence of the N source. The existing model was verified by measured ¹⁵N of soil inorganic N and it explained the variations of plant ¹⁵N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare ¹⁵N signatures from different ecosystems and to understand N dynamics.     

Inorganic nitrogen source utilization byFagus crenata on different soil types

The nitrogen source utilization by Fagus crenata distributed on soils with different forms of inorganic nitrogen in a cool-temperate deciduous forest in central Japan was determined by measuring foliar ¹⁵N. Two soil habitat types along a slope were delineated based on nitrogen transformation patterns, i.e., soils with high net nitrification rates and with no or low net nitrification, respectively. Despite differences in soil types, the study species, F. crenata, was distributed along the entire slope. The foliar ¹⁵N value of F. crenata from the lower slope area was significantly lower than that from the upper slope. Given the finding of a previous study that the ¹⁵N of NO₃^– was lower than that of NH₄⁺, our results indicate that reliance on NO₃^– as a nitrogen source was greater in the lower slope area than in the upper slope area. Differences in the values of foliar ¹⁵N were about 1, which is far less than the 10 ¹⁵N value of soil inorganic N reported in the previous study. This discrepancy might suggest that the study species utilized NO₃^– even in the upper site where net nitrification had not been detected. Measurements of nitrate reductase activity, an index of NO₃^– uptake, also supported this interpretation. Nitrate reduction occurred in leaves and roots at both the lower and the upper sites. Thus, the study species may be able to use NO₃^– even in soils with no net nitrification, a factor that could allow the distribution of F. crenata along the entire length of the slope.     

Nation-wide litter fall data from 21 forests of the Monitoring Sites 1000 Project in Japan

This data paper reports litter fall data collected in a network of 21 forest sites in Japan. This is the largest litter fall data set freely available in Japan to date. The network is a part of the Monitoring Sites 1000 Project launched by the Ministry of the Environment, Japan. It covers subarctic to subtropical climate zones and the four major forest types in Japan. Twenty-three permanent plots in which usually 25 litter traps were installed were established in old-growth or secondary natural forests. Litter falls were collected monthly from 2004, and sorted into leaves, branches, reproductive structures and miscellaneous. The data provide seasonal patterns and inter-annual dynamics of litter falls, and their geographical patterns, and offer good opportunities for meta-analyses and comparative studies among forests.     

Using food network unfolding to evaluate food–web complexity in terms of biodiversity: theory and applications

Food–web complexity often hinders disentangling functionally relevant aspects of food–web structure and its relationships to biodiversity. Here, we present a theoretical framework to evaluate food–web complexity in terms of biodiversity. Food network unfolding is a theoretical method to transform a complex food web into a linear food chain based on ecosystem processes. Based on this method, we can define three biodiversity indices, horizontal diversity (D_H), vertical diversity (D_V) and range diversity (D_R), which are associated with the species diversity within each trophic level, diversity of trophic levels, and diversity in resource use, respectively. These indices are related to Shannon's diversity index (H′), where H′ = D_H + D_V ? D_R. Application of the framework to three riverine macroinvertebrate communities revealed that D indices, calculated from biomass and stable isotope features, captured well the anthropogenic, seasonal, or other within‐site changes in food–web structures that could not be captured with H′ alone.     

Differences between height- and light-dependent changes in shoot traits in five deciduous tree species

The effects of tree height on shoot traits may in some cases differ in magnitude and direction from the effects of light. Nevertheless, general patterns of change in shoot traits in relation to variations in height and light have not so far been revealed. A comprehensive analysis of the differences between the effects of height and light on a range of leaf and shoot traits is important for the scaling of these traits to individual trees. We investigated the biomass allocation and structure of current-year shoots at the top of the crowns of five deciduous tree species in Japan. Height effect was investigated by comparing shoot traits among trees of different heights growing under a high light environment. The effects of light were examined by comparing saplings growing in high- and low-light environments. The effects of light were significant for most traits, while those of height were not significant for some traits. The magnitudes of the effects of light were larger than those of height for most traits related to biomass allocation. There was an extreme difference between the effects of height and light in the direction of change in the length of current-year shoots and in the number of standing leaves. The measures of both parameters increased with the increase in light, but decreased with the increase in tree height. Thus, the effects of height and light on diverse traits at the level of current-year shoots were not always similar. These results suggest that great care must be taken when scaling shoot traits from small trees to tall trees because the effects of height and light can be complex.     

The effects of herbivory and soil fertility on the growth patterns of <Emphasis Type="Italic">Quercus serrata</Emphasis> and <Emphasis Type="Italic">Q. crispula</Emphasis> saplings at the shoot and individual levels

We investigated the combined effects of herbivore damage and soil fertility on shoot growth patterns of Quercus serrata and Q. crispula saplings at both the shoot and individual levels. Saplings were grown in herbivore-damaged or undamaged areas of the greenhouse with the two fertilization treatment levels, low or high. We measured the leaf area loss, number of flushes, length of extension units (EUs; the first vs the higher), number of leaves on each individual, and number of EUs. At the shoot level, the leaf area loss at high soil fertility was significantly greater than that at low soil fertility among the highest EUs of Q. serrata, while this difference was not significant in Q. crispula, suggesting that effect of soil fertility on leaf area loss is species-specific. Furthermore, herbivore damage was associated with a significant increase in the number of EUs and a reduction in the length of the higher EUs under both soil fertility treatments, although saplings had a tendency to produce significantly more flushes and longer individual EUs under the high soil fertility. At the individual level, herbivore-damaged saplings exhibited a significant increase in leaf numbers; however, the total length of the EUs in Q. serrata or Q. crispula was not significantly affected by herbivore damage, regardless of soil fertility. These results suggest that Q. serrata and Q. crispula saplings produce shorter EUs in response to herbivore damage in order to reduce the cost of mechanical support and spread the risk for any subsequent herbivore damage.     

The potential of NO3--N utilization by a woody shrub species Lindera triloba: a cultivation test to estimate the saturation point of soil NO3--N for plants

Responses of seedlings of a shrub species, Lindera triloba, grown in perlite culture medium, to nitrate (NO3--N) supply were investigated to estimate the saturating point of available NO3--N for plant utilization. NO3--N concentration and nitrate reductase activity (NRA) in leaves and roots were used as indicators of NO3--N uptake and assimilation by L. triloba. Root NRA increased with NO3--N supply when concentrations were low and reached a plateau at high NO3--N concentrations. On the other hand, root NO3--N concentration increased linearly with NO3--N supply; therefore, it is suggested that NO3--N uptake did not limit NO3--N assimilation by L. triloba. In contrast, leaf NRA and leaf NO3--N concentration were low and were not influenced by NO3--N supply. This may be caused by the lack of transport of NO3--N from roots to leaves. The NO3--N retained in perlite was compared with NO3--N pool sizes in soils from a forest where L. triloba occurs naturally to estimate the level of NO3--N availability to plants in the forest soil. The maximum NO3--N pool size in the forest soil was comparable to concentrations at which root NRA reached a plateau in perlite cultures. These results indicate that soil NO3--N availability is below the saturation point for NO3--N uptake by L. triloba, and it is the limiting factor of NO3--N utilization by L. triloba under field conditions in which this species naturally occurs.     

Soil N fluxes in three contrasting dry tropical forests

A comparative study of N fluxes in soil among a dry dipterocarp forest (DDF), a dry evergreen forest (DEF), and a hill evergreen forest (HEF) in Thailand was done. N fluxes in soil were estimated using an ion exchange resin core method and a buried bag method. Soil C and N pools were 38 C Mg/ha/30 cm and 2.5 N Mg/ha/30 cm in DDF, 82 C Mg/ha/30 cm and 6.2 N Mg/ha/30 cm in DEF, and 167 C Mg/ha/30 cm and 9.3 N Mg/ha/30 cm in HEF. Low C concentration in the DDF and DEF sites was compensated by high fine soil content. In the highly weathered tropical soil, fine soil content seemed to be important for C accumulation. Temporal and vertical fluctuations of N fluxes were different among the sites. The highest N flux was exhibited at the onset of the wet season in DDF, whereas inorganic N production and estimated uptake of N were relatively stable during the wet season in DEF and HEF. It is suggested that N cycling in soil becomes stable in dry tropical forests to intermediate in temperate forests. N deposition may result in large changes of N cycling in the DDF and DEF due to low accumulations of C and N.     

Impacts of moso bamboo (Phyllostachys pubescens) invasion on dry matter and carbon and nitrogen stocks in a broad‐leaved secondary forest located in Kyoto,western Japan

In western and central Japan, the expansion of exotic moso bamboo (Phyllostachys pubescens Mazel ex J. Houz.) populations into neighboring vegetation has become a serious problem. Although the effects of bamboo invasion on biodiversity have been well studied, shifts in nutrient stocks and cycling, which are fundamental for ecosystem functioning, are not fully understood. To explore the effects of P. pubescens invasion on ecosystem functions we examined above‐ and below‐ground dry matter and carbon (C) and nitrogen (N) stocks in a pure broad‐leaved tree stand, a pure bamboo stand, and two tree–bamboo mixed stands with different vegetation mix ratios in the secondary forest of Kyoto, western Japan. In the process of invasion, bamboo shoots offset broad‐leaved tree deaths; thus, no clear trend was apparent in total above‐ or below‐ground biomass or in plant C and N stocks during invasion. However, the ratio of above‐ground to below‐ground biomass (T/R ratio at the stand level) decreased with increasing bamboo dominance, especially in the early stages of invasion. This shift indicates that rapid bamboo rhizomatous growth is a main driver of substantial changes in stand structure. We also detected rises in the C/N ratio of forest‐floor organic matter during bamboo invasion. Thus major impacts of P. pubescens invasion into broad‐leaved forests include not only early shifts in biomass allocation, but also changes in the distribution pattern of C and N stored in plants and soil.     

Nitrate-use traits of understory plants as potential regulators of vegetation distribution on a slope in a Japanese cedar plantation

Background and aims

Plant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan.

Methods

At the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO ₃ ^? -N) use traits, and compared the results with the soil traits.

Results

Our results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO ₃ ^? -N. Pieris japonica lacks the capacity to use NO ₃ ^? -N as a N source, but other species do use NO ₃ ^? -N. Lindera triloba, whose distribution is unrelated to soil NO ₃ ^? -N availability, changes the extent to which it uses NO ₃ ^? -N in response to soil NO ₃ ^? -N availability.

Conclusions

Our results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO ₃ ^? -N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions.