Noboru Fujita

Importance of permafrost as a source of water for plants in east Siberian taiga

Stable oxygen isotope ratios of plant water (sap water) were observed at Spasskaya Pad experimental forest near Yakutsk, Russia in 1997–1999. The δ18O of sap water in larch trees (Larix gmelinii) decreased soon after leaf unfolding every year, indicating that snowmelt water was used in the beginning of summer. During mid to late summer, a clear difference in the water source used by plants was observed between wet summers and severe drought summers. The δ18O values of water in larch trees were high (−17.8 to −16.1‰) in August 1999 (wet summer), but low (−20.4 to −19.7‰) in August 1998 (drought summer). These results indicated that plants used rainwater during a wet summer, but meltwater from permafrost was used by plants during a drought summer. One important role of permafrost is to provide a direct source of water for plants in a severe drought summer; another role is to keep surplus water in the soil until the next summer. If this permafrost system is disturbed by future global warming, unique monotypic stands of deciduous larch trees in east Siberia might be seriously damaged in a severe drought summer.

Studies on theAbies population of Mt. Shimagare

The survivorship curve is estimated for theAbies population of the subalpine wave-regenerated forest on Mt. Shimagare in the northern Yatsugatake Mountains, Central Japan. The repetition of decay and regeneration of nearly even-aged stands is the nature of this forest in a steady-state, so that the survivorship curve is given as the decrease of density with stand age. Four distinct stages are recognized on the curve. The mortality is high in the first 10 years (stage i). Though germination is observed from the floor under dense canopy to the forest edge, only a portion of the seedlings can survive and form a nearly even-aged young regeneration. The established regeneration enjoys a low mortality before it attains a full density state (stage ii, until 32 years). When full density is reached, as indicated by the −3/2 power law, the mortality increases again due to an intensive self-thinning (stage iii). However, mortality rate declines with age. This decline is thought to occur because canopy trees become uniform in size and spatial distribution. In the last stage the aggregative dying-off of trees takes place (stage iv, about 90 years). The uniformity among trees is believed to promote the collapse of the stand.

Different distribution patterns of woody species on a slope in relation to vertical root distribution and dynamics of soil moisture profiles

Distribution patterns along a slope and vertical root distribution were compared among seven major woody species in a secondary forest of the warm-temperate zone in central Japan in relation to differences in soil moisture profiles through a growing season among different positions along the slope. Pinus densiflora, Juniperus rigida, Ilex pedunculosa and Lyonia ovalifolia, growing mostly on the upper part of the slope with shallow soil depth had shallower roots. Quercus serrata and Quercus glauca, occurring mostly on the lower slope with deep soil showed deeper rooting. Styrax japonica, mainly restricted to the foot slope, had shallower roots in spite of growing on the deepest soil. These relations can be explained by the soil moisture profile under drought at each position on the slope. On the upper part of the slope and the foot slope, deep rooting brings little advantage in water uptake from the soil due to the total drying of the soil and no period of drying even in the shallow soil, respectively. However, deep rooting is useful on the lower slope where only the deep soil layer keeps moist. This was supported by better diameter growth of a deep-rooting species on deeper soil sites than on shallower soil sites, although a shallow-rooting species showed little difference between them.

Significance of rooting depth in mire plants: Evidence from natural ^15N abundance

Variation in stable nitrogen isotope ratios (δ15N) was assessed for plants comprising two wetland communities, a bog-fen system and a flood plain, in central Japan. δ15N of 12 species from the bog-fen system and six species from the flood plain were remarkably variable, ranging from −5.9 to +1.1‰ and from +3.1 to +8.7‰, respectively. Phragmites australis exhibited the highest δ15N value at both sites. Rooting depth also differed greatly with plant species, ranging from 5 cm to over 200 cm in the bog-fen system. There was a tendency for plants having deeper root systems to exhibit higher δ15N values; plant δ15N was positively associated with rooting depth. Moreover, an increasing gradient of peat δ15N was found along with depth. This evidence, together with the fact that inorganic nitrogen was depleted under a deep-rooted Phragmites australis stand, strongly suggests that deep-rooted plants actually absorb nitrogen from the deep peat layer. Thus, we successfully demonstrated the diverse traits of nitrogen nutrition among mire plants using stable isotope analysis. The ecological significance of deep rooting in mire plants is that it enables those plants to monopolize nutrients in deep substratum layers. This advantage should compensate for any consequential structural and/or physiological costs. Good evidence of the benefits of deep rooting is provided by the fact that Phragmites australis dominates as a tall mire grass.

Recovery of a Sasa tsuboiana population after mass flowering and death

The recovery process of a Sasa tsuboiana population after a mass flowering and death in 1977 was investigated by 15 years of observation in the Hira Mountains, Kinki district, western Japan. Seed production was high (6600–13 800 seeds m−2 in Sasa plots and 3900 seeds m−2 in a forest plot) but emergent seedling density was low (14–21 seedlings m−2), probably because of seed predation by Microtus montebelli occurring between seed shedding and the next spring. The seedling density had decreased further by the next year and the S. tsuboiana population recovered from only a limited number of seedlings. In spite of such a low initial density, the S. tsuboiana population was able to regenerate successfully and attained the previous full stand height in 7–16 years. Miscantbus sinensis invaded and delayed the recovery of S. tsuboiana in one plot, but S. tsuboiana became dominant as it caught up with the height of M. sinensis. Seedling growth patterns, such as frequent tillering, the onset of rhizome extension in the early stage of seedling growth and frequent culm production from rhizomes, played important roles in the successful regeneration of S. tsuboiana.

Food webs in Mongolian grasslands: The analysis of 13C and 15N natural abundances

Overgrazing often lowers species richness and productivity of grassland communities. For Mongolian grassland ecosystems, a lack of detailed information about food-web structures makes it difficult to predict the effects of overgrazing on species diversity and community composition. We analysed the δ13C and δ15N signatures of herbaceous plants, arthropods (grouped by feeding habit), wild and domestic mammals, and humans in central Mongolia to understand the predominant food-web pathways in this grassland ecosystem. The δ13C and δ15N values of mammals showed little variation within species, but varied considerably with slope position for arthropods. The apparent isotopic discrimination between body tissue and hair of mammals was estimated as 2.0 ‰ for δ13C and 2.1 ‰ for δ15N, which was large enough to cause overestimation of the trophic level of mammals if not taken into account when using hair samples to measure isotopic enrichment.

A proposal of the method of deer density estimate without fecal decomposition rate: a case study of fecal accumulation rate technique in Japan

The accuracy of estimating deer density using the fecal pellet count method is greatly limited by variability of the fecal decomposition rate. The fecal accumulation rate technique can avoid the issue of decomposition rate. However, the precision of this technique is not clear when the decomposition rate is relatively high, such as in Japanese forests. We estimated deer population densities on Yakushima Island by the fecal accumulation rate technique and compared them between seasons. The estimated densities were similar to reported estimates, and did not differ seasonally, in accord with reports that deer on Yakushima do not migrate seasonally. Thus, we conclude that the fecal accumulation rate technique is applicable in Japanese forests.

Vegetation Interactions for the Better Understanding of a Mongolian Ecosystem Network

We examined vegetation interactions throughout the Mongolian ecosystem network to understand its present status. The existing forest mosaic in the forest-steppe zone is the result of the combined effects of climate, slope direction, livestock grazing and browsing, and human activities. Pasture production was positively correlated with precipitation, and at least 10 mm over a period of 15 days is required for good pasture plant production. Mixing with grazing-tolerant plants is not a disadvantage for edible plants in terms of production in the drier habitat. Coverage of grazing-tolerant plants increased with soil alkalization caused by pasture degradation. The recovery of edible pasture plant species diversity after removing disturbances, such as overgrazing and farming, depends on the degree of soil alkalization. In dry regions, shrubs contribute more to livestock nutrition than do herbs because of their production is higher and more consistent. Soil texture, whether sand or silt, affects shrub distribution. The taproot depth of Caragana species was especially deep compared with other shrubs. Human activities contribute to shrub disappearance in pastureland. Moreover, shrub patch degradation accelerates wind erosion in pastures. In the Caragana site, goats preferred Caragana leaves but sheep chose herbs, especially Stipa. Both goats and sheep grazed Caragana leaves completely and even ate the underground parts of herbs under strong hunger.

Distribution Patterns of Vegetation as a Fundamental Factor in Mongolian Ecosystems

In central Mongolia, south of Ulan Bator up to Dalanzadgad, vegetation is classified into three zones from north to south: the forest-steppe, steppe, and dry steppe zones, with trees and shrubs specific to each of these zones. The deciduous conifer Larix sibirica is the dominant tree in the forest-steppe zone; the shrub Caragana microphylla (Fabaceae), which fixes nitrogen, is widespread in the steppe zone between Ulan Bator and Mandalgobi; and two shrubs, Caragana stenophylla on sandy soils and a mixture of Kalidium foliatum and Reaumuria soongorica on silty soils, dominate the dry steppe zone between Mandalgobi and Dalazadgad. In the steppe and dry steppe zones, species of Allium and Stipa differ between zones, with Allium mongolicum and Stipa glareosa, and Allium polyrrhizum and Stipa gobica, in the former and latter, respectively. In each zone, shrubs that are palatable for livestock are decreasing and becoming extinct through overgrazing by livestock for a long time. In degraded pasture, Artemisia species dominate, changing from Artemisia dracunculus and Artemisia adamsii to Artemisia pectinata with decreasing altitude. Desertification does not advance in the sense of the complete loss of vegetation, but is progressing in the sense of vegetation and soil degradation.

Distributions of pH and chemical components in Mizorogaike, a pond with a floating-mat bog

Distribution patterns of pH and concentrations of chemical components were studied in Mizorogaike, a pond with a floating-mat bog, which is naturally acidified by the Sphagnum community. Considering the distribution of pH, sampling sites were selected on the floating mat (two sites), beneath the floating mat (one site), in the open water around the floating mat (two sites), and at the mouths of inflows (two sites). Monthly observations from April 1994 through March 1995 revealed that the distribution patterns of chemical components could be classified into three groups according to their pH dependence: (1) concentrations of dissolved organic carbon, organic phosphorus, aluminum, iron, and manganese increase with decrease in pH; (2) concentrations of dissolved orthophosphate and silicon increase with increase in pH; (3) concentrations of suspended particulate aluminum, iron, manganese, phosphorus, and silicon do not depend on the pH. Thus in Mizorogaike, the distribution of dissolved components depends on the pH.