Yoshimichi Hori

Roles of leaf water potential and soil-to-leaf hydraulic conductance in water use by understorey woody plants

Diurnal changes of leaf water potential and stomatal conductance were measured for 12 deciduous shrubs and tree saplings in the understorey of a temperate forest. Sunflecks raised the leaf temperature by 4°C, and vapor pressure deficit to 2 kPa. Although the duration of the sunflecks was only 17% of daytime, the photon flux density (PFD) of sunflecks was 52% of total PFD on a sunny summer day. Leaf osmotic potential at full turgor decreased in summer, except in some species that have low osmotic potential in the spring. Plants that endured low leaf water potential had rigid cell walls and low osmotic potential at full turgor. These plants did not have lower relative water content and turgor potential than plants with higher leaf water potential. There were three different responses to an increase in transpiration rate: (i) plants had low leaf water potential and slightly increased soil-to-leaf hydraulic conductance; (ii) plants decreased leaf water potential and increased the hydraulic conductance; and (iii) plants had high leaf water potential and largely increased the hydraulic conductance.

Flowering Phenology of Understory Herbaceous Species in a Cool Temperate Deciduous Forest in Ogawa Forest Reserve, Central Japan

Observations of the flowering phenology and measurements of the heights of understory herbaceous plants were made in a cool temperate deciduous forest, where light availability is relatively low and fluctuates markedly through the year, and it is too cold for many plants to grow in winter. Ninety-one species were recorded between April and October. The number of flowering species as a function of date showed a bimodal distribution. The plants flowering in spring and those flowering from late summer to early autumn each accounted for about 40% of the number of species. The plants that flowered in spring were smaller than those that flowered later in the season. The springflowering plants would produce flower buds in the previous growing season, as the time from the appearance of the aerial part to flowering was transient. Some species completed their main life history events during the spring as spring ephemerals. The small sizes in these species may result from bud formation in the previous 8eason and/or the short period of growth. The plants that flowered in late summer and early autumn, by contrast, were large. These plants should have relatively long periods of vegetative growth and flower at the end of the growing season using matter produced in that year. A long vegetative growth period would tend to make plants firm and/or large. It was suggested that flowering phenology was separated into two periods by the climatic and environmental constraints of a cool temperate deciduous forest.

Spatial heterogeneity in a grassland community: Use of power law

A new regression analysis was proposed to evaluate the degree of spatial heterogeneity for individual species comprising a plant grassland community. The weighted average of the heterogeneity value of all the species comprising the community provides a measure of community-level heterogeneity. A field survey was carried out, as an example, in order to analyze the spatial heterogeneity of a pasture with grazing cows, using 100 quadrats 50 cm × 50 cm, each of which was divided into four smaller quadrats 25 cm × 25 cm, on a 50 m long line-transect. The frequency of occurrence for all the species in each small quadrat was recorded. The regression associated with the ratio of the theoretical and observed variances of occurence counts was used to analyse the frequency distribution of species in a pasture community. A good fit to the regression for the whole community was obtained. These results indicate that (i) each species in the example was distributed more heterogeneously than a random pattern; and (ii) the regression could well describe the spatial heterogeneity of the grassland plant community. In most of the observed species, spatial heterogeneity is often characterized by species-specific propagation traits and the architecture of plant bodies. Thus, the spatial patterns of a grassland community can be evaluated in detail by this power-law approach. This measure is suitable for field surveys and comparative studies of grassland communities, and for other plant communities that are generally short in height.

Life history and population dynamics of the Japanese Yam, dioscorea japonica Thunb. II: Adaptive significance of the emergence mode

The adaptive significance of the emergence mode ofDioscorea japonica was studied with respect to initial plant size (seed, bulbil and tuber) and light intensity, using mathematical simulation based on Yokois (1976) model.Under 1.5% full sunlight conditions, plants emerging with only one leaf did not develop a shoot system throughout the growing period (Hori and Oshima, 1986). Simulation indicated that, for this species of plant under poor productive conditions, the optimal time for switch-over from the vegetative to reproductive growth phase to maximize the tuber weight at the end of the growing period, occurred immediately following the start of autotrophic growth. By means of shoot growth patterns, small and large size plants acquired the ability of shade tolerance and shade avoidance, respectively.Further, the life history ofD. japonica could be expressed as a flow chart based on plant size and light intensity data.

Photosynthesis of bract and its contribution to seed maturity inCarpinus laxiflora

Light saturated net photosynthesis was measured in bracts and leaves ofCarpinus laxiflora, the major species in secondary forests in cool and intermediate temperate zones in Japan. The maximum net photosynthesis of leaves and bracts was essentially constant from May to early August and decreased gradually thereafter. For bracts, it was 3.2 μmol m−2s−1, approximately half that for the leaves. The photosynthesis of bracts would thus appear to contribute significantly to seed maturity. The estimated production of bract based on the photosynthesis would make seeds (3 mg dry weight) mature for 37 days, assuming all photosynthate of the bracts to have been distributed in the seeds only. This was quite consistent with the growth curve for the seeds. A mast year phenomenon is discussed in relation to bract photosynthesis and leaf number.

Production dependence of vegetative propagation inDisporum smilacinum A. Gray

The dependence of vegetative propagation on the production of individual plants was examined inDisporum smilacinum A. Gray on the basis of shading experiments and field surveys. This species typically showed four types of reproductive behavior: sterile plants producing one plantlet (no propagation), sterile plants producing more than two plantlets (vegetative propagation), fertile plants producing one plantlet (sexual reproduction) and fertile plants producing more than two plantlets (both sexual reproduction and vegetative propagation).The propagation ofD. smilacinum was clearly related to the annual net production of each individual plant. The probability of a mother plant producing more than two vegetative propagules (plantlets) increased with net production of the plant in the current year. The number of propagules per plant and runner length increased with net production.It was possible to explain the types of reproductive behavior of this species on the basis of both the initial plant size before sprouting and its net production during the growing season. There was a critical initial plant size for sexual reproduction and a critical level of production for vegetative propagation.

Photosynthesis and water-relation traits of the summer annual C4 grasses, Eleusine indica and Digitaria adscendens, with contrasting trampling tolerance.

Two summer annual C4 grasses with different trampling susceptibilities were grown as potted plants, and diurnal leaf gas exchange and leaf water potential in each grass were compared. The maximum net photosynthetic rate, leaf conductance and transpiration rate were higher in the trampling-tolerant Eleusine indica (L.) Gaertn. than in trampling sensitive Digitaria adscendens (H. B. K.) Henr. Leaf water potential was much lower in E. indica than in D. adscendens. There were no differences in soil-to-leaf hydraulic conductance and leaf osmotic potential at full turgor as obtained by pressure–volume analysis. However, the bulk modulus of elasticity in cell walls was higher in E. indica leaves than in D. adscendens leaves. This shows that the leaves of E. indica are less elastic. Therefore, the rigid cell walls of E. indica leaves reduced leaf water potential rapidly by decreasing the leaf water content, supporting a high transpiration rate with high leaf conductance. In trampled habitats, such lowering of leaf water potential in E. indica might play a role in water absorption from the compacted soil. In contrast, the ability of D. adscendens to colonize dry habitats such as coastal sand dunes appears to be due to its lower transpiration rate and its higher leaf water potential which is not strongly affected by decreasing leaf water content.

Differences in Field Gas Exchange and Water Relations Between a C3 Dicot (Plantago Asiatica) and a C4 Monocot (Eleusine Indica)

Field gas exchange and water potential in the leaves of a C3 dicot, Plantago asiatica L., and a C4 monocot, Eleusine indica Gaertn., which dominate in trampled vegetation in eastern Japan were surveyed during the growing periods for two consecutive years. Net photosynthetic rate (PN) of E. indica increased with photosynthetic photon flux density (PPFD) and leaf temperature (TL). PN was not saturated at PPFDs above 1500 µmol m−2 s−1 and at TL above 30 °C. On a sunny day in mid summer, maximum PN was two times higher in E. indica than in P. asiatica [42 vs. 20 µmol(CO2) m−2 s−1], but their transpiration rate (E) and the leaf water potential (ΨL) were similar. Soil-to-leaf hydraulic conductance, which probably plays a role in water absorption from the trampled compact soil, was higher in E. indica than in P. asiatica. The differences in photosynthetic traits between E. indica explain why E. indica communities more commonly develop at heavily trampled sites in summer than the P. asiatica communities.

Differences in sapling architecture between Fagus crenata and Fagus japonica

The ecological significance of architectural patterns for saplings of Fagus crenata and Fagus japonica co-occurring in a secondary oak forest were evaluated by comparing the size and shape of leaves, trunks and crowns. Fagus japonica saplings were different from F. crenata saplings in some architectural properties: (i) the leaf area and specific leaf area were larger; (ii) the ratio of sapling height to trunk length was lower, indicating greater leaning of the trunk; and (iii) the projection area of the crown was larger and the leaf area index lower indicating less mutual shading of leaves. These architectural features indicated that F. japonica saplings were more shade tolerant than F. crenata and F. crenata saplings were superior to F. japonica for growth in height and could, therefore, utilize sunlight in the upper layer. An erect trunk in F. crenata and a leaning trunk in F. japonica may be important characteristics associated with the regenerations patterns of each species; regeneration from seedlings under canopy gaps in the former and vegetative regeneration by sprouting in the latter.

Effect of soil moisture on leaf ecophysiology of Parasenecio yatabei, a summer-green herb in a cool–temperate forest understory in Japan

Leaf physiological and gas-exchange traits of a summer-green herbaceous perennial, Parasenecio yatabei, growing along a stream were examined in relation to leaf age. In its vegetative phase, the aerial part of this plant consists of only one leaf and provides an ideal system for the study of leaf longevity. Volumetric soil water content (SWC) decreased with increasing distance from the stream, whereas relative light intensity was nearly constant. The light-saturated net CO2 assimilation rate (Asat) and leaf stomatal conductance (gs) were approximately 1.5-fold and 1.4-fold higher, respectively, in the lower slope near the mountain stream than in the upper slope far from the mountain stream. The lifespan of aerial parts of vegetative plants significantly increased with decreasing SWC. The leaf mass-based nitrogen content of the leaves (Nmass) was almost constant (ca. 2.2%); however, the maximum carboxylation rate by ribulose-1,5-biphosphate carboxylase/oxygenase (rubisco) (Vcmax) and photosynthetic nitrogen use efficiency (PNUE, Asat/Narea) decreased more slowly in the upper slope than in the lower slope. The higher leaf photosynthetic activity of P. yatabei plants growing lower on the slope leads to a decrease in Vcmax and PNUE in the early growing season, and to a shorter leaf lifespan.