We studied the relationship between the light environment and distribution patterns of understory woody species in a natural beech forest in Japan. Stem locations and diameter at breast height (dbh) of 2,039 understory trees comprising of 30 species (1 cm≦dbh<cm) were investigated in a 0.4-ha plot. Relative photosynthetic photon flux density (rPPFD) at 0,2 and 4 m above ground was measured at 189 points in the plot in September and November. The understory vegetation was covered with heavy snow until early June, when the overstory trees had already leafed out. Using the rPPFD data at 4m height in September, the light condition above each stem was evaluated for the understory trees. The rPPFD value varied within a wide range, 5.4〜65.7%. Higher rPPFD values were related to the presence of canopy gaps of ≧100 m2. Two types of understory species were recognized from their distribution patterns, according to the relationship between stem locations and rPPFD distribution. The Prunus grayana type was distributed widely throughout the plot without any change in its local density, irrespective of the rPPFD level. The Corylus sieboldia type showed an aggregated distribution at sites with a high rPPFD value, although it also appeared throughout the plot. These results suggest that the varied light environment resulting from a heterogeneous canopy structure is an important factor affecting local density for some components of the understory community.
Inputs, outputs, and retention of leaf litter from deciduous riparian forest were examined in a headwater section of Horonai Stream, a low-gradient, gravel-bed stream in southwestern Hokkaido, Japan. During the leaf-fall season (late September-early November), total inputs exceeded total outputs, and 56% of the former was stored in the stream section. In contrast, during the post-leaf-fall season (mid November-early March), total outputs amounted to 609% of total inputs, suggesting that leaf litter stored during the leaf-fall season was decreased by fluvial transport after all of the leaves had fallen from the riparian trees. Correspondingly, leaf litter retention was highest in late autumn (November), followed by early spring (March), and the lowest in early summer (June). Irrespective of season, pools were more retentive than riffles. Further more, leaf litter retention in riffles and pools increased with woody debris abundance in autumn and early summer, respectively. Pools and woody debris were suggested to be important retention structures in the low-gradient, gravel-bed stream.
Although various leaf traits (such as photosynthetic capacity, leaf mass per area (LMA), longevity, nitrogen content and toughness) are highly influenced by changes in environmental conditions, strong correlations exist among leaf traits. Photosynthetic productivity and leaf persistence are negatively correlated, i.e., short-lived leaves often have higher photosynthetic activity and lower defensive ability than long-lived leaves. There are two major hypotheses for explaining the observed variation of leaf longevity: the nutrient use efficiency theory and the carbon balance theory. The theories differ in their viewpoints of the factors limiting photosynthetic activity, i.e. mineral limitation and carbon limitation. In general, plants living in tundra environments suffer from a short growing season, and the extent of this stress changes along the altitudinal, latitudinal and snowmelt gradients in arctic and alpine ecosystems. As season length decreases, deciduous plants produce shorter-lived, lower-LMA and nitrogen-richer leaves, whereas evergreen plants produce longer-lived leaves within and among specics. This contrasting pattern of changes in leaf traits can be explained by a simple graphic model based on the carbon balance theory. The ecological significance of leaf-trait variations should be applied to the fields of reproductive ecology and biological interactions between plants and herbivores.