Nitrogen source utilization in co-existing canopy tree and dwarf bamboo in a northern hardwood forest in Japan

Nitrogen (N) competition among co-existing plant species utilizing different mycorrhiza types was explored through the investigation of N sources of oak trees and dwarf bamboo. Vertical distribution of fine roots, soil N pools, δ¹⁵N of leaves, and possible soil N sources and nitrate reductase activity (NRA) were all quantified. The fine roots of canopy trees were more concentrated in the surface soils than roots of the understory dwarf bamboo. Soil NH₄+ and extractable organic N (EON) content (based on unit weight) decreased from the organic horizon (O horizon) to the deep soils, the size of the NH₄+ pool per unit volume increased with soil depth, and the EON was approximately constant. Soil NO₃− was not detected at any soil depth or was not significant in value, while NO₃− captured by ion-exchange resin (IER) buried at a 10 cm soil depth and net nitrification were observed via laboratory incubation at all soil depths. The δ¹⁵N of the NH₄+ and EON pools increased with soil depth and the δ¹⁵N of NO₃− of IER was lower than that of other N forms, except for the δ¹⁵N of NH₄+ in the O horizon. Furthermore, root NRA tended to be lower in canopy trees than in the understory, implying lower dependency on NO₃− by canopy trees. The pattern of root distribution and mycorrhizal fungi association of the understory vegetation (as well as the high root NRA) suggested that dependence on N in deeper soils was higher in understory plants than in canopy trees. These findings indicate that understory vegetation mitigates soil N competition against co-existing canopy trees via the use of alternative N sources

Timing of forest fine root production advances with reduced snow cover in northern Japan : implications for climate-induced change in understory and overstory competition

To investigate the effect of reduced snow cover on fine root dynamics in a cool-temperate forest in northern Japan because of decreases in snowfall at high latitudes due to global warming, we monitored root length, production, and mortality before and after snow removal with an in-ground root scanner. We measured root dynamics of both overstory deciduous oak (Quercus crispula) and understory evergreen dwarf bamboo (Sasa nipponica), the two major species in the forest. Snow removal advanced the timing of peak root production by a month both in total and in Sasa, but not in oak. There was a significant interaction between snow removal and plant form on root production; this indicates that enhanced Sasa root production following snow removal might increase its ability to compete with oak. In contrast, snow removal did not enhance root mor-tality, suggesting that the roots of these species tolerate soil freezing. The earlier snow disappearance in the snow removal plot expanded the growing season in Sasa. We speculate that this change in the understory environment would advance the timing of root production by Sasa by extending the photosynthetic period in spring. We propose that different responses of root production to reduced snow cover between the two species would change the competitive interactions of overstory and understory vegetation, influencing net primary production and biogeochemistry (e.g., carbon and nitrogen cycles) in the forest ecosystem

Presence of understory dwarf bamboo determines ecosystem fine root production in a cool-temperate forest in northern Japan

Fine root biomass (FRB) and production (FRP) are crucial in forest carbon and nutrient cycling, but the factors controlling FRB and FRP are not well understood. Here, we examined FRB, FRP, aboveground environmental and stand factors, and soil environmental factors in four stands in a forest covered with dense understory vegetation of dwarf bamboo, Sasa senanensis (hereafter, Sasa). The four stands had different tree species composition and included a primary forest (PF), secondary forest (SF), conifer plantation (CP), and Sasa area (SA). We quantified the FRB and FRP of trees and Sasa separately using the ingrowth core method. Total FRP was higher in stands with substantial presence of Sasa (99–130 g m−2 yr−1) than in CP with scarce Sasa (69 g m−2 yr−1). Despite being occupied by Sasa alone, SA had high FRP, suggesting that the presence of Sasa regardless of trees is a key determinant of ecosystem FRP. Tree FRB increased with increasing tree aboveground biomass, tree density, or basal area at breast height, but Sasa FRB and total FRB decreased. Total FRP was also lower at higher values of these aboveground stand factors. In Sasa, specific root length was significantly higher, and root tissue density was significantly lower, than in trees, indicating the capacity of Sasa for explosive growth. Positive correlations between Sasa FRB or FRP and soil inorganic N or ammonium contents (i.e. N availability) were detected. We conclude that Sasa is important in determining FRB and FRP in this northern forest with understory vegetation.</p