Roles of dominant understorySasabamboo in carbon and nitrogen dynamics following canopy tree removal in a cool-temperate forest in northern Japan

To clarify the role of dense understory vegetation in the stand structure, and in carbon (C) and nitrogen (N) dynamics of forest ecosystems with various conditions of overstory trees, we: (i) quantified the above- and below-ground biomasses of understory dwarf bamboo (Sasa senanensis) at the old canopy-gap area and the closed-canopy area and compared the stand-level biomasses of S. senanensis with that of overstory trees; (ii) determined the N leaching, soil respiration rates, fine-root dynamics, plant area index (PAI) of S. senanensis, and soil temperature and moisture at the tree-cut patches (cut) and the intact closed-canopy patches (control). The biomass of S. senanensis in the canopy-gap area was twice that at the closed-canopy area. It equated to 12% of total biomass above ground but 41% below ground in the stand. The concentrations of NO3− and NH4+ in the soil solution and soil respiration rates did not significantly change between cut and control plots, indicating that gap creation did not affect the C or N dynamics in the soil. Root-length density and PAI of S. senanensis were significantly greater at the cut plots, suggesting the promotion of S. senanensis growth following tree cutting. The levels of soil temperature and soil moisture were not changed following tree cutting. These results show that S. senanensis is a key component species in this cool-temperate forest ecosystem and plays significant roles in mitigating the loss of N and C from the soil following tree cutting by increasing its leaf and root biomass and stabilizing the soil environment

Temporal variation in fine-root biomass, production and mortality in a cool temperate forest covered with dense understory vegetation in northern Japan

To understand the temporal pattern of fine-root dynamics and the factors that affect it, we investigated the seasonal and interannual variation in fine-root production (FRP) and fine-root mortality (FRM) rates, as well as fine-root biomass (FRB) and necromass in a cool temperate forest in northern Japan that was covered with dense understory vegetation of Sasa senanensis. We measured the root length density (RLD) and the rate of root production and mortality over 3 yr using minirhizotrons, and compared these rates with temperature, precipitation, soil moisture, and plant area indices (PAI). We also measured the FRB and the necromass of fine roots four times per year for 2 yr using soil cores and calculated dry weight-based FRP and FRM. FRB in the uppermost 15 cm of the surface-soil layer accounted for 41–61% of the biomass up to 60 cm soil depth, and decreased with increasing soil depth. The biomass of fine roots with root diameters <0.5 mm was almost equivalent to that of roots measuring 0.5–2 mm in diameter. Sasa roots accounted for 59–88% of the total FRB. FRB did not fluctuate seasonally, whereas RLD did. The FRP rate was high in mid- to late summer and correlated significantly with air and soil temperatures, indicating that temperature affects FRP. However, the relationship between FRP and soil moisture was weak. FRP was significantly correlated with the PAI of oak trees and the increment in the PAI of Sasa, suggesting that endogenous factors also affect FRP. Depending on the method used to calculate turnover, mean FRP for the 3-year study period was 589 or 726 g m−2 yr−1, accounting for 36% or 41% of forest net primary production, respectively. The results of this study illustrate the substantial seasonal and interannual fluctuations in FRP, and indicate that a significant proportion of assimilated carbon was allocated to below-ground root systems in an oak-Sasa stand

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

Effects of clear-cutting on nitrogen leaching and fine root dynamics in a cool-temperate forested watershed in northern Japan

Stream and soil solution chemistry, fine root biomass and soil nitrogen processing before and after clear-cutting of trees and subsequent strip-cutting of understory vegetation, dwarf bamboo (Sasa senanensis), were investigated to understand the effect of these disturbances on biogeochemical processes on forested watershed in northern Japan. Tree-cutting did not cause a significant increase of nitrate (NO3−) concentration in stream water during the growing season after the cutting. Subsequent Sasa-cutting caused significant increase of stream NO3− concentration to ca. 15 μmol L−1. There was no significant change of stream pH following both cuttings. NO3− concentration in soil solution increased after both cutting, but the change of concentration was higher (>100 μmol L−1) after the Sasa-cutting than after the tree-cutting. In a riparian conserved area, on the other hand, NO3− in soil solution remained low after tree-cutting, suggesting the riparian area acted as a NO3− sink after the tree-cutting. There was no significant change in total biomass of fine roots after the tree-cutting because of an increase in Sasa root biomass despite a decrease in tree roots. The subsequent Sasa-cutting caused a 50% decrease of fine root biomass compared to that in the un-cut Sasa site. These results suggested that nitrogen uptake by Sasa was very important in preventing nitrogen leaching after tree-cutting, and decline of this nitrogen uptake after Sasa-cutting lead to marked NO3− leaching to the stream

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