Effects of resource heterogeneity on nitrogen translocation within clonal fragments of Sasa palmata: an isotopic (15N) assessment

Abstract

Background and aims: Clonal fragments of the rhizomatous dwarf bamboo Sasa palmata, which widely predominates in temperate regions of Japan, were grown under heterogeneous resource conditions such as gap understories or nutrient-patchy grassland. Clonal fragments develop multiple ramets with long rhizomes and appear to be physiologically integrated by the translocation of assimilates. The glasshouse experiment reported here was designed to clarify the mechanisms of physiological integration of nitrogen more precisely.

Methods: To assess how resource conditions influence the amount of nitrogen translocation, and which organ acts as the strongest sink, two experiments were conducted that traced movement of 15N label between interconnected pairs of ramets to compare homogeneous and heterogeneous light and soil nitrogen conditions.

Key results: The amount of 15N translocated to leaves was between 9% and 11% greater in high-N and high-light ramets in the heterogeneous compared with homogeneous treatments. Under heterogeneous soil nitrogen conditions, translocation increased from individual ramets in resource-rich patches to ramets in resource-poor patches, while the reverse was true under heterogeneous light environments, reflecting differences in the positions of leaves that act as the strongest sinks. Neither the mass increments nor the total mass of clonal fragments was significantly affected by heterogeneity of either light or nutrients, possibly because the experimental period was too short for differences to manifest themselves.

Conclusions: This study clearly demonstrated that nitrogen is readily translocated between ramets, particularly under heterogeneous resource conditions. The translocation patterns were governed by functional 'division of labour' mechanisms that resulted in net nitrogen movement from understory sites to gaps, thereby enhancing the carbon acquisition of the whole fragment. Thus, physiological integration may provide benefits for S. palmata when it is growing under heterogeneous conditions in which there are deficits of certain environmental resources.

Fig. 1.

Accumulation of dry mass by ramets within clonal fragments in Sasa palmata when heterogeneous and homogeneous resource conditions are compared for the effects of (A) soil nitrogen (expt 1) or (B) light (expt 2). Bars show means; vertical lines show s.e. for total mass of ramets (up) and mass of ramet components within clonal fragments (down).

Fig. 2.

Total N content (open bars) and amount of ¹⁵N (shaded bars) per (A and C) ramet and per (B and D) current-year leaves in Sasa palmata when heterogeneous and homogeneous resource conditions are compared for the effects of (A and B) soil nitrogen (expt 1) or (C and D) light (expt 2). ¹⁵N was applied to the soil around the ramets indicated by arrows. See Table 1 for results of significance tests.

Fig. 3.

Partitioning of ¹⁵N among ramet components within clonal fragments in Sasa palmata on comparison of heterogeneous and homogeneous resource conditions for the effects of (A) soil nitrogen (expt 1) or (B) light (expt 2).The amount of ¹⁵N applied to the soil around the ramets is shown on the vertical axis. Downward error bars show s.e. for ramet components within clonal fragments. See Table 1 for results of significance tests.