Intraclonal Nitrogen Allocation in the Bunchgrass Schizachyrium scoparium Hubb.: An Assessment of the Physiological Individual

Abstract

The size of the physiological individual within the bunchgrass Schizachyrium scoparium Hubb. was investigated by evaluating the pattern and magnitude of 15N allocation within and between individual ramet hierarchies (sequences of connected ramet generations). Nitrogen-15 was acropetally allocated throughout the three generation hierarchies within 24 h regardless of the ramet generation or organ labelled. Both primary and secondary ramet generations allocated nitrogen to juvenile, tertiary ramets. Basipetal nitrogen allocation, from secondary to ontogenetically older primary ramets, was observed, but accounted for less than 1% of the 15N mass in the ramet hierarchies at the end of the 5-day experiment. Foliarly labelled ramets exported less nitrogen intraclonally than did root labelled ramets indicating that a greater proportion of the nitrogen was rapidly incorporated into metabolic compounds. A majority (>93%) of the 15N introduced into the ramet hierarchies remained within the labelled hierarchies as opposed to being allocated to associated hierarchies within individual clones. Clones of this bunchgrass consist of an assemblage of autonomous physiological individuals, composed of a minimum of three connected ramet generations, as opposed to a system of completely integrated ramet hierarchies. The propensity for acropetal resource allocation appears to be the predominant factor limiting resource allocation between ramet hierarchies within young clones possessing complete vascular continuity. Key-words: Acropetal allocation, caespitose growth form, clonal biology, 1 5N, physiological integration, resource allocation Introduction The bunchgrass or caespitose growth form consists of an assemblage of genetically identical ramets growing in close proximity. The potential for physiological integration between and among ramets is established by the vascular connections formed when juvenile ramets are initiated from axillary buds of parental ramets. Acropetal resource allocation within ramet hierarchies (sequences of connected ramet generations) has been documented in several clonal graminoids with isotopic tracers (Anderson-Taylor & Marshall, 1983; Welker et al., 1985; Welker, Briske & Weaver, 1987). Although these data clearly demonstrate the capacity for physiological integration within the clone, the number of connected ramet generations participating in the acquisition, allocation and utilization of resources to form the physiological individual has received relatively little attention (Watson & Casper, 1984; but see Jonsdottir & Callaghan, 1988). The capacity for resource allocation between and among ramet hierarchies is a prerequisite for complete clonal integration and may be an important constraint regulating the size of the physiological individual. However, interhierarchical resource allocation requires basipetal resource allocation through older intervening ramet generations including the common progenitor of the clone. Basipetal resource allocation has been observed in several clonal species, but the ecological significance of this allocation pattern is obscured by the minimal quantities of resource allocation and the apparent inconsistency of the process (Clifford, Marshall & Sagar, 1973; Lauer & Simmons, 1988; Magda, Warembourg & Labeyrie, 1988; Jonsdottir & Callaghan, 1989). The pattern of resource allocation may also influence the rate and extent of intraclonal allocation. Nutrients may be absorbed and recycled between the root and shoot system of individual ramets (Simpson, Lambers & Dalling, 1982), absorbed, assimilated and eventually remobilized to connected ramet generations (Jonasson & Chapin, 1985; Thorne & Wood, 1987) or absorbed and This content downloaded from 157.55.39.111 on Sat, 17 Sep 2016 05:45:22 UTC All use subject to http://about.jstor.org/terms 434 allocated to connected ramets without being assiJ. M. Welker milated by the ramet which acquired the nutrient et al. from the environment (Marshall & Sagar, 1968; Welker et al., 1985, 1987). The predominant allocation route is presumably determined by the relative sink strengths among ramet generations and organs (i.e. roots, leaves, culms and inflorescences) within the clone and resource availability in the immediate environment (Pitelka & Ashmun, 1985; Lang & Thorpe, 1986). This investigation was conducted to evaluate the size of the physiological individual within intact clones of Schizachyrium scoparium Hubb. by identifying the pattern and magnitude of nitrogen allocation within and between ramet hierarchies. Root and foliar absorbed nitrogen were evaluated to contrast the importance of soil-absorbed and remobilized nitrogen, respectively, on the potential size of the physiological individual. S. scoparium possesses an architecture representative of the perennial bunchgrass growth form and is distributed throughout the eastern two-thirds of the USA