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- Materials and Methods
Physiological integration between ramets of clonal plants is beneficial for processes such as ramet establishment, sustaining stressed ramets, and for resource uptake over large or heterogeneous areas (Ong & Marshall, 1979; Pitelka & Ashmun, 1985; Salzman & Parker, 1985; Headley et al., 1988; Jónsdóttir & Callaghan, 1988, 1990; Evans, 1992; Alpert, 1999; Hutchings et al., 2000). Physiological integration is suggested to occur when benefits of resource sharing outweigh the costs for ramets to stay interconnected (e.g. maintenance costs of keeping spacers or increased risk of pathogen spread among interconnected ramets, Pitelka & Ashmun, 1985; Caraco & Kelly, 1991; Wennstrom & Ericsson, 1992; D’Hertefeldt & van der Putten, 1998). Clonal plants show varying degrees of integration between interconnected ramets, ranging from short-period initial subsidy to developing ramets to long-period, extensive integration in clonal ramet systems (Marshall, 1990; Jónsdóttir & Watson, 1997; Marshall & Price, 1997). Integration patterns and the importance of physiological integration for ramet growth and survival has been studied either by severing ramet connections or by using tracers (Watson & Casper, 1984; Pitelka & Ashmun, 1985; Marshall, 1990).
Pitelka & Ashmun (1985) addressed the advantages of physiological integration in different habitats, concluding that different types of integration should occur, depending on environmental heterogeneity and stability. For example, the extensive integration demonstrated in the sand sedge, Carex arenaria (i.e. full integration in large clonal systems; Jónsdóttir & Watson, 1997) has been suggested to be beneficial in poor, heterogeneous habitats (Tietema & van der Aa, 1981; Noble & Marshall, 1983; Pitelka & Ashmun, 1985; Hutchings & de Kroon, 1994; Jónsdóttir & Watson, 1997; D’Hertefeldt & Jónsdóttir, 1999; Hutchings et al., 2000). In the field, resources are, in general, heterogeneously distributed (Oborny & Cain, 1997; Hutchings et al., 2000) and high integration is shown from ramets in rich to ramets in poor patches (Ong & Marshall, 1979; Jónsdóttir & Callaghan, 1990; Alpert & Stuefer, 1997; Stuefer, 1998). Although resource heterogeneity affects physiological integration, it is not clear if extensive integration is dependent on resource heterogeneity (i.e. that plants in relatively homogeneous habitats would demonstrate more restricted integration). In the present experiment, we therefore studied whether high and low resource availability itself affected physiological integration in C. arenaria and the closely related brown sedge, Carex disticha.
We compared physiological integration in high- and low-resource systems of C. arenaria, which grows in resource-poor sand dunes, and of C. disticha, which grows in more productive habitats (lake shores and productive pastures). Plants were grown at high and low resource levels to study specifically if resource availability affected the pattern and amount of resource sharing. The resources that were manipulated and traced were water and potassium (K) because water can be scarce in the sand dunes and potassium allows us to study integration for a macronutrient.
We hypothesized that plants grown at low resource availability would show higher physiological integration, due to high, acropetal demand by young, unestablished (i.e. nonrooted) ramets, than plants grown at high resource availability. Specifically, we investigated whether a basipetal ramet exported more K towards the apex, in terms of amount and the transport distance, when the plant was previously grown with restricted access to K than when it had received ample K. We also investigated whether water taken up by a below-ground, basipetal ramet of a plant grown at low water availability was transported further towards the apex than when plants were well watered. Finally, we compared the distance and amount of integration for K and water in C. arenaria, from a poor site, with that in C. disticha, from a more productive site, in order to study physiological integration in species from contrasting habitats.