Figure 14-1. Model I: determinants of potential resorption.

relations and nutrient use strategies, and long-term site fertility (Fig. 14-1). Hypotheses describing these determinants of potential resorption are outlined below. Other factors also influence potential and realized resorption efficiency and proficiency (e.g., CO2 enrichment, Norby et al, 2000; leaf size and specific leaf mass, Killingbeck and Whitford, 2001; life-form class, Killingbeck et al, 2002; temperature, Nordell and Karlsson, 1995), but because it is beyond the scope and intent of these models to consider every factor that has ever been related to resorption, only the best documented, or theoretically most compelling relationships will be addressed.

Potential resorption that is low (inefficient and unproficient) in phylogenetic precursors decreases potential resorption, and potential resorption that is high in phylogenetic precursors increases potential resorption. Similarities in resorption among related taxa, and dissimilarities among less related taxa, suggest that phylogeny (taxonomic related-ness) influences potential resorption (Killingbeck, 1996). Unusually high or low potential resorption in a species could conceivably be the result of a close taxonomic affiliation with precursors in which evolutionary selection was particularly strong, or weak, for efficient and proficient resorption.

Weak nutrient sinks present during senescence decrease potential resorption, and strong sinks present during senescence increase potential resorption. Species have inherent differences in the strength of nutrient sinks present during leaf senescence. Strong sinks, such as large masses of maturing fruits or developing roots, should act to elevate potential resorption, while weak sinks should decrease potential resorption. Phenology is a critical component of this relationship because, for example, flowers and fruits produced in early summer add nothing to autumnal sink-strength.

High long-term site fertility decreases potential resorption, and low long-term site fertility increases potential resorption. In seven of 11 studies that examined the effects of fertility on resorption in natural sites that differed in long-term fertility, resorption was higher on low fertility sites than on high fertility sites (Small, 1972; Stachurski and Zimka, 1975; Boerner, 1984a, 1984b; Kost and Boerner, 1985; Ralhan and Singh, 1987; Pugnaire and Chapin, 1993: but see Ostman and Weaver, 1982; Staaf, 1982; del Arco et al., 1991; Minoletti and Boerner, 1994). This suggests the existence of evolutionary tradeoffs between resorption and long-term site fertility.

Well-developed nutrient conservation and acquisition adaptations decrease potential resorption, and poorly-developed nutrient conservation and acquisition adaptations increase potential resorption. Species differ considerably in their abilities to acquire and conserve nutrients. Well-developed nutrient acquisition and/or conservation adaptations such as exceptionally efficient nutrient uptake or symbiotic nitrogen-fixation should decrease potential resorption, while the lack of such adaptations should increase potential resorption. Plants harboring nitrogen-fixing symbionts have consistently low nitrogen resorption efficiency (Dawson and Funk, 1981; Rodriguez-Barrueco et al., 1984, Coté and Dawson, 1986; Coté et al., 1989; Killingbeck, 1993) suggesting that the evolution of nutrient acquisition and nutrient resorption may be guided by a degree of reciprocity. Additionally, embedded within this determinant is a cadre of other plant adaptations that may influence potential resorption. Leaf habit is perhaps the most studied and debated of these, yet the questions of if and how potential resorption differs between evergreen and deciduous species remain unresolved (Killingbeck, 1996, 1998; Craine and Mack, 1998; Aerts and Chapin, 2000).

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