INTERSPECIFIC AND INTRASPECIFIC VARIATIONS IN PRODUCTION OF SPINES AND PHENOLS IN PROSOPIS-CALDENIA AND PROSOPIS-FLEXUOSA

In central Argentina the leguminous tree Prosopis caldenia is more abu ndant in sites of relatively high productivity (lowlands), whereas P. flexuosa is more abundant in sites of relatively low productivity (slo pes and uplands). Based upon current antiherbivore defense theory, we predicted: (1) a higher investment in defenses in P. flexuosa than in P. caldenia, and (2) that limitations in resources would result in an increase of the defenses in both species. Our approach for testing the se predictions was to estimate leaf phenol concentration and spinescen ce in adults (field study) and seedlings (greenhouse study) of both sp ecies growing at different levels of resource availability. In adult p lants, the concentration of phenols was higher (P < 0.01) in P. flexuo sa than in P. caldenia, but the opposite relationship was observed in seedlings. The amount of biomass invested in spines was similar IP > 0 .10) in both species, whereas spine density was higher (P < 0.05) in P . caldenia than in P. flexuosa. In both species, limitations in resour ces did not result (P < 0.05) in increases in leaf phenol concentratio n, amount of biomass invested in spines, spine length, or spine densit y, except for the increase (P < 0.05)of spine density in seedlings of P. caldenia at low water and nutrient availability. In general, our re sults did not support current hypotheses on the production of antiherb ivore defenses. It is argued that factors such as herbivore behavior ( e.g., habitat selection, trampling, branch breakage) and alternative s inks for carbon (e.g., N-2 fixation, carbohydrate reserves), in additi on to resource availability in evolutionary and ecological time, shoul d be considered for a more complete understanding of the inter-and int raspecific variations in the production of both physical and chemical antiherbivore defenses.