DEFOLIATION AND WOODY PLANT (PROSOPIS-GLANDULOSA) SEEDLING REGENERATION - POTENTIAL VS REALIZED HERBIVORY TOLERANCE

Herbivory by rodents, lagomorphs and insects may locally constrain woo dy plant seedling establishment and stand development. Recruitment may therefore depend either upon plant tolerance of herbivory, or low her bivore abundance, during seedling establishment. We tested potential h erbivory tolerance by quantifying growth, biomass allocation, and surv ival of defoliated Prosopis glandulosa seedlings under optimal abiotic conditions in the absence of competition. Realized tolerance was asse ssed by clipping seedlings of known age grown in the field with and wi thout herbaceous competition. At 18-d (= 'young') or 33-d (= 'old') of age, seedlings in the growth chamber were clipped just above the firs t (cotyledonary) node, above the fourth node, or were retained as non- clipped controls. Potential tolerance to defoliation was high and neit her cohort showed evidence of meristematic limitations to regeneration . Clipping markedly reduced biomass production relative to controls, e specially belowground, but survival of seedlings defoliated 5 times wa s still greater than or equal to 75%. Contrary to expectations, surviv al of seedlings defoliated above the cotyledonary node 10 times was gr eater (P < 0.10) for 'young' (75%) than 'old' (38LTo) seedlings. Under field conditions, survival of defoliated 11-month-old P. glandulosa s eedlings was less than or equal to 59% after one defoliation and only less than or equal to 13% after six defoliations. Results indicate P. glandulosa is potentially tolerant of repeated shoot removal early in its life cycle. Seedling tolerance to defoliation under field conditio ns therefore appears dependent upon abiotic stresses or resource limit ations rather than a lack of intrinsic adaptations for shoot replaceme nt or a depletion of the seedlings' bud bank. Curtailment of root grow th, a consequence of top removal observed in the growth chamber experi ment, may reduce the capacity of P. glandulosa seedlings to acquire so il resources needed for meristem activation and shoot growth under fie ld conditions. The importance of resource availability to post-defolia tion regeneration was implicated in the field experiment, where surviv al, shoot elongation, and aboveground productivity of clipped seedling s was greatest in plots without herbaceous interference. In light of t he marked increases in P. glandulosa abundance in grasslands in recent history, our results suggest that (1) utilization of Prosopis seedlin gs by herbivores may be infrequent or sporadic, (2) periodic episodes of seed production and germination may satiate herbivore populations, and/or (3) Prosopis seedling establishment occurs during periods of lo w herbivore density.