Blackbrush acacia (Acacia rigidula) and guajillo (Acacia berlandieri) are major browse species in diets of white-tailed deer (Odocoileus virginianus) in southern Texas and northern Mexico. We conducted both field and greenhouse experiments to test the hypotheses that (1) fertilization reduces tannins and spinescence in black-brush acacia and guajillo seedlings, (2) thorn removal increases utilization of shrub seedlings by deer, and (3) fertilization alters seedling nutrient content, chemical composition, and growth characteristics. Guajillo seedlings grown in a greenhouse with high soil fertility were lower (P < 0.05) in protein-precipitating tannin content relative to guajillo seedlings grown with low fertility. In the field, tannin content and spinescence of fertilized and unfertilized seedlings were similar (P > 0.05). Fertilization increased the levels of nitrogen and digestible protein in shrub seedlings of both species grown in the greenhouse (P < 0.05), but low rainfall during the field study may have precluded a similar response. Intensity of browsing was similar on fertilized and unfertilized seedlings. Seedlings with thorns removed were browsed more heavily than those with thorns (P < 0.05) in the field, where several species of herbivores were present. However, in feeding trials with tame deer, thorns did not affect browsing intensity. Thorns may be more important than tannins in defending seedlings against herbivory, and may be more important in defending against browsing by lagomorphs and small mammals than against browsing by white-tailed deer. Transplanted seedlings of both species should be protected from herbivores during early stages of establishment.
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Vol. 69 • No. 2