Nutrient Enrichment, Tannins, and Thorns: Effects on Browsing of Shrub Seedlings

Abstract

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 blackbrush 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 whitetailed deer. Transplanted seedlings of both species should be protected from herbivores during early stages of establishment. JOURNAL OF WILDLIFE MANAGEMENT 69(2):782-793; 2005

Fertilization—Effects on deciduous tree growth and browsing by moose

“…Distribution and feeding by snow-shoe hare (Lepus americanus Erxleben) was also affected by nitrogen fertilization in the way that they were attracted to the area with fertilized plots, but fed more intensively only within the fertilized plots (Nams et al, 1996). However, Cash and Fulbright (2005) did not find any support of increased browsing from white-tailed deer on two Acacia species after fertilization.…”

“…The resources available for plants may determine not only growth rate but also the amounts of secondary compounds in the plant tissues. For example, better growth conditions may cause the plant to use its carbon resources for growth, and thereby reduce carbon available for compounds acting as deterrents (Bryant et al, 1983;Coley et al, 1985;Cash and Fulbright, 2005). Plant growth rate has been coupled with herbivory in the plant vigour hypothesis, which states that herbivory is positively correlated to growth rate of plants (Coley et al, 1985;Price, 1991;Kytö et al, 1996;Cash and Fulbright, 2005).…”

“…For example, better growth conditions may cause the plant to use its carbon resources for growth, and thereby reduce carbon available for compounds acting as deterrents (Bryant et al, 1983;Coley et al, 1985;Cash and Fulbright, 2005). Plant growth rate has been coupled with herbivory in the plant vigour hypothesis, which states that herbivory is positively correlated to growth rate of plants (Coley et al, 1985;Price, 1991;Kytö et al, 1996;Cash and Fulbright, 2005). Resource availability changes do not only affect tissue nutritional quality in plants, but also other plant traits such as tree growth, twig biomass and shoot length to which herbivores also respond (Danell et al, 1991;Edenius, 1993;Cash and Fulbright, 2005).…”

“…Plant growth rate has been coupled with herbivory in the plant vigour hypothesis, which states that herbivory is positively correlated to growth rate of plants (Coley et al, 1985;Price, 1991;Kytö et al, 1996;Cash and Fulbright, 2005). Resource availability changes do not only affect tissue nutritional quality in plants, but also other plant traits such as tree growth, twig biomass and shoot length to which herbivores also respond (Danell et al, 1991;Edenius, 1993;Cash and Fulbright, 2005). For example, growth rate was a determinant of food selection by moose Alces alces L. both between and within plant species (Danell et al, 1985;Bergströ m and Danell, 1987).…”

Spiny east Mediterranean plant species flower later and in a drier season than non-spiny species

“…Thus, there is a permanent evolutionary arms race between the plants and their herbivores, in which, on an evolutionary time scale, plants acquire better defenses and herbivores partly or fully overcome them (Cornell and Hawkins, 2003). Spines, thorns and prickles are well known as physical protection against herbivory (e.g., Cash and Fulbright, 2005;Cooper and Owen-Smith, 1986;Grubb, 1992;Gowda, 1996;Janzen, 1986;Janzen and Martin, 1982;Myers and Bazely, 1991;Rebollo et al, 2002) because they can wound mouth parts and digestive systems (Janzen and Martin, 1982;Janzen, 1986) as well as other body parts of the herbivores. In addition, spines inject pathogenic bacteria and fungi into large herbivores (Halpern et al, 2007a(Halpern et al, , 2007bLev-Yadun and Halpern, 2008).…”