An analysis of nitrogen content in central Texas Quercus species as a function of herbivory

Background/Question/Methods

Studies have documented that many plant populations undergo recruitment failure. The loss can be due to many factors which act on various life stages of a plant species. Recruitment failure may be due to seed production, but can also be due to factors which operate post germination on seedlings, such as herbivory or the seedlings response to abiotic factors, including light and moisture. The Quercus genus has received attention because of its lack of recruitment. This genus possesses a number of attributes that are considered relatively important to the successful support of wildlife communities, including nutritional support in the form of nitrogen. The effects of simulated herbivory on nitrogen content in the leaves of Quercus durandii var. breviloba, Q. fusiformis, and Q. rubra var. texana were evaluated at the Kerr Wildlife Management Area in central Texas. Juvenile (seedlings or saplings) within enclosures were left intact, or the stems clipped at 33%, 66%, or 100%. In addition, the natural herbivory was monitored by sampling seedlings outside of the enclosures. Percent nitrogen was determined using Kjeldahl analysis and compared between herbivory treatments and species.

Results/Conclusions

Quercus durandii var. breviloba had the highest percent mortality (10-63%) across all herbivory treatments, while Q. rubra var. texana generally had the lowest mortality (5-25%) among the herbivory treatments. Analysis of variance indicated significant differences in nitrogen content among the three species of Quercus, however no differences were found between the simulated herbivory treatments. There was not a significant interaction between species and simulated herbivory. Quercus durandii var. breviloba  had the highest percent nitrogen (1.32%), which was significantly greater than the Q. texana (0.99%), which was the lowest of the three species. Quercus fusiformis had 1.17% nitrogen. Preliminary findings suggest that herbivory may not affect the nitrogen content in these central Texas Quercus species, however nitrogen content does vary among the species.