THRESHOLD MODEL IN STUDIES OF ECOLOGICAL RECOVERY IN BERMUDAGRASS (CYNODON DACTYLON L.) UNDER NUTRIENT STRESS CONDITIONS

Bermudagrass (Cynodon dactylon L.), an important turfgrass as well as a grazing species, was used as a case study for the threshold model in this paper. The growth of this grass in terms of root and shoot lengths and general quality (i.e., color) were evaluated under various levels (full, 1/4, 1/8, and 1/16 N) of nutrients in the culture medium. The concept of a simplified threshold model was described and applied and the conclusions were drawn on the bases of the results of this study. Bermudagrass shoot length and the grass general quality (i.e., color) significantly decreased at lower (1/8 and 1/16 N) nutrient levels. However, root length was stimulated at lower (1/4, 1/8, and 1/16 N) nutrient levels. The values of 1/8 and 1/16 N are significant, because the shoot length at the 1/8 N is reversible and this can be considered the conservation zone in the threshold model. However, the 1/16 N is irreversible or as an indicator of unrecoverable period of life span for bermudagrass which is equal to the critical threshold value. This can be a management guide for bermudagrass production for grazing purposes. Root length was stimulated at lower (1/4, 1/8, and 1/16 N) nutrient levels. Since the bermudagrass roots were not adversely affected by the stress and were somehow stimulated, this grass can be used for reclamation purposes. The grass general quality followed the same pattern as the shoot length, decreased only at the lower levels of nutrients (1/8, and 1/16 N). These three features of bermudagrass are important indicators for the life cycle of this species. These indicators would help the manager for an early warning phase between "healthy" and "unhealthy" states of this grass for production or grazing purposes.