LONG-TERM VEGETATION DYNAMICS MEDIATED BY HERBIVORES, WEATHER AND FIRE IN A JUNIPERUS-QUERCUS SAVANNA

Long-term (45-yr) basal area dynamics of dominant graminoid species we re analyzed across three grazing intensity treatments (heavily grazed, moderately grazed and ungrazed) at the Texas A&M University Agricultu ral Research Station on the Edwards Plateau, Texas. Grazing intensity was identified as the primary influence on long-term variations in spe cies composition. Periodic weather events, including a severe drought (1951-1956), had little direct influence on composition dynamics. Howe ver, the drought interacted with grazing intensity in the heavily graz ed treatment to exacerbate directional changes caused by grazing inten sity. Species response to grazing was individualistic and noisy. Three response groups were identified. Taller, more productive mid-grasses were most abundant under moderate or no grazing. Short grasses were mo st abundant under heavy grazing. Intermediate species were most abunda nt under moderate grazing and opportunistic to weather patterns. Grami noid diversity increased with the removal or reduction of grazing inte nsity. The moderately and ungrazed treatments appeared most resistant to short-term weather fluctuations, while the heavily grazed treatment demonstrated significance resilience when grazing intensity was reduc ed after over 110 yr of overgrazing. Identification of a 'climax' stat e is difficult. Significant directional change, which took nearly 20 y r, appears to continue in the ungrazed treatment after 45 yr of succes sion. The observed, relatively linear patterns of perennial glass comp osition within the herbaceous patches of this savanna were generally e xplained by traditional Clementsian succession. However, when dynamics of the herbaceous community are combined with the woody component of this savanna, the frequency and intensity of fire becomes more importa nt. Across the landscape, successional changes follow several pathways . When vegetation change is influenced by several factors, a multi-sca le model is necessary to demonstrate interactions and feedbacks and ac curately describe successional patterns. Absence of fires, with or wit hout grazing, leads ultimately to a Juiziperus/Quercus woodland with,o rating intensity primarily influencing the fuel load and hence fire in tensity.