851  2001 Estuarine Research Federation Estuaries Vol. 24, No. 6A, p. 851–861 December 2001 Response of Estuarine Marsh Vegetation to Interannual Variations in Precipitation KENNETH H. DUNTON 1,* ,BEAU HARDEGREE 2 , and TERRY E. WHITLEDGE 3 1 The University of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port Aransas, Texas 78373 2 Texas Parks and Wildlife Department, 6300 Ocean Drive, Corpus Christi, Texas 78412 3 University of Alaska at Fairbanks, Institute of Marine Science, Fairbanks, Alaska 99701 ABSTRACT: The response of deltaic emergent marsh vegetation to increases in precipitation was examined over a 14- mo period at three sites in the lower Nueces Estuary in south Texas. At all three sites, significant changes in plant biomass, percent cover, and allocation of aboveground and belowground tissues were associated with more than double the rainfall in late winter and early spring 1992 compared to the previous year and the 50-yr average for this region. Water column salinities, which ranged from 10‰ to 35‰ at all three sites in 1991, remained below 10‰ through August 1992. Significant changes in marsh vegetative structure included decreases in bare space, increases in the percent cover and aboveground biomass of a relatively less salt tolerant halophytes (Borrichia frutescens), and significant increases in root:shoot ratios in B. frutescens, Batis maritima and Suaeda linearis (in Salicornia virginica root:shoot ratios decreased significantly). Higher precipitation generally led to an overall increase in the biomass of most marsh perennials, but these increases were not statistically significant. For one species, Lycium carolinianum, additional rainfall extended its growing season through August 1992, 2 mo longer than in the previous year. The expansion (58%) of B. frutescens at one site was also coincident with the significant loss of B. maritima, whose cover decreased nearly 20%. In an ecological context, these responses suggest that precipitation events in arid environments may be considered a major physical disturbance that can result in large changes in the composition and relative abundance of emergent vascular plants over a relatively short period. The long-term significance of these changes is unknown and demonstrates the value of ecological studies that are conducted over several years for a more complete understanding of the dynamic processes that regulate marsh productivity. Introduction Coastal deltaic marshes form a valuable and pro- ductive interface between terrestrial and marine habitats. The mosaic of vegetation assemblages and the persistence of these coastal landscapes are products of uninterrupted inputs of riverine nutri- ents and detrital matter that sustain habitats essen- tial for estuarine-dependent species. Despite their recognized importance (see Valiela et al. 1978; Wil- cox and Childress 1981; Mitsch and Gosselink 1986), wetlands have historically been regarded as a hindrance to productive land use, and in the con- tiguous United States, about 53% of the original wetland area has been destroyed (Dahl and John- son 1991). This loss has accelerated in recent years as diversion and channelization of river systems has reduced sediment deposition, resulting in wide- spread subsidence (Baumann et al. 1984). The insidious diversion of freshwater inflow to support the water demands of rapidly expanding metropolitan areas is becoming a serious problem for many coastal marsh systems. In south Texas, * Corresponding author; tele: 361/749-6744; fax: 361/749- 6777; e-mail: dunton@utmsi.utexas.edu. diversion of the Nueces River from the deltaic marsh system in combination with continuous de- clines in freshwater inflow since 1968 have resulted in higher salinities and reversal of delta progra- dation to active erosion (Morton and Paine 1984; White and Calnan 1990; Solis 1994). Net aerial pri- mary production for this marsh is only 60% to 70% of that measured in two adjacent river deltas in south Texas (Ward 1985; Solis 1994). The lower productivity of the Nueces marsh is attributed to the nearly complete diversion of freshwater inflow from the marsh, which has resulted in reduced nu- trient input and increased salinities. Based on experimental evidence, increased sa- linities have been noted to reduce photosynthesis and aboveground biomass in some marsh peren- nials (Naidoo and Mundree 1993; Hacker and Bertness 1995). Seed germination and survival are strongly suppressed by high salinities (Shumway and Bertness 1992), which ultimately affects vege- tative patterns and plant distributions (Bertness et al. 1992). There is some field evidence that plant species respond to increased precipitation by in- creasing biomass, but these studies have mainly fo- cused on one species, Spartina alterniflora (Dame