Sharon J. Lite

RESPONSE OF HERBACEOUS RIPARIAN PLANTS TO RAIN AND FLOODING ON THE SAN PEDRO RIVER, ARIZONA, USA

Herbaceous species comprise most of the floristic diversity in semi-arid region riparian zones, yet little is known about their response to river flooding. We compared cover, richness, and distribution of six functional groups of herbaceous plants after a large fall flood (pre-vs. post-flood year comparison) and after small monsoon floods and rains (dry vs. wet season contrast), and compared richness across a longitudinal (upstream-downstream) gradient of flood intensity. Herbaceous cover and richness increased significantly (p≤0.05, ANOVA) from the pre-flood to post-flood year and from the dry to wet season. Overall, the post-flood increases in richness and cover were related to the combined effects of disturbance (as indicated by strong increases of annual plants) and increased water availability (as indicated by response patterns of hydric perennials and other functional groups). All annuals showed strong increases in richness and cover in the year following the large fall flood, with hydric annuals increasing in richness by 43%, mesic annuals by 52%, and xeric annuals by 75%. Hydric perennials had a small net increase in richness following the large flood, reflecting a positive response to increased flow permanence, countered by low richness at sites with very high flood intensity (total stream power). Mesic and xeric perennials did not change significantly in richness from the pre- to post-flood year. However, across the spatial flood intensity gradient, the richness response pattern of the annuals and perennials alike peaked at intermediate levels of disturbance. In response to seasonal rains and moderate flooding, hydric perennials did not change in abundance, reflecting their primary association with shallow ground water and perennial stream base flows, but mesic perennials increased in cover and xeric perennials increased in both cover and richness. All three annual groups increased in cover and richness and in distribution across the flood plain following the summer monsoon floods and rains: hydric annuals had peak cover in inundated zones, suggesting positive response to river flooding, while xeric annuals peaked in cover above inundation zones, suggesting positive response to icreased rainfall; mesic annuals had intermediate patterns. During the dry season, in contrast, annuals had low richness and cover and were restricted to low elevation fluvial surfaces adjacent to the stream channel and/or underlain by shallow ground water. Overall, both disturbance and increased moisture conditions caused by floods, as well as moisture from seasonal rains, contribute to increased richness and cover of herbaceous plants within the flood plain of the San Pedro River.

Vegetation, soils, and hydrogeomorphology of riparian patch types of a dryland river

Abstract From a landscape perspective, riparian corridors can be viewed as mosaics of vegetation patches. We delineated 10 patch types within the floodplain of the San Pedro River (Arizona) on the basis of physiognomy, dominant overstory species, and tree size class; and we assessed differences in hydrogeomorphology, vegetation structure, plant species richness, and soil chemistry and texture. Patches of tamarisk (Tamarix), an introduced species, fell within the continuum of variation shown by other patch types in the landscape mosaic. Among the tree-dominated types, cottonwood-willow (Populus-Salix) and tamarisk patches were inundated more frequently than mesquite (Prosopis) patches, while cottonwood-willow patches had shallower groundwater than tamarisk or mesquite patches. Due to the wetter conditions, cottonwood-willow patches had a high relative abundance of wetland and exotic species in the understory. Tamarisk patches and wet shrublands (Baccharis salicifolia–Salix exigua) had high woody stem densities while cottonwood-willow patches had dense canopy cover. In association with differences in canopy cover, cottonwood-willow patches had low herbaceous species richness but high woody species richness, while tamarisk patches had high herbaceous and low woody species richness. Soil electrical conductivity, silt content, organic matter content, and available phosphorus increased from young to old stands of both tamarisk and cottonwood-willow, often resulting in greater differences between patches of different size/age class than between patches with different dominant species. Surface soil salinity (electrical conductivity) was low in all patches, including those dominated by tamarisk. Nitrate was abundant in soils of tamarisk patches (perhaps reflecting their high clay content) and wet shrubland patches. Dry shrublands (Hymenoclea-Ericameria) and wet shrublands were similar to young forest patches in having coarse soils with little organic matter.