Juliet C. Stromberg

Don't judge species on their origins

Conservationists should assess organisms on environmental impact rather than on whether they are natives, argue Mark Davis and 18 other ecologists.

RIPARIAN VEGETATION RESPONSE TO ALTERED DISTURBANCE AND STRESS REGIMES

River damming and flow regulation can alter disturbance and stress regimes that structure riparian ecosystems. We studied the Bill Williams River in western Arizona, USA, to understand dam-induced changes in channel width and in the areal extent, structure, species composition, and dynamics of woody riparian vegetation. We conducted parallel studies along a reference system, the Santa Maria River, an unregulated major tributary of the Bill Williams River. Flood magnitude on the Bill Williams River has been dramatically reduced since the closure of Alamo Dam in 1968: the 10-yr recurrence interval flood in the pre-dam era was 1397 m3/s vs. 148 m3/s post-dam. Post-dam average annual flows were higher due to increased precipitation in a few years, but increases in post-dam May–September flows are largely attributable to dam operation. An analysis of a time series of aerial photographs showed that channels along the Bill Williams River narrowed an average of 111 m (71%) between 1953 and 1987, with most narrowi...

Establishment of woody riparian vegetation in relation to annual patterns of streamflow, Bill Williams River, Arizona

Previous studies have revealed the close coupling of components of annual streamflow hydrographs and the germination and establishment ofPopulus species. Key hydrograph components include the timing and magnitude of flood peaks, the rate of decline of the recession limb, and the magnitude of base flows. In this paper, we retrospectively examine establishment of four woody riparian species along the Bill Williams River, Arizona, USA, in the context of annual patterns of streamflow for the years 1993–1995. The four species examined were the nativePopulus fremontii, Salix gooddingii, andBaccharis salicifolia and the exoticTamarix ramosissima. We modeled locations suitable for germination of each species along eight study transects by combining historic discharge data, calculated stage-discharge relationships, and seed-dispersal timing observations. This germination model was, a highly significant predictor of seedling establishment. Where germination was predicted to occur, we compared values of several environmental variables in quadrats where we observed successful establishment with quadrats where establishment was unsuccessful. The basal area of mature woody vegetation, the maximum annual, depth to ground water, and the maximum rate of water-table decline were the variables that best discriminated between quadrats with and without seedlings. The results of this study suggest that the basic components of models that relate establishment ofPopulus spp. to annual patterns of streamflow may also be applicable to other woody riparian species. Reach-to-reach variation in stage-discharge relationships can influence model parameters, however, and should be considered if results such as ours are to be used in efforts to prescribe reservoir releases to promote establishment of native riparian vegetation.

Marsh development after large floods in an alluvial, arid-land river

Large expanses of riverine marsh are rare in the desert Southwest, given the dry surface of many floodplain soils. Along the Hassayampa River, riverine marsh underwent a 5-fold increase (from 2% to 9% of the floodplain-channel area) after a large winter flood in 1993. Flood waters eroded terraces that had aggraded during frequent, smaller floods, widened the channel from about 3 to 50 m, and recharged the floodplain aquifer. The net effect of these changes was a lowering of the floodplain surface relative to the water table, a variable of critical importance to riparian plant composition in arid-land rivers. Olney’s bulrush (Scirpus americanus Pers.), southern cattail (Typha domingensis Pers.), jointed rush (Juncus articulatus L.), and other obligate wetland species were abundant in 1993 and 1994 on areas with saturated surface soils or shallow water tables and often were intermixed with seedlings of early-seral tree species, including Fremont cottonwood (Populus fremontii S. Watson), Goodding willow (Salix gooddingii Ball), and salt cedar (Tamarix chinensis Loureiro and related species). The gain in riverine marsh and young cottonwood-willow stands occurred at the expense of mature cottonwood-willow forests and deep-rooted, velvet mesquite (Prosopis velutina Woot.) woodlands. Another large flood in 1995 scoured the channel of most existing vegetation and aggraded the 1993 flood channel. Early-seral tree species again established in moist soils exposed by the slowly receding flood waters. However, redevelopment of extensive marsh habitat was precluded by sediment deposition that increased the elevation of the floodplain surface relative to the water table. These changes highlight the transitory nature of riverine marsh and other vegetation patch types in the dynamic floodplains of alluvial, arid-land rivers and underscore the importance of maintaining flood flows of varying magnitude to maintain patch type diversity.

Instream flow and cottonwood growth in the eastern Sierra Nevada of California, USA

Dendro-ecological studies indicated that radial growth of Populus trichocarpa was significantly related to annual streamflow at 20 riparian sites in the eastern Sierra Nevada of California. The strength of the relationship varied among sites, depending on geomorphology and tree cover. The strongest correlation between streamflow and tree growth occurred at sites in wide, unconfined valleys, where alluvial groundwater typically fluctuates directly with surface water. In such areas, trees on streambanks and in the floodplain showed equally strong relationships between flow and growth. In narrow mountain canyons, relationships between tree growth and streamflow were weaker and showed more within-site variability. Streambank trees in the canyon settings generally showed stronger relationships with streamflow than did floodplain trees. These data suggest that P. trichocarpa trees in confined canyons, in comparison with those in wide alluvial valleys, may rely to a greater extent on water sources that are not in direct hydraulic connection with surface water. Flow-growth models were also stronger at sites where tree basal area and density were low, including sites where flow diversion has caused tree mortality. Sparse tree cover may allow for a greater expression of flow-growth relationships by minimizing the effects of competition for light and other resources, and allowing for greater control of growth by abiotic rather than biotic factors.

Vegetation‐Hydrology Models: Implications for Management of Prosopis Velutina (Velvet Mesquite) Riparian Ecosystems

Prosopis velutina (velvet mesquite) forests are one of many types of aridland riparian ecosystems that are threatened by groundwater pumping and other types of water development. Empirical models developed using both hydrological and vegetational data sets have potential uses in the management of these threatened ecosystems. To this end, we developed models for Prosopis velutina stands across a xeric-to-mesic moisture gradient. The models expressed canopy height, basal area, leaf area index, vegetation volume, and leaflet area as functions of plant water potential, and they expressed plant water potential and riparian stand structure as functions of water table depth. These data indicated that stand structure was strongly related to water availability. Management applications of the models include the ability (1) to identify minimum water-table depths for riparian stand maintenance and (2) to detect stressful hydrological conditions, via water potential measurements, before the onset of structural degradation.

Functional equivalency of saltcedar (Tamarix Chinensis) and fremont cottonwood (Populus fremonth) along a free-flowing river

Saltcedar (Tamarix chinensis) reportedly has altered ecosystem function in many riparian areas of the American Southwest. I investigated the functional role of this exotic species relative to native Fremont cottonwood (Populus fremontii) along the free-flowing middle San Pedro River in southern Arizona. Floodplains in this river reach are co-dominated by young stands of saltcedar (<44 yrs old) and Fremont cottonwood (<67 yrs old). Mean values for 22 of 30 soil, geomorphology, and vegetation structure traits did not differ significantly between saltcedar and Fremont cottonwood stands. Twenty-six of the 30 traits had similar patterns for change over time (infereed from a succession gradient). Saltcedar was functionally equivalent to Fremont cottonwood for about half of the traits construed as indicators of riparian ecosystem function. Several functions or traits that are theoretically or observably influenced by saltcedar on other rivers (e.g., sedimentation rates, electrical conductivity of soils) did not differ between the two species along the San Pedro. This suggests that the functional role of saltcedar is context-specific and variable among rivers. Also in contrast to the working paradigm, saltcedar appeared to enhance the maintenance of floristic biodiversity. Understory herbaceous cover and species richness were significantly greater than in cottonwood stands, perhaps due to soil differences that developed between the two stand types (e.g., higher clay content in saltcedar soils). Stem densities of velvet mesquite (Prosopis velutinu) and other woody successional species did not differ between saltcedar and cottonwood stands. However, stem densities for this group increased with stand age only for cottonwood, raising the possibility that salicedar may disrupt successional pathways. Other notable differences included greater basal area and canopy height in cottonwood stands, and accumulation of more phosphorus in cottonwood soils than in saltcedar soils.

Coupling groundwater and riparian vegetation models to assess effects of reservoir releases

Although riparian areas in the arid southwestern United States are critical for maintaining species diversity, their extent and health have been declining since Euro-American settlement. The purpose of this study was to develop a methodology to evaluate the potential for riparian vegetation restoration and groundwater recharge. A numerical groundwater flow model was coupled with a conceptual riparian vegetation model to predict hydrologic conditions favorable to maintaining riparian vegetation downstream of a reservoir. A Geographic Information System (GIS) was used for this one-way coupling. Constant and seasonally varying releases from the dam were simulated using volumes anticipated to be permitted by a regional water supplier. Simulations indicated that seasonally variable releases would produce surface flow 5.4–8.5 km below the dam in a previously dry reach. Using depth to groundwater simulations from the numerical flow model with conceptual models of depths to water necessary for maintenance of riparian vegetation, the GIS analysis predicted a 5- to 6.5-fold increase in the area capable of sustaining riparian vegetation.

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.

Soil seed banks of two montane riparian areas: implications for restoration

Understanding the role of seed banks can be important for designing restoration projects. Using the seedling emergence method, we investigated the soil seed banks of two montane, deciduous riparian forest ecosystems of southeastern Arizona. We contrasted the seed banks and extant vegetation of Ramsey Canyon, which is the site of riparian restoration activities, with that of Garden Canyon, which has been less affected by human land uses. Fewer plant species were found at Ramsey Canyon than Garden Canyon, for both the seed bank and extant vegetation, and the vegetation at Ramsey Canyon (seed bank and extant) had consistently drier wetland indicator scores. As well, vegetation patterns within sampling zones (channel margins and adjacent riparian forests) differed between canyons. At Garden Canyon channel margins, the seed bank and extant vegetation had relatively high similarity, with herbaceous wetland perennial species dominating. Extant vegetation in the floodplain riparian forest zone at Garden Canyon had a drier wetland indicator score than the seed bank, suggesting that the floodplains are storing seeds dispersed from wetter fluvial surfaces. Vegetation patterns for Ramsey Canyon channel margins were similar to those for Garden Canyon floodplains. Vegetation patterns in the Ramsey Canyon riparian forest zone were indicative of non-flooded conditions with an abundance of upland species in the soil seed bank and extant vegetation. Channel geomorphology measurements indicated that much of the riparian forest zone at Ramsey Canyon is functionally a terrace, a condition that may be a legacy of channel erosion from historic land uses. Steep, erodible channel slopes may contribute to the low seed bank germinant density at Ramsey Canyon channel margins, and narrower flood-prone area may explain the greater terrestrialization of the vegetation in both sampling zones. We recommend testing the use of donor soils from more diverse stream reaches to restore biodiversity levels at Ramsey Canyon, following restoration activities such as channel-widening. Seed banks from Garden Canyon, for example, although predominantly consisting of herbaceous perennials, would supply species with a range of moisture tolerances, life spans, and growth forms. We also recommend that restorationists take care not to harm seed banks exposed during removal of introduced species; at Ramsey Canyon, soil seed banks were equally diverse in areas with high and low cover of the introduced Vinca major (a legacy of Ramsey Canyon land use).