Platanus wrightii

Abstract: 1. The growth of riparian trees in semi-arid regions is influenced by stream flow regime, but the relative importance of base flow and seasonal floods on growth has not been explored. I examined abiotic influences on the growth of Platanus wrightii in four stream reaches in Arizona. All reaches had a bimodal pattern of discharge, but only two had continuous flow throughout the growing season. 2. In two reaches of Sycamore Creek without perennial flow, a large percentage of the annual variation in radial growth rate of P. wrightii was explained by annual and growing season flow rate. Growth was related to these same variables in a perennial reach of Sycamore Creek, but trees maintained higher growth during drought years than they did in the temporary reaches. At Oak Creek, a larger perennial stream, P. wrightii growth showed a bell-shaped relationship with flow. These data suggest that growth rate is frequently limited by water availability at Sycamore Creek, but not at Oak Creek. 3. At both rivers, much of the annual surface flow occurs as winter floods. Oak Creek, however, maintains a high summer base flow even during years with no floods. Platanus wrightii growth was significantly related to winter flood frequency only at Sycamore Creek. The positive relationship of growth with stream flow and winter flood frequency at Sycamore Creek presumably occurs because the P. wrightii trees are dependent on the winter flows to recharge the shallow alluvial aquifer and to raise the level of ground water within the root zone. 4. Frequent summer floods increased the growth of trees in perennial and non-perennial reaches alike. At perennial Oak Creek, summer flood frequency was the only variable linearly related to growth of P. wrightii. Summer flood frequency was a significant, but secondary, component of multiple-regression growth models for trees in the perennial and non-perennial reaches of Sycamore Creek. Summer floods may stimulate growth, in part, by replenishing limiting nutrients. 5. High temperature was negatively associated with the growth of P. wrightii at Sycamore Creek. The combination of drought and high temperature resulted in very low growth rate. 6. These results have implications for the management of flood and base flow regimes on regulated, diverted and pumped rivers.

Abstract: Nonnative plant invasions are a management concern, particularly in riparian forests, but little is known about mechanisms through which they influence vertebrate communities. In the American Southwest, native trees such as cottonwood (Populus spp.) are thought to provide better habitat for breeding birds than nonnative plants, which are more tolerant of human-altered conditions. To evaluate effects of riparian forest composition on riparian-nesting birds, we examined nest plant use along two rivers in New Mexico that differed in abundance of nonnative vegetation. Of the nests we observed, 49% along the Middle Rio Grande were constructed in nonnative plants, compared with 4% along the Gila River. Birds in the canopy and cavity-nesting guilds constructed less than 5% of their nests in nonnative plants along either river. At the Middle Rio Grande, birds in the subcanopy/shrub guild constructed 67% of their nests in nonnative plants. Despite the relatively low availability of cottonwoods, they were used by greater numbers of species than any other woody plant at either river. Riparian obligates and species of conservation concern in the canopy and cavity guilds were especially dependent on cottonwood and Arizona sycamore (Platanus wrightii). Our results show that, although nonnative trees and shrubs support large numbers of nests for certain birds, cottonwoods and other large native trees are disproportionately important to riparian bird communities. Copyright © 2013 John Wiley & Sons, Ltd.

Abstract: Identifying habitat characteristics that influence selection of nest sites can provide information necessary for understanding and managing songbird populations. We quantified patterns of nest-site selection of 7 species in a riparian songbird community (n = 162 nests) at 2 spatial scales in southeastern Arizona, USA: Bell's vireo (Vireo bellii), verdin (Auriparus flaviceps), phainopepla (Phainopepla nitens), summer tanager (Piranga rubra), northern cardinal (Cardinalis cardinalis), blue grosbeak (Guiraca caerulea), and hooded oriole (Icterus cucullatus). We compared vegetation characteristics at nests to points chosen at random both near each nest (nest-patch scale) and within the entire study area (canyon scale). At the nest-patch scale, riparian vegetation-particularly Arizona sycamore (Platanus wrightii) and netleaf hackberry (Celtis reticulata)-was selected strongly by most species. At the canyon scale, most species nested in areas with higher vegetation density and volume than available at random. Managing riparian areas to foster high vegetation density and key structural components, such as sycamore and hackberry trees, is essential for meeting the habitat requirements necessary to maintain abundant and diverse songbird communities in the arid southwestern United States and adjacent Mexico.