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Northern Arizona University (2013)

Interspecific variation in functional rooting depth in plant communities along an elevational gradient in the southwestern United States

Guo, Jessica Sijia

Titre : Interspecific variation in functional rooting depth in plant communities along an elevational gradient in the southwestern United States

Auteur : Guo, Jessica Sijia

Etablissement de soutenance : Northern Arizona University,

Grade : Master of Science (MS) 2013

Résumé
In semi-arid environments, co-existing plant species may vary in rooting depth, reflecting functional differences in water sources. In mountains of the southwestern U.S., moisture availability increases with elevation and winter and summer precipitation inputs differ isotopically. Examining variation in stem water isotopes among different plant communities and seasons is important for understanding how niche partitioning may respond to the predicted warming and drying of the Southwest. The goal of this study was to assess the water partitioning of the woody plant community along an elevational aridity gradient using stem and soil water isotopes to estimate rooting depth. We hypothesized that niche overlap in functional rooting depth would be lower at more arid sites because of greater interspecific competition for water. Moreover, we expected that plants from more arid sites would show a larger seasonal shift in functional rooting depths between the dry and monsoon seasons. Five plots were established in each of five biotic communities : upland Sonoran desert, pinyon-juniper woodland, ponderosa pine forest, mixed-conifer forest, and spruce-fir forest. Soil samples from three depths (0-2, 19-21 cm, and 34-36 cm) and stem samples of dominant woody perennials were collected during the late spring dry season and during the summer monsoon season, water was extracted using a cryo-vacuum line, and δD was determined by off-axis cavity ringdown spectroscopy. Sites were characterized by an aridity index based on annual precipitation to potential evapotranspiration. Functional rooting depth of plants was calculated from plant δD using the logarithmic regression of soil depth and soil δD for each site and season. Niche overlap was determined using the functional rooting depths of all species pairs at each site and was regressed against aridity, with season tested as a co-factor. Soil moisture content increased with elevation and after the onset of monsoon rains across all sites, and soil δD decreased with increasing depth. Plant δD differed significantly among species in all sites and seasons except in June in the mixed-conifer forest. Niche overlap decreased with increasing aridity, but aridity only explained 9.5% of the variation in niche overlap. Desert plants did not shift to shallower water in August, although pinyon-juniper woodland plants did. Our findings support the hypothesis that niche overlap in functional rooting depth is lower at more arid sites. This study did not support the hypothesis that the seasonal shift in functional rooting depth is greater at more arid sites.

Mots Clés : Aridity, Coexistence, Functional rooting depth, Niche partitioning, Water isotopes

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