Water source niche overlap increases with site moisture availability in woody perennials
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Classical niche partitioning theory posits increased competition for and partitioning of the most limiting resource among coexisting species. Coexisting plant species may vary in rooting depth, reflecting niche partitioning in water source use. Our goal was to assess the soil water partitioning of woody plant communities across northern Arizona along an elevational moisture gradient using stem and soil water isotopes from two sampling periods to estimate the use of different water sources. We hypothesized that niche overlap of water sources would be higher and monsoon precipitation uptake would be lower at sites with higher moisture availability. Pairwise niche overlap of coexisting species was calculated using mixing model estimates of proportional water use for three sources. Across the moisture gradient, niche overlap increased with site moisture index (precipitation/potential evapotranspiration) across seasons, and site moisture index explained 37% of the variation in niche overlap of intermediate and deeper sources of water. Desert trees utilized more winter source water than desert shrubs, suggesting the partitioning of water sources between functional groups. However, seasonal differences in surface water use were primarily found at intermediate levels of site moisture availability. Our findings support classical niche partitioning theory in that plants exhibit higher overlap of water sources when water is not a limiting resource.
KeywordsCoexistence Niche overlap Water source Stable isotopes Plant communities Moisture gradient
The authors thank Hannah Russell for her assistance in the field and lab, Jaimie Brown and Kimberly Samuels-Crow for their expertise in stable isotopes, Derek Sonderegger for his statistical advice, and the Ogle Lab for comments on this manuscript.
JG was supported by a Science Foundation Arizona Graduate Research Fellowship under Award GRF 0449-10.
JG, TK, BH, and GK conceived and designed the study; JG conducted fieldwork and lab analyses; JG and BH analyzed data; JG wrote the manuscript; and BH, TK, and GK provided editorial advice.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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