Photosynthetic responses of riparian tree taxa to monsoonal moisture in the American Southwest: large-scale patterns across three warm-desert biomes.
Williams, David1, 1 Departments of Renewable Resources and Botany, Laramie, WY, USA
ABSTRACT- Riparian trees in desert environments intercept water flowing from large drainage basins and potentially respond physiologically to both local and watershed-level hydrologic perturbations. Integrated photosynthetic responses to moisture in the soil and atmosphere can be assessed using stable carbon and oxygen isotope ratios in leaf and tree-ring tissues. Isotopic variation in riparian trees in desert landscapes therefore potentially records photosynthetic responses to stream flow, humidity, water table depth and/or soil moisture depending on the relative sensitivity of photosynthetic parameters (stomatal conductance, chloroplast CO2 demand) to variation in moisture conditions. Leaf samples from one obligate phreatophyte (Populus fremontii) and two facultative phreatophytes (Prosopis spp., Celtis reticulata) were collected before (June) and after (September) the summer monsoonal period from eleven sites spanning a summer monsoon gradient across the Mojave, Sonoran and Chihuahuan Deserts. Monsoonal precipitation amounts (July-September) in the year of the study varied from 11 to 249 mm and average maximum daily vapor pressure deficits ranged from 3.1 to 6.7 kPa across the 11 sites. Leaf 13C values across all sites and sampling dates ranged from −25.6 to −30.9 ‰ in Populus, from −25.9 to −28.9 ‰ in Celtis, and from −25.4 to −28.5 ‰ in Prosopis. The change in 13C values from pre- to post-monsoon sampling periods within a population was significantly correlated with monsoonal precipitation across the 11 sites in Prosopis and Celtis, but not in Populus, indicating species-specific stomatal responses to rainfall in these riparian communities. 13C values in Populus and Celtis were strongly correlated with vapor pressure deficit, but no such relationship was observed in Prosopis. These and other physiological and morphological responses show that riparian tree taxa respond to the monsoonal environment in fundamentally different ways and record in the isotopic composition of their tissues, very different aspects of climatic change in warm desert regions.−
Key words: carbon isotope ratio, humidity, precipitation, riparian ecosystem
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