We measured δ13C of tree ring holocellulose to assess intra- and interannual variation in integrated leaf gas exchange responses of Frémont cottonwood (Populus fremontii) to monsoonal moisture inputs in southeastern Arizona. We predicted that δ13C of trees growing along drought-susceptible intermittent reaches of this semiarid river system would be more responsive to monsoonal moisture inputs than trees found along perennial reaches, where groundwater is consistently available. We sampled stem xylem cores from 7 trees, each at an intermittent and perennial reach of the San Pedro River near Tombstone, Arizona. We identified and subdivided individual rings from 1990 to 2000. δ13C of holocellulose from these subdivisions was compared with precipitation amount, atmospheric vapor pressure deficit (Da), and 90% exceedence flows (Q90) calculated from seasonal flow duration data. δ13C values were higher at the intermittent reach than at the perennial reach. Furthermore, annual ring δ13C values at the perennial reach were not correlated with stream flow, precipitation, or Da. δ13C values for trees at the intermittent reach were negatively correlated with monsoon stream flow, precipitation, or Da. δ13C values for trees at the intermittent reach were negatively correlated with monsoon season (1 July–15 September) Q90 (r2 = 0.50, P = 0.015) and positively correlated with Da (r2 = 0.45, P = 0.03). Shifts in δ13C between the inner- and outer-third of the annual ring were used as a measure of intra-annual variation. These shifts were correlated with monsoon season Da (r2 = 0.57, P = 0.01) and Q90 (r2 = 0.59, P = 0.005) for trees growing along the intermittent reach. Intra- and interannual variation in integrated photosynthetic response exists at the population-scale for these native, riparian forests. Changes in monsoonal precipitation and stream flow may differentially alter photosynthetic gas exchange of P. fremontii and function of these riparian ecosystems.
Potts, Daniel L. and Williams, David G.
"Response of tree ring holocellulose δ13C to moisutre availability in Populus fremontii at perennial and intermittent stream reaches,"
Western North American Naturalist: Vol. 64
, Article 4.
Available at: http://scholarsarchive.byu.edu/wnan/vol64/iss1/4