Table 9.
d(%) VSMOW (Vienna Standard Mean Ocean Water) of saguaro resources and surface water as a function of date sampled.  Means ± SE, and sample sizes (n) are presented.  An asterisk denotes the onset of summer monsoon rains.  Note that surface water enriches in dD slightly as a result of evaporative fractionation before the onset of monsoon rains.  After rains, the composition of surface water approaches that of rainfall.  The temporal variation in isotopic composition in both saguaro resources and rainwater is small relative to the turnover of the deuterium pool in doves.  However, the use of deuterium as a resource tracer for saguaro consumption requires monitoring the composition of both sources.