Intra- and Inter-Annual Variability in Surface Hydrology in Northern Arizona from δ18O of Tree-Ring Cellulose

The primary goal of our research is to better understand how the surface hydrology of semi-arid sites in the Southwestern U.S. is affected by the annual cycles of precipitation and evaporation. Both are tied to relative strength of the North American Monsoon, El Niño-Southern Oscillation and, on shorter timescales, the occasional passage of tropical cyclone remnants. To achieve this we aim to develop high-resolution stable oxygen isotope ratio (δ18O) profiles of tree-ring cellulose for much of the last 20 years that can be ground-truthed to direct meteorological observations. It is well documented that δ18O of alpha-cellulose extracted from wood reflects hydrological conditions of a trees’ environment at the time the tree grew. Primary controls on isotopic variability are changes in source waters and relative humidity during the growing season. We sampled rings from ≥ 10 Pinus ponderosa (Douglas) at six stands along an east-west transect across northern Arizona. Annual precipitation at these sites has a bimodal distribution with almost all annual rainfall occurring during the summer monsoon (Jul, Aug) and winter storms. At Flagstaff, in the center of our study area, monthly mean precipitation δ18O values are enriched ~6‰ during the monsoon relative to winter storms. P. ponderosa (Dougl.) rings display distinct early- and latewood bands. Earlywood typically forms using winter storm precipitation that has resided within the soil until the tree began growing and ought to reflect the isotopic composition of this water. Latewood δ18O reportedly reflect summer rainfall isotopic values. We investigate the eleven year period 1994-2004. This range encompasses the transition into the present ‘drought’, the intense 1997/98 El Niño, and the passage of the remnants of Hurricanes Nora (1997) and Javier (2004). Individual rings are sliced into subsamples of mass ~1.5 mg (yielding 3-13 samples/ring). Early isotopic data from these samples display three significant trends. First, isotopic variability in a given annual ring is closely matched at intra- and inter-tree scales in a single stand (inter-site comparisons unavailable at time of writing). Second, isotopic values demonstrate that trees growing within meters of each other do not begin/cease growing simultaneously, which has implications for low-resolution isotope cross-dating studies. Third, and most significantly, earlywood samples are consistently enriched in 18O relative to latewood samples by on average ~6‰. This result is unexpected based on the isotopic composition of local precipitation and suggests that rates of evaporative enrichment of 18O in soil and leaf moisture during the growing season vary and with significant effect. Further investigation of this phenomenon will incorporate IsoGSM model output of growing season precipitation and water vapor δ18O for the period of study.