Southwestern U.S. tree-ring carbon isotope indices as a possible proxy for reconstruction of greenness of vegetation

Southwestern U.S. pinyon tree-ring carbon-isotope indices showed promise two decades ago as an indicator of moisture and drought. However, because those isotopic indices were developed on 5-year ring groups rather than annual rings, the full extent of their effectiveness as environmental proxies was not established. Recent resampling of the sites and subsequent availability of annual isotopic indices for 1985 through 1999 has allowed us to more definitively characterize environmental relationships across 14 sites in six southwestern states. Principal component (PC) analysis reveals a significant positive relationship (p ≤ 0.02) between isotope indices and summer-season Normalized Difference Vegetation Index (NDVI), a satellite-derived "greenness index". Correlation and PC analysis also show a negative relationship of isotope indices with cumulative monthly precipitation (p ≤ 0.05) and confirm a strong positive relationship with Palmer Drought Index (especially spring and summer, p ≤ 0.0002). Although the drought and NDVI relationships are not as simply interconnected as hypothesized, the NDVI link reveals the potential of the existing multi-century record of isotope indices for long-term assessment of southwestern ecology and carbon cycling.