Exploring seasonal climate variability in East African trees: High-resolution oxygen and carbon isotope records of cellulose of vitex payos from Chyulu Hills, Kenya

A nearly continuous, annually resolved record of cellulosic delta 18O and delta13C is presented for the East African tree Vitex payos for the recent through the late 19th century. The isotope data of individual rings is cross-calibrated against atmospheric variables during the late 20th century to ascertain how and when during the year cellulose obtains its delta18O and delta13 C value in response to environmental conditions at the tree site. During the late 20th century the delta18O and delta 13C obtained from alpha cellulose from annual rings of Vitex payos correlates strongly with precipitation amount and with relative humidity, with depleted (enriched) delta18O and depleted (enriched) delta13C values associated with increased (reduced) precipitation and relative humidity. The strongest correlations are found between cellulose delta18O and the amount of precipitation and the relative humidity during December-February, although temperature contributes to the observed isotopic variance as well. Relative humidity in December-February appears to have the strongest correlation to annual cellulosic delta 13C. The results of this cross-correlation study are used to develop a preliminary seasonal growth model for Vitex payos that reflects variations in precipitation amount and relative humidity during the December-February growing season. During the late 20th century the East Africa continent and the western Indian Ocean basin experienced anomalously wet conditions in summer (December-February) when sea surface temperatures (SSTs) were anomalously warm in the western Indian Ocean and zonal winds were anomalously strong in association with positive phases of the Indian Ocean Dipole (IOD) and/or El Nino Southern Oscillation (ENSO). Using the correlation between cellulosic isotopic values and hydroclimate variability over East Africa and Indian Ocean SSTs during the late 20th century a preliminary climate reconstruction is proposed for the pre-20th century fromVitex payos delta 18O and delta13C. The isotopic record of Vitex payos indicates the late 19th century was anomalously wet over East Africa when compared to the late 20th century, consistent with lake level changes and ice core isotopic data. Climate model simulations indicate the wetter pre-20th century climate was a response to enhanced warming in the western Indian Ocean basin, generating a low pressure center and easterly surface winds that brought moisture to East Africa. The annually resolved isotopic data from Vitex payos suggest much of the observed and modeled hydroclimate variability at multidecadal to century scales is a response to SST anomalies in the western Indian Ocean. The results of the present study should help guide future phenological studies of Vitex payos and other Vitex species in equatorial East Africa. Importantly, stable isotope data obtained from individual growth rings suggests that the annual production of cellulose in Vitex payos occurs during a short seasonal period from January to February, information that could not be inferred directly from tree ring width data alone. Further improvements in sampling tree rings at intra-annual resolution and in-situ monitoring of weather/climate variables, including isotopic variability of moisture would be necessary in order to fully utilize cellulosic delta 18O as a climate proxy, with the potential for becoming a continent-wide hydrologic monitoring network, the first of its kind from Africa. The isotopic database for Vitex payos is among the few available terrestrial climate records provides estimates of how hydrologic budgets varied over East Africa during the past several centuries.