Co-occurring species differ in tree-ring δ18O trends

The stable oxygen isotope ratio (delta O-18) of tree-ring cellulose is jointly determined by the delta O-18 of xylemwater, the 8180 of atmospheric water vapor, the humidity of the atmosphere and perhaps by species-specific differences in leaf structure and function. Atmospheric humidity and the delta O-18 of water vapor vary seasonally and annually, but if the canopy atmosphere is well mixed, atmospheric characteristics should be uniform among co-occuffing trees. In contrast, xylem water 8180 is determined by the delta O-18 of water being drawn from the soil, which varies with depth. If co-occuffing trees draw water from different soil depths, this soil-water delta O-18 signal would be manifest as differences in delta O-18 among the trees. We examined the variation in tree ring delta O-18, over eight decades during the 20th Century, among three species co-occurring in natural forest stands of the northern Rocky Mountains in the USA. We sampled 10 Douglas-firs (Pseudotsuga menziesii (Mirb.) Franco var. glauca), 10 ponderosa pines (Pinus ponderosa Laws.) and seven western white pines (Pinus monticola Dougl.). As expected, variation in atmospheric conditions was recorded in the delta O-18 of the cellulose produced in a given year, but observed climatic correlations with delta O-18 were weak. Significant correlations with June climate data included: daily maximum temperature (r = 0.29), daily minimum temperature (r=-0.25), mean temperature (r = 0.20), mean daily precipitation (r = -0.54), vaporpressure deficit (r = 0.32) and solar radiation (r = 0.44). Lagged effects were observed in Douglas-fir and western white pine. In these species, the delta O-18 of a given annual ring was correlated with the 8180 of the previous ring. Ponderosa pine showed no significant autocorrelation. Although the species means were correlated among years (r 0.67 to 0.76), ponderosa pine was consistently enriched in delta O-18 relative to the other species, differences were close to 2 parts per thousand and they are steadily increasing. Relative to the mean for the three species, ponderosa pine is becoming steadily more enriched (-1.0 parts per thousand). In contrast. Douglas-fir is being steadily depleted and western pine is intermediate, with an enrichment of 0.5 parts per thousand. Because all trees were exposed to the same atmospheric conditions, the differences in delta O-18 observed between species are likely due either to differences in the depth of water extraction or leaf function. If the former, presumably ponderosa pine has steadily taken up more water from near the soil surface and Douglas-fir has shifted uptake to a greater depth. If the latter, we suggest the pronounced changes in leaf-water delta O-18 are a result of changes in leaf structure and function with tree size and age.