Co - occurring species differ in tree - ring 8 18 0 trends

The stable oxygen isotope ratio (6 0) of treering cellulose is jointly determined by the 6 0 of xylem water, the 8' 8 0 of atmospheric water vapor, the humidity of the atmosphere and perhaps by species-specific differences in leaf structure and function. Atmospheric humidity and the 6 0 of water vapor vary seasonally and annually, but if the canopy atmosphere is well mixed, atmospheric characteristics should be uniform among co-occurring trees. In contrast, xylem water 6 0 is determined by the 60 of water being drawn from the soil, which varies with depth. If co-occurring trees draw water from different soil depths, this soil-water 6 0 signal would be manifest as differences in 8 18 0 among the trees. We examined the variation in tree ring 6 0, 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 pondemsa Laws.) and seven western white pines (Pinus monticola Doug!.). As expected, variation in atmospheric conditions was recorded in the 6 0 of the cellulose produced in a given year, but observed climatic correlations with 60 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), vapor pressure deficit (r = 0.32) and solar radiation (r = 0.44). Lagged effects were observed in Douglas-fir and wes tern white pine. In these species, the 6! 8 0 of a given annual ring was correlated with the 6 180 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 6 0 relative to the other species; differences were close to 2%o and they are steadily increasing. Relative to the mean for the three species, ponderosa pine is becoming steadily more enriched (-1.0%o). In contrast, Douglas-fir is being steadily depleted and western pine is intermediate, with an enrichment of 0.5%0. Because all trees were exposed to the same atmospheric conditions, the differences in 8' 0 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 8' 8 0 are a result of changes in leaf structure and function with tree size and age.