CORRELATING DELTA-C-13 AND DELTA-O-18 IN CELLULOSE OF TREES

We measured the carbon and oxygen isotopic composition of stem cellulo se of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus exce lsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions, At every site , six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed, For all species, the mean site delta(13)C and delta(18)O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites, This result is consistent with isotope fractiona tion models when assuming enhanced stomatal resistance (thus higher de lta(13)C of incorporated carbon) and increased oxygen isotope enrichme nt in the leaf water (thus higher delta(18)O) at the dry sites, delta( 18)O-delta(13)C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose, To interpret this relationship we de veloped an equation which combines the above-mentioned fractionation m odels, An important new parameter is the degree to which the leaf wate r enrichment is reflected in the stem cellulose, In the combined model the slope of the delta(18)O-delta(13)C plot is related to the sensiti vity of the p(i)/p(a) of a plant to changing relative humidity.