delta13C and water-use efficiency in Australian grasstrees and South African conifers over the last century.

ABSTRACT

Annual or biannual time courses of plant delta13C (delta13C(p)) over the last century (70-100 years) were recorded for leafbases of four grasstrees (Xanthorrhoea preissii) at four sites in mediterranean Australia and wood of four conifers (Widdringtonia cedarbergensis) at two sites in mediterranean South Africa. There was a strong downward trend of 2-5.5(per thousand ) from 1935 to 1940 to the present in the eight plants. Trends were more variable from 1900 to 1940 with plants at two sites of each species showing an upward trend of 1-2.5 per thousand. Accepting that delta13C of the air (delta13C(a)) fell by almost 2 per thousand over the last century, the ratio of leaf intercellular CO2 to atmospheric CO2 (c(i)/c(a)) rose in five plants and remained unchanged in three over that period. Changes in c(i)/c(a) rather than delta13C(a) were more closely correlated with changes in delta13C(p) and accounted for 6.7-71.8% (22.6 c(i)/c(a)) and 28.2-93.3% (delta13C(a)) of the variation in delta13C(p). We doubt that possible changing patterns of rainfall, water availability, temperature, shade, air pollution or clearing for agriculture have contributed to the overall trend for c(i)/c(a) to rise over time. Instead, we provide evidence (concentrations of Fe and Mn in the grasstree leafbases) that decreasing photosynthetic capacity associated with falling nutrient availability due to the reduced occurrence of fire may have contributed to rising c(i)/c(a). Intrinsic water-use efficiency (W(i)) as a function of (c(a)-c(i)) usually increased linearly over the period, with the two exceptions explained by their marked increase in c(i)/c(a). We conclude that grasstrees may provide equivalent delta13C(p )and W(i) data to long-lived conifers and that their interpretation requires a consideration of the causes of variation in both c(i)/c(a )and delta13C(a).