?13C and water-use efficiency in Australian grasstrees and South African conifers over the last century

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Abstract
Annual or biannual time courses of plant δ13C (δ13Cp) 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‰ 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‰. Accepting that δ13C of the air (δ13Ca) fell by almost 2‰ 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 δ13Ca were more closely correlated with changes in δ13Cp and accounted for 6.7–71.8% (22.6c i/c a) and 28.2–93.3% (δ13Ca) of the variation in δ13Cp. 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 δ13Cp 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 δ13Ca.