Leaf δ13C signatures in heath and lowland evergreen rain forest species from Borneo

Abstract

Two contrasting forest formations (sensu Whitmore 1984), heath forest (HF) of a range of stature and tall lowland evergreen rain forest (LERF) occur frequently in close proximity in Borneo (Brtinig 1974). The two forest formations differ in floristic composition, stature and structure with the LERF being taller with emergent trees. One hypothesis to explain the striking differences between the two forest formations has been that the podsolized sandy soils of the HF (the LERF occurs on ultisols) together with the irregularly recurring periods of drought impose certain adaptation constraints on the component species of the HF (Richards 1996). Xeromorphic features such as small leaf size, high specific leaf weight and even canopy structure have been proposed by Brtinig (1974) as adaptive characters. The evidence presented so far to support the 'drought hypothesis' has been circumstantial (e.g. Becker & Wong 1993) and Proctor (1999) has argued that the main cause of HF is hydrogen ion toxicity in the very acid soils. Stomatal limitation of photosynthesis by drought is expected to result in higher than normal uptake of the heavier '3C isotope caused by decreased discrimination during drought (Farquhar et al. 1982). Therefore, isotopic signatures which integrate responses over the life-time of a leaf can be used to study the history of stomatal limitation. This paper describes the findings of a comparative study of the 8'3C signatures of some component species from a stand of short HF (c. 9 m tall) and tall LERF (c. 35 m). The study was carried out near the Barito Ulu Base Camp (1 14?01'E