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Abstract:Precise estimation of root biomass is important for understanding carbon stocks and dynamics in tropical rain forests. However, limited information is available on individual root masses, especially large trees. We excavated 121 root systems of various species (78) and sizes (up to 116 cm in dbh), and estimated both above- and below-ground biomass in a lowland primary dipterocarp forest in the Pasoh Forest Reserve, Peninsular Malaysia. A tree census was conducted in four research plots (each 0.2 ha) and stand-level biomass was estimated. We examined relationships between tree size parameters and masses of coarse roots (roots ≥5 mm in diameter) and derived a dbh-based allometric equation. The amounts of coarse roots that were lost during excavation were corrected. Coarse-root biomass before and after correction for lost roots was estimated to be 63.8 and 82.7 Mg ha−1, indicating that significant amounts of roots (23%) were lost during the sampling. We also estimated the biomass of small root (<5 mm) by applying pipe-model theory. The estimate, 13.3 Mg ha−1, was similar to another estimate of small roots, 16.4 Mg ha−1, which was obtained directly by the soil-pit sampling method. Total below-ground (BGB) and above-ground biomass (AGB) was estimated to be 95.9 and 536 Mg ha−1, respectively. The biomass-partitioning ratio (BGB/AGB) was about 0.18. In conclusion, the dbh-based allometric equation for coarse roots developed in this study, which kept good linearity even including the data of larger trees, might be useful for evaluating below-ground carbon stocks in other stands of similar forest (old-growth dipterocarp) in South-East Asia.

The oxygen isotope enrichment of bulk leaf water (D b ) was measured in cotton (Gossypium hirsutum) leaves to test the CraigGordon and Farquhar-Gan models under different environmental conditions. D b increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (e a /e i ) and to stomatal conductance (g s ). The oxygen isotope enrichment of lamina water relative to source water ð D 1 Þ; which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. Recently, the analysis of the oxygen isotope composition (d 18 O) of leaf water became of increased interest as a result of efforts to obtain information on the global carbon cycle Gillon and Yakir, 2001) and because of applications in agriculture (Barbour et al., 2000a). These and other applications were recently updated (Barbour, 2007;Farquhar et al., 2007). The d 18O of atmospheric CO 2 and of plant organic matter depends strongly on the extent of leaf water enrichment that occurs during transpiration (Barbour et al., 2000b) because the diffusivity and vapor pressure of heavier H 2 18O are less than that of lighter H 2 16 O (Craig and Gordon, 1965). A large portion of the CO 2 that enters the leaf equilibrates with evaporatively enriched leaf water via the catalytic activity of carbonic anhydrase, then retrodiffuses out of the leaf, increasing the d

We used stable isotope techniques to investigate water utilization of two native trees, Sabina vulgaris Ant. and Artemisia ordosica Krasch., and one introduced tree, Salix matsudana Koidz., in the semiarid Mu‐Us desert, Inner Mongolia, China. The study site was in a region where there has been a decline in agricultural productivity, caused by severe desertification over the past several decades. S. matsudana is used extensively for reforestation to protect farms and cultivated lands from shifting sand dunes. We identified water sources for each tree species by comparing the stable isotopes δD and δ18O in water in stems, soil, and groundwater. We also measured δ13C levels in leaves to evaluate the intrinsic water‐use efficiency (WUE) of each plant. Comparison of isotopes showed that S. vulgaris and S. matsudana consume relatively deep soil water as well as groundwater, whereas A. ordosica uses only shallow soil water. The δ13C measurements indicated that S. vulgaris has exclusively high WUE, whereas that of the other species was typical of temperate‐region C3 plants. The water source data plus WUE data suggest that planted S. matsudana uses groundwater freely, whereas native plants conserve water. Thus, reforestation with S. matsudana might cause irreversible groundwater shortages. Corresponding Editor: E. A. Holland.