Verification of the accuracy of the recent 50 years of tree growth and long-term change in intrinsic water-use efficiency using xylem Δ¹⁴C and δ¹³C in trees in an aseasonal tropical rainforest

Growth analysis based on tree-ring chronology is difficult in trees in aseasonal tropical rain forests, because annual growth rings may be unclear or completely absent. Fortunately, tree growth history recorded in xylem tissue is capable of providing valuable information on the responses of trees and forests to past and present environmental changes, including global warming. We have developed a new technique for aseasonal tropical forest trees which derives their growth rates from xylem Δ¹⁴C, and verified its accuracy. We also determined, from xylem δ¹³C, the intrinsic water-use efficiency (iWUE) in the past 50 years. We analysed changes in xylem Δ¹⁴C and δ¹³C in 23 canopy trees of 12 species in 6 families growing in Pasoh Forest Reserve, Malaysia; each stem diameter at breast height (DBH) was recorded 14 times from 1969 to 2011. We found a significant positive relationship between the growth rates determined by ¹⁴C dating and the past DBH data. On the other hand, leaf-internal CO₂ (C_i) content did not change with increasing atmospheric CO₂ (C_a). Thus, the iWUE increased significantly over the last 50 years in all the families and species tested. This study showed that the simultaneous measurements of xylem Δ¹⁴C and δ¹³C could reveal a long-term change in tree growth and iWUE during the past 50 years with high accuracy in various species and/or individuals in aseasonal tropical rainforests exhibiting high species diversity.