1. Growth rate is a fundamental property of organisms. In trees, growth is indeterminate and varies in space and time depending on resource availability, genetic constraints, competition and stress from biotic and abiotic environmental factors. 2. The ratio between the abundance of RNA and DNA in the tissue has been used to indicate recent growth rates in many systems, but not for trees. We assessed the applicability of using RNA : DNA ratios for assessing intraspecific and interspecific differences in growth rates in two species of mangrove trees under field conditions. We manipulated growth by fertilizing mangrove trees over a period of 4 years and measured tree growth as increments in linear extension of tagged shoots. The C, N and P contents per unit biomass were measured to test the hypothesis that faster growing organisms require more P per unit biomass (the Growth Rate Hypothesis). 3. We found that interspecific differences in the RNA : DNA ratio clearly reflected the difference in shoot elongation rates between the species. Intraspecific differences in RNA : DNA were significantly correlated with growth rates only for Avicennia marina and not for Ceriops australis. C : N and C : P ratios were lower in trees with higher growth rates and exhibited a negative correlation with RNA : DNA ratios. 4. Our results indicate that RNA : DNA ratios can reliably predict interspecific differences in growth rates between the two mangrove species and that RNA : DNA ratios can be used as an indication of variation in growth rate for Avicennia marina. The interaction between C : N : P ratios, RNA : DNA ratios and growth rates supported the Growth Rate Hypothesis on an interspecific level, but not on an intraspecific level.
