The carbon isotope ratio of plant organic material reflects temporal and spatial variations in CO2 within tropical forest formations in Trinidad

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Abstract
A method of monitoring and collecting CO2 samples in the field has been developed which has been used to study both temporal and spatial variations in canopy CO2 isotopic signatures in two contrasting tropical forest formations in Trinidad. These have been related to vertical gradients in the carbon isotope ratio (δ13C) of organic material in conjunction with measurements of other environmental parameters. The δ13C of leaf material from two canopies showed a gradient with respect to height, more negative values being found low in the understorey. The deciduous secondary forest, (Simla) showed a difference of 4.6‰ and the semi-evergreen seasonal canopy (Aripo), 2.8‰. The range of δ13C values at Simla was 4‰ less negative than those at Aripo. In order to relate these measurements to the interaction between diffusion or carboxylation limitation, and source CO2 effects, variations in environmental parameters through the canopy have been compared with changes in CO2 partial pressure (P a) and isotopic composition δ13C throughout the day during the dry season. Values of P a20 m above the ground at Aripo varied from 380 vpm at dawn to 340 vpm at midday, at which time the partial pressure 15 cm above the ground was 375 vpm. The CO2 partial pressure did not stabilise during the course of the day, and there was good correlation (r 2=0.82) between δa and P a, with more negative values of δa occuring in the understorey. Diuraal changes of 2‰ were evident at all canopy positions. In the more open canopy at Simla, these gradients were similar, but less marked. Leaf-air vapour pressure deficit (VPD) showed no relationship with height, possibly as a result of minimal water flux from both the soil and the canopy due to low soil water content; VPD was 1.5 kPa higher at midday than dawn. A 3° C temperature gradient between the understorey and upper canopy was observed at Aripo but not in the more open Simla canopy. CO2 partial pressure stabilised for only 4 h in the middle of the day, while other parameters showed no stable period. The proportion of floor respired CO2 reassimilated at Aripo has been calculated as 26%, 19%, and 8% for the periods 0600–1000, 1000–1400, and 1400–1800 hours. In order to quantify source CO2 effects, measurements of the environmental parameters and assimilation rate must be made at all canopy positions and throughout the day.