SCREENING OF SELECTED FOREST TREE SPECIES OF SRI LANKA FOR THEIR RESPONSE TO INCREASING ATMOSPHERIC CARBON DIOXIDE

Abstract

Investigating the effects of increasing atmospheric CO2 on forest tree species is important asforests playa major role in maintaining the ecological balance of an ecosystem. The objective ofthis study was to determine the response of photosynthesis, and it related physiological parameters,Growth and morphological parameters of selected forest tree species such as Teak (Tectonograndis), J3k (Artocarpus heterophyllusy; Satinwood iChloroxylon swieteniay, and two varieties ofMahogany (Swietenia macrophylla arid Swietenia mahogany) to elevated C02.

The experiment was carried out at the Rice Research and Development Institute (RRDI),Batalagoda,lbbagamuwa, from October 2002 to February 2003. Three experimental treatmentswere defined as elevated CO2 (i.e., 570 ppm) in open top chambers, ambient CO2 (i.e., 370 ppm)in open top chambers and ambient C~ (i.e., 370 ppm) under open field conditions. Theexperiment was conducted over a period of 161 days.

The total dry weight was higher under elevated C~ as compared to ambient C02 in all speciesexcept Chloroxylon swietenia. The response was highest in Swietenia macrophylla and thehighest mean total dry weight was in the elevated C~ treatment. The mean root dry weight, meanleaf dry weight and mean stem dry weight were significantly higher under elevated CO2 ascompared to the ambient. The height increment during the experimental period was highest inChloroxylon swietenia under elevated C02. The girth increment during this period was highest inSwietenia macrophylla and it was observed in the elevated CO2 treatment. When averaged acrossall species, the mean relative growth rates and absolute growth rates were higher under elevatedCO2 than under ambient. When individual species were considered, only Chloroxylon swieteniashowed a slight decrease in absolute and relative growth rates under elevated C02 while the othersshows an increase. Net photosynthetic rate (Pn) was higher in the elevated CO2 treatment inArtocarpus heterophyllus, Swietenia mahogany and Tectona grandis. But a slight decrease of Pnwas observed in Swietenia macrophylla under elevated C02 in comparison to ambient C02. Leafwater potential was higher in all five species under elevated C02 and the highest response toelevated C~ was observed in Chloroxylon swietenia. The stomatal diffusive resistance washighest in Tectona grandis and the value was 16.86 s em". The lowest was observed inChloroxylon swietenia and the value was 2.87 s em", 'The lowest diffusive resistance wasobserved in the elevated CO2 treatment. The transpiration rate was highest in Chloroxylonswietenia and the value was 12.71~g cm,2 S'l. The lowest was observed in Artocarpusheterophyllus and there was no significant difference among the three treatments. The mean leaftemperature was highest in Artocarpus heterophyllus and the lowest leaf temperature was found inChloroxylon swietenia.

Based on the above results, it can be concluded that elevated C(h increases growth of a majority oftree species tested. This could have favourable implications for both natural and plantationforestry in Sri Lanka. Results of the present study showed that the capacity of forests for carbonsequestration would be higher in a future climate with higher atmospheric C02 levels.