RESPONSE OF SELECTED FOREST TREE SPECIES IN SRI LANKA TO INCREASING AIR TEMPERATURE

Abstract

Increasing air temperature is an important aspect of long-term global climate change. It has beenbrought about by increased emissions of greenhouse gases such as carbon dioxide, methane,nitrous oxides and chlorofluourocarbons into the atmosphere and the consequent retention ofoutgoing thermal radiation in the earth's atmosphere. Global climatic models have predicted thatthe mean air temperature of the earth will increase by 1.4 - 5. 8°C in the next 100 years and thiswill be the most rapid episode of warming in the earth's history.

Temperature is a key environmental variable that determines the physiology and growth of forests.Therefore, it is important to know how different forest tree species would respond to the expectedincrease in air temperature. This is especially relevant in view of the long life cycle durations ofmany forest tree species which will force them to experience the phenomenon of global warming.The primary objective of this study was to determine the response of some key physiologicalprocesses and growth parameters of selected forest tree species to increasing air temperature at theseedling stage.

The experiment was carried out in the controlled environmental growth chambers at the Faculty ofAgriculture of the University of Peradeniya, Sri Lanka during a 100-day period from August toNovember, 2003. Seedlings of five tree species, i.e. Teak (Tectona grandis), Jak (Artocarpusheterophyllus), Satinwood (Chloroxylon swietenia'i, and two varieties of Mahogany (Swieteniamacrophylla and Swietenia mahogany) were grown at two daily mean temperatures, i.e. 28°C(day/night regime of 300126°C) and 32° (day/night regime of 34°/30°C).

The majority of tree species tested showed lower leaf net photosynthetic rates at the highertemperature Swietenia mahogany was a consistent exception to this trend. The leaf chlorophyllcontent was lower at the higher temperature in all species except Swietenia macrophylla. Allspecies showed decreases in stomatal conductance and increases in leaf temperature when airtemperature increased from 28° to 32°C. Despite reduced stomatal conductance, transpiration rateper unit leaf area increased in all species in response to the temperature increase. This wasbecause of the greater leaf-air water vapour concentration gradients brought about by the higherleaf temperatures at 32°C. As a result of increased transpiration rates, the leaf water potential ofall tree species except teak, showed decreases when temperature increased from 28°C to 32°C

While both mahogany species and satinwood showed greater plant heights at the highertemperature, jak and teak showed the opposite trend. On the other hand, higher stem girths wereobserved at the higher temperature in S. macrophylla, satinwood and teak while the opposite wasobserved in the rest. Number of leaves and leaf area per plant were greater at the highertemperature in all species tested except jak. Jak, S. mahogany and teak showed decreases in totalbiomass (W), absolute growth rate (AGR) and relative growth rate (RGR) in response to thetemperature increase from 28°C to 32°C. In contrast, S. macrophylla and satinwood showedgreater W, AGR and RGR at the higher temperature. The leaf weight ratio (fraction of leaf dryweight in total biomass) increased while the root weight ratio (fraction of root dry weight in totalbiomass) decreased in response to increasing temperature in all species.

Based on the above results, we conclude that the response of forest tree species to increasing air temperature isspecies-specificand that the directionofresponsc dependson the optimum temperature (i.e. the temperature at whicha given physiological or growth parameter is maximum), which varies from tree species to species. If the airtemperature increase due to global warming does not exceed the optimum for a given species, it would show apositive response to warming. On the other hand, if the temperature increase goes beyond the optimum of a givenspecies, its response would be negative. Accordingly,the temperature increase from 28·C to 32·C in the presentstudy probably went beyond the optimum for bionws productionofjak, S. mahogany and teak, but stayed under theoptimum for satinwood and S. macrophylla. These results have important implications on the selection of treespeciesfor future plantation forestry and on the futureproductivityof existing plantation forests