TRANSPIRATION BY TREES FROM CONTRASTING FOREST TYPES

Temperate rainforests and eucalypt forests of coastal south-eastern Au stralia are distributed differentially with aspect. Rainforests, in wh ich Ceratopetalum apetalum D.Don and Doryphora sassafras Endl. are the dominant tree species, occur on slopes of southerly aspect and along gully bottoms, whereas eucalypt forests, dominated by Eucalyptus macul ata Hook., occur on upper slopes of northerly aspect and on ridge tops . Whether transpiration rates of trees differed across the rainforest- eucalypt forest boundary on north and south facing aspects was tested by measuring stem sap flow in trees in a single catchment during winte r, summer and autumn. Differences in transpiration rate by trees in th ese stands were due to various combinations of biological and physical factors. Firstly, mean maximum transpiration rate per tree (crown are a basis) was greater in rainforest on the gully bottom where deep soil water from down-slope drainage was greater than in eucalypt forest lo cated upslope on the northern aspect. By contrast, there was no differ ence between maximum transpiration rates in rainforest and eucalypt fo rest on the southern aspect. Variation in transpiration rate between s easons was not related to variation in surface soil moisture content ( < 0 . 35 m depth). Secondly, transpiration rates per unit crown area i n rainforest at the gully bottom were associated with higher leaf area indices than upslope on the northern aspect. However, in rainforest u pslope on the southern aspect, higher transpiration rates were not ass ociated with higher leaf area indices. Thirdly, trees in eucalypt fore st maintained similar sapwood moisture contents in summer as in winter and autumn, whereas sapwood moisture contents declined in rainforest trees in summer, suggesting that eucalypts had access to water from de ep within the soil profile which was unavailable to more shallow rooti ng rainforest trees. Fourthly, higher modal and maximal sap velocities in eucalypt trees were partly due to wider xylem vessels and resulted in faster maximum sap flow and greater daily total water use in all s easons on both aspects than in rainforest species. Finally, as atmosph eric demand for water increased from winter to summer, transpiration r ates were mediated by stomatal closure as indicated by lower average m idday shoot conductance to water vapour during summer than other seaso ns. The interaction between microenvironment, which determines water a vailability, and physiological attributes, which determine tree water acquisition and use, may contribute to the differential distribution o f rainforest and eucalypt forest with aspect in south-eastern Australi a.