REDEFINING THE ECOLOGICAL NICHE OF A TROPICAL RAIN-FOREST CANOPY TREESPECIES USING AIRBORNE IMAGERY - LONG-TERM CROWN DYNAMICS OF TOONA-CILIATA

Past controlled growth experiments indicate that the seedling and sapl ing responses of the tropical rainforest canopy tree species Toona cil iata are most consistent with a light-demanding, early successional pi oneer. This ecological niche assignment was tested in the mature stage of its life cycle after it achieves a position in the upper canopy. M ortality, survivorship and crown growth rates over the 18-y period 197 6-1994 were measured using co-registered repeat airborne stereophotogr aphic coverage of a representative forest stand in northeast Queenslan d, Australia, where T, ciliata had the fourth highest relative importa nce in a population of 46 co-occurring canopy tree species. The airbor ne re-inventory was conducted in a 3.6-ha sample area and limited to o nly canopy trees. The results were compared with a ground-based invent ory of both canopy and subcanopy trees greater than or equal to 10 cm dbh in a 0.5-ha permanent plot. Over the period 1976-1994,, there was no mortality and no evidence of decline among T. ciliata conspecifics having crown areas >60 m(2) and trunk diameters >30 cm. In the 3.6-ha airborne sample area, more than 85% of T. ciliata survivors experience d positive crown growth, in contrast to only 57% of the other co-occur ring canopy trees. Toona ciliata's crown growth rates were highest in the 60-80-m(2) crown size class. Upon reaching an upper canopy positio n, T. ciliata not only persisted as a dominant canopy tree species, bu t it also achieved some of the largest crown areas (> 100 m(2)). Toona ciliata mortality in the ground-based plot involved mainly subcanopy trees of 10-30 cm dbh that had not yet assumed a canopy position and w ere not detectable in the aerial stereopairs. Both the crown and dbh g rowth rates of T. ciliata indicate enhanced vigour in the later stage of its life cycle. Its long-term survivorship and growth patterns are indicative of a persistent canopy tree species that fits the niche of a long-lived, shade-intolerant pioneer. These results demonstrate the usefulness of a long-term airborne database on the life-histories of c anopy tree crowns for more clearly defining their ecological niches.