Redefining the ecological niche of a tropical rain forest canopy tree species using airborne imagery: long-term crown dynamics of Toona ciliata

Abstract

Past controlled growth experiments indicate that the seedling and sapling responses of the tropical rainforest canopy tree species Toona ciliata are most consistent with a light-demanding, early successional pioneer. This ecological niche assignment was tested in the mature stage of its life cycle after it achieves a position in the upper canopy. Mortality, survivorship and crown growth rates over the 18-y period 1976–1994 were measured using co-registered repeat airborne stereophotographic coverage of a representative forest stand in northeast Queensland, Australia, where T. ciliata had the fourth highest relative importance in a population of 46 co-occurring canopy tree species. The airborne re-inventory was conducted in a 3.6-ha sample area and limited to only canopy trees. The results were compared with a ground-based inventory of both canopy and subcanopy trees ≥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 m² and trunk diameters >30 cm. In the 3.6-ha airborne sample area, more than 85% of T. ciliata survivors experienced positive crown growth, in contrast to only 57% of the other co-occurring canopy trees. Toona ciliata's crown growth rates were highest in the 60–80-m² crown size class. Upon reaching an upper canopy position, T. ciliata not only persisted as a dominant canopy tree species, but it also achieved some of the largest crown areas (>100 m²). 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 were not detectable in the aerial stereopairs. Both the crown and dbh growth 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 canopy tree crowns for more clearly defining their ecological niches.