Sr. Herwitz et al., REDEFINING THE ECOLOGICAL NICHE OF A TROPICAL RAIN-FOREST CANOPY TREESPECIES USING AIRBORNE IMAGERY - LONG-TERM CROWN DYNAMICS OF TOONA-CILIATA, Journal of tropical ecology, 14, 1998, pp. 683-703
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.