PARENT SESSION
Organized Oral Session 50: Dynamics of disturbed and undisturbed tropical rain forests
Organizer(s): A Huth and J Groeneveld
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 511 B, Level 5, Palais des congrès de Montréal

Deterministic changes in species composition in primary forest in the Eastern Congo basin.

Thomas, Sean^*,1, Makana, Jean-Remy^{1, 2, 3}, Hart, Terese², Hart, John², Condit, Richard⁴, Ewango, Corneille^{2, 5}, ¹ University of Toronto, Toronto, ON, Canada² Wildlife Conservation Society, Bronx, NY, USA³ Centre de Formation et de Recherche en Conservation Forestière, Epulu, Eastern Province, Dem. Republic Congo⁴ Center for Tropical Forest Science, Panama City, Panama⁵ University Missouri - St. Louis, St. Louis, MO, USA

ABSTRACT- 'Monodominant' forests dominated by a single species are found in large areas of primary forest in the Congo basin; the existence of such forests, and their co-occurrence with species-diverse mixed forests, are of great interest as a means of understanding the processes that determine species diversity in tropical forests generally. Data on static and dynamic patterns of forest structure community composition in four 10-ha mapped, repeatedly-censused forest plots (including ∼300,000 stems ≥1.0 cm dbh) in the Ituri Forest were used to address the following questions: (1) Is monodominant 'mbau' forest (dominated by Gilbertiodendron dewevrei) a distinct community type? Does G. dewevrei act as a 'super-competitor' which locally increases in abundance at the expense of mixed forest species? And more generally, does undisturbed primary forest in the Congo basin exhibit deterministic changes in species composition through time? A total of ∼420 tree species were enumerated in the 40-ha area as a whole; ordination analyses suggest three distinct forest community types: monodominant, mixed and riparian forests. A small number of understory trees had a positive association with G. dewevrei-dominated areas, including trees in the genera Scaphopetalum, Diospyros, Garcinia, and Aidia. Both static and dynamic spatial analyses suggest an 'invasion front' structure of the interface between mbau and mixed forest, with regenerating G. dewevrei trees found dispersed up to 30 m within mixed forest areas. G. dewevrei and some of its associated understory tree flora showed increased abundance though time, mainly along mixed forest margins. Among commercial timber tree species for which shade-tolerance classes are known, light-demanding species showed decreased abundance while shade tolerant species increased. In conclusion, monodominant mbau forest in the Ituri does constitute a distinct community type, and observed patterns are consistent with the idea that G. dewevrei is spreading into adjacent mixed forests. However, there are pronounced changes in species composition in all major forest types. Such changes appear to driven by successional processes, rather than climate change or other extrinsic agents.

Key words: tropical forests, community dynamics, Central Africa, succession