Big tree fall as driving factor in the gap dynamics in a mixed Dipterocarp forest

Background/Question/Methods

Studying the dynamics of trees in tropical rain forests is a major challenge in tropical forest ecology, because i) these forests are structurally complex, ii) they are extremely species rich, iii) there is a lack of long-term and accurate data. A standard approach to study dynamics of tropical forests is measuring basic tree parameters in permanent plots, using repeated surveys. There are just a few plots resurveyed more than twice. We analyze data from two 1ha plots in a mixed Dipterocarp forest in Brunei Darussalam (Borneo, SE Asia), resurveyed four- to five times from 1991 to 2011. Our aim was to reveal the temporal patterns in trees with dbh >5 cm over the past 20 years. The main question was, how fast is the turover of tree individuals and at what spatial scale does it appear? Spatial distribution of trees was measured in x and y coordinates, initially with tape and compass, and from 2007 with digital laser technology. This enabled precise positioning of each tree in a x-y-z coordinates space. Trees were classified in three groups reflecting dynamics of the forest: dead, slow and fast growing trees. Groups constituted of the groups were related using spatial pattern analysis.

Results/Conclusions

Clumping of the dead and fast growing trees, and regular patterns between the slow and fast growing, and dead and slow growing trees, suggest that tree radial growth was driven with some events causing mortality. Our analysis revealed that trees were dying in two basic modes: i) abruptly as a consequence of fall of a big tree, and ii) slowly as a consequence of long-term competition with other trees. Big tree fall was encountered only a few times within the 1ha plots. Despite of its relatively low frequency, big tree fall is a major driving factor creating gaps of 200-400 m² at each occasion. In average, 120 trees die and about the same number occurs a new, reaching over >5 cm in dbh every five years (period of resurveys). Competition for light (and possibly also for nutrients) is the strongest at distances of 10-15 m, eventually leading to deaths of many understory trees. Some of these trees can withstand the competition for long, not increasing their trunk diameter for two decades at all. The mixed Dipterocarp forest dynamics can be therefore denoted the gap dynamics with big tree falls as acting factors.