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Drainage, hydrology
and Biota

Drainage and hydrology

The island is drained by low Strahler order headwater streams radiating from the centre (Figure 2.3).  The higher order channels downstream have been flooded under the lake.  Most streambeds on the island are gently or moderately sloping, but many are bouldery. By the end of the dry season there may be pools but little flow in larger streams, whilst the beds of minor streams are dry.

Data on soil moisture for the 50 ha LTER plot and its environs (Becker et al., 1988; Daws et al., 2002), and for five years in two 2.25 ha control plots in a large dry season irrigation experiment on the Poacher peninsula (Kursar et al.1995) indicate that soils can dry to tensions of 3 MPa by the end of the dry season.  However, the intensity and duration of high moisture tensions vary with topographic position. Daws et al. (2002) showed that moisture stress is more intense and prolonged on the main dipslope/plateau upper tread surface than on the connecting riser slopes. This may be due to significant lateral transfers downslope by subsurface throughflow.

 

Biota

Apart from the small areas for the lab complex, sight lines for shipping lights, and other minor infrastructures, BCI is now entirely covered by tropical moist forest.  The forest cover was disturbed during the construction of the Canal, and there were patches of farming on the north and south coasts.  The younger regrowth is mostly in the north and east of the island.  Much old growth forest, possibly dating back to the 17th century, has been identified in the west and south (Foster & Brokaw, 1996).

Leigh (1999) and Leigh et al. (1996) give overviews of the tropical moist forest ecosystem of BCI.  Aspects of the biota that directly affect soil formation include:

  • There is much soil excavation, especially by ants.  They may contribute for the burial of stone sheets.
  • As seen during our fieldwork at the end of the dry season, there are common worm casts on the soil surface.   Pockets of endogeic casting were seen in some subsoils (e.g. Profile PF02 in Appendix B). However, the pedogenic importance of earthworms on BCI is as yet unquantified.
  • Treefalls are unevenly distributed on the island (Brokaw, 1996; Putz, 1983; Putz et al., 183 & 1985).  They appear to be frequent on the Bohio formation, probably because of the generally steep topography.  Frequencies also seem to be high on the western side of the island, possibly due to windthrow by westerlies (Foster & Brokaw, 1996). The soils in areas of frequent treefall are subject to patchy profile truncation, retarded weathering and pedogenesis, and other disturbances.