Barro Colorado Island
Barro Colorado Island (BCI) was created in 1914 by the construction of the Panama Canal, during which the lower valley of Rio Chagres was flooded to form the freshwater Gatun Lake. The flooding isolated the upper and middle slopes of some low hills to form BCI, which is now separated from the mainland by open fresh water, nowhere less than 200 m wide.
In 1914 the island was largely under moist deciduous tropical forest, some of it regrowth after cutting and disturbance during the construction of the Canal in 1880 –1905. There were also substantial areas of old growth forest (Foster & Brokaw, 1996) and small areas of cultivation. The few smallholder farmers were bought out when the island was designated as a Biological Reserve in 1923. The reserve was dedicated to watershed protection, conservation and scientific research. A committee of the US National Academy of Science administered it until 1946, when the Smithsonian Institution (SI), specifically the Smithsonian Tropical Research Institute (STRI), assumed responsibility. STRI extended the range of research, especially into forest ecology (Leigh et al., 1982 & 1996). The post-1979 phased transfer of sovereignty of the Canal Zone from U.S.A to the Republic of Panama (RoP) is now complete. RoP designated BCI and five nearby mainland peninsulae as the Barro Colorado Nature Monument (BCNM) in 1993. This status gives rigourous protection under Panamanian law, and also internationally under the 1940 hemispheric Convention on Nature Protection and Wildlife Preservation (STRI, 1987). The adjacent Soberania National Park provides a physical buffer that should further protect the Monument and BCI. Under agreement with the Government of RoP, management of BCI remains with STRI, whose activities are mainly resourced through US Congress budget appropriations and by endowments from other sources in the USA.
Soil – Related Ecological Research on BCI
BCI is the most intensively researched tropical forest in the world (Ocana et al. 1988; Rubinoff & Leigh, 1990). Work done on BCI has greatly contributed to current understanding of the ecology of neotropical moist forests and of the tropical forest biome in general. There are overviews of scientific investigations and publications on BCI in Leigh (1999) and Leigh et al. (1996).
One of the activities pioneered on BCI was the setting up of large long-term ecological research (LTER) plots in tropical forests. There is now a pantropical network of 16 such plots, coordinated by the Center for Tropical Forest Science (CTFS), an entity within STRI, and, for the nine Asian plots, by the Arnold Arboretum of Harvard University. The plots are large (ideally 50 ha, with up to a third of a million stems on each), so that they contain statistically robust populations (ideally > 100 stems) of all but really rare species. The freestanding vegetation (i.e. excluding climbers) down to 1 cm diameter at reference height (1.3 m above ground level or 0.6 m above the highest buttress) is initially inventoried and then monitored at five-year intervals, so that the life history of every tree in the forest can be traced from young sapling to death and disappearance.
The first CTFS plot was established on BCI, and the CTFS standard methodologies were pioneered, tested and codified on the BCI 50 ha LTER plot (Condit, 1998). The monitoring period for BCI is longer than for any other CTFS plot, with the initial inventory in 1981-2, and the sixth quinquennial re-census in 2005. This enables analyses of the dynamics of the forest on BCI to extend over decades rather than just years, and a picture of the effects of medium term climatic variations on tropical forest is starting to emerge (Condit et al.1995 & 1996; Chave et al. 2003).
One of the themes that consistently interests plant ecologists on BCI, and in CTFS and in other tropical forests, is the origin and maintenance of high tree species diversities at local, regional and biome scales. Possible influences and determinants include:
- Density–dependent biological processes and pressures, such as predation by species-specialist pests and infection by specific pathogens;
- Stochastic assembly-dispersal processes;
- Periodic intense disturbances (e.g. hurricanes, seismic, volcanic);
- Refined specialisation for diverse abiotic niches
(Leigh et al., 2004).
In general, less importance has been accorded to abiotic niche specialisation on BCI than in some Asian forests. This is true for island–wide studies (e.g. Knight, 1975; Svenning et al., 2004), and on the BCI 50 ha LTER plot (e.g. Harms et al., 2001).
Research into the possibility of abiotic niche specialisation cannot progress far without detailed characterisation of the physical environment, including soils. Soil data for BCI are limited. Edaphic habitats have hitherto been characterised as topographic classes, and by topographic attributes, such as elevation, slope angle, and hydrological indices. BCI soils have so far been differentiated only as generalised classes (Brenes-Arguedas & Coley, 2005; Croat 1978; Harms et al., 2001; Knight, 1975; Svenning et al. 2004).
Aims of BCI Soil Survey
This survey aims to clarify the pedospatial structure of BCI. It is paralleled by several other current activities that will substantially enhance understanding of BCI’s soils. These include: soil hydrological studies; systematic soil nutrient characterisation of the 50 ha LTER plot; microbial rock mineral weathering; follow-up of the long term dry-season irrigation experiment; and fertiliser and necromass manipulation trials on pedologically similar areas on adjacent mainland peninsulae.
The objective of this survey is to compile a soil map of the island. The emphasis is pedological but the map should facilitate:
- Characterisation and spatial differentiation of edaphic environments on BCI.
- Pedospatial interpretation of data on nutrients, water, aeration and other soil-related features.
- Comparison of BCI soils with those of other tropical forests; particularly the soils of the BCI 50 ha LTER plot with those of other CTFS plots.
The survey is at semi-detailed scale, with a density of field observations of ca 1 per 2 ha, which supports a map scale of 1:15 000.
The spatial patterns elucidated by soil survey are largely based on morphological features and emphasise mineral components of the soil. Ecologically they are more helpful with respect to meso-scale variations in soil stability, hydrature and mineral-derived nutrients, such as the basic cations, and less for atmospherically-sourced and biotically mediated nutrients, such as C, N and S. Soil surveys can sometimes be related to spatial distribution and variations in growth in vegetation (e.g. Veldkamp et al., 1990), but not all soil maps are immediately and obviously useful. Nonetheless, because the pedogenic characteristics of the mineral components of soils change only slowly, soil maps are valid for long periods, and their implications may become only slowly apparent. The long-term value of soil maps is enhanced if the original data are retained and available for re-interpretation to meet future research needs. Our data will be posted on the STRI and Universität Potsdam websites.