Masters Plan of Work submitted 2000


A comparison of light regimes under human-made multi-species tropical ecosystems, and their effect on plant growth

Miguel Cifuentes


Abstract

My objective is to determine the extent to which light intensity affects understory plant growth in multi-species stands of native tropical tree species at La Selva Biological Station in Costa Rica. In 1991, three native tree species with fast growth and high timber value were planted alone and in combination with two monocotyledonous plants to determine the ability of human-made ecosystems to sustain soil fertility under contrasting conditions of plant diversity and stand rotation time. This is experiment is known at La Selva as the Huertos Project (“huertos” is the Spanish word for orchards). The light characteristics of these tree plantations where 3 plant life forms (monocotyledonous tree, a palm, and a giant herbaceous perennial) are growing simultaneously will be determined. The influence of light intensity on understory plant growth in these ecosystems will be assessed.

Both the horizontal and vertical variation of light availability will be measured in a 5 x 5 m grid within each of the study plots. I will use a series of quantum sensors placed at 0.5, 5, 10, and 16 m above the ground to measure the vertical variation of light over a point within the experimental plots. By repeating this procedure for every grid cell within a plot, the horizontal variation of light characteristics at different heights above the ground will also be determined.

Project significance

According to FAO (1999), between 1980 and 1995, the extent of the world’s forests decreased some 180 million hectares. The major causes of change in forest cover in the tropics appear to be expansion of subsistence agriculture in Africa and Asia and large economic development programs in Latin America and Asia (FAO 1996). A decline in soil fertility is typically observed in areas where the original forest cover has been converted to agricultural lands. Plantations can be used to foster regeneration of these degraded lands (Powers et al. 1997; FAO 1992). Although there is some evidence that more species-rich communities are better at conserving soil nutrients than monocultures, their complexity and dynamics make it difficult to identify the mechanisms that might be involved (Haggar and Ewel 1997).

The Huertos project experimental setup is designed so that the roles of plant diversity and rotation frequency over sustainability of soil fertility and plant productivity can be better understood. This endeavor has proven to be more complex than it was anticipated, as intricate interactions between plant species have been observed since the research started. Within this project, I will focus on the influence of light in human-made ecosystems when used to restore degraded ecosystems since not a lot of research has been made in this area. Results from my research will have important ecological implications for succession theory, for designing ecologically meaningful stages in restoration efforts, and for constructing ecologically sustainable systems of land use.

References

FAO. 1992. Mixed and pure forest plantations in the tropics and subtropics. FAO forestry paper 103. Food and Agriculture Organization of the United Nations. Rome, 152 pp.

FAO. 1996. FRA 1990: survey of tropical forest cover and study of change processes. FAO Forestry Paper No. 130. Rome. Italy. 154 p.

FAO. 1999. State of the world’s forests 1999. FAO. Rome, Italy.

Haggar, J.P. & Ewel, J.J. 1997. Primary productivity and resource partitioning in model tropical ecosystems. Ecology 78(4):1211-1221.

Powers, J.S.; Haggar, J.P. and Fisher, R.F. 1997. The effect of overstory composition on understory woody regeneration and species richness in 7-year-old- plantations in Costa Rica. Forest Ecology and Management.