Degree Name

Doctor of Philosophy


Department of Physics


The work described in this thesis involved development of a predictive tool for characterizing the light climate within a plant stand canopy under clear sky conditions. Characterization of the light climate within plant canopies has been the focus of a considerable number of measuring and modelling programs. Measurement programs have tended to involve the use of large numbers of sensors with enormous amounts of data collected over lengths of time. Many of these programs have not taken account of the spectral characteristics of the incident radiation, the optical properties of the radiated plant elements or the mechanism of the photosynthetic process itself. Modelling programs have tended to focus on crop plants and total PAR measurements rather than consider the spectral characteristics of the incident radiation in complex plant stands such as rainforests.

This indirect method of determining the in-canopy light climate, described here, consists of:- i. collecting hemispherical canopy photographs over a number of sites in a plant stand and determining the plant stand parameters. ii. measuring the optical properties of the characteristic species of the site. iii. applying a canopy penetration model, which incorporates the optical properties of the site species, to the canopy hemispherical photograph data and predicting the in-canopy light climate.

In order to validate the predictive tool a set of portable spectroradiometers were constructed, calibrated and tested and then used to measure the light climate encountered at a number of sites in sub-tropical rainforest stands.

The value of this predictive tool is in investigation of the characteristics of in-canopy irradiance levels. A range of investigations that the tool can be effectively applied to, have been outline, eliminating the need for extensive measuring programs when determining in-canopy light climates under conditions of clear skies.