This thesis examines how, and to what extent, do light regimes control the distribution of terrestrial orchids, through field studies of orchid populations under different forest types, manipulated light environment trials, and seasonal studies of orchids growing under different light environments. The significance of understorey light environment as a determining factor on terrestrial orchid occurrence and distribution is examined. A survey is performed over two primary (Nothofagus and Podocarp) and two secondary (Broadleaf and Manuka) forest types at selected South and Stewart Island sites. Hemispherical photography is used to record canopy architecture. Four parameters are used to represent canopy and understorey light characteristics. Broadleaf sites are significantly darker than the other three forest types. Of the 23 orchid species identified at these forest sites, the thirteen most abundant are classified into three categories based on the four light parameters. Pour species are considered to be low-light plants, all of which are of the genus Pterostylis. A further three species are classified as occurring under medium light conditions, with the remaining six species assigned to a high-light group. Light environment is a key factor controlling orchid occurrence under the sampled forests (R2 = 0.37 - 0.74). Even stronger correlations are identified for plant density (R2 = 0.73 - 0.93). The adaptability of Pterostylis banksii, P. graminea, Thelymitra longifolia and T. pauciflora to different light environments is examined using transplanting trials. Plant occurrence, morphology and physiology is compared under three different light regimes, with comparisons made to the original field populations. P. banksii is able to successfully adapt from the low-light Broadleaf environment to significantly higher enclosure light regimes. T. longifolia specimens are able to successfully adapt to both higher and lower light regimes than the field conditions. These plants also exhibit modified metabolism with leaf efficiency (Pmax/Rdark) of T. longifolia enclosure plants significantly lower than field plants. Morphological changes are also evident with the production of multiple replacement tubers in enclosure individuals. Results for P. graminea and T. pauciflora are inconclusive due to high mortality rates. It is believed that P. graminea is unable to adapt to the significantly higher light conditions, with T. pauciflora intolerant of transplanting. The relationship between understorey light environment and the morphology and phenology of natural populations of P. banksii, P. graminea, T. longifolia and T. pauciflora is examined. Dry weight and total non-structural carbohydrate concentrations of organs are recorded over a 12 month period. The two Pterostylis species have a similar phenology and morphology, with biomass and carbohydrate allocation patterns generally following similar trends. Inflorescence production precedes replacement tuber development, suggesting that floral induction is based on cues from the previous season. Replacement tuber development occurs late in the growing season, only two months prior to plant senescence, making November and December key months for carbohydrate acquisition. The two Thelymitra species have similar morphology and phenology, with equivalent biomass and carbohydrate allocation patterns. Photosynthetic characteristics vary significantly with plant age for T. longifolia, but not T. pauciflora. Inflorescence production occurs several months after replacement tuber initiation. The results of this study demonstrate that understorey light environment is a key environmental factor for terrestrial orchids, inducing changes in orchid morphology and physiology. The influence of light regime on orchid occurrence and density is significant, but only fully apparent when understorey conditions are represented using multi-parameter models.