Both Nothofagus solandri var. cliffortioides and Nothofagus menziesii have a wide natural distribution in New Zealand, comparable in latitude and altitude and overlapping throughout much of their range. Fossil records show that the two species diverged genetically at least 70 million years ago. The objectives of this thesis were to examine and compare the genetic and physiological characteristics of N. solandri var. cliffortioides and N. menziesii; and to evaluate the extent to which the present distribution of the two species can be attributed to differences in their physiology. Results from isozyme analysis indicated that the levels of variability within species in both N. solandri var. cliffortioides and N. menziesii were comparable to that reported for other woody plants native to New Zealand. In both species, diversity was found to reside mainly within provenances; and no important isozyme diversity was shown between the major disjunct geographical populations representing the three broad endemic-rich areas. Marked differentiation in water relations between N solandri var. cliffortioides and N. menziesii was revealed through a series of experiments: The leaf and stomatal structures in N. solandri var. cliffortioides were characteristically more xeromorphic than those in N. menziesii; N. solandri var. cliffortioides was clearly more tolerant to both water stress and water logging than N. menziesii. The relative water stress tolerance was estimated to be approximately -1.2 MPa in soils and -7 MPa in plants for N. solandri var. cliffortioides, compared respectively to -0.7 MPa and -4 MPa for N. menziesii. The greater tolerance to water stress in N. solandri var. cliffortioides was shown not only in its response to decreasing soil water potential, but was also exhibited in its recovery following re-watering, and in its water use efficiency. The difference between the two species in tolerance to water stress and water logging is generally consistent with the pattern of the present distribution of N. solandri var. cliffortioides and N. menziesii. Response differences between N. solandri var. cliffortioides and N. menziesii in relation to several attributes of light and temperature were clearly demonstrated. N. menziesii was shown to be morphologically and photosynthetically more adapted to a low light environment than N. solandri var. cliffortioides, in terms of the partitioning of photosynthates, the efficiency of the photosynthetic system, and in the level of light compensation. While both species showed a maximum net photosynthetic rate at about 20°C, optimum growth in N. solandri var. cliffortioides occurred approximately at a day temperature of 27°C and a night temperature of 22°C; optimum growth in N. menziesii, in contrast, was achieved at a day temperature of around 21°C and a night temperature of 16°C. Tolerance to both winter and summer frosts was comparable between the two species. N. menziesii, however, was shown to have more capacity to recover from summer frost than N. solandri var. cliffortioides. In contrast with the results of the isozyme analysis, significant differences between provenances were shown in growth, shade tolerance and frost tolerance. In both species, seedlings from the two disjunct endemic-rich areas (the 'northern region' and the 'southern region') in the South Island were clearly differentiated in growth. A difference between provenances in N. solandri var. cliffortioides and N. menziesii was shown in their photosynthetic light response. Provenance variation in winter frost hardiness could be related to altitude, mean annual minimum temperature, and mean number of frost days per annum. For optimum growth, N. solandri var. cliffortioides required higher levels of nutrients in the growing medium, especially phosphorus and magnesium, than did N. menziesii. The greater tolerance of N. menziesii to low levels of phosphorus and magnesium in the growing medium was associated with a difference between the two species in uptake efficiency of those elements. A pronounced difference in tolerance to salt spray was shown between N. solandri var. cliffortioides and N. menziesii. The leaves of N. solandri var. cliffortioides were relatively tolerant to direct spraying with sea water, whilst in N. menziesii, spraying with a 50% concentration of sea water resulted in severe injury to leaves. Results showed consistently that in the absence of profound environmental restraints, N. solandri var. cliffortioides grew significantly faster, and had a considerably higher rate of net photosynthesis, than N. menziesii. Several physiological characteristics, i.e. high requirement for mineral nutrients in soils, low shade tolerance, and tolerance to both water stress and water logging, suggest that N. solandri var. cliffortioides has more of the attributes of a pioneer species than N. menziesii. Use of the climate model for New Zealand, to determine whether the present distribution of N. solandri var. cliffortioides and N. menziesii is climate-based, did not produce conclusive results. The predicted distribution based on the climate profiles departed considerably from the actual distribution of the two species. Results suggest that the absence of both species in the "beech gap" of central Westland in the South Island is unlikely to be a consequence of climate alone. The physiological differences between N. solandri var. cliffortioides and N. menziesii were examined in relation to their contribution to the present distribution of the two species. It was concluded that the present distribution pattern of N. solandri var. cliffortioides and N. menziesii reflected in large part real physiological differences between species. In part, however, it was also the result of a "snap-shot in time".