Holloway's climatic change hypothesis : a quantitative evaluation in the Longwood forests
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Holloway's (1954) hypothesis of widespread cooling and drying in South Island climates c. 800 years ago is evaluated through examination of some of its key evidence - unbalanced podocarp populations and their replacement by Nothofagus in the Longwood forests, Southland. Analysis of gradients in species' dominance, vigour and variety, and in site factors, indicates correspondence broadly between patterns of forest and those of soil and climate; in detail between the distribution of silver beech and air temperatures. Modelling of forest stand successions suggests that the narrow beech-podocarp ecotones on the seaward flanks of the Range are stable, silver beech is slowly encroaching upon podocarp stands in broader ecotones on the eastern-lee, while small angiosperm trees are supplanting the sparsely-stocked podocarps of the northern hinterland. Inventories of podocarp stem populations show that small stems are relatively scarce, particularly farthest inland from the coast. Aging of rimu stems reveals that fewer of these date to establishment as seedlings during the 17th to early 19th Centuries A.D., than to establishment during prior and subsequent periods. Analysis of rimu growth rings provides a positive relationship between increment and recorded air temperatures (since 1853) enabling climatic inference from lengthy chronologies. These feature significantly slow diameter growth, indicating a major cool period from the 17th to the 19th Centuries A.D., relating apparently to depressed rimu regeneration then. Pollen diagrams from peat bogs on the crest of and beside the Range show that the broad pattern of forest on the Range has not changed in the last thousand years. Major changes from matai-kahi-katea types to rimu and silver beech types were initiated c. 4000 years B.P., apparently by climatic changes in the direction of coolness and increased rainfall. Holloway's hypothesis therefore requires considerable modification.