The current status, comparative ecology and spatial meso-scale climatic modelling of lowland native grass species

Abstract

The Poaceae or grass family comprises approximately 20% of New Zealand's vascular plant biodiversity. Despite the importance of this family to New Zealand agriculture, lowland native grass research has been relatively ignored. This thesis provides a broad comparative introduction to New Zealand grass ecology with a specific focus on lowland native grass species. It synthesizes historical and autecological reviews, grass community association analyses and an investigation of species distribution patterns, as determined by climate. Prior to human settlement, there was little unforested land in northern lowland New Zealand. Native grasses were generally restricted to harsh environments maintained by climate extremes or seral environments created by natural disturbance. At present croplands or pastures of European grasses largely replace the forest and disturbed lands. Exotic plant and animal species, and habitat modification now threaten native lowland grasses. It is possible that current farming practices may have also inadvertently endangered their continued survival. Natural grass and grassland distributions are largely determined by climatic conditions and are subsequently modified by a hierarchy of edaphic factors and biotic interactions; anthropogenic influences further alter these patterns. To explore the relationship between climate and grass distributions, climate profiles were developed for 25 native grass species using the BIOCLIM procedure and species presence data. The species profiles differed in maximum and minimum thresholds for 27 solar radiation, temperature and rainfall parameters. The profiles were also cross matched with national climate estimates to predict and map geographic distributions for each species. Complementary classification and ordination analyses identified climate profile groups or 'plant functional types' that reflected broad climate threshold differences between C3 and C4 species. Results show that seasonal rainfall thresholds have a greater effect on distribution patterns than temperature and solar radiation. The geographical distributions for these types show distinct latitudinal and/or altitudinal boundaries. This work presents a systematic, interdisciplinary investigation into the ecological factors affecting New Zealand lowland grass species and is part of a growing public awareness of grasses and grasslands. The study thus establishes a foundation for the sustainable management of this major component of the native flora and joins international trends towards grassland conservation and research.