Local adaptation and phenotypic plasticity in manuka (Leptospermum scoparium)

Abstract

This study tested for local adaptation to moisture regime in manuka (Leptospermum scoparium), a New Zealand native that is found in a diverse range of habitats. My primary interest was whether manuka plants from dry or swamp habitats were locally adapted to their specific environments, in particular the moisture regime, thus potentially limiting gene flow between populations. In a series of reciprocal transplant experiments, manuka plants grown from seed and seedlings transplanted from swamp and dry habitats were grown in the field at swamp and dry sites. Measurements of survival, plant height and leaf number were taken to indicate fitness. Evidence for local adaptation was found in both the plants grown from seed and the transplanted seedlings. In a dry environment, seedlings grown from seeds that had been collected from dry environments exhibited higher survivorship, were taller and had more leaves than seeds that were collected from swamp environments. However, while seedlings transplanted into different environments did not survive better or grow taller in their "home" environment, seedlings sourced from dry environments had significantly fewer leaves in dry environments, suggesting that plants are locally adapted in terms of leaf number. Morphological traits varied plastically across all environments and may act to reduce selection for adaptation to specific moisture regimes, and thus may reduce local adaptation. Selection pressures acted differently on seedlings planted as seeds and seedlings transplanted into the field. Seedlings planted as seeds were more fit in dry environments and seedlings transplanted in to the field more fit in swamp environments. In conclusion, while manuka does show some evidence for local adaptation, continuing studies are needed to examine the cumulative fitness of adults in these (and other) environments.