LOCALIZED ECOTYPES OF THUJA OCCIDENTALIS L. IN WISCONSIN

A bstract. Northern white cedar (Thuja occidentalis L.) sometimes occurs in wet lowlands, and adjacent, relatively dry upland sites suggesting potential localized ecotypic differentiation as a response to selection for moisture stress tolerance. This study examined morphological and physiological responses to root-zone water potential (RWP) of adjacent upland and lowland northern white cedar populations, and related the responses to soil moisture of the habitats as a basis for ecotypic differentiation. In common environment experiments, seedlings from one pair of upland and lowland populations separated by less than 0.7 km illustrated separate ecotypes presumably resulting from natural selection for response to moisture stress. Variation between two study areas in degree of ecotypic differentiation suggested a polytopic origin of the upland and lowland ecotypes. Lowland soils had higher moisture-holding capacity than did upland soils. A study of seedling growth in different soil media showed that as moisture-holding capacity increased in the soil, seedling tops increased in size and root form changed from a long taproot with few laterals to shorter, thicker roots with many laterals. In tests of photosynthetic response to temperature, irradiance, and RWP, 6-mo-old northern white cedar seedlings maintained high rates of net photosynthesis under a wide range of environmental conditions.