Local adaptation and microsite limitation

Abstract

For local adaptation to occur, there must be heritable variation among individuals in response to specific environmental factors. However, relatively few studies have assessed differentiation among plant populations as well as heritable variation within populations in response to experimentally manipulated environmental factors. I examined differentiation in response to environmental conditions at different scales: among species, populations and maternal families (i.e. offspring from different plants). To investigate local adaptation of different populations and maternal families of a native New Zealand herb, Phormium tenax, to microsites with varying moisture and substrate conditions, I contrasted germination and seedling establishment traits in a common garden environment. I found genetic differentiation of populations and significant variation among maternal families in response to the moisture level for germination and seedling emergence and the substrate type for seedling establishment. These results provide evidence for heritable variation that could lead to adaptive differentiation both among P. tenax populations and within populations at the microsite level. To investigate variation among species at the microsite level, I examined germination and seedling establishment of three coexisting native New Zealand forest species (Libertia grandiflora, Melicytus ramiflorus and Piper excelsum) in response 10 different substrate types under both field and glasshouse conditions. Germination responses to different substrates were similar among species but optimal microsites for seedling establishment differed. Patterns of seeding establishment also varied between glasshouse and field environments suggesting that responses to substrate type interact with other environmental conditions. These results suggest that microsite limitation is a strong force influencing patterns of recruitment of these species. The scale of environmental heterogeneity can therefore influence patterns of variation and adaptation among species, populations and maternal families.