Geoecology of limestone-hosted dryland calcicolous plants, North Otago, New Zealand

Dryland calcicolous plants are drought-tolerant species that are adapted to survive on alkaline low-nutrient bare substrates derived from limestone around the world. Outcrops of the Otekaike Limestone of North Otago host some rare low-growing (<10 cm) endemic calcicoles, and conservation reserves have been created at some localities. The calcicoles occupy microhabitats in solution cavities in limestone outcrops, growing in thin (<10 cm) limestone debris accumulations, including detrital sand and silt. Calcicoles also grow in coarse and fine disaggregated limestone talus deposits at tops and bottoms of outcrops, commonly with the low-growing exotic hawkweed but the calcicoles are displaced by taller species, especially grasses. Calcicole substrates have pH 8 to 9, with a balance between calcite dissolution and calcite re-precipitation. Electrical conductivity is typically <500 µS/cm, but can be as high as 7 mS/cm with evaporative salts and salt weathering enhancing porosity. The limestone contains variably oxidised authigenic glaucony and pyrite, which provide some plant nutrients (K, Mg, Fe, S, N) as roots penetrate porous limestone, with apatite providing phosphate. Persistence of these rare calcicolous communities in soil-free settings is strongly affected by the underlying mineralogy and geochemistry, and some similar microhabitats may occur on other rock types.