The Role of Regeneration Stages in the Distribution of Edaphically Restricted Fynbos Proteaceae

Regeneration stage requirements can be determinants of adult plant distributions. We investigated this in two species—pairs: Protea abtusifolia and Leucadendron meridianum occurring on shallow soils overlying limestone, and P. susannae and L. coniferum on the adjacent, deep, weakly acidic, colluvial sands. We tested the hypothesis that abiotic regeneration stage requirements at the seed germination and/or seedling establishment stages determine adult distribution patterns. We excluded competitive effects in our experiments. We preformed reciprocal seed transplants in the field, and monitored seedling emergence, establishment, and mortality over two summers. We determined iron, aluminum, calcium, and magnesium contents of 1—yr—old shoots, as well as soil temperature and moisture and midsummer water stress measurements of seedlings. There was higher seedling emergence of seed of all species on the limestone soils, negating the idea that seeds had germination requirements specific to their natural soil types. We suggest that this higher emergence is related to the higher moisture contents of the limestone soils. Seedlings of the limestone species performed better on their own soil type, with lower seedling mortality and overall increased survival (taking into account seed germination and seedling establishment) on their own soil type. Seedlings of the colluvial sands species, on the other hand, had slightly increased seedling mortality, and overall less survivorship, on their own soil type. Despite this, seedlings of these two species growing on the colluvial sands were larger than their counterparts on the limestone soils. There were both more and larger seedlings than those of P. obtusifolia and L. meridianum on the colluvial sands. Thus competitive exclusion is invoked to explain the sole persistence of the colluvial sands species on their native substrate.