Dead frond “skirts” as tree fern defence: what is the evidence?

Many tree fern taxa have a skirt, an encircling structure of persistent dead fronds or stipes around the growing crown at the top of the trunk. Page and Brownsey (1986) hypothesised that the function of these skirts was to protect tree ferns against damage from large epiphytes, hemiepiphytes, and climbing plants. Tree fern trunks provide both suitable establishment surfaces for a range of woody epiphytes and hemiepiphytes in New Zealand, as well as attachment surfaces for climbing rātā (Metrosideros spp.). We collected detailed occurrence and cover data of woody epiphytes (including hemiepiphytes), climbing rātā, and skirts from 350 Cyathea smithii and 350 Dicksonia squarrosa across New Zealand. We also collected frequency data on epiphyte, climbing rātā and skirt occurrence from an additional 1212 tree ferns. While skirts reduce the density of woody epiphytes on tree fern trunks, they neither prevent woody epiphytes from establishing on trunks nor prevent epiphytes establishing and growing on areas of the trunk covered by skirts. Envelopment of the growing crown of tree ferns was not observed in any of the 1912 individuals surveyed; incidental observations suggest that climbing rātā crown-envelopment is extremely rare and may occur only when tree ferns are exposed in high-light environments. Tree fern skirts occur on species that are frost-tolerant and occur more frequently in higher elevations and latitudes than non-skirted species. We suggest a new defence hypothesis: skirts protect the growing meristem from freezing conditions.