Maintenance costs of serotiny do not explain weak serotiny

Considerable variation in the duration of serotiny exists among species of both Australian and South African Proteaceae. 'Weak' serotiny (pre-fire loss after < 3 years) could be dictated by the costs (water or carbon) of cone/fruit retention or by benefits accruing from pre-fire seed establishment. We determined that cones/fruits of a range of Australian and south western Cape Proteaceae species (Leucadendron xanthoconus, Aulax umbellata, L. linifolium, L. gandogeri, Hakea drupacea, H. sericea) are not sealed dead wood, but that they continuously lose H2O and CO2. Water loss from cones/fruits was poorly controlled, occurring in both light and dark. The rates of both H2O and CO2 loss from mature cones/fruits were negatively correlated with the degree of serotiny (r(2) = 0.59 and 0.18, respectively, P < 0.001 both). However, the amounts of H2O and CO2 lost per weight were small relative to the fluxes from leaves (13-29% for H2O and 3-10% for CO2). The [N] and [P] in the cones/fruits and seeds was substantial. Despite 25% of N and 38% of P being recovered from the cones/fruits following maturation, the loss of the cones/fruits and seeds would still incur a substantial nutrient cost. The seed [P] was positively correlated with the degree of serotiny (r(2) = 0.24, P = 0.001). We suggest that maintenance costs (water and carbon) of serotiny, although exceeding those of soil stored seeds, are relatively low. The correlation between the degree of serotiny and seed [P] indicates that stronger serotiny is required, much like sclerophylly, for survival under low nutrient availability in frequently burnt vegetation.