Population Structure and Fecundity in the Putative Sterile Shrub, 'Grevillea rhizomatosa' Olde & Marriott (Proteaceae)

Abstract

'Grevillea rhizomatosa' Olde & Marriott (Proteaceae) is a threatened species of shrub known only from 12 populations within a 7 x 8 km area within Gibraltar Range and Washpool National Parks, northern New South Wales, Australia. Prior to this study it was believed that the species only reproduced from rhizomatous suckers as seed and fruit were never detected in the wild. A concern for the reproductive and evolutionary potential of the species in the event of a catastrophic disturbance was the basis for an investigation into the reproductive ecology of 'G. rhizomatosa'. Such an event occurred in October 2002 with an intense wildfire affecting most of the populations. Five populations were studied in detail for demography and fecundity prior to this fire and two populations were resurveyed in August 2005. In 2000, 916 individual stems were recorded across these populations and only small to large shrubs were found; no seedlings were recorded. Post-fire response was documented in two populations where plants were found to be resprouting and suckering from underground stems. In the pre-fire surveys of 2000 and 2001 flowering occurred in all populations, but since the fire of October 2002 flowering has only occurred in unburnt habitats. Flowers on shrubs in two of the five populations failed to produce fruit, but low fruit-set (7-13% of flowers) occurred in three populations. Seeds collected from two populations (n = 14) were tested for viability using tetrazolium chloride and were 100% viable. Ramets were detected in all populations and resprouting from underground stems was observed after wildfire. This is the first record of viable seed in this species and fertile populations require specific management to prevent loss of fertile plants. Loss of fertile plants could occur if repeated burning selects for vegetative reproduction and sterile plants.