DOI: 10.5061/dryad.r2280gb96
title: Long inter-fire intervals do not guarantee a large seed bank in a serotinous shrub (Banksia spinulosa Sm.)

description: 1. It is often assumed that long-lived woody perennials with canopy-stored
 seed banks steadily accumulate seeds over time since fire. Trends in
 flowering and fruiting have usually been inferred from synchronic studies
 of sites of different post-fire ages or counting stored seeds in apparent
 age classes, and mostly in obligate-seeder species. Long-term longitudinal
 studies on a broader range of species are needed to fully understand the
 dynamics of flower and fruit production and accumulation of viable seeds.
 Key questions are: What is the annual trend in flowering and cone
 production? Is this matched by accumulation of cones and seeds? How is
 seed germinability related to cone age at time of a fire? 2. We counted
 inflorescences and tagged cones produced annually by the resprouting shrub
 Banksia spinulosa in 315 plants over 13 years at one site and in 46 plants
 over 20 years at another. At the end of the study, we harvested all
 accumulated cones, burned them and assessed seed viability using
 germination trials. 3. We detected enormous inter-plant variation in
 reproductive effort and output. 50% of inflorescences were produced by
 only 10-15% of plants. There was potential for accumulation of massive
 seed banks. However, (i) only 8-10% of inflorescences became cones; (ii)
 only 44-50 % of these were retained until harvest; (iii) many retained
 cones suffered seed predation; and (iv) viability of retained seeds
 declined with cone age. The result of these processes meant that the
 accumulated seed bank was only 2 to 4 seeds per plant, 82% to 94% of the
 viable seeds had been produced in 6 years prior to harvest, and only 12%
 to 26% of plants contributed to this viable seed bank. 4. Synthesis: Cone
 and seed losses and declining seed viability in Banksia spinulosa mean
 that almost all viable seeds come from the past few years of flowering
 even though apparently intact cones may be retained for decades. If this
 is typical for resprouting serotinous shrubs, it is important to
 understand trends in flowering and fruit set over time because the
 magnitude of recruitment will depend on fecundity in the few years prior
 to a fire.
description: We established a long-term
 study using two populations of <i>Banksia spinulosa</i> var.
 <i>spinulosa</i> in south eastern Australia: Barren Grounds
 Nature Reserve (34° 40’ S, 150° 43’ E) and Kioloa State Forest (35° 34’ S,
 150° 19’ E). At Barren Grounds (BG), a random sample of 49 mature plants
 was selected from a population of about 1,000. The Kioloa State Forest
 population (KSF) was added nearly 10 years later and comprised a sample of
 349 mature plants from within a similarly sized population. All study
 plants were tagged and mapped. Towards the end of the flowering season
 each year we counted the numbers of new inflorescences and the numbers of
 cones resulting from the previous year’s flowering. Inflorescences that
 failed to produce any follicles were removed to simplify future censuses
 and each cone was tagged with a stainless-steel tag identifying the
 flowering year. We harvested all accumulated cones on the BG plants in
 December 2005 and on the KSF plants in mid 2006.
description: No data on numbers of cones were recorded for the 1995 flowering
 season at KSF, though flowering data were recorded in all years.
 The timing of this harvest
 for BG meant that the most recent cones in the sample were from the 2004
 flowering season; the mid-2006 harvest for KSF captured the fruit
 production from the 2005 lowering season. By the time of harvest, one of
 the 49 BG plants had died, which we excluded from the data set. There were
 two instances of plants initially tagged as discrete individuals but later
 discovered to be emanating from the same lignotuber; the data from these
 pairs of plants were combined, thus producing a total data set of 46
 plants.  At KSF, there were 315 extant plants at the time of the harvest.
 Losses of initially tagged plants were attributable to three main causes:
 (i) plants initially tagged as discrete individuals but later discovered
 to be emanating from the same lignotuber – these were merged in the data
 set; (ii) track-widening works at KSF resulting in the removal of about 20
 plants near the edges of two of the plots; and (iii) crushing of plants by
 large tree trunks or branches falling in storms. The latter plants could
 not contribute to flowering and fruiting for several years although most
 eventually resprouted from their lignotubers. We excluded plants within
 all three categories from the final data set.