Long inter‐fire intervals do not guarantee a large seed bank in a serotinous shrub (Banksia spinulosa Sm.)

Abstract

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 as follows: 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 the 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 was produced by only 10%-15% of plants. There was a potential for accumulation of massive seed banks. However, (a) only 8%-10% of inflorescences became cones; (b) only 44%-50% of these were retained until harvest; (c) many retained cones suffered seed predation; and (d) viability of retained seeds declined with cone age. The result of these processes meant that the accumulated seed bank was only two to four seeds per plant, 82%-94% of the viable seeds had been produced in 6 years prior to harvest and only 12%-26% of plants contributed to this viable seed bank. 4. Synthesis: Cone and seed losses and declining seed viability in B. 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.

Evaluation of seven indices of on‐plant seed storage (serotiny) shows that the linear slope is best

“…From a practical point of view, the chosen approach may depend on how easy it is to gauge cone/fruit/seed age versus the importance of distinguishing any differences between the variables under study. If the objective is to understand how serotiny contributes to post‐fire recruitment it would seem essential to include an estimate of the rate at which seed viability is lost as part of that (Crawford et al., 2011; Whelan & Ayre, 2020).…”

“…Note that viability is based on all seeds produced but excludes those attacked by granivores or pathogens, as this is unrelated to the genetic control of serotiny. Thus, % closed cones/fruits is always higher than % seed viability at any given age, as seeds almost never start from an initial viability of 100%, unlike cones/fruits (Whelan & Ayre, 2020; Figure 5; Table 3).…”

“…Most species retain their old open cones/fruits or the stumps can be recognized (Goubitz et al., 2004; Talluto & Benkman, 2014) enabling their open condition at that plant age still to be determined. Since the method only requires a representative sample at each crop age, it is not necessary to record the entire crop, which has the advantage that it does not matter if some cones/fruits have been lost due to removal by granivorous animals provided these were not atypical (Talluto & Benkman, 2013; Whelan & Ayre, 2020; Witkowski et al, 1991). Or, if visits by florivores/granivores are fitful, then the affected years could be omitted, that is not possible with the non‐temporal methods.…”

“…Calculated from data in Lamont, Connell, and Bergl (1991), Lamont and Barker (1988) and Lamont and van Leeuwen (1988) (Goubitz et al, 2004). As a solution to a possible post hoc ageing problem, Whelan and Ayre (2020) undertook the long-term commitment of tagging inflorescences in each crop year as they occurred.…”

“…Note that Crawford et al (2011) used viability of the seeds still held at a given age but, for calculating the rate of loss on a viable seed basis, this needs to be multiplied by the fraction of cones/ fruits still closed in that year, as in Table 3. The alternative is to have the number of seeds produced each year (for a sample of cones/fruits) and the number of viable seeds retained in that year at the time of the study, both excluding seeds lost from granivory or disease, and convert to a percentage (Lamont & Enright, 2000;Whelan & Ayre, 2020). When this is done, it becomes clear that the slope value cannot be used routinely as an index of serotiny.…”