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

Abstract

On‐plant storage of seeds (serotiny) is a feature of many fire‐prone dominant trees and shrubs in North America, Mediterranean Basin, South Africa and Australia. Understanding how it has responded to the prevailing fire regime and recruitment/growing conditions, and its genetic basis and adaptive significance, depends on the use of an accurate index of the level of serotiny. Seven indices, three measuring ‘apparent’ serotiny and four measuring ‘inherent’ serotiny, were evaluated for their ability to reflect the extent to which seeds are stored in the crown for later release. The most widely used indices ignore the temporal aspects of seed attrition and are vulnerable to plant size, age, fecundity and local growing‐condition effects, while age of the oldest crop year with closed cones/fruits depends on the accuracy of a single observation. Presence/absence of serotiny at the individual tree level ignores the effect of plant age on the expression of serotiny. The slope of the line through the percentage of cones/fruits that still retain their seeds in progressively older crop years, the ‘degree of serotiny’ (100/slope), is shown to be unbiased by annual fluctuations in crop size, differences in plant age or variations in local growing conditions. It also corresponds to a probability, and thus more accurate, estimate of age of the oldest unopened structures. Since seeds have a lower % viability than the % of supporting cones/fruits closed at any age, there is a case for applying the slope method to the fraction of viable seeds retained to arrive at an estimate of the age of the oldest viable seeds on the plant, as the ultimate index of serotiny. If the level of serotiny is to be related to the fire regime, then care needs to be taken to use records over a sufficiently long time to have had an evolutionary impact, unless this is being used against controls to examine a genetic response to the new fire regime. Synthesis. Among seven currently available indices of serotiny, the slope measure is shown to be the most accurate for studies that relate the level of serotiny to the fire regime and habitat conditions under which the species evolved, as well as for examining its heritability. Attention needs to shift from just recording cones/fruits closed to include the viability of seeds that remain as a better basis for understanding the role of serotiny in post‐fire recruitment.

Does recent fire activity impact fire-related traits of Pinus halepensis Mill. and Pinus sylvestris L. in the French Mediterranean area?

“…After the fire, the seeds are released into an environment without competition for light, water, and nutrients as most plants are burned, allowing the seedlings to grow in better environmental conditions. Given its primordial implication in the fire adaptive strategy of numerous fire-embracer species, serotiny is one of the most studied functional fire-related traits (Causley et al, 2016;Keeley et al, 2011;Lamont & Enright 2000;Lamont 2020;Martín-Sanz et al, 2016). In addition, many species have seed germination stimulated by smoke or high temperature (Tieu et al, 2001), or plant resprouting (Ne'eman et al, 2012).…”

Fire as a Selective Agent for both Serotiny and Nonserotiny Over Space and Time

“…Nevertheless, even the slope measure of serotiny will be susceptible to say, increases in mean temperatures due to climate change which have yet to lead to differential survival and adaptation, so that the best estimates of the level of serotiny will still have an environmental component (Hernández-Serrano et al, 2014). Ideally, viability of the seeds should be included in any estimates of inherent serotiny, for example, age of oldest viable seeds retained on the plant (Lamont, 2020), although this is rarely done.…”

“…Two initial gene supports for serotiny were used, P x ðseeds produced x S x viabilityÞ: It is 1 P 0.3 and 0.5 (30 and 50% of genes for serotiny are in a homozygous state). Two degrees of serotiny were used, 4.0 (seeds held for up to 4 years) and 12.0 (seeds held for up to 12 years), based on 100/ b where b is the linear slope of fraction of seeds retained or fruits/cones closed per year (Cowling and Lamont, 1985a;Lamont, 2020). Progressive seed store, H ¼ x (seeds produced x fraction of seeds retained x viability) where x is the number of years prior to, but including, the current year, and progressive total gene support, G ¼ 1 assumed that seeds released are genetically nonserotinous (Hern, andez-Serrano et al, 2013 so that G gets reallocated to the retained seeds.…”

“…Alternatively, the position of the cone within the plant, or fruit within the cone, might affect its vascular supply or exposure to the 'elements' and thus its tendency to open (congenital basis). Nevertheless, assuming that the method of estimating serotiny is sound (Lamont, 2020), there are many findings that (a) the postfire population is often more serotinous than the prefire population (Goubitz et al, 2004;Raffaele et al, 2016) and (b) offspring from different populations show very different levels of serotiny (previous section). These imply rapid Fire-caused death of at least some parents and interfire recruits, en masse release of serotinous seeds and seedling establishment, extensive seedling/juvenile death but many recruits still reach adulthood (fire promotes selection of serotinous phenotypes) Fire-caused death of at least some parents and interfire recruits, en masse release of both seed types and seedling establishment, extensive seedling/juvenile death but many recruits still reach adulthood, more than interfire (fire promotes selection of both phenotypes)…”

“…Indices of serotiny are most meaningful when calculated on a viable seed basis but this is almost never done. A critical look at what exactly the indices of serotiny currently available measure, and how they can be improved, is now available (Lamont, 2020).…”