Seedling performance covaries with dormancy thresholds: maintaining cryptic seed heteromorphism in a fire‐prone system

Abstract

Abstract. The production of morphologically different seeds or fruits by the same individual plant is known as seed heteromorphism. Such variation is expected to be selected for in disturbance-prone environments to allow germination into inherently variable regeneration niches. However, there are few demonstrations that heteromorphic seed characteristics should be favored by selection or how they may be maintained. In fire-prone ecosystems, seed heteromorphism is found in the temperatures needed to break physical dormancy, with seeds responding to high or low temperatures, ensuring emergence under variable fire-regime-related soil heating. Because of the relationship between dormancy-breaking temperature thresholds and fire severity, we hypothesize that different post-fire resource conditions have selected for covarying seedling traits, which contribute to maintenance of such heteromorphism. Seeds with low thresholds emerge into competitive conditions, either after low-severity fire or in vegetation gaps, and are therefore likely to experience selection for seedling characteristics that make them good competitors. On the other hand, high-temperature-threshold seeds would emerge into less competitive environments, indicative of stand-clearing high-severity fires, and would not experience the same selective forces. We identified high and low-threshold seed morphs via dormancy-breaking heat treatments and germination trials for two study species and compared seed mass and other morphological characteristics between morphs. We then grew seedlings from the two different morphs, with and without competition, and measured growth and biomass allocation as indicators of seedling performance. Seedlings from lowthreshold seeds of both species performed better than their high-threshold counterparts, growing more quickly under competitive conditions, confirming that different performance can result from this seed characteristic. Seed mass or appearance did not differ between morphs, indicating that dormancy-breaking temperature threshold variation is a form of cryptic heteromorphism. The potential shown for the selective influence of different post-fire environmental conditions on seedling performance provides evidence of a mechanism for the maintenance of heteromorphic variation in dormancy-breaking temperature thresholds.

Achene heteromorphism in Bidens pilosa (Asteraceae): differences in germination and possible adaptive significance

“…Differences in seed dormancy/germination may contribute to seedling recruitment by allowing plants to emerge in a broad range of regeneration niches (Grubb 1977; Coomes and Grubb 2003; Liyanage et al 2016). In B. pilosa , freshly matured central achenes have a relatively low level of dormancy and may germinate across a wide range of temperatures similar to temperature of their natural habitat (Fig.…”

“…Diaspore heteromorphism, a condition where a genotype produces different morphophysiological types of seeds, has been interpreted as an adaptive trait in heterogeneous environments (Venable 1985; Imbert 2002; Baskin and Baskin 2014). Not only do the diaspores of heteromorphic species look different, but they may differ in numerous other ways, including size/mass (Ellner and Shmida 1984; Venable and Levin 1985; Rocha 1996; Liyanage et al 2016); dispersal ability (Venable 1985; Rocha 1996; Mandák and Pyšek 2001; Zhang et al 2015); dormancy and germination (Corkidi et al 1991; Wang et al 2008; Baskin and Baskin 2014; Zhang et al 2015; Liu et al 2018); and the reproductive allocation of plants grown from heteromorphic seeds (Cheplick and Quinn 1988; Liyanage et al 2016; Zhang et al 2017). These differences range from autecological and population traits to genetic and molecular aspects of biology (Baskin and Baskin 2014).…”

Heteromorphism in seeds ofLeptocereus scopulophilus(Cactaceae) from Pan de Matanzas, Cuba

“…Other variations between seed morphs also occur in seed behaviour without visible morphological differences (Venable, 1985). This type of seed heteromorphism, termed cryptic, has been observed in various species (Leverett and Jolls, 2014; Liyanage et al ., 2016), and is a common, yet rarely documented phenomenon (Venable, 1985; Leverett and Jolls, 2014). The ecological advantages of seed heteromorphism include differences between morphs in competitive ability, dispersion, dormancy, germination time, vulnerability to predators and seedling growth (Matilla et al , 2005; Lu et al ., 2010; Leverett and Jolls, 2014).…”

Do dormancy-breaking temperature thresholds change as seeds age in the soil seed bank?

“…Fire-related dormancy-breaking temperature thresholds vary between PY species (Trabaud and Oustric, 1989; Auld and O'Connell, 1991) and within populations of single species (Liyanage and Ooi, 2015). It has been proposed that such variation has been selected for because of the inherent variability of fire (Trabaud and Oustric, 1989; Ooi et al ., 2014; Liyanage et al ., 2016), which results in variation in soil heating over the area burnt (Bradstock et al ., 1992; Penman and Towerton, 2008). Having different dormancy-breaking temperature thresholds among species can therefore contribute to species co-existence, by distributing germination over space (Trabaud and Oustric, 1989; Ooi et al ., 2014).…”

“…Having different dormancy-breaking temperature thresholds among species can therefore contribute to species co-existence, by distributing germination over space (Trabaud and Oustric, 1989; Ooi et al ., 2014). It has also been suggested that within-species variation can operate as a bet-hedging mechanism, ensuring that at least some germination is possible across a range of fire severities, and that PY thresholds are maintained by co-varying with seedling characteristics, which ensure that seedling performance matches the post-fire conditions (Liyanage and Ooi, 2015; Liyanage et al ., 2016). Dormancy-breaking threshold temperatures have been used to estimate the potential magnitude of post-fire seedling establishment, depending on the fire severity and/or the amount of soil heating that occurs (Wright et al ., 2015).…”