Temperature thresholds of physically dormant seeds and plant functional response to fire: variation among species and relative impact of climate change

Abstract

Variation in dormancy thresholds among species is rarely studied but may provide a basis to better understand the mechanisms controlling population persistence. Incorporating dormancy-breaking temperature thresholds into existing trait frameworks could improve predictions regarding seed bank persistence, and subsequently species resilience in response to fire, climate change and anthropogenic management. A key ecological strategy for many species from fire-prone ecosystems is the possession of a long-lived seed bank, ensuring recovery after fire. Physical dormancy is dominant in these ecosystems and maintaining this dormancy is directly linked to seed bank persistence. We identified a suite of seed-related factors relevant to maintaining populations in fire-prone regions for 14 co-occurring physically dormant species. We measured variation in initial levels of dormancy and then applied experimental heating treatments, based on current seasonal temperatures and those occurring during fires, to seeds of all study species. Additionally, higher seasonal temperature treatments were applied to assess response of seeds to temperatures projected under future climate scenarios. Levels of germination response and mortality were determined to assess how tightly germination response was bound to either fire or seasonal cues. Six species were found to have dormancy cues bound to temperatures that only occur during fires (80°C and above) and were grouped as having obligate pyrogenic dormancy release. The remaining species, classified as having facultative pyrogenic dormancy, had lower temperature dormancy thresholds and committed at least 30% of seeds to germinate after summer-temperature treatments. Evidence from this study supports including dormancy-breaking temperature thresholds as an attribute for identifying functional types. High temperature thresholds for breaking dormancy, found in our obligate pyrogenic group, appear to be a fire-adapted trait, while we predict that species in the facultative group are most at risk to increased seed bank decay resulting from elevated soil temperatures under projected climate change.

To resist or to germinate? The effect of fire on legume seeds in Brazilian subtropical grasslands

“…Keeley et al 2011). Several species from Mediterranean ecosystems show fire-related cues to germi- nation (see Moreira et al 2010), being fire-dependent, while other species can germinate after temperature fluctuations (gap-dependent, Ooi et al 2014). Our study showed that only seeds of Stylosanthes montevidensis had a fire-related increase in germination after the exposure to 120ºC, showing breakage of physical dormancy of at least 20% of the seeds.…”

“…Fire-related germination is found in different types of vegetation, but most studies describe this relationship in Mediterranean ecosystems (see Reyes & Trabaud 2009;Moreira et al 2010) and in Australia (see Auld & O'Connell 1991;Ooi et al 2014). Keeley et al (2011) concluded thus, that breaking the dormancy or stimulating the germination by fire cues should be considered as evidence for selective pressures in plant evolution.…”

“…Keeley et al (2011) concluded thus, that breaking the dormancy or stimulating the germination by fire cues should be considered as evidence for selective pressures in plant evolution. In a plant functional approach, researchers have classified physically dormant seeds as having pyrogenic obligate or facultative dormancy-breaking mechanisms (Ooi et al 2014). On the other hand, fire may be irrelevant for some species, so they survive high temperatures but do not show a specific dormancy-breaking bounded by higher temperatures (see Hanley & Lamont 2000;Fichino et al 2016).…”

“…Most studies addressing the effects of high temperatures on the germination of Fabaceae species found peak germination between 70 -100ºC (Martin et al 1975;Auld & O'Connel 1991;Williams et al 2003). Luna et al (2007) reported that maximum germination for Mediterranean plants was after the exposure to 80ºC, and that these temperatures can be reached by the soil during summer, being these species classified as having facultative pyrogenic dormancy, or as gap dependent (Ooi et al 2014).…”

Seeds at the forefront: synthesis of the inaugural National Seed Science Forum and future directions in Australian seed science

“…Seed traits and environmental thresholds for regeneration can also further understanding of species' vulnerabilities to climate change. For example, recent studies have found that as soil temperatures increase or fire regimes change there will be measurable changes in the level and extent of seed dormancy within-species, as well as possible deleterious changes in the capacity of seeds to persist in the soil seed bank (Ooi et al 2012;Hoyle et al 2013;Ooi et al 2014). Australia's alpine regions are predicted to experience significant changes in temperature over the next few decades, with the effects on soil seed banks and seed regeneration potentially resulting in profound changes to the composition of vegetation communities (Hoyle et al 2013;Sommerville et al 2013;Hoyle et al 2014).…”