Seed traits determine species responses to fire under varying soil heating scenarios

1) Many plant species in fire-prone environments maintain persistence through fire via soil seedbanks. However, seeds stored within the soil are at risk of mortality from elevated soil temperatures during fire. Seeds may be protected from fire-temperature impacts by burial, however those buried too deeply may germinate but fail to emerge. Thus, successful post-fire seed regeneration is contingent upon a trade-off between burial depth and survival through fire.

2) We examined the relationships between seedling emergence behaviour, seed survival, and soil temperatures during fire in 13 native and four non-native woodland species in southwestern Australia. We assessed total seedling emergence per depth, maximum seedling emergence depth and seedling emergence speed from seeds planted at seven depths (0, 1, 2, 3, 4, 5, 7 10 cm). Soil temperatures were quantified using distributed temperature sensing in optic fibre (DTS), measured continuously between 1 and 10 cm in depth (temperatures were subsequently categorised into 1 cm increments for analysis) during five experimental fires in beds with fine fuels manipulated between 8–20 t ha-1. Using seed survival and emergence success relative to soil temperatures, , we determined vulnerability of seedling emergence relative to soil temperatures generated by combustion of fuel quantities typically observed in woodlands.

3) Maximum depth of emergence varied between species from 2 to >10 cm, with a positive linear correlation to seed mass. Maximum soil temperatures from the two highest fuel masses exceeded seed lethal thresholds (T50 - representing temperatures lethal to 50% of  seeds) of at least five species. Lethal temperatures  exceeded at all potential emergence depths for all three grass species, and all four non-native species studied. Of the remaining 10 species, temperatures did not exceed the lethal thresholds under any of the fuel mass levels tested. We found no relationship between lethal temperature thresholds and maximum emergence depth.

4) Our data demonstrate that seeds exhibit variation in their response to soil heating and capacity to emerge from depth, with three distinct functional responses amongst our study species, which enable persistence through, and recruitment following, fire. Such variation in species attributes and fuel mass may lead to heterogeneity (within fires) or divergent trajectories (among fires) in community response under changed fire regime.

1) 易发生火灾的生境中，多数植物类群可通过土壤种子库（soil seedbank）在火灾后维持种群存续。然而，储存在土壤中的种子面临火灾时土壤高温引发的死亡风险。适当埋藏可使种子免受火温影响，但埋藏过深的种子虽可萌发却无法成功出土。因此，火灾后成功的种子更新取决于埋藏深度与火灾存活率之间的权衡关系。
2) 本研究以澳大利亚西南部的13种本土林地植物与4种非本土林地植物为研究对象，探究了幼苗出土行为、种子存活率与火灾时土壤温度之间的关联。我们在7个埋藏深度（0、1、2、3、4、5、7、10 cm）下播种，评估了各深度的总幼苗出土量、最大幼苗出土深度以及种子出苗速率。采用分布式光纤温度传感（Distributed Temperature Sensing, DTS）对土壤温度进行量化，在5场模拟火灾实验中，于燃料载荷为8–20 t ha⁻¹的细燃料床层中，连续监测1~10 cm深度的土壤温度（后续分析将温度按1 cm增量进行分类）。结合种子存活率与对应土壤温度下的出苗成功率，我们明确了林地常见燃料燃烧产生的土壤温度条件下，幼苗出土的温度脆弱性。
3) 不同物种的最大幼苗出土深度介于2~>10 cm之间，且与种子质量呈正线性相关。两个最高燃料载荷组产生的最高土壤温度，超过了至少5个物种的种子致死中温（T50，即导致50%种子死亡的温度阈值）。所有3种草本植物以及本次研究中的4种非本土物种，在所有潜在出土深度下的土壤温度均超过了其致死阈值。剩余的10个物种中，所有测试燃料载荷下的土壤温度均未超过其致死阈值。本研究未发现种子致死温度阈值与最大出土深度之间存在关联。
4) 本研究数据表明，不同物种的种子对土壤加热的响应以及从深处成功出土的能力存在显著差异，研究物种可划分为三类独特的功能响应模式，这些模式使其能够在火灾中存续并在火灾后完成种群更新。物种属性与燃料载荷的这类差异，可能在火灾制度改变的背景下，引发火灾内部的群落响应异质性，或是不同火灾间群落响应的分化轨迹。