Abstract
Changes in fire regimes due to climate change and fire management practices are affecting the timing, length, and distribution of vegetation fires throughout the year. Plant species responses and tolerances to fire differ from season to season and are influenced by species-specific phenological processes. The ability of seeds to tolerate extreme temperatures associated with fire is one of these processes, with survival linked to seed moisture content at the time of exposure. As fire is more often occurring outside historic dry fire seasons, the probability of fire occurring when seeds are hydrated may also be increasing. In this study, we set out to understand the seasonal dynamics of seed hydration for seeds of Banksia woodland species, and how certain seed traits interact with environmental conditions to influence survival of high temperatures associated with fire. We measured the moisture content of seeds buried to 2 cm in the soil seed bank for four common native species and one invasive species on a weekly basis throughout 2017, along with soil moisture content and environmental correlates. We determined water sorption isotherms at 20°C for seeds of each species and used these functions to model weekly variation in seed water activity and predict when seeds are most sensitive to soil heating. Using Generalised additive models (GAMs), we were able to describe approximately 67% of the weekly variance in seed water activity and explored differences in seed hydration dynamics between species. Seed water activity was sufficiently high (i.e., ≥ 0.85 aw) so as to have created an increased risk of mortality if a fire had occurred during an almost continuous period between May and November in the study period (i.e., 2017). There were brief windows when seeds may have been in a dry state during early winter and late spring, and also when they may have been in a wet state during summer and late autumn. These data, and the associated analyses, provide an opportunity to develop approaches to minimize seed mortality during fire and maximize the seed bank response.
Highlights
Post-fire recruitment from seeds is a fundamental mechanism for the persistence, regeneration, and expansion of plant populations in fire-prone ecosystems (Pausas and Keeley, 2014)
Using seeds from five species common in Banksia woodlands, which produce seeds that will be present within soil seed banks at periods when fires may occur, our aim was to quantify the role of soil moisture content and local weather patterns including rainfall and evaporation rate in driving patterns of seed hydration
We included A. asparagoides which is a common weed species within Banksia woodlands to assess whether native and non-native species were differentiated in their hydration dynamics in a way that might alter their risk of mortality
Summary
Post-fire recruitment from seeds is a fundamental mechanism for the persistence, regeneration, and expansion of plant populations in fire-prone ecosystems (Pausas and Keeley, 2014). The ability of seeds in the soil to persist through fire and sense their environment to time germination, and seedling emergence, to coincide with periods of the year when conditions are most suitable for seedling survival is key to successful regeneration (Baskin and Baskin, 2001). Soil temperatures during fire moderate with depth due the insulating properties of soil (Tangney et al, 2020a), increasing the chances of survival for seeds buried deeper within the soil. Increased moisture content of the seeds at the time of exposure to fire temperatures strongly reduces the chances of seed survival (Fer and Parker, 2005; Ruprecht et al, 2016; Tangney et al, 2019)
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