Environmental Factors Driving Seed Hydration Status of Soil Seed Banks and the Implications for Post-fire Recruitment

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.

Soil moisture and temperature drive emergence delays associated with fire seasonality in eucalypt forests

“…The results of this study suggest that changes to fire season have immediate adverse effects on recruitment by initiating a delay and potentially shifting community composition over time, as species from different seed dormancy classes show variable responses to the shifting regime. For example, the volumetric soil moisture was 7% to 7.4% on the day of the spring burn following a 10.6 mm rainfall event 4 days earlier, likely high enough to hydrate non-PY seeds and cause excess mortality from temperatures experienced during a cool fire ( Tangney et al, 2021 ). Such a rainfall event preceding fire would be very unlikely in a regime dominated by late summer/early autumn burns before the onset of anthropogenic climate change.…”

Seed Moisture Isotherms, Sorption Models, and Longevity

“…Vashisth and Nagarajan ( 2009 ) also reported shifts in water binding properties in magnetically primed maize seeds. Vertucci and Roos ( 1993 ) used isotherms to determine the optimum storage moisture content of pea, soya bean and peanut seeds over a wide temperature range; Socorro García et al ( 2010 ) took a similar thermodynamic approach to relate sorption properties with long-term storage; Tangney et al ( 2022 ) determined isotherms to assess risk of fire-induced death of buried seeds.…”