The Response of Nassella Trichotoma (serrated Tussock) Seeds and Seedlings to Different Levels of Fire Intensity

Context. Fire is an important disturbance regime in grassland communities, since it is responsible for stimulating the regeneration of many species and for maintaining levels of biodiversity. When invasive plants, such as Nassella trichotoma, establish and become widespread in a grassland community, these important fire events can be altered in intensity and frequency, which means that they are able to facilitate the establishment of the exotic species. Therefore, before fire can be recommended as a suitable control technique for invasive species, or alternatively to be integrated into grassland restoration programs, understanding the response of the seeds of exotic species to high temperatures, such as those experienced during a fire, should be well understood.Aims. Our aim was to identify their response to a gradient of temperatures associated with different levels of fire intensity. We examined how increased duration of exposure affects their response, and whether seed age or seed moisture content affect the germination response of this species.Methods. To gain a fuller understanding of the fire response of N. trichotoma's seedbank, seeds were collected in 2016, 2017, 2018 and 2019 and then stored until the commencement of the experiments in 2020. Selected seeds were first subjected to an increasing temperature gradient (80 degrees C, 100 degrees C, 120 degrees C, 140 degrees C, and a control), and an increasing duration of exposure (of 1, 3, 6, and 9 min). In the second experiment, one population was selected to test these same temperatures and duration of exposure after the seeds were hydrated to 15%, 50%, or 95%. Last, seedlings were grown for 3 months under glasshouse conditions and then exposed to increasing temperatures (20 degrees C, 60 degrees C, 80 degrees C, 100 degrees C, and 120 degrees C), and an increasing duration of exposure (3, 6, and 9 min). The seedlings were assessed 2 weeks after the heat exposure for signs of damage.Key results. It was found that increased temperatures and duration of exposure had a subtle negative effect on germination parameters, including reduced total germination and increased time to 50% germination. The 140 degrees C treatment was seen to be a significant threshold because it killed all the seeds at any duration of exposure. A significant difference among the ages of each seed lot was observed to be a factor on the tested germination metrics, with the oldest tested population (2016) demonstrating the highest germination percentage, uniformity, and rate. Seed germination percentage was significantly reduced for seeds hydrated to 95% compared with the control treatment, whereas no significant difference was observed for the seeds hydrated to 15% and 50%. For the heat treatment of the seedlings, damage to the leaves was observed in the 80 degrees C, 100 degrees C, and 120 degrees C treatments, with some plants in the 120 degrees C treatment experiencing extensive damage prior to resprouting. No seedlings were killed at the tested temperatures.Conclusions. Results of this study indicated that fire may be a useful tool for reducing seedbank density by killing a high proportion of the seeds on the soil surface, or located within the top 1 cm of the soil profile, but not for seeds buried more deeply. Efficacy of fire on surface and shallow-buried seeds is improved with high seed moisture content; however, these seeds buried below this depth are still protected by the soil from the lethal effects of temperature.Implications. Fire implemented before seed set could be used to effectively kill a large proportion of N. trichotoma seeds. However, for more comprehensive control, it is recommended that chemical treatment is integrated with the fire treatment to improve the overall control efficiency.