Abstract: Phragmites australis is an invasive reed species originating from Europe that takes over and outcompetes native species in many areas of Suisun Marsh, California. Phragmites has become a major threat to biodiversity, and it diminishes ecosystem services of the marsh by impeding navigation, hunting, and wildlife-watching activities. Current control techniques include herbicide spraying, mowing, burning, and discing. These techniques have had limited success, as remote sensing data show Phragmites' extent increased by 325% between 1999 and 2015. Here we sought to understand more about the resistance of Phragmites to local control by measuring the viability of the seeds from areas that have been heavily sprayed in the last two decades and areas with minimal spraying. We characterized parcels of the marsh as having been subject to high or low levels of control, based on historical data from Suisun Resource Conservation District. From each parcel, we sampled 5 patches, collecting 5 representative inflorescences in August 2022, while seeds were ripening. We then stripped 0.05 g of mature spikelets from each inflorescence and combined them (n=24 patches). We also weighed aliquots of 60 spikelets and counted seeds per spikelet to understand how seed mass and number affected germination rate. Spikelets were placed on No. 1 Whatman filter paper, wetted, and cold stratified for 2 weeks at 4℃ to break physiological dormancy. After cold stratification, seeds were germinated in a growth chamber on a tray of wet sand, and checked daily after 5 days for germination. Conditions in the growth chamber were on a diurnal cycle set to match spring temperatures and day length in Suisun Marsh (9°C and dark for 12h, 21°C and light for 12h). Preliminary results indicated no significant difference in germination rate with both groups being around 3% germination. There was a significant difference between the weight of an aliquot of 60 spikelets, with high-intensity treatment sites being 12% heavier than low-intensity treatment sites (P< 0.001). High-intensity had an average weight of 0.064 g and low-intensity sites having an average of 0.057 g. Spikelets from the high-intensity sites also had 6% more seeds per spikelet than the low-intensity sites (P< 0.01) with low-intensity having 5.5 seeds per spikelet on average and the high-intensity sites having 5.8 on average. This indicates that areas that have undergone years of heavy spraying are associated with more robust inflorescences with heavier, more numerous seeds.
