The role of grassland seedbanks
Limitations in funding and technology makes it difficult to manage grassland restoration projects over long time scales (Freudenberger & Gibson-Roy, 2011; Lengyel et al ., 2012). Without follow-up management, weeds can re-establish either from regeneration from the seedbank or from migration of seeds from surrounding sites (Gibson-Royet al ., 2007). By knowing how long the dominant weeds seeds remain viable within the soil seedbank, we can make recommendations as to how long a site should be actively managed.
Weed species are often prolific seed producers, therefore, to protect a rehabilitated grassland from being reinvaded by a dominant weed species, an understanding of its seedbank persistence is required. Gardeneret al ., (2003) identified that after three years of no seed migration within a Nassella neesiana dominated grassland, there was still, on average, 1457 viable seeds per square metre of this species, indicating that ongoing management of only three years would not be effective for controlling this invasive grass. Seed longevity studies are useful for investigating how long a species can persist at different depth or soil types. The common trend is that seed viability declines with shallow burial and increased duration within the seedbank. This is seen in Andropogon gayanus (Bebawi et al ., 2018),R. raphanistrum (Reeves et al ., 1981), Conyza canadensis (Vargas et al ., 2018) and Artemisia tridentate(Wijayratne & Pyke, 2012). Seeds located on or just below the soil surface are exposed to more intense fluctuations in soil moisture and temperature compared to deeper buried seeds, and these fluctuations can result in the shallow buried seeds drying out. A two-year study foundA . tridentate had only 0-11% of seeds remaining viable at the soil surface compared with almost half of the seeds maintaining viability at 3cm depth (Wijayratne & Pyke, 2012). Seed predation by mammals, birds, or soil microbes is also enhanced at these shallow depths (Dalling et al ., 2011). Seeds that are buried deeper into the soil profile, are often better protected from these devitalizing and predatory pressures (Bebawi et al ., 2018). The trade-off, however, is that at these depths seed dormancy is usually prolonged, particularly for photoblastic seeds (Benvenuti et al. , 2005; Ahmed et al. , 2015), making these weeds troublesome for managing cropping systems utilizing tillage, since this can resurface viable invasive seeds, resulting in reinvasion.