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.