Fire, aridity and seed banks. What does seed bank composition reveal about community processes in fire‐prone desert?

Abstract

Questions: The relationship between fire, aridity and seed banks is poorly understood in plant community ecology. We tested whether there was a close correspondence between the seed bank and standing vegetation composition with time‐since‐fire in a desert. We also examined whether longer‐lived species showed seed limitation relative to more ephemeral species, as this could influence grass‐woody ratios in a major biome. Location: Dune hummock grasslands/shrublands of central Australia. Methods: The effects of time‐since‐fire on floristic and functional group composition were examined by comparing plots unburned since 1984 against plots that had been burned in 2002. Three methods were used to quantify seed abundances: a germination trial using heat and smoke application, a flotation method, and a sieving method. Results: Seed bank densities were very low (<3000 m−2). Species similarity between the seed bank and standing vegetation was high at sites recently burned (0.86) and low in sites long‐since burned (0.52). The relative abundance of ephemeral species in the seed bank peaked in recently burned plots, but the relative abundance of seeds of woody species did not match the pattern of abundance in the standing vegetation. Remarkably, the dominant perennial grasses and woody species were either absent from the seed bank or present at extremely low abundances. Discussion: Differences in the relative abundance of ephemeral species between standing vegetation and seed bank relate to the post‐fire succession process. The small soil pool of seed from woody species may be explained by allocation to belowground carbohydrate storage over seed production. Field observations suggest, however, that production of strongly dormant seed can be prolific and that high levels of seed predation make this system strongly seed‐limited. The discovery of this seed bank syndrome indicates that shifts in grass‐woody ratios can be driven by the juxtaposition of unpredictable seed rain and fire events in these desert dunes. However, estimates of grass‐woody ratios due to changing fire regimes will be difficult to predict.

Seed banks are biodiversity reservoirs: species–area relationships above versus below ground

“…The top 3 cm of soil was excavated from each of these seed bank plots, keeping the material from each subplot separate. While additional germinable seeds may occur below the sampled depth, up to 80–98% of seeds in grasslands and heath seed banks can be found in the top 2–5 cm (Pywell et al , Ma et al , ) Further, several grassland studies have used comparable sample depths (Caballero et al , Standish et al , Gonzalez and Ghermandi , Wright and Clarke ). Sampling was done in late August (after germination, before dispersal), to avoid sampling transient seed banks as well as the current year's seed rain, and thereby reliably assess the persistent seed bank pools (Thompson et al , Baskin and Baskin ).…”

Comparison of seedling emergence and seed extraction techniques for estimating the composition of soil seed banks

“…When seeds resisted this pressure, they were deemed viable and recorded. Although this testing procedure was less rigorous than other viability tests, such as tetrazolium and germination‐based tests, it was quicker and cheaper and is commonly used in seed bank research (Ball & Miller 1989; Wright & Clarke 2009). Exposure to K 2 CO 3 can reduce germination of some species depending on length of exposure time and hence, if viability of seeds is to be determined, then the effects of exposure must be tested for the species of interest (Buhler & Maxwell 1993).…”

“…Indeed, the seedling emergence method can dramatically underestimate the density of the seed bank due to errors associated with seed dormancy and specific environmental requirements for germination (Brown 1992; Bernhardt et al. 2008; Wright & Clarke 2009). This may be particularly so for wetland species, in which emergence from the soil seed bank is largely determined by the water regime (Casanova & Brock 2000; Crossle & Brock 2002).…”

Potential aboveground biomass in drought‐prone forest used for rangeland pastoralism

“…An even longer period of grazing relief would be necessary with cattle grazing, because young mulga trees (as opposed to seedlings) are also palatable to cattle, which have a longer browse reach than sheep. Mulga does not seem to have a substantial long-lived seed bank (Wright and Clarke 2009), and the lack of recruitment in some longterm grazing exclosures (Fig. 3) may reflect a lack of coincidence between high rainfall and seed production.…”