Seedbank mortality

2.5.4.1 Background and mathematical representation

Seed banks decline due to dispersal, post-dispersal seed predation, fatal germination, successful emergence, pathogen attack and embryo death due to ageing (Gallandt et al., 1999; Forcella, 2003). The persistence of weed seed banks ranges from transient (<1 yr), through short-term persistent (between 1 and 5 years) to long-term persistent (>5 years) (Thompson et al., 1993). By mixing a known quantity of seeds with a volume of soil, retrieving the viable seeds in the soil over time whilst preventing emerged weeds from reproducing, decline rates have been established for many weed species, see for example Wilson and Lawson (1992). A negative exponential function is by far the most used model to estimate the numbers of viable seed numbers over time (Roberts and Feast, 1973; Roberts and Boddrell, 1983; Lawson et al., 1993; Sanchez del Arco et al., 1995) though occasionally other declining negative functions have been fitted as well (Donald, 1993). Discontinuous or linear declines have been reported, however, for within-year observations (Puricelli et al., 2005; Sester et al., 2006).

Seedbank decline has been assessed through various ways but it is important to appreciate that the nature of the assessment can affect the decline rate through eliminating, or not distinguishing between some of the factors responsible for seedbank decline. Seed predators predominantly target seeds on the soil surface (Scopel et al., 1988; Orrock and Damschen, 2007). Hence, persistence studies that mix seeds through the soil do not account for losses of fresh seeds due to seed predation. Some persistence studies do not record emerged seedlings and therefore establish an ‘all-in’ decline rate, not a ‘mortality-only’ rate (e.g. 2002; Westerman et al., 2003a).

Seed predation, emergence and fatal germination are accounted for in ECOSEDYN by other model components. Hence, for ECOSEDYN seedbank decline-rates that include all factors responsible for seedbank decline are not useful. Instead, the decline of the seedbank due to seed embryo death caused by ageing and seed death from pathogen attack is the parameter required. In practice it is often impossible to distinguish seed mortality due to fatal germination from seed mortality due to decay. Although it could be argued that fatal germination is already being accounted for by the ‘Germination and Emergence’ model (see Section 2.5.2), the fact is that in reality

emergence is assumed to happen in ECOSEDYN. Even if these seeds successfully emerge, then the seedlings are likely to die or be killed prior to setting seeds and therefore represent a substantial extra mortality factor. Seedbank mortality studies in which emergence was explicitly recorded and seeds were not scattered freely on the surface were considered the most relevant.

Empirical studies evaluating the fate of weed seeds over the soil volume found no (consistent) relationship between seed mortality and depth in the soil (Lapham and Drennan, 1990; Mohler, 1993) except for seeds close to the soil surface that either die faster (Carmona and Boas, 2001; Gulden et al., 2004; Puricelli et al., 2005; Peachey and Mallory-Smith, 2007) or slower (Taylor et al., 2005). Due to the inconsistency in results, it was assumed that seed mortality is independent of depth in the soil. Three more assumptions were made in the ‘seedbank mortality’ component of ECOSEDYN: • the proportion mortality in each year is the same regardless of the age

distribution or the proportional dormancy of the seed bank. • seedbank mortality is independent of crop type.

• seeds in Array 1, i.e. freshly produced seeds that are on the surface, are only dying because of seed predation and not from decay.

If the value for annual seedbank mortality is extrapolated to daily seed mortality then the net annual seedbank mortality is lower, since seeds that disappear from the seedbank due to seed predation, fatal germination or successful emergence can not die from ‘seedbank mortality’. To minimise this effect, yet at the same time acknowledge that seeds die throughout the season, seedbank mortality is calculated on a weekly basis. The proportion weekly mortality is calculated from the annual seedbank persistence: Equation 2-6:

( )

( )

and the proportion weekly survival multiplied with the seedbank resulting in an exponential decline.

Equation 2-7: S_tot

(

)

(

( )

)

( )

(

)

( )

( )

S d tot + = −m −

2.5.4.2 Parameterisation and implementation

Seedbanks decline due to mortality and successful emergence. When considering studies for parameterisation it is important to discern to which degree these two processes have been distinguished.

Measurements on persistence in the soil have been conducted in various ways: buried in mesh envelopes at certain depths (Method 1), mixed with soil and buried in pots (Method 2), spread over soil surface followed by cultivation (Method 3). Interestingly the various methods gave rather different results for annual decline rates. When seeds are stored in nylon mesh envelopes (Method 1), the annual decline values are much lower than for the other methodologies (see Table 2.4). Van Mourik

T. inodorum

et al

The highest ‘all-in’ decline rates are reported from studies where seeds were broadcast in the field and the field then cultivated (Method 3). Barralis

. (2005) warned that high seed densities in mesh envelopes could overestimate decline rates, but from the review here it seems that mesh envelopes may exclude certain mortality factors and therefore lead to an underestimation of the depletion rate.

et al.

Intermediate values are reported from Harold Roberts’ experiments where he mixed seeds with soil in pots that were buried in the field (Method 2) (Roberts, 1964; Roberts and Feast, 1972; Roberts and Feast, 1973). In these experiments there was evidence for an exponential decrease of viable seeds and therefore annual decline values were calculated as follows: if ‘t’ is the length of the experimental period in years then over the course of the experimental period a proportion of the seeds emerge, e

(1988) reported a staggering 88% decline for T. inodorum in the first year and Roller and Albrecht (2006) found values in the same range, with, on average the seedbank declining 75% after 25 months under various cultivation regimes.

Table 2-4 Experimental results from literature, where annual decline rates of T. inodorum were measured. C/U stands for cultivated vs uncultivated soil. Shading implies the use of these values for the parameter in ECOSEDYN.

1

Based on experimental results obtained via literature it was assumed that the annual emergence rate was 10%

The aim is to calculate the annual proportion of the seedbank that dies to causes other than emergence and seed predation, most likely decay, md-S

Equation 2-8:

(a), which can be calculated as: