What types of species survive better in soil5

Species vary greatly in their ability to survive in the soil seed bank, and certain broad patterns exist that predict which species survive well in the soil. Understanding trends in seed survival across species provides a means for targeting ecological management strategies at particular classes of weeds. It also provides a basis for guessing the likely seed persistence of unstudied species.

A first broad pattern across species is that many broadleaf weeds are able to survive in the seed bank for several decades (Chancellor, 1986), whereas seeds of only a few grass species survive in substantial numbers for more than 5 to 10 years (Dawson & Bruns, 1975; Froud-Williams, 1987, Baskin & Baskin, 1998b). Moreover, the seeds of some grass weeds do not persist longer than a single year (e.g., Bromus diandrus - Harradine, 1986; Gleichsner & Appleby, 1989; Bromus sterilis - Roberts, 1986), whereas few, if any, broadleaf agricultural weeds appear to have seed survival times as short as that. Both of the annual grasses tested by Conn & Deck (1995) (Avenafatua and Hordeum jubatum) were reduced to < 1% viability by 3.7 years of burial and were completely gone after 9.7 years. In contrast, mean survival of the 13 annual broadleaf species they tested was 32% after 3.7 years, and all but one of the broadleaf species had at least a few surviving seeds after 9.7 years. Burnside et al. (1996) counted seedlings emerging from seed samples that had been left in the soil for 1 to 17 years. After 12 years of burial, the 14 annual broadleaf species averaged 10% germination, whereas germination of the 11 annual grasses averaged only 2%. Nevertheless, some economically important weedy annual grasses, notably Echinochloa crus-galli, Bromus secalinus, and several species of Setaria, retained moderate viability into their second decade of burial.

A second broad pattern among species is that annual and stationary perennial weeds tend to form persistent seed banks, whereas wandering perennials and woody weeds usually do not. For example, of the 22 terrestrial grass species tested by Roberts (1986), the weedy, short-lived Avena fatua, A. sterilis ssp. ludoviciana, and Poa annua had the greatest emergence 2 years after sowing, whereas many of the rhizomatous species were completely gone by that time. In the study of Burnside et al. (1996), seven stationary perennial broadleaf species (including biennials) averaged 37% germination after 8 years ofburial and 25 annual broadleaf species averaged 15%, whereas nine wandering broadleaf perennials had an average germination of only 5%. In a study of the flora of northwestern Europe that emphasized arable weeds and grassland species, Thompson et al. (1998) showed greater persistence of monocarpic (single fruiting - in this context, annual and biennial) species relative to poly-carpic (multiple fruiting) species.

A third pattern across species is that those with weak dormancy mechanisms often have shorter longevity than species with well-developed dormancy mechanisms (Bostock, 1978; Roberts, 1986). The greater persistence of species with dormancy is due to reduction of inappropriate germination rather than to greater retention of viability (Roberts, 1972). Experimental support for the importance of innate dormancy for seed survival comes from studies showing that seed lots of several weed species that were dormant when buried retained viability in the soil better than initially nondormant seed lots (Taylorson, 1970; Naylor, 1983; Zorner, Zimdahl & Schweizer, 1984b).

A fourth general pattern is that small, round-seeded species tend to persist in the seed bank longer than species with large or elongate seeds. Thompson, Band & Hodgson (1993) surveyed 97 species and found that few species with seeds larger than 2 mg or with variance in diaspore relative dimension greater than 0.2 persisted longer than 5 years in the soil. In the non-agricultural conditions in which seed persistence evolved, incorporation into the soil was probably more difficult for large or elongate seeds than for small, round ones. The latter can wash into cracks, or be ingested by earthworms more easily than the former. Since seed survival is lower on the soil surface than deep in the profile, species with a low probability of incorporation into the seed bank probably experienced little selection favoring mechanisms that allow long persistence. Large seeds are also more likely to be eaten by small mammals (Hulme, 1994), and possibly selection rarely favors mechanisms that allow long residency in the soil of large-seeded species for this reason. The exceptions to the general pattern occur primarily in the Leguminosae, Convolvulaceae, and Malvaceae, which include many species with large, hard seeds (Taylorson, 1987). Seeds of most of these species have toxins that may help defend them against mammalian seed predators (e.g., Senna obtusifolia, Ipomoea purpurea) (Kingsbury, 1964, p. 314; Friedman & Henika, 1991).

Growing Soilless

Growing Soilless

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