Why high seed densities within buried mesh bags may overestimate depletion rates of soil seed banks

Abstract

Summary 1.Estimates of seed bank depletion rates are essential for modelling and management of plant populations. The seed bag burial method is often used to measure seed mortality in the soil. However, the density of seeds within seed bags is higher than densities in natural seed banks, which may elevate levels of pathogens and influence seed mortality. The aim of this study was to quantify the effects of fungi and seed density within buried mesh bags on the mortality of seeds. Striga hermonthica was chosen as the study species because it has been widely studied but different methods for measuring seed mortality in the soil have yielded contradictory estimates. 2. Seed bags were buried in soil and exhumed at regular time intervals to monitor mortality of the seeds in three field experiments during two rainy seasons. The effect of fungal activity on seed mortality was evaluated in a fungi exclusion experiment. Differences in seed-to-seed interaction were obtained by using two and four densities within the seed bags in consecutive years. Densities were created by mixing 1000 seeds with 0, 10, 100 or 1000 g of coarse sand. 3. The mortality rate was significantly lower when fungi were excluded, indicating the possible role of pathogenic fungi. 4. Decreasing the density of seeds in bags significantly reduced seed mortality, most probably because of decreased seed-to-seed contamination by pathogenic fungi. 5. Synthesis and applications . Models of plant populations in general and annual weeds in particular often use values from the literature for seed bank depletion rates. These depletion rates have often been estimated by the seed bag burial method, yet seed density within seed bags may be unrealistically high. Consequently, estimates of seed mortality rates may be too high because of an overestimation of the effects of soil or seedborne pathogens. Species that have been classified from such studies as having short-lived seed banks may need to be re-assessed using realistic densities either within seed bags or otherwise. Similarly, models of seed bank dynamics based on such overestimated depletion rates may lead to incorrect conclusions regarding the seed banks and, perhaps, the management of weeds and rare species.

Seed longevity of Eragrostis plana Nees buried in natural grassland soil

“…Van Mourik et al (2005) observed that high seed densities placed in bulk in nylon bags lost viability rapidly due to the attack of soil fungi. In accordance with that author, at densities around 1,000 seeds per nylon bag, the mortality rate is extremely high.…”

“…In accordance with that author, at densities around 1,000 seeds per nylon bag, the mortality rate is extremely high. In the present study, using the same permeable nylon bags recommended by Van Mourik et al (2005) with seeds placed in five different vertical positions, instead of bulk, the attack of fungi was supposed to be reduced. However, the idea of removing the forage mass, in an attempt to keep the pasture height similar to the adjoining grazing area, was not completely achieved.…”

“…These attempts indicate that it was prudent to increase the number of nylon bags in order to control the value of the STDEV. Part of the higher deviations can occur due to activity of fungi (Kremer, 1993;Van Mourik et al, 2005) and other microorganisms of the soil habitat (Baskin and Baskin, 2001). Nonpersistent seeds are lost from the seed bank through mortality because of aging, attacks by seed predators, seed decay and fatal germination (Baskin and Baskin, 2001).…”

“…Fifty-four permeable nylon bags were buried. The seeds were placed in the soil profile inside nylon bags (Van Mourik et al, 2005), and buried along the transect line, 20 cm distant from each other. The nylon bags, with five vertical divisions, containing 100 seeds in each division, were buried in three steps: 1) V-shaped hole cut vertically using a shovel in a friable soil; 2) the bags were inserted in the V-shaped hole using a wood ruler; 3) V-shaped hole was closed with the shovel.…”

Germination Ecophysiology

“…In drier, unimodal rainfall areas 1 of Benin, S. hermonthica seed viability declined from 90 to 15 % over 6 months (Gbèhounou et al 1996a), with almost total loss of viability over two wet seasons (Gbèhounou 1998). Reasons for the discrepancy were resolved by showing that burial of S. hermonthica seeds in small mesh packets, as in the Benin and Western Kenyan reports, leads to rapid loss of viability associated with microbial decay (van Mourik et al 2005).…”