Low Seed Availability May Limit Recruitment in Grazed Atriplex vesicaria and Contribute to Its Local Extinction

• Published in: Plant ecology 2001-11, Vol.157 (1), p.53-67
• Main Author: Hunt, L. P.
• Format: Article
• Language: English
• Subjects: Applied ecology; Atriplex vesicaria; Availability; Flowering; Flowers; Germination; Grazing; Growth rate; Plant ecology; Plants; Rain; Recruitment; Seed banks; Seeds; Sheep; Shrubs; Soil conditions; Soils; Species extinction; Trampling
• ISSN: 1385-0237; 1573-5052
• DOI: 10.1023/A:1014515122231

Poor recruitment in Atriplex vesicaria Hewd ex Benth (bladder saltbush) under sheep grazing in the chenopod shrublands of southern Australia contributes to a decline in the shrub's population growth rate. This may lead to local extinction of the species over large areas around watering points. This study investigated whether low seed availability may contribute to poor recruitment. It examined the incidence of flowering and seed bank size at sites distributed across a large grazed paddock, and examined the longevity of seed in the soil. Grazing by sheep reduced the incidence of flowering and input to the seed bank. The proportion of shrubs with flowers increased with distance from water, showing the characteristic piosphere response. Shrubs on grazed sites closer to water experienced extended periods when they failed to flower or flowered poorly. The seed bank was negligible at three of the sites within 1650 m of water for all three years of sampling. In contrast, the seed bank at the most distant site sampled (2800 m from water) was small in 1990 ($37\pm 5.1\text{seeds}/\text{m}^{2}$) but in 1991 and 1992 seed numbers were substantial ($626\pm 315.2\text{seeds}/\text{m}^{2}$and 318 ± 169.0, respectively). Soil seed was short-lived in this study, with only 34% and 17% of the original seed remaining as viable ungerminated seed after 12 months for the under-canopy and exposed treatments respectively. Whilst recruitment may also be limited by altered soil conditions due to grazing and trampling and the availability of safe sites, the results of this study suggest that low seed availability may be an important factor contributing to poor recruitment and may limit the ability of the population to recover from the loss of established plants. Management of grazing must take into account the need for A. vesicaria populations to flower and set seed on a regular basis.