Pre-emergence processes limit seedling recruitment in two direct seeded Acacia spp.

Abstract

The use of direct seeding for revegetation often results in poor recruitment outcomes. For many species, it is unclear where recruitment bottlenecks occur in the transitions between early life-history stages and how soil moisture conditions affect these bottlenecks. Thus, we asked: (1) which life-history stage transitions are most limiting to seedling recruitment? and (2) how do soil moisture levels affect recruitment? Using a field-based trial, we quantified the recruitment process from a seed to seedling for two woody Acacia species. Using a novel technique, in which seeds were confined to germination caches (small baskets), along with the use of germination bags, we assessed pre-emergence processes at regular intervals. Seeds sown directly into the soil in adjacent rows were intensively monitored to assess post-emergence processes. To investigate the effects of soil moisture on seedling recruitment, a glasshouse experiment assessed transitions between life-history stages under three different soil moisture treatments. In the field, the transition between a seed and germinated seed limited recruitment more than all other life-history stage transitions combined, with 32%–50% of seeds not germinating for the two species. Approximately one third of seeds of both species died prior to germinating, with few seeds remaining dormant. In the glasshouse trial, seed germination increased with increasing soil moisture, however, so did the extent of seed death. Our results suggest that the transition from a seed to germinated seed was the most limiting bottleneck to recruitment, mostly due to seed mortality rather than dormancy processes, with pathogen attack the most likely cause. As increased soil moisture both promoted germination but also seed mortality, understanding the optimal soil moisture thresholds that maximise germination and the transition to an established plant is essential in maximising direct seeding outcomes. Identifying the life-history stage transitions most limiting to plant recruitment may allow management to target specific bottlenecks in order to improve direct seeding outcomes.