Soil rock content influences the maximum seedling emergence depth of a dominant arid zone grass

Aims: We aimed to understand the influence of differing soil rock content on seedling emergence of a dominant arid zone grass critical to mine restoration (Triodia pungens R.Br.). We assessed whether emergence mortality resulting from seedlings failing to navigate rocky soils contributes meaningfully to failed recruitment and the maximum emergence depth. Methods: Cleaned seeds and florets (the natural dispersal unit) were buried at discrete depths of 0, 5, 10, 15, 20, 30, and 40 mm in three soil-rock matrices of varying rock content (soil sieved to <5 mm, sieved to <25 mm, or left un-sieved). Results: Rock content reduced emergence of seeds buried below 15 mm and truncated the maximum emergence depth. When seeds were buried at 5 mm; near the modelled 6.0 mm optimal depth, any failure to emerge was largely attributable to failed seed germination. However, when seeds were buried greater than 20 mm, failure of pre-emergent seedlings to reach the surface became the significant factor contributing to failed emergence. Florets failed to recruit under all scenarios. Conclusions: Soil rock content can influence critical seed germination and emergence life stages and therefore the optimal seed recruitment depth. Emergence of deeply (>20 mm) buried seeds is likely restricted by rocks imposing a physical barrier to recruitment.