Variable seed bed microsite conditions and light influence germination in Australian winter annuals

Isaac R Towers,Margaret M Mayfield,John M Dwyer,David J Merritt,Todd E Erickson

doi:10.1007/s00442-021-05091-7

Abstract

Environmentally cued germination may play an important role in promoting coexistence in Mediterranean annual plant systems if it causes niche differentiation across heterogeneous microsite conditions. In this study, we tested how microsite conditions experienced by seeds in the field and light conditions in the laboratory influenced germination in 12 common annual plant species occurring in the understorey of the York gum-jam woodlands in southwest Western Australia. Specifically, we hypothesized that if germination promotes spatial niche differentiation, then we should observe species-specific germination responses to light. In addition, we hypothesized that species’ laboratory germination response may depend on the microsite conditions experienced by seeds while buried. We tested the laboratory germination response of seeds under diurnally fluctuating light and complete darkness, which were collected from microsites spanning local-scale environmental gradients known to influence community structure in this system. We found that seeds of 6 out of the 12 focal species exhibited significant positive germination responses to light, but that the magnitude of these responses varied greatly with the relative light requirement for germination ranging from 0.51 to 0.86 for these species. In addition, germination increased significantly across a gradient of canopy cover for two species, but we found little evidence to suggest that species’ relative light requirement for germination varied depending on seed bank microsite conditions. Our results suggest that variability in light availability may promote coexistence in this system and that the microsite conditions seeds experience in the intra-growing season period can further nuance species germination behaviour.

Highlights

Mediterranean-climate regions are characterized by hot, dry summers and cool, wet winters, and support disproportionally high levels of global vascular plant diversity relative to their land area (Kreft and Jetz 2007)
Winter annual plants, which are a significant component of the diversity in Mediterranean ecosystems, capitalize on the winter months throughout the vegetative phase of their life cycle and avoid unfavourable conditions during the dry summer months as seeds in the seed bank (Cowling et al 1996)
The median percentage of germinating seeds ranged from 12.8% (T. ornata) to 93.5% (Daucus glochidiatus) under dark conditions and 15.5% (Podolepis aristata) to 100% (Hypochaeris glabra) under light conditions (Table S3)

Summary

Introduction

Mediterranean-climate regions are characterized by hot, dry summers and cool, wet winters, and support disproportionally high levels of global vascular plant diversity relative to their land area (Kreft and Jetz 2007) Plant species in these regions typically exhibit an ecological strategy which permits them to either avoid or tolerate seasonal droughts (Bernhardt 2007). Spatial variation in species’ seed germination is often invoked to explain how diversity is maintained in ecological communities, because it is another axis along which species can partition their niches (i.e., the “regeneration niche”; Chesson 2000b; Grubb 1977) This partitioning depends on seeds of different species exhibiting unique germination behaviour in response to different microsite conditions

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Results

Conclusion