Variable seed bed microsite conditions and light influence germination in Australian

Environmentally-cued germination may play an important role in promoting coexistence in 24 Mediterranean annual plant systems if it causes niche differentiation across heterogenous 25 microsite conditions. In this study, we tested how microsite conditions experienced by seeds 26 in the field and light conditions in the laboratory influenced germination in twelve common 27 annual plant species occurring in the understorey of the York gum-jam woodlands in 28 southwest Western Australia. Specifically, we hypothesized that if germination promotes 29 spatial niche differentiation then we should observe species-specific germination responses to 30 light. In addition, we hypothesized that species’ laboratory germination response may 31 depend on the microsite conditions experienced by seeds while buried. We tested the 32 laboratory germination response of seeds of species under diurnally fluctuating light and 33 complete darkness which were retrieved from microsites spanning local-scale environmental 34 gradients known to influence community structure in this system. We found that seeds of six 35 out of the twelve focal species exhibited significant positive germination responses to light 36 but that the magnitude of these responses varied greatly. In addition, maximum germinability 37 increased significantly across a gradient of canopy cover for two species, but we found little 38 evidence to suggest that species’ relative light requirement for germination varied depending 39 on microsite conditions. Our results suggest that variability in light availability may promote 40 coexistence in this system and that the microsite conditions seeds experience in the intra- 41 growing season period can further nuance species germination behaviour.


Abstract 23
Environmentally-cued germination may play an important role in promoting coexistence in 24 Mediterranean annual plant systems if it causes niche differentiation across heterogenous 25 microsite conditions. In this study, we tested how microsite conditions experienced by seeds 26 in the field and light conditions in the laboratory influenced germination in twelve common 27 annual plant species occurring in the understorey of the York gum-jam woodlands in 28 southwest Western Australia. Specifically, we hypothesized that if germination promotes 29 spatial niche differentiation then we should observe species-specific germination responses to 30 light. In addition, we hypothesized that species' laboratory germination response may 31 depend on the microsite conditions experienced by seeds while buried. We tested the 32 laboratory germination response of seeds of species under diurnally fluctuating light and 33 complete darkness which were retrieved from microsites spanning local-scale environmental 34 gradients known to influence community structure in this system. We found that seeds of six 35 out of the twelve focal species exhibited significant positive germination responses to light 36 but that the magnitude of these responses varied greatly. In addition, maximum germinability 37 increased significantly across a gradient of canopy cover for two species, but we found little 38 evidence to suggest that species' relative light requirement for germination varied depending 39 on microsite conditions. Our results suggest that variability in light availability may promote 40 coexistence in this system and that the microsite conditions seeds experience in the intra-41 growing season period can further nuance species germination behaviour.

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Mediterranean-climate regions are characterized by hot, dry summers and cool, wet winters, 46 and support disproportionally high levels of global vascular plant diversity relative to their 47 land area (Kreft and Jetz 2007). Plant species in these regions typically exhibit an ecological 48 strategy which permits them to either avoid or tolerate seasonal droughts (Bernhardt 2007). 49 For example, winter annual plants, which are a significant component of the diversity in 50 Mediterranean ecosystems, capitalize on the winter months throughout the vegetative phase 51 of their life cycle and avoid unfavourable conditions during the dry summer months as seeds 52 in the seed bank (Cowling et al. 1996). Environmentally-cued germination is an important  Light availability is one the major environmental factors that is perceptible by seeds and is 65 often required to elicit germination in annual plants (Grime et al. 1981 where harsh solar irradiation is attenuated by nurse plant coverage, or to prevent germination 77 on a rapidly drying or impenetrable soil surface  conditions such as the amount of shade and leaf litter as well as scattered woody debris. Thus, 100 in addition to testing light sensitivity for germination, we also tested how variation in seed 101 bank conditions influenced germination responses amongst species. In contrast to many 102 studies which simulate variation in the seed bank environment under laboratory conditions, 103 our aim in this study was to test species' germination after experiencing natural, field-based 104 variation in seed bank conditions. As such, we assessed whether microsite conditions 105 experienced in the seed bank in situ are associated with variation in: i) species' maximum 106 germinability and ii) the magnitude of species' germination responses to light relative to 107 complete darkness.

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Study system and species 110 Winter annual plants in the York gum-jam woodlands germinate following the onset of cool, 111 wet conditions at the beginning of winter (June) and set seed in spring (October-November).

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Dispersed seeds spend the hot, dry summer-autumn period in the seed bank. Experimental  (Table 1).  Prior to being buried, seeds were separated from chaff and placed into thin nylon mesh bags 130 mixed with 20 mL of coarse sand to reduce seed-seed contact and associated risks of    The germination test was conducting by randomly dividing the seeds for a given species and 157 patch into two equal groups. In a few cases, some seeds were lost from the mesh bag prior to 158 the start of the experiment so that the total number of seeds across both groups was less than 159 50. There were no obvious signs that germination had occurred in any of the mesh bags prior 160 to extraction from the field. Each group of seeds was placed on a sheet of filter paper  Overall, seed fill was high for all species (>90%) and was mostly unresponsive to measured 181 environmental variables (Table S1). Nevertheless, we adjusted germination percentages for 182 each species to account for seed fill by multiplying the total number of seeds in each Petri 183 dish by the species and patch-specific seed fill fractions obtained from X-ray analysis 184 (rounding up to the next whole number). In addition, individuals which had broken their seed 185 coat but either had no radicle or an unhealthy-appearing radicle were considered non-viable 186 and were subtracted from the total number of viable seeds in each Petri dish.

Species overall germination responses to light vs. complete darkness 231
Six out of the twelve species exhibited significant germination responses to the light 232 treatment, and in all these cases, they were positive (Figure 1) where P. aristata and T. ornata had significantly lower germination as the amount of canopy 243 cover increased (Figure 2). The presence of CWD had no effect on maximum germinability 244 for any species. canopy cover above the seed bed was found to influence maximum germinability for two 263 species; although only seeds of one of these species, T. ornata, responded significantly in its 264 germination to both the amount of canopy cover and light treatment. There was only minor 265 evidence to suggest that seed bank conditions may also influence species' relative response to 266 light and dark conditions (Table 3). . Interestingly, after conducting further regression analysis, we found that larger-285 seeded species instead tended to exhibit stronger, positive germination responses to light than 286 smaller-seeded species, although this relationship was weak ( Figure S2; Table S2). Given the 287 relatively small number of focal species and plant families that we studied, and the fact that 288 we did not observe species with significantly higher germination under dark conditions, it is 289 not possible to infer whether this pattern is more broadly applicable to our system. found that germination under light differed for seeds of some annual species depending on 406 whether they were exposed to light for only five seconds or for the entire incubation period.

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This phenomenon warrants further investigation in our system given the strong light 408 responses that we have already observed (Milberg and Andersson 1997).

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In another study in our system, Dwyer and Erickson (2016)