Seed germination is a critical stage in the life cycle of most plants and is defined by specific tolerance thresholds beyond which rates and success of germination rapidly decline. Previous studies have demonstrated that widespread plant species commonly germinate over a broad range of temperatures and water stress levels, whereas range-restricted species often exhibit a narrower germination window in terms of temperature and moisture. We investigated the relationships of the key germination traits of maximum germination (Gmax) and time to 50% germination (t50) in response to temperature (5–35°C) and water stress (−1.5–0 MPa) in four co-occurring Western Australian native Eucalyptus species with widely varying biogeography. Eucalyptus caesia subsp. caesia and E. ornata exhibit a highly localized distribution and a narrow geographical range, being restricted either to granite outcrops or the upper slopes and tops of lateritic rises, respectively. These two species were compared with the two widespread and dominant congenerics E. salmonophloia and E. salubris. There was a distinctive hump-shaped response of t50 to temperature and an exponential response to water stress, characteristic of rate- and threshold-limited processes, but no consistent pattern in the response of Gmax. The four species were significantly different in their thermal performance of t50, with E. caesia and E. ornata displaying narrower thermal tolerance ranges than the two widespread species. In terms of mean final germination percentage, the two range-restricted endemic taxa exhibited higher lability in their response to thermal stress and drought stress compared to the two broadly distributed congenerics. These findings indicate a link between distributional extent, temperature and water stress tolerance and may have implications for identifying ecological filters of rarity and endemism.

The return of vegetation to mined lands often requires broadcast seeding of diverse native seed mixes. However, seeds are highly adapted to germination windows with specific hydrothermal thresholds that maximize the likelihood of seedling survival, and post-mining landscapes typically offer markedly different hydrothermal conditions than pre-disturbance ecosystems. According to niche theory, generalist species should exhibit broader hydrothermal performance niches than specialist taxa, which may influence the success of recruitment from seeds in post-mining ecological restoration. To test this assumption, the impact of hydrothermal stress (incubation temperature (10-30 C) and osmotic potential (À0.8 to 0 MPa)) on the time to 50% germination (t 50 ) and maximum germination (G max ) was compared between two narrow-range species of conservation concern (Acacia woodmaniorum and A. karina) restricted to mining-impacted Banded Ironstone Formations (BIF) and three broadly distributed congenerics (A. assimilis, A. exocarpoides, and A. ramulosa). The hydrothermal germination niches of the study species were broadly congruent with hydrothermal conditions of their habitats. The two range-restricted taxa were more tolerant of hydrothermal stress compared to the three widely distributed taxa, suggesting that tolerance of greater hydrothermal stress by both range-restricted Acacia species is likely to be adaptive to establishment in uncontested niche space. Complex interactions between thermal and water stress suggest these environmental gradients may shape the germination niche as well as patterns of plant diversity in BIF ecosystems. This study highlights the importance of quantifying interactions between niche dimensions and their implications for species performance, which will aid future restoration efforts for micro-endemic species impacted by mining.