Abstract
Water availability is a critical driver of population dynamics in arid zones, and plant recruitment is typically episodic in response to rainfall. Understanding species’ germination thresholds is key for conservation and restoration initiatives. Thus, we investigated the role of water availability in the germination traits of keystone species in an arid ecosystem with stochastic rainfall. We measured seed germination responses of five arid species, along gradients of temperature and water potential under controlled laboratory conditions. We then identified the cardinal temperatures and base water potentials for seed germination, and applied the hydrotime model to assess germination responses to water stress. Optimum temperatures for germination ranged from 15 to 31 °C under saturated conditions (0 MPa), and three species had low minimum temperatures for germination (<3 °C). A small proportion of seeds of all species germinated under dry conditions (Ψ ≤ −1 MPa), although base water potential for germination (Ψ b50) ranged from −0.61 to −0.79 MPa. Species adhered to one of two germination traits: (i) the risk-takers which require less moisture availability for germination, and which can germinate over a wider range of temperatures irrespective of water availability (Casuarina pauper and Maireana pyramidata), and (ii) the risk-avoiders which have greater moisture requirements, a preference for cold climate germination, and narrower temperature ranges for germination when water availability is low (Atriplex rhagodioides, Maireana sedifolia and Hakea leucoptera). High seed longevity under physiological stress in H. leucoptera, combined with a risk-avoiding strategy, allows bet-hedging. The hydrotime model predicted lower base water potentials for germination than observed by the data, further supporting our assertion that these species have particular adaptations to avoid germination during drought. This study provides insights into the complex physiological responses of seeds to environmental stress, and relates seed germination traits to community dynamics and restoration in arid zones.
Highlights
Plant recruitment in the arid zone is infrequent and episodic due to water limitation (Schwinning and Sala 2004; Wiegand et al 2004)
No germination occurred at 40 °C for any species, while H. leucoptera and M. sedifolia showed no germination at 35 °C
While temperature envelopes for germination were wide under saturated conditions (0 MPa), they were much narrower in dry conditions, for A. rhagodioides, M. sedifolia and H. leucoptera (Fig. 2)
Summary
Plant recruitment in the arid zone is infrequent and episodic due to water limitation (Schwinning and Sala 2004; Wiegand et al 2004). Some arid plant species adopt a risk-taking strategy, and produce seeds with low moisture thresholds that germinate in response to small rainfall events (Ramírez-Tobías et al 2014), while others adopt a risk-averse strategy so germination occurs only in wet soils (Sfairi et al 2012; Mollard and Naeth 2015; Merino-Martín et al 2017). Small rainfall events may only affect species with a risk-taking strategy and fast response times, while larger rainfall events are required to stimulate germination and support the establishment of risk-avoiders—generally higher vascular plants and slow-growing species, including tree species (Noy-Meir 1973; Schwinning and Sala 2004). We assume that seed germination in arid zones occurs mostly in response to large rainfall events (Gutterman 1994), yet species responses to different-sized rainfall events are rarely quantified (but see Meyer and Allen 2009), even though they determine community dynamics in arid zones (Reynolds et al 2004)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
