Risk-takers and risk-avoiders: germination sensitivity to water stress in an arid zone with unpredictable rainfall

How much water does it take to turn an arid land into a hotbed of new life?

Water availability is a key driver of plant population dynamics in arid ecosystems. Plant recruitment (the formation of new populations or the addition of new individuals to an existing population) is typically episodic in response to rainfall, largely due to the fact that seed germination of arid plant species is controlled by water availability. Some arid plant species adopt a risk-taking strategy and produce seeds with low moisture thresholds that germinate in response to small rainfall events, while others adopt a risk-avoidance strategy such that germination occurs only in wet soils. It is generally assumed that seed germination in arid zones occurs in response to large rainfall events, yet species responses to different-sized rainfall events are rarely quantified. Understanding species’ germination thresholds in such ecosystems is key for conservation and restoration initiatives.

Examples of species from this study show two germination strategies, including the risk-takers—(a) Casuarina pauper and (b) Maireana pyramidata—and the risk-avoiders—(c) Atriplex rhagodioides and (d) Maireana sedifolia. Image credit: Duncan et al.

In a recent study published in AoBP, Duncan et al. investigated the germination traits of keystone trees and shrubs in an arid ecosystem with unpredictable rainfall. Seeds were collected from arid, south-west New South Wales from remnant populations targeted for restoration. Species adhered to one of two germination strategies: (1) the risk-takers which require less moisture availability for germination and have wide temperature ranges for germination (Casuarina pauper and Maireana pyramidata), and (2) the risk-avoiders that have greater moisture requirements, a preference for cold climate germination, and narrower temperature ranges for germination (Atriplex rhagodioides, Maireana sedifolia and Hakea leucoptera). We suggest that these species have particular adaptations to avoid germination during drought, and this is supported by low base water potentials for germination. The authors highlight that large-scale ecosystem restoration methods could be improved by sowing seed in cooler months, rather than the common practice of sowing in the autumn when soil temperatures are still warm. Whilst germination times may be marginally lengthened by cooler temperatures, overall germination rates will likely be improved due to lower evaporation rates ensuring water retention in the soil.