Hydrothermal thresholds for seed germination in winter annual forbs from old‐field Mediterranean landscapes

Abstract

Under Mediterranean climates with dry-hot summers and cool-wet winters, many forbs with potential for habitat restoration are winter annuals, but there is little information about their germination. We performed laboratory germination experiments on 13 ruderal dicots native to Andalusia (southern Spain). We measured the germination of recently harvested seeds from natural populations across nine temperature treatments (from 5 to 35 °C, constant and alternate); two storage periods; and eight water stress treatments (from 0 to -1.0 MPa). We then calculated the hydrothermal thresholds for seed germination. Final germination ranged from 0-100% and results were mixed in response to temperature. Base temperature was below 6 °C, optimal temperature was around 14 °C and the ceiling temperature around 23 °C. For five species, 10 months of storage improved total germination, indicating a dormancy-breaking effect, but the other species did not respond or had their germination reduced. All species were relatively tolerant to water stress, with base water potential ranging from -0.8 to -1.8 MPa. Our results suggest that hydrothermal germination thresholds, rather than physiological dormancy, are the main drivers of germination phenology in annual forbs from Mediterranean semi-dry environments. The variation in germination responses of these forb species differs from winter annual grasses, but their seeds are all suitable for being stored before restoration.

Native plants for greening Mediterranean agroecosystems

“…In the study region, natural populations available for initial collection of foundation seeds are not threatened or legally protected, and they are relatively abundant. In a set of germination experiments, we confirmed that all the study species behave as winter annuals and they are expected to germinate in autumn 34,35 , a desired condition for avoiding water competition with olive trees in summer. The remaining seeds were stored under ambient conditions until field experiments started.…”

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

“…From the two linear regressions in each segmented model, the intercepts with the temperature axis provided estimates for T b and T c , respectively, and T opt is the temperature at which the two linear regressions intercept (e.g. Frischie et al 2019). Temperatures from the base ( T b ) to optimum ( T opt ) were species suboptimal temperatures for germination.…”

“…Although departures of actual seed germination from the hydrotime model are frequently reported at suboptimal and supra-optimal temperatures (Kebreab and Murdoch 1999; Grundy et al 2000), it can be a useful tool in defining seed responses to microclimate conditions, and the germination niche of seeds (Bloomberg et al 2009; Watt et al 2011). Few studies have defined the physiological thresholds for seed germination from wild species in arid zones (Arnold et al 2014b; Hu et al 2015; Lewandrowski et al 2016; Frischie et al 2019); hence, our understanding of the role of water availability in the germination strategy of native species in the arid zone is limited.…”

Maximising the use of native seeds in restoration projects

“…Adding a layer of environmental complexity, Frischie et al . () found for 13 ruderal dicots from southern Spain that germination efficiency was affected by a combination of water availability and temperature rather than physiological dormancy. Finally, across a wide spectrum of 23 key herbaceous species for use in European habitat restoration, Lopez del Egido et al .…”