Thermal Requirements Underpinning Germination Allude to Risk of Species Decline from Climate Warming

Abstract

The storage of seeds is a commonly used means of preserving plant genetic diversity in the face of rising threats such as climate change. Here, the findings of research from the past decade into thermal requirements for germination are synthesised for more than 100 plant species from southern Western Australia. This global biodiversity hotspot is predicted to suffer major plant collapse under forecast climate change. A temperature gradient plate was used to assess the thermal requirements underpinning seed germination in both commonly occurring and geographically restricted species. The results suggest that the local climate of the seed source sites does not drive seed responses, neither is it indicative of temperatures for optimal germination. The low diurnal phase of the temperature regime provided the most significant impact on germination timing. Several species germinated optimally at mean temperatures below or close to current wet quarter temperatures, and more than 40% of species were likely to be impacted in the future, with germination occurring under supra-optimal temperature conditions. This research highlights both species vulnerability and resilience to a warming climate during the regeneration phase of the life cycle and provides vital information for those aiming to manage, conserve and restore this regional flora.

Keywords: degree-days; germination rate; global warming; seed germination; temperature.

Figure 1

The relationship between the diurnal temperature conditions (°C) required for highest and most rapid germination (T_opt—optimum temperature for germination) in common (○, r² = 0.4384) and restricted species (●, r² = 0.088) from southern Western Australia.

Figure 2

Box and whisker plot showing the amplitude of temperature fluctuations for optimum germination performance in common (□) and restricted species (■) from southern Western Australia. The X represents the mean, the horizontal line is the median, upper and lower boundaries of the box represent the interquartile range (75th and 25th percentiles), and whiskers extend from maximum to minimum.

Figure 3

A comparison of the relationship between time to commence germination (lag time, T₀) and night temperature for most rapid and highest germination in common (○) and restricted species (●) from southern Western Australia.

Figure 4

The impact of (a) mean diurnal temperature (b) day temperature and (c) night temperature for optimum germination on mean time to germination (MTG) in common (○) and restricted (●) species from southern Western Australia.

Figure 5

The relationship between mean optimum night temperature for germination and germination rate (inverse of time to 50% germination, 1/_T50) in common (○) and restricted (●) species from southern Western Australia.

Figure 6

The difference (Δ) in observed mean temperature for optimum germination (empirical data) and current (1970–2000) temperature in the wettest quarter plotted against the mean temperature for optimum germination (T_opt) for (a) restricted species and (b) common species. Fourteen collections are within 1 °C or below of the mean optimum temperatures for germination of the current wettest quarter. By 2061–2080, 50 collections are predicted to be within 1 °C or below threshold mean temperatures for germination in the wettest quarter (c) restricted species and (d) common species.

Figure 7

The relationship between common and restricted species for the degree days (as measured by mean temperature x time to 50% germination, T₅₀) in common (○) and restricted (●) species from southern Western Australia.

Figure 8

Locations of species investigated from southern Western Australia.