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Australasian Plant Conservation

Originally published in Australasian Plant Conservation 16(4), March - May 2008, pp 16-18

How will warming temperatures affect recruitment of southern Western Australian narrow range mountain endemics?

A. Cochrane1 and M.I. Daws2
1Department of Environment and Conservation, WA. Email: anne.cochrane@dec.wa.gov.au
2Seed Conservation Department, Royal Botanic Gardens, Kew, UK.




Top: Banksia brownii. Photo: Sarah Barrett
Bottom:
Andersonia echinocephala. Photo: Anne Cochrane

Introduction

Climatic conditions, especially rainfall and temperature, are critically important for the successful recruitment, establishment, survival and reproduction of plants. Climate can influence plants directly (through changes in temperature, rainfall, winds and extreme events) and indirectly (through changes in factors such as fire frequency and behaviour, the spread of disease such as Phytophthora cinnamomi and competition with introduced weeds and other native plants). Climate can impact on the distribution and health of plant populations and any change may lead to major range shifts in some species and regional declines in others. One predicted consequence of climate change, for which there is increasing evidence emerging worldwide (e.g. Parolo and Rossi 2008) is the movement of plants to higher altitudes and latitudes as the climate to which they are adapted is displaced.

Current climate models predict that temperatures will rise while winter rainfall decreases across much of fire-prone southern Western Australia (WA). Obligate seeding species in fire-prone environments typically germinate in response to conditions that match the most suitable period for germination. As a result of greater water availability, the periods most favourable for plant recruitment in this environment are autumn and winter (Bellairs and Bell 1990; Bell 1994). However, of concern is the fact that these areas have already experienced a reduction in rainfall since the 1970’s (Dracup et al. 2005) with the potential to impact negatively on the region’s rich plant diversity.

Data on the conditions required for optimum germination of most WA mountain species are lacking. This is particularly the case for species from the botanically diverse but threatened Stirling Range. Consequently, this paper reviews the germination response of 10 plant species from the Stirling Range, including lowland populations of four of these species (Table 1). Because the Stirling Range is located near the coast, is isolated in a flat landscape and has a limited height (max. 1080 m), species can neither migrate upwards nor south in response to climate warming and hence may be highly sensitive to climate change. As many Stirling Range species are narrow range endemics we hypothesised that their narrow distribution might reflect very specific requirements for germination. In particular, we expected that such species would germinate best at comparatively low temperatures consistent with their mountain distribution. In addition, for four species with both mountain and lowland distribution ranges, we compared germination responses for seedlots from both locations. Since establishment depends not only on germination but also seedling growth, we compared the sensitivity of germination and early radicle growth to temperature. The improved knowledge of the risk posed by climate change to southern WA mountain flora will act as a basis for conservation planning and management.

Table 1. Temperature (°C) limits for germination and root growth of 10 species from SW Australia. To = optimum germination temperature, ToRGR = optimum temperature for root growth

Species

Family

Altitude

To

ToRGR

Calothamnus crassus (Benth.) Hawkeswood

Myrtaceae

800 m

14

20

Andersonia echinocephala (Stschegl.) Druce

Epacridaceae

450 m

14

13

 

 

80 m

17

17

Sphenotoma drummondii

Epacridaceae

1000 m

13

15

Deyeuxia drummondii (Steud.) Vickery

Poaceae

1000 m

17

15

Velleia foliosa (Benth.) K.Krause

Goodeniaceae

800 m

15

25

Gastrolobium leakeanum J.Drumm.

Papillionaceae

900 m

19

21

Banksia brownii R.Br.

Proteaceae

650 m

13

20

 

 

10 m

17

20

Kunzea montana (Diels) Domin

Myrtaceae

1000 m

15

27

Eucalyptus megacarpa F.Muell.

Myrtaceae

800 m

22

27

 

 

50 m

23

27

Allocasuarina decussata (Benth.) L.A.S.Johnson

Casuarinaceae

800 m

19

17

 

 

50 m

20

17

Methods

A two-way temperature gradient plate (Model GRD1, Grant Instruments, Cambridge, UK) was used to provide 12 constant or 30 fluctuating and 6 constant temperatures ranging from 5oC to 35oC with a 12 hour photoperiod. Seeds were sown either on the surface of filter paper or 1% agar in water in 50 mm or 90 mm Petri dishes. Seeds of Gastrolobium leakeanum required scarification of the water-impermeable seed coat to stimulate germination. Germination was scored every two days and was determined by visible radicle emergence.

The optimum temperature for germination (To – the temperature which allows the maximum level of germination in the shortest time) was calculated for each germination test at constant temperatures. Radicle growth rates were recorded at each constant temperature by measuring radicle growth over five days. Calculating the mean root growth rate for each temperature provided the temperature for optimum root growth (ToRGR). Examples of fluctuating temperature germination response are presented using contour diagrams.




Top to bottom: Figure 1 (No temperature limits to recruitment) and Figure 2 (Temperature limits to recruitment) are graphs depicting the different scenarios for temperature limits for germination exhibited by species investigated from the mountains of southern WA. Shades of grey show varying percent germination.

Results and Discussion

Although restricted to the Stirling Range peaks, a number of species (e.g. Kunzea montana, Velleia foliosa, Deyeuxia drummondii and Gastrolobium leakeanum) had a remarkably wide temperature range for germination (as per the example provided by Fig. 1) with optima between 15 and 19°C. Consequently, a narrow temperature range for germination does not account for these species having a narrow distribution restricted to the Stirling Range peaks. A number of small-seeded species (Calothamnus crassus, Andersonia echinocephala and Sphenotoma drummondii) displayed a limited and comparatively low optimum temperature for germination (as per the example provided by Fig. 2) and a wider range of temperatures for root growth than for germination (data not shown). Thus these species may prove to be highly vulnerable, at the seed germination stage, to the effects of rising temperatures.

There were only minor differences in optimum temperature responses between low and high altitude populations of more widespread common species like Allocasuarina decussata and Eucalyptus megacarpa (Table 1). In contrast, Andersonia echinocephala and Banksia brownii, conservation-listed species but not restricted to the peaks of the Stirling Range, displayed different optimum temperatures for germination for montane and lowland populations, with higher temperature optima for seeds from lowland populations (Table 1).

For some species (e.g. Bansksia brownii, Calothamnus crassus, Velleia foliosa, Kunzea montana and both populations of Eucalyptus megacarpa) seedling performance, measured by ToRGR, diverged from To for germination and there was typically a lower optimum temperature for germination than for seedling performance (Table 1). As a result, climate change may have a greater impact on seed germination than on relative seedling performance for these species.

Although southern WA has experienced climate variation in the past and its native species may have a broad tolerance to extreme climate conditions, we have little knowledge of climate thresholds for southern WA’s flora. It is therefore vital that we begin to gather accurate data on species requirements for recruitment as just one small piece of the jigsaw puzzle that will assist our understanding of climate change impacts on native flora.

References

Bell, D.T. (1994). Interaction of fire, temperature and light in the germination response of 16 species from the Eucalyptus marginata forest of South-western Australia. Australian Journal of Botany 42: 501-509.

Bellairs, S.M. and Bell, D.T. (1990). Temperature effects on the seed germination of ten Kwongan species from Eneabba, Western Australia. Australian Journal of Botany 38: 451-458.

Dracup, M., McKellar R. and Ryan, B. (eds) (2005). Living with our changing climate. Indian Ocean Climate Initiative, Perth.

Parolo, G. and Rossi, G. (2008). Upward migration of vascular plants following a climate warming trend in the Alps. Basic and Applied Ecology 9: 100-107.

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