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- Materials and Methods
There are many differences in the structure, species diversity and climate of tropical and temperate forests. However, there have been few direct comparisons of the physiology of their component species. Furthermore, these direct comparisons are often complicated by the inclusion of species with different growth forms (Schreiber & Riederer, 1996; Franks & Farquhar, 1999). The predominant changes in climate from temperate to tropical forests are the increase in temperature and the reduction in seasonality of temperatures (Archibold, 1995). Comparisons of species native to temperate and tropical forests will provide an understanding of how plants are adapted to these contrasting climates.
Rainforests occur across a latitudinal range of 33° in Australia, which includes climates from cool-temperate to tropical. These forests have a disjunct distribution along the eastern margin of Australia, being restricted to areas that have a high annual rainfall (> 1300 mm) and low fire frequency (Webb & Tracey, 1994; Specht & Specht, 1999). Therefore, they provide an opportunity to study the temperature responses of temperate and tropical species within the same forest type.
A recent study of some Australian rainforest tree species found that temperate species show maximum net photosynthesis at lower growth temperatures (the temperature at which leaves are developed) but maintain close to this rate over a larger range of growth temperatures than tropical species (Cunningham & Read, 2002). However, in the field, trees do not produce leaves continuously. Instead, leaves are produced during certain periods of the year and then exposed to the seasonal changes of temperature. The few studies of phenology in rainforest trees of Australia show that tropical species produce new leaves throughout the wet season from late spring to early autumn, whereas temperate species produce the majority of their leaves during spring and early summer (e.g. Read, 1989; Lowman, 1992). The ability to acclimate to new temperature conditions is likely to be more important in temperate trees due to their shorter growth period and the larger seasonal variation in temperature of their climates.
In the field, temperate species tend to maintain a similar rate of maximum net photosynthesis over the warmer months (e.g. Drew & Ledig, 1981; Pereira et al., 1986). In contrast, tropical species commonly show peaks in maximum net photosynthesis that are related to rainfall and not to temperature (Doley et al., 1987; Lugo et al., 1978). The seasonal responses of net photosynthesis in temperate species have been reproduced in plants exposed to a series of acclimation temperatures in controlled environment cabinets (e.g. Strain et al., 1976; Slatyer & Ferrar, 1977b; Mooney et al., 1978). Many of these studies have concluded that species from more variable climates have a higher acclimation potential – the ability to maintain maximum net photosynthesis when exposed to a wide range of acclimation temperatures (temperatures to which leaves are exposed after development, Berry & Björkman, 1980). Therefore, temperate species are likely to have a higher acclimation potential due to their exposure to larger seasonal variations in temperature than tropical species. However, most of these studies have only used two to three acclimation temperatures (e.g. Battaglia et al., 1996; Goldstein et al., 1996; Teskey & Will, 1999), which does not give an indication of their full acclimation potential.
Previous studies of Australian rainforest trees, using six to eight acclimation temperatures, have shown distinct differences in photosynthetic responses among species from different latitudes. A study of temperate rainforest trees found that species from lower latitudes showed maximum photosynthesis at higher acclimation temperatures than species from higher latitudes (Hill et al., 1988). Tropical and temperate species of Nothofagus showed little difference in the acclimation temperature for maximum photosynthesis (Read, 1990). Instead, the difference was the ability of temperate species to acclimate to a wider range of temperatures than tropical species. This is consistent with the tropical species of Nothofagus being from high altitude areas that experience similar maximum temperatures of the warmest month to temperate regions of Australasia where Nothofagus grows, but have smaller seasonal variation in temperature (Read, 1990). However, there is a positive relationship between the acclimation temperature for maximum photosynthesis and the maximum temperature of the warmest month among the temperate and tropical Nothofagus species of Australasia (Read & Hope, 1996). Whether these trends in acclimation are true of a broad range of temperate and tropical genera is the focus of this paper.
The majority of previous studies have used leaves developed under constant temperature conditions. However, in the field, plants are exposed to variation in day-to-day conditions. In addition, day-to-day variation in temperature is larger in temperate than tropical climates (Table 1). Therefore, leaves of temperate species in particular would be predicted to show a higher acclimation potential when developed under ambient conditions compared with constant conditions. A few studies have investigated leaves developed under ambient conditions and then exposed to a series of constant acclimation temperatures in controlled environment cabinets (e.g. Tranquillini et al., 1986; Hill et al., 1988). The acclimation potential of some species has been measured separately in leaves developed under ambient conditions and in controlled environment cabinets (e.g. Hill et al., 1988; Read & Busby, 1990; Gunderson et al., 2000). However, the effect of constant and fluctuating temperatures on their acclimation responses can not be discerned, as other factors besides temperature are likely to have varied between the growth conditions. Therefore, this study presents the first investigation under carefully controlled conditions of the effect of development under fluctuating vs. constant temperature on the acclimation potential of net photosynthesis. The study aimed to answer the following two questions:
Table 1. Weekly standard deviations (WSD) in maximum temperature for different locations in eastern Australia
|Location||Maximum temperature (°C)|
|Annual WSD||Summer WSD|
|Strahan (42° S)||2.4 (0.1)||3.0 (0.2)|
|Noojee (38° S)||3.3 (0.0)||4.2 (0.2)|
|Coffs Harbour (30° S)||2.2 (0.1)||2.2 (0.1)|
|Rockhampton (23° S)||2.0 (0.0)||2.1 (0.2)|
|Cairns (17° S)||1.2 (0.1)||1.4 (0.1)|
Do temperate species show a greater ability to acclimate to new temperatures than tropical species?
Do leaves developed under fluctuating temperature conditions show a greater ability to acclimate to new temperatures than leaves developed under constant temperature conditions?