A project undertaken at The University of Queensland, and supervised by John Dwyer
It is well understood that different tree species occur in wet versus dry types of rainforests in Australia. Our project revealed the fundamental reasons why. We surveyed rainforests in 45 locations across a massive rainfall gradient (600 to 2500 mm/yr) from Mallanganee in the south to Kroombit Tops in the north. On more than 90% of the tree species that we recorded (285/315 species), we took measurements that describe how each species moves water from roots to canopies (hydraulic strategies), how they assimilate carbon and how they disperse their seeds.
Using sophisticated models, we found that the distribution of each species along the rainfall gradient was strongly determined by their hydraulic strategy, with “safe” strategies prevailing in the drier rainforests and “efficient” strategies prevailing in reliably wet forests. In this context, “safe” strategies have dense wood, small leaves and small xylem vessels, which allows these species to tolerate dry soils without experiencing catastrophic damage to their water-conducting system. At the other end of the spectrum, “efficient” strategies have less dense wood, large leaves and larger xylem vessels. These species can move water quickly when it is plentiful which helps them compete for light in dense forests, but they run the risk of damaging their water-conducting system during extended dry periods.
We also revealed how soil fertility (plant-available phosphorus) interacts with moisture availability to further refine which strategies occur where. We even found strong signatures of long-past climate fluctuations (over hundreds of thousands of years). Areas with more stable climates that maintained rainforest during these fluctuations (termed “refugia”) comprised more tree species with large fruits that are typically dispersed by only a few highly specialised rainforest bird species. Less stable areas where rainforest presumably disappeared and re-appeared during these fluctuations comprised more tree species with smaller fruits that are dispersed by many bird species, including those not restricted to rainforest habitats.
We validated these findings using glasshouse experiments that simulated acute droughts and confirmed that species with safe strategies do indeed tolerate drought much more effectively than efficient strategies. As expected, they achieve this tolerance at the expense of growth rates, which makes safe strategies poor competitors for light in reliably wet climates.
Overall, we revealed how contemporary and past climates have shaped Australia’s rainforests, which is central to predicting how future climates will impact their composition and function.