Digging below the surface: Belowground adaptations to drought in Australian eucalypt woodlands

Helen M. Bothwell, Research School of Biology, Australian National University, Canberra, ACT, Australia and Justin Borevitz, Research School of Biology, Australian National University, Acton, ACT, Australia

Background/Question/Methods
Our world’s forests are experiencing elevated stress and mortality as a result of hotter, drier climate, including increased frequency and severity of drought events. While considerable research efforts have investigated aboveground responses to drought stress, most models treat belowground components as a black box. Using several Australian tree species that occur along an aridity gradient, we investigated inter- and intraspecific variation in root growth strategies, as well as above- and belowground interactions that may constrain a tree’s capacity to respond to climate change. We predicted that genotypes from cooler, wetter homesite environments would exhibit drought avoidance strategies, whereas genotypes from hotter, drier homesites would exhibit drought tolerance strategies. We first tested the effect of increasing temperatures on seedling germination and initial radicle and hypocotyl elongation using a temperature gradient experiment. Early stage (i.e., first three months) root growth rates, architecture, and above- and belowground biomass production were then assessed in response to well-watered and water-limited treatments, using rhizoplates to observe real-time changes in root growth. We further assessed trait heritability to identify which morphological and physiological characteristics hold the most promise for managing forest productivity and resilience under increasing aridity.
Results/Conclusions
From assessment of genotype-trait associations in an integrated, whole plant research model, five key findings emerged. 1) Testing germination and initial growth across a temperature gradient revealed that a 1°C increase in temperature relative to the response function optimum resulted on average in a ~40% reduction in radicle elongation, but no similar reduction was observed for hypocotyl elongation. Findings from the rhizoplate experiment tracking subsequent seedling growth during the first three months generally supported our hypothesis of drought tolerance and avoidance strategies segregating by environment. 2) Eucalypts from cooler, wetter regions displayed nearly 2X the rate of root elongation compared to those inhabiting the most arid provenances, and similarly 3) exhibited 2-3X greater shoot height. 4) Yet, more arid homesites displayed significantly greater fine root to course root ratios, and 5) a greater proportion of total biomass allocated to belowground relative to aboveground growth. Furthermore, our findings revealed heritable variation for key root growth traits that can be utilized to enhance restoration of Australian woodlands and production forestry globally. By advancing our understanding of belowground adaptations, we can better harness the capacity of our world’s forest to draw down atmospheric carbon and mitigate the effects of climate change.