Growth and nitrogen fixation of inland and coastal tropical Acacia species

Nitrogen is mainly lost from most Australian tropical savanna ecosystems via fire and replenished by nitrogen fixation. Fires in Australian tropical savanna woodlands may become more frequent and severe under climate change and thus nitrogen loss may be exacerbated. This research partly examines how climate change can affect the seed germination of Acacias (family: Mimosaceae) in terms of changing rainfall and fire regimes. The research also examines the effect of climate change on the nitrogen fixation and growth of tropical Acacias, in terms of changing rhizobia sources (in case of range shifts under climate change), responses to drought (which are predicted to become more frequent and severe), and factors affecting the nodulation of mature Acacia trees/shrubs (since the dry season may be prolonged and soil properties may change). By comparison to coastal Acacias, the following key hypotheses were proposed: • seeds of inland Acacias have weaker physical dormancy when no pretreatment was applied; • inland Acacias display co-adaptation with sympatric rhizobia in nitrogen fixation; • inland Acacia seedlings cope with drought better; and • mature inland Acacias produce fewer nodules and rely proportionally less on symbiotic nitrogen fixation in obtaining nitrogen from the surrounding environment. When no pretreatment was applied, we found no significant difference between inland and coastal Acacias in the seed germination responses. Thus the original hypothesis was not supported. We also could not find any clear differences in the heat tolerance and sensitivity between inland and coastal species. But in another study by Congdon et al. (in prep.), which the current experiment supplements, inland Acacias had a lower heat tolerance level (80°C dry heat for five minutes) than coastal species (100°C dry heat for 5 minutes). It appears that inland Acacia species have adapted to fire in years of average rainfall as 80°C is the soil temperature recorded at 3 mm soil depth in an early dry season fire. More intense fire can drive the temperature at 3 mm to 182°C and hence can cause substantial mortality of seeds of both inland and coastal Acacia. In the provenance experiment, nodulation always enhanced seedling growth. Acacia hosts did not necessarily grow best with sympatric soil rhizobia, implying that tropical Acacias can grow at least equally well in soil outside their current ranges as in sympatric soil. Thus any climate-induced or assisted range shifts of tropical Acacia species under climate change might not be constrained by mutualism. Three, of the four Acacia species tested, established and/or grew better with inland rhizobia. Hence species expanding into inland areas might even benefit from more effective mutualistic symbiosis. Adding inland soil as a source of rhizobia might benefit the growth of tropical Acacia seedlings in a nursery situation. Absence of watering for 8 weeks had a negative effect on the biomass and nitrogen contents of Acacia seedlings. The inland Acacias were more adapted to drought than coastal ones by having smaller changes in foliage water content, foliage thickness (deduced from specific foliage weight), and higher water use efficiency under drought than under the normal watering regime (deduced from foliage ¹³C values). Secondly, nodule nitrogen contents and the relative importance of symbiotic nitrogen fixation in acquiring nitrogen under drought were lower than those under the normal watering regime. The percentage reduction was found to be more severe in inland Acacias than coastal Acacias and might be of adaptive significance. Furthermore, the decreases in foliage and nodule biomass, stem to root ratio and foliage nitrogen content in all the four Acacia species under drought were smaller in the presence of inland rhizobia than in the presence of coastal rhizobia, suggesting symbioses formed with the inland source of rhizobia were less affected by drought. In short, Acacia species restricted to inland areas or species growing on inland soil might therefore have higher resistance to a single drought event. Using the ¹⁵N natural abundance method, it was found that symbiotic nitrogen fixation and/or mycorrhizal uptake of soil nitrogen was likely to be the primary mechanism(s) of all four tropical Acacia species for acquiring nitrogen in the wild. Abundant nodules were found growing on the roots of coastal A. crassicarpa in the top 24 cm of soil while few nodules were found on the roots of coastal A. aulacocarpa, and inland A. elachantha and A. ramiflora. Their nodules, if any, might have developed in deeper soil (below 24 cm). Deeper soil (12-24 cm vs top 12 cm), greater crown width and root biomass of the Acacia hosts, the change from dry to wet season, decreasing soil moisture and soil bulk density were positively related to the development of effective nodules of A. crassicarpa. Total soil nitrogen and phosphorus were, however, not correlated with effective nodulation. Prolonged periods of drought under climate change can be detrimental to plant growth. Based on the field study results, it is also likely to inhibit effective nodulation in surface soil directly, or indirectly through slower tree growth and hence smaller tree size. It is possible that effective nodules can develop in deeper soil in the dry season to compensate for the decrease in surface soil. Deeper soil should be sampled in the future to verify this possibility.