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In higher plants, leaf waxes provide a barrier to non-stomatal water loss, and their composition varies both between and within species. Characteristics of n-alkanes, a suite of ubiquitous compounds in these waxes, are thought to be influenced by the availability of water and the temperature in a plant’s growing environment. Longer n-alkane distributions with less variability in chain length are hypothesised to confer greater resistance to non-stomatal water loss and thus are expected in higher abundance in desiccating environments. Relationships between the distribution of n-alkanes and both precipitation and temperature have previously been observed. Despite this, it is unclear whether n-alkane chain length distributions vary plastically in response to climate, or whether they are fixed within populations in different climate settings. To better understand this, we examine the relationship between n-alkane characteristics of Melaleuca quinquenervia and both spatial and temporal climate variation. Across eastern Australia, we find that n-alkane homolog concentrations and distributions in leaves of M. quinquenervia do not vary with climate where samples are proximate, even when climate shows significant variability. However, the concentration and distribution of n-alkane homologs do differ considerably between geographically separated populations in very different climate regimes. These results suggest n-alkane characteristics are not a plastic response to climate variability, and instead are likely fixed and could be driven by genetic differences between populations. This has important implications for the use of n-alkane characteristics as palaeoenvironmental proxies.

Allison T. Karp, Jake W. Andrae, Francesca A. McInerney, Pratigya J. Polissar and Katherine H. Freeman

The photosynthetic pathway of plants is a fundamental trait that influences terrestrial environments from the local to global level. The distribution of different photosynthetic pathways in Australia is expected to undergo a substantial shift due to climate change and rising atmospheric CO2; however, tracking change is hindered by a lack of data on the pathways of species, as well as their distribution and relative cover within plant communities. Here we present the photosynthetic pathways for 2428 species recorded across 541 plots surveyed by Australia’s Terrestrial Ecosystem Research Network (TERN) between 2011 and 2017. This dataset was created to facilitate research exploring trends in vegetation change across Australia. Species were assigned a photosynthetic pathway using published literature and stable carbon isotope analysis of bulk tissue. The photosynthetic pathway of species can be extracted from the dataset individually, or used in conjunction with vegetation surveys to study the occurrence and abundance of pathways across the continent. This dataset will be updated as TERN’s plot network expands and new information becomes available., Measurement(s) oxidative photosynthetic carbon pathway Technology Type(s) digital curation • elemental analysis isotope ratio mass spectrometry Factor Type(s) date • geographic location Sample Characteristic - Organism Viridiplantae Sample Characteristic - Environment terrestrial natural environment Sample Characteristic - Location Australia Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.14134994

Leaf wax n-alkanes provide a valuable palaeoecological proxy, but their interpretation requires an understanding of the scale of temporal and spatial integration in soils. Leaf wax n-alkanes are continually deposited into soils directly from local plants as well as from more distant plants via wind or water transport. In addition, n-alkanes can persist in soils for thousands of years, and tend to decrease in age with shallower depth. To explore whether the uppermost soils reflect recent leaf fall inputs we compared surface soils and modern vegetation from 20 sites along a transect across Australia. At each site, the three most dominant plant species and a soil sample from the top 3 cm were analysed for n-alkane concentration, average chain length (ACL), proportional abundance of C33 and C29 (Norm33) and carbon preference index (CPI). Chain length distributions differ between trees and grasses, with a higher proportion of C29 in trees and C33 in grasses. Norm33 in soils correlates with proportional grass to tree cover across the transect. To model n-alkane inputs for each site, we calculated a predicted ACL, Norm33 and CPI using the dominant plants at that site, weighted by proportional species cover and n-alkane concentration. Predicted ACL, Norm33 and CPI inputs were generally higher than the soils, demonstrating that recent and local inputs do not dominate soil n-alkanes at our study sites. Thus, n-alkane distributions in surface soils do not correlate with local, current vegetation, but do correlate with proportional grass and tree cover, suggesting they provide a faithful record of large scale ecosystem structure.

Since the late Miocene, plants using the C4 photosynthetic pathway have increased to become major components of many tropical and subtropical ecosystems. However, the drivers for this expansion remain under debate, in part because of the varied histories of C4 vegetation on different continents. Australia hosts the highest dominance of C4 vegetation of all continents, but little is known about the history of C4 vegetation there. Carbon isotope ratios of plant waxes from scientific ocean drilling sediments off north-western Australia reveal the onset of Australian C4 expansion at ~3.5 Ma, later than in many other regions. Pollen analysis from the same sediments reveals increasingly open C3-dominated biomes preceding the shift to open C4-dominated biomes by several million years. We hypothesize that the development of a summer monsoon climate beginning in the late Pliocene promoted a highly seasonal precipitation regime favorable to the expansion of C4 vegetation.