Untangling liana responses to elevated CO2

Lianas are an important physiognomic feature of forests globally and are increasing in abundance and biomass in some tropical and temperate forests. Lianas often negatively affect tree growth and survival, therefore increases may cause significant changes to forest diversity, carbon storage and ecosystem dynamics. The first chapter of this thesis addresses and synthesises the putative mechanisms that may explain the current observed increases in lianas in some forests and attempts to determine if such increases are a global or regional phenomenon. Increasing atmospheric CO2, prolonged dry seasons, and heightened disturbance are the three main drivers attributed to the observed increases in lianas. There is ongoing debate around the significance of each driver and their individual contributions to liana increases. It is also likely that these drivers act in combination at global, regional and local scales to produce the reported trends. To further examine the importance of increasing atmospheric CO2 as a driver of liana increases, the second chapter of this thesis contains primary research on the effects of elevated CO2 (including interactions with drought) on the comparative responses of lianas and trees and is formatted as a manuscript for submission for publication to Global Change Biology. Using a Free Air CO2 Enrichment facility in Richmond, Australia, I had the unique opportunity to test the effects of elevated CO2 on co-occurring liana and tree responses at the adult life stage – research which has never been conducted previously. In contrasting ambient and elevated CO2 conditions I investigated the water-use and growth trait responses of the single dominant liana and tree species at the experimental site. Elevated CO2 benefitted liana water status during drought, although additional CO2 did not confer a greater liana growth response compared to trees. Lianas were less water-stressed than trees overall, thus prolonged dry seasons under future climates may benefit lianas relative to trees and lead to liana increases, although this may not depend on elevated CO2. The knowledge generated herein can assist to untangle and resolve questions of the relative responses of mature lianas and trees to elevated CO2.