Vegetation response to climate change: a functional traits-based approach
Keywords: functional traits
Fynbos
South Africa
plant response
roots
dryland
climate change
belowground allocation
remote-sensing

Climate change influences all aspects of plant biology. The responses of vegetation to climate changes (particularly water availability in the world’s drylands) constitute a critical and timely research agenda, with potentially significant ecological consequences. This doctoral thesis aimed to investigate vegetation response to climate change using a plant functional traits-based approach, with a specific focus on root traits in Southern Africa by undertaking three interlinked research objectives:
i) Determination of globally important plant functional traits for coping with climate change (Chapter 4; Research Paper 1). Here the aim was to synthesize the knowledge to date from the published literature on which traits are important in determining a positive response in plant performance and fitness to climate and associated environmental changes. A systematic review of 148 studies published between 2000-2017 was carried out. Results from this work present a suite of eight key traits that best predict positive plant responses: greater water-use efficiency (WUE), greater resprouting ability, lower relative growth rate, greater clonality/bud banks/below-ground storage, higher wood density, greater rooting depth, lower or higher specific leaf area (SLA) and lower or higher plant height (the latter two varying across biomes). These findings illustrate important and general trait-climate responses within and between biomes that enhance understanding of which plant phenotypes may cope with or thrive under current and future climate change. They also highlight the importance of generally understudied belowground traits in conferring plants the ability to cope with climate change.
ii) Determination of how root traits vary within a dry biome (Chapter 5; Research Paper 2). Here the aim was to quantify the contribution of belowground traits to overall trait variation in the semi-arid Fynbos biome of South Africa and analyse how this changed along regional and local water availability gradients. Fieldwork was conducted to collect root and aboveground traits of 124 individuals of dominant woody shrub species. Results from this work show that drier regions have greater root investment (rooting depth, length, dry matter content and root:shoot ratio) which was consistent intra-specifically and in post-fire environments. Additionally, roots accounted for significant whole-plant trait variation and, importantly, in drier conditions increased root allocation (at the expense of shoot allocation) deviated from expected global allometric relationships. These findings suggest that root investment will be especially crucial for plant performance and survival in a drier and warmer future predicted for dryland biomes. Chapter 5 (Research Paper 2) thus contributes to the still deficient field data on belowground traits in drylands.
iii) Determination of the role that roots play in reducing sensitivity to climate variability in drylands (Chapter 6; Research Paper 3). Here the aim was to use empirical belowground trait data and remote sensing imagery to explore belowground processes with space-borne, spatially continuous data that allow for regional assessments. A statistical analysis was conducted on the relationship between root depth data and remotely derived vegetation sensitivity to climate variability (VSI, after Seddon et al. (2016)) in Southern Africa. Results from this work show that a significant negative relationship between root depth and vegetation sensitivity exists in Southern Africa, as well as a significant positive relationship between root depth and temporal autocorrelation in vegetation productivity. These relationships were influenced by both biome and growth form, but generally imply that deeper roots reduce vegetation responses to concurrent climate variability and dampen temporal variability in aboveground productivity. These findings suggest that accessing deeper water resources during times of water stress through deeper roots is a potential resilience mechanism for drylands under future climate change.
In this thesis I conclude that traits play a key role in determining vegetation response to climate change. Specifically, I conclude that in dry biomes often-neglected root traits contribute significantly to overall plant trait variation and are thus key in reducing sensitivity to climate variability and determining positive plant responses to climate change. The novelty of this body of work includes but is not limited to the following findings: i) there is a global set of traits important across biomes to cope with multiple climate changes, ii) both local and regional drivers of water availability are significant drivers of belowground trait variation in the semi-arid Fynbos biome of South Africa and, iii) aboveground plant-climate interactions are reflected by belowground trait.