Seeing roots from space: aboveground fingerprints of root depth in vegetation sensitivity to climate in dry biomes

Abstract

With predicted climate change, drylands are set to get warmer and drier, increasing water stress for the vegetation in these regions. Plant sensitivity to drier periods and drought events will largely depend on trait strategies to access and store water, often linked to the root system. However, understanding the role of below-ground traits in enhancing ecological resilience to these climate changes remains poorly understood. We present the results of a study in southern Africa where we analysed the relationship between root depth and the Vegetation Sensitivity Index (VSI) (after Seddon, Macias-Fauria et al. (2016)). VSI demonstrates remotely-sensed aboveground vegetation responses to climate variability; thus our study compares aboveground vegetation responses to belowground root traits. Results showed a significant negative relationship between root depth and vegetation sensitivity. Deeper roots provided greater resistance to climate variability as shown by lower sensitivity and higher temporal autocorrelation in vegetation greenness (as measured by the Enhanced Vegetation Index, EVI). Additionally, we demonstrated a link between deeper roots and depth to groundwater, further suggesting that it is the ability of deeper roots to enable access to groundwater that provides ecological resistance to climate variability. Our results therefore provide important empirical evidence that the ability to access deeper water resources during times of lower water availability through deeper roots, is a key trait for dryland vegetation in the face of future climate change. We also show that belowground traits in drylands leave a fingerprint on aboveground, remotely-sensed plant-climate interactions, an important finding to aid in scaling up data-scarce belowground research.

Spectral asynchrony as a measure of ecosystem response diversity

“…One possible explanation for the positive relationship between spectral asynchrony and ecosystem resistance could be that certain species are able to maintain photosynthetically active leaves for extended periods toward the end of the wet season (Oehri et al., 2017). Differences in exposure to water stress/deficit, such as high hydraulic resistance or access to groundwater, may contribute to their ability to maintain ecosystem functioning (Kühn et al., 2022; Oliveira et al., 2021).…”

Root trait variation along water gradients in the Cape Floristic Region

“…The greater root depth and length observed at the drier sites in our study suggest that deep vertical exploration of the root profile in search of deeper water resources is prioritised as compared with the wetter sites. Although reflected globally (Schenk & Jackson, 2002b) and regionally (Kühn et al, 2022) for drier biomes, our results indicate that this pattern holds true at a local scale within a dry biome, with mean root depths of 88.54 vs 57.67 cm, and mean root lengths of 142.63 vs 93.66 cm at the drier (200–300 mm precipitation) and wetter site (600–700 mm annual precipitation) respectively. This highlights the importance of considering root variation when investigating vegetation's contribution to water dynamics and associated ecosystem processes.…”

“…At a global scale, it has been demonstrated that plants in semi‐arid ecosystems tend to have deeper roots (Schenk & Jackson, 2002b, 2005), reflecting a tendency in water‐limited ecosystems for plants to access water that was stored at depth during occasional or seasonal wet periods (Schenk & Jackson, 2002b; Fort et al, 2017). It has also been shown that in drier biomes in Southern Africa, deeper roots are linked to groundwater depths and reduced vegetation sensitivity to climate variability (Kühn et al, 2022). However, what is still not well understood is whether trait variation occurs across a water availability gradient within a dry biome.…”