What lies beneath? Botanists find a disconnection between how plants behave above and below ground.

Plants are incredibly complex organisms capable of responding to both biotic and abiotic drivers. To understand how plants respond, we traditionally focus on above-ground traits like stem size or foliage. However, understanding below-ground traits such as root morphology and nutrient uptake are just as important. In a recent paper by Mengesha Asefa and colleagues, botanists examined how above-ground and below-ground plant traits interact with each other. Their paper, published in Annals of Botany, also examines how these traits respond to soil moisture gradients and intraspecific competition, competition with plants of the same species. 

The study tackles the ‘plant economics spectrum’. This is a theory that says plants have a choice. They can expand fast to gather resources, or conserve their reserves, growing slowly but also prepared for harsh times. Writing in a commentary, Monique Weemstra and Oscar Valverde-Barrantes say:

The ‘plant economics spectrum’ theory predicts that trait expressions across species reflect a fundamental trade-off between efficient resource uptake and conservation (Reich, 2014), resulting in two distinct ecological strategies. It further assumes that the success of these strategies is contingent on the resource environment, and that these trade-offs present themselves in similar ways above and below ground. For example, inherently fast-growing species in high-resource environments would have efficient, fast leaves (e.g. high specific leaf area and roots (e.g. high specific root length) for rapid resource acquisition, whereas stress-tolerant species would have slow, long-lived leaves and roots to conserve resources and increase survival rates on low-resource sites (Reich, 2014). Evidence is accumulating, however, that these relationships are more intricate than assumed; e.g. specific leaf area and specific root length often do not correlate across species (Weigelt et al., 2021).

Weemstra & Valverde-Barrantes 2022

Asefa and colleagues examined how plants responded by growing five different tropical tree species in a greenhouse and monitoring them for 16 weeks. To create a soil moisture gradient, they provided the seedlings with varying levels of soil moisture content. The low level was 10% (drought), medium was 25%, and high was 35% (well-watered). They also included intraspecific competition to see how the plants reacted to their neighbours. After the 16 weeks were up, they measured a variety of traits both above and below ground to analyze the results.

The botanists found that the above-ground traits of the species were consistent with the leaf economics spectrum, while the below-ground traits showed contrasting patterns depending on the soil moisture. In well-watered conditions, they observed high specific leaf area and total leaf area, whereas in drought conditions, they found high leaf dry matter content, leaf thickness and stem dry matter content. Intraspecific competition was also been found to influence both above- and below-ground traits, but interacted with soil moisture to affect only below-ground traits. These findings demonstrate that soil moisture can have a significant impact on both above- and below-ground traits, and can be used to better understand the functional responses of plants to their environment.

The research has revealed that the relationship between above-ground and below-ground plant traits is more complex than previously thought. In particular, they found that plant traits which are associated with below-ground traits, such as root growth, cannot be inferred from above-ground traits such as leaf size. This disconnect indicates that multiple resource use axes are needed to understand how plants interact with their environment. The study also revealed that the correlation between leaf and root traits is highly variable, suggesting that plant trait relationships are highly multi-dimensional. The authors conclude:

Lack of consistent correlations between above- and below-ground traits across soil moisture gradients may limit the utility of the plant economics spectrum across plant communities. Our findings are consistent with the functional equilibrium hypothesis that plants allocate relatively more biomass to the organ that encounters limited resources for growth. Overall, our results demonstrate that plants may use a range of ecological strategies in response to varying environmental changes, which is consistent with the finding of Castorena et al. (2022) that species have different ways to co-ordinate traits to use resources in which, on average, they gain a similar amount of carbon per body mass over their life spans. Placing more above- and below-ground physiological and anatomical traits in the context of multiple biotic and abiotic factors will help to better understand plant functional strategies under global environmental change.

Asefa et al. 2022

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Asefa, M., Worthy, S.J., Cao, M., Song, X., Lozano, Y.M. and Yang, J. (2022) “Above- and below-ground plant traits are not consistent in response to drought and competition treatments,” Annals of Botany, 130(7), pp. 939–950. Available at: https://doi.org/10.1093/aob/mcac108.