Small and slow is safe: On the drought tolerance of tropical tree species
Joannès Guillemot, Nicolas K. Martin-StPaul, Leticia Bulascoschi, Lourens Poorter, Xavier Morin, Bruno X. Pinho, Guerric le Maire, Paulo R. L. Bittencourt, Rafael S. Oliveira, Frans Bongers, Rens Brouwer, Luciano Pereira, German Andrés Gonzalez Melo, Coline C. F. Boonman, Kerry A. Brown, Bruno E. L. Cerabolini, Ülo Niinemets, Yusuke Onoda, Julio V. Schneider, Serge Sheremetie, Pedro H. S. Brancalion
Summarized by Habiba Rabiu, a student of environmental geosciences at Fort Hays State University. Habiba is interested in all aspects of environmental science and conservation & sustainability. She would like to work in educating others about those topics. In her free time, she likes to read, write, and bake.
What data were used? In this study, data concerning 601 tree species were examined. To determine what characteristics of a tree would make it more drought resistant, three qualities were assessed: resistance of xylem to embolism, which is the blocking of water from moving through the plant (designated as P50 by the authors), leaf turgor loss point or the ability of a plant to maintain turgor pressure and operate under water stress (TLP), and the hydraulic safety margin (HSM) which is the risk that a plant will experience hydraulic failure in the driest conditions it could normally face. HSM can also be defined as the difference between turgor loss point and resistance to embolism (HSM=TLP-P50).
The researchers compiled data from previous meta-analyses on the TLP and P50 values of the chosen tree species. The species were further divided based on leaf habit, meaning whether they were evergreen or deciduous. Additionally, seven traits of the species were considered: leaf mass per area (LMA), leaf size, leaf nitrogen concentration (leaf N), leaf phosphorus concentration (leaf P), wood density, maximum height, and seed mass. The type of forest the species lived in, whether dry or moist, was also a factor that was considered.
Methods: To organize these data in a way that would allow classification of the tree species based on drought resistance, the researchers found two major axes (traits) that contributed to drought resistance. The most important factor (labeled the “fast-slow” axes) showed the difference in rate of resource attainment and processing between the tree species. The second most important factor was the “stature-recruitment” axes, which compared the relationship between preference for (that is more energy and resources are allotted to) growth and survival of individual plants, to preference for new seedling propagation.
LMA, wood density, and leaf N and leaf P concentrations are features that determine where the species falls on the fast-slow axes, while maximum height, seed mass, and leaf size indicate their position on the stature-recruitment axes. TLP and P50 values (plus the calculated HSM values) demonstrate how well the species respond to lack of water and the accompanying stress. Lower values of HSM, TLP, and P50 (which are expressed as negative numbers) indicate more drought resistance.
Results: The research determined that TLP and P50 (blocking of hydraulic action and the ability of the plant to maintain water pressure) were more negative in dry forests, and evergreen species tended to exhibit more negative TLP and smaller TLP- based HSM (risk that a plant will experience hydraulic failure) in dry forests than deciduous forests. The species that had the more negative TLP/P50 values and smaller HSM values tended to be smaller (leaned more to the recruitment side of the “stature-recruitment” axes) and slower to get and use resources (leaning towards “slow” rather than “fast” on those axes). In other words, smaller and slower evergreen trees were more drought resistant, and dry forests were naturally better suited to survive water stress than moist ones.
Why is this study important? One significant takeaway from this study is that it shows that drought resistance is not an independent quality that can be assessed on its own, it’s a complex mix of many traits. Isolating which traits are possessed by the most drought-resistant trees is valuable information when contending with ecosystems that are becoming hotter and drier as global warming becomes a bigger threat.
The big picture: Planting trees to restore tropical forests could be a great tool to combat the ill effects of climate change. However, care has to be taken to ensure that the trees planted are equipped to deal with the increased temperature of the atmosphere and presence of greenhouse gasses that come with global warming.
Citation: Guillemot, J., Martin- StPaul, N. K., Bulascoschi, L., Poorter, L., Morin, X., Pinho, B. X., le Maire, G., Bittencourt, P. R. L., Oliveira, R. S., Bongers, F., Brouwer, R., Pereira, L., Gonzalez Melo, G. A., Boonman, C. C. F., Brown, K. A., Cerabolini, B. E. L., Niinemets, Ü., Onoda, Y. Schneider, J. V., … Brancalion, P. H. S. (2022). Small and slow is safe: On the drought tolerance of tropical tree species. Global Change Biology, 28, 2622– 2638. https://doi.org/10.1111/gcb.16082