What is it about?
How mature forest trees will respond to water deficit is poorly known. We investigated wood anatomy and leaf traits in lowland tropical forest trees after 24 months of experimental rainfall exclusion. Sampling sun‐exposed young canopy branches from target species, we found species‐specific systematic variation in hydraulic‐related wood anatomy and leaf traits in response to drought stress. Relative to controls, drought‐affected individuals of different tree species variously exhibited trait measures consistent with increasing hydraulic safety. These included narrower or less vessels, reduced vessel groupings, lower theoretical water conductivities, less water storage tissue and more abundant fiber in their wood, and more occluded vessels. Drought‐affected individuals also had thinner leaves, and more negative pre‐dawn or mid‐day leaf water potentials. Future studies examining both wood and leaf hydraulic traits should improve the representation of plant hydraulics within terrestrial ecosystem and biosphere models, and help fine‐tune predictions of how future climate changes will affect tropical forests globally.
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Why is it important?
Increased drought in the future may severely affect tropical regions, with severe implications for the health and function of forest ecosystems. It is therefore important for scientists, conservationists and a sympathetic public to underestand how drought may affect precious and fragile trropical ecosystems, in particular biodiversity-rich tropical rainforests.
Perspectives
Mature rainforest trees are not easy to study in the field. To study drought impacts on plants, most reseachers have resorted to glasshouse studies, using seedlings. In this experiment, we were privileged to have the opportunity to conduct a study that simulated a drought in the field, and thus study how mature trees will respond to environmental stresses..
Dr David YP Tng
School for Field Studies
Read the Original
This page is a summary of: Rainforest trees respond to drought by modifying their hydraulic architecture, Ecology and Evolution, December 2018, Wiley,
DOI: 10.1002/ece3.4601.
You can read the full text:
Resources
Drought transforms trees internally
Droughts lead to transformed trees. At an experimental tropical rainforest study plot in the Daintree, Australia, researchers from the James Cook University led by Professor Susan Laurance set up a "Drought Experiment" and studied what happens to trees in the forest after two years of artificially reduced rainfall. What they found was that tree change in their water conducting mechanisms during that time.
Transformed trees! Drought changes the plumbing system of rainforest trees
A blog post on the article
Rainforest trees respond to drought by modifying their hydraulic architecture
This is the downloadable open-access version of the paper. Abstract Increased drought is forecasted for tropical regions, with severe implications for the health and function of forest ecosystems. How mature forest trees will respond to water deficit is poorly known. We investigated wood anatomy and leaf traits in lowland tropical forest trees after 24 months of experimental rainfall exclusion. Sampling sun‐exposed young canopy branches from target species, we found species‐specific systematic variation in hydraulic‐related wood anatomy and leaf traits in response to drought stress. Relative to controls, drought‐affected individuals of different tree species variously exhibited trait measures consistent with increasing hydraulic safety. These included narrower or less vessels, reduced vessel groupings, lower theoretical water conductivities, less water storage tissue and more abundant fiber in their wood, and more occluded vessels. Drought‐affected individuals also had thinner leaves, and more negative pre‐dawn or mid‐day leaf water potentials. Future studies examining both wood and leaf hydraulic traits should improve the representation of plant hydraulics within terrestrial ecosystem and biosphere models, and help fine‐tune predictions of how future climate changes will affect tropical forests globally.
Fig 1 - Graph of anatomical trait means
Plant trait means (±SE) of four tree species of lowland tropical rainforest trees in a rainfall exclusion experiment in a tropical lowland rainforest, Cape Tribulation, Australia. Species were replicated within a control and drought treatment areas (Argyrodendron: 4, 4; Endiandra: 4, 2; Myristica: 6, 6; Syzygium: 7, 4 individuals, respectively). Branch wood traits include (a) vessel density; (b) vessel fraction; (c) theoretical stem water conductivity; and (d) vessel connectivity (vessel grouping index). Leaf‐related traits include (e) leaf thickness, and (f) pre‐dawn and (g) mid‐day leaf water potentials. Only traits with significant responses within species are featured, based on post hoc Tukey test comparisons (p < 0.05*, <0.01**, <0.001***). See Table 1 for means (±SD) of all traits
Fig 2 - Size classes of vessels that show how vessels sizes change
Vessel area class distribution within canopy branch sapwood for four species in a rainfall exclusion experiment in a tropical lowland rainforest, Cape Tribulation, Australia. Species are as follows: (a), Argyrodendron peralatum; (b), Endiandra microneura; (c), Myristica globosa; (d), Syzygium graveolens and replication follows Figure 1. The median vessel area classes of the three control and drought‐affected individuals are shown as dashed gray and red lines, respectively
Fig 3 - Changes in proportions of water conducting tissues
Tissue proportions within branch sections of trees from tropical lowland rainforest, Cape Tribulation, Australia. (a) Mean (±SE) proportions of vessels, parenchyma, and fibers from replicated individuals of the four study species within the control and drought treatment (replication as per Figure 1). Asterisks in parenthesis denote Tukey's test significance between control and drought‐affected individuals within each species (p < 0.05*, p < 0.01**, p < 0.001***). An example branch cross sections of (b) a control and (c) a drought‐affected individual of Syzygium illustrates the relative stem tissue fractions of vessels (v), storage parenchyma (p), and fibers (f)
Fig 4 - Occlusions in tree plumbing systems!
Drought‐induced vessel occlusions in tree branch cross sections. (a) Branch cross sections of Myristica globosa (replication as per Figure 1) demonstrate a lower frequency of vessel occlusion in control individuals of Myristica than in (b) drought‐affected individuals in a rainfall exclusion experiment in a tropical lowland rainforest, Cape Tribulation, Australia. At higher magnification (c), the occlusions are discernable as dark tyloses (white “T”s). All study species exhibited some degree of vessel occlusions (d). Asterisks denote significant results of Tukey's post hoc comparisons between control and drought‐affected individuals within each species (p < 0.05*, p < 0.01**, p < 0.001***)
Fig 5 - A graphical overview of what happens when trees are exposed to drought
Summary of directional trait acclimation of all studied trees in trait space (principal components analysis axes) in a rainfall exclusion experiment in a tropical lowland rainforest, Cape Tribulation, Australia. The length of the vectors (dark lines radiating from the center) reflects the strength of the influence of individual variables on the overall variance of the sample set. Only traits with significant differences between control and drought‐affected individuals were used for the ordination (See Table 2), and vessel lumen fraction was not included in the analysis due to intercorrelation with relative vessel fraction. The first three axes accounted for 74.63% of the trait variation across all individuals (only first two axes shown). The control and drought‐affected individuals show segregation of trait composition in their wood and leaf traits, and also some species‐specific responses driven chiefly by single traits (e.g., high vessel occlusion by drought‐affected individuals of Myristica globosa) (for axis loadings, see Supporting Information Table S3)
Images of the four study species
Images of the four study species
Trees change inside as drought persists
A press release on the drought work by James Cook University's official media platform.
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