As trees mature, they enter different microenvironments, each with its own characteristics concerning shade, temperature, and humidity. In response to the varying surroundings, leaves slowly change their phenotype.
These adaptations are of particular interest to Mark Ashton, Morris K. Jesup Professor of Silviculture and Forest Ecology, who believes that knowledge of a tree’s leaf plasticity can lend information crucial to regenerating rainforests.
“I foresee that upland forests in humid regions of the world will become critical resources for the sustenance of global services and products,” notes Ashton. However, given their degradation over the past two centuries, it is difficult for rain forests to meet these demands.
Ashton hypothesizes that future needs can be met if rainforests are “artificially” augmented with certain trees. By planting trees with carefully selected behaviors, entire patches of rain forest would have ideal growing conditions. In theory, this would dramatically quicken restoration.
It has long been accepted that knowledge of trees’ behaviors can be used to recreate the environment in which they will thrive. For example, “sun-loving” trees may be planted in strategic locations so that they can provide an “umbrella” for “shade-loving” trees.
However, detailed studies on other phenotypes such as leaf change due to size class have not been conducted. This large gap in knowledge, crucial for the development of restoration techniques, spurred Ashton to examine leaves from tropical tree species in southwest Sri Lanka.
Ashton and colleagues picked four species to examine: Shorea worthingtonii and Mesua nagassarium, which were labeled as “valley” species, and Shorea megistophylla and Mesua ferrea, which were labeled as “ridge” species. The two valley species are from a moisture-rich site with fluctuations in light. The other two, labeled as “ridge” species, are from a steep, rocky site with fluctuations in soil moisture. They were selected for study because their genera are dominant in rain forests of south and south-east Asia, where a significant portion of the world’s rain forests lie.
Since trees in forests have to grow in different microenvironments as they reach maturity, the leaves were taken from each species with attention paid to height. The researchers defined four size classes: seedling (<50 cm), sapling (4-5 m), pole (10-20 m), and mature tree (25-35 m).
Each class size is characterized by a unique microenvironment. Trees would begin as seedlings on the forest floor with little lighting due to the thick foliage of the upper layers. Then, as the seedlings become saplings and then poles, the increase in height allows them to reach the understory, where sunlight is less faint and humidity is thick. As they mature, they enter the sunny canopy with higher temperatures and less moisture than in the previous growth stages.
Due to its changing surroundings, a tree must adapt by changing its phenotype. For instance, a leaf must make changes to offset the increase in temperature as they grow into the sunny canopy. If it does not sufficiently adapt, the change in heat levels will denature its metabolic enzymes.
By examining the leaves, Ashton was able to understand how the phenotype of plants with similar growth habits changes depending on the environment. In his studies, it was noted that leaf thickness, regardless of the species, increased as trees approached the canopy. Additionally, leaf size was found to be a universal transformation; leaf area and length both increased from seedling to sapling, and then they both decreased from pole to mature trees.
Therefore, the researchers concluded that the microenvironment is most likely responsible for the anatomical and morphological changes in leaves. This discovery has dramatic implications in the design of regeneration techniques and refocuses several modern theories.
Ashton hopes to utilize the new data to create better restoration conditions at various test sites located around the world.
(Image courtesy of Tropical Plants Library Online)