Reserach topics

My research encompasses plant physiological ecology, forest ecology, functional ecology, biomechanics, and remote sensing. Driven by a deep passion for nature, I have focused on uncovering the intricate adaptive strategies of diverse organisms and the mechanisms behind biological production in plants. Additionally, I enjoy developing new technologies and exploring novel research frontiers. In recent years, I have actively applied remote sensing technologies such as UAVs (Unmanned Aerial Vehicles) and LiDAR (Light Detection and Ranging) to evaluate and analyze forest structure.

During my PhD studies (completed in 2005), I specialized in plant physiological ecology, investigating plant responses to environmental factors—such as temperature, CO2, and light—with particular emphasis on photosynthesis and resource use, including both acclimation and adaptation processes. Afterward (since 2005), I recognized the critical role of biomechanics in understanding plant form and function and subsequently expanded my research to include this field. Over time, my scope has broadened further to encompass macro-level topics, including biodiversity assessment, species coexistence, vegetation succession, competition, evolutionary ecological dynamics, and macroecology. Building on my physiological ecology expertise, I also conduct applied research on tree breeding and enhancing forest productivity.

Forest Dynamics Analysis Using Drones (UAV)
Since 2015, when drones started becoming more widely available, I have been using various types of drones to assess forests. Recently, drones equipped with LiDAR have enabled highly precise measurements of forest structure at centimeter-scale resolution. In the past, it was only possible to measure a few dozen trees per day for attributes like tree height and crown area. Now, with just an hour of data collection and subsequent analysis, we can analyze several thousand trees in a single day.

By overlaying data collected at different time points, we can now accurately and broadly quantify changes in forests—such as gap formation and crown growth—that were previously difficult to measure. I aim to make effective use of these technological advancements to enhance forest resource assessments and improve our understanding of forest dynamics.

The figure on the left shows orthophotos and canopy height models (CHMs) of the Higashiyama forest from May 2024 and May 2025, along with a difference map highlighting changes in canopy height. We have been conducting annual measurements in the Higashiyama area since 2015 (a 10-year dataset).

Why Are Photosynthetic Efficiency and Leaf Longevity Incompatible?

We see a wide variety of plants—from roadside weeds to deciduous trees that shed their leaves in some time of a year, and evergreen trees that retain their leaves year-round. Generally, longer-lived leaves tend to be thicker and contain more photosynthetic tissue, yet paradoxically, they do not exhibit higher photosynthetic rates. In other words, long-lived leaves typically have lower photosynthetic efficiency. This trade-off is observed globally, but its underlying causes have remained elusive.

Onoda et al. (2004) were the first to demonstrate, using the perennial herb Fallopia japonica, that long-lived leaves allocate a greater proportion of their nitrogen (a proxy for protein content) to the cell wall rather than to the photosynthetic machinery. This nitrogen allocation pattern leads to reduced photosynthetic efficiency and highlighted the previously underappreciated role of cell walls in photosynthesis. Since then, numerous studies have investigated the trade-off in nitrogen allocation between cell walls and photosynthetic proteins. There is also a separate line of research exploring how cell walls influence CO₂ diffusion within the leaf.

To synthesize these findings and test the generality of the mechanisms driving this trade-off, Onoda et al. (2017) assembled an international research team and compiled data from a wide range of plant species worldwide. Their analysis demonstrated two key mechanisms associated with the robust structure and thick cell walls required for leaf longevity:

A larger fraction of nitrogen is allocated to the cell wall, reducing the nitrogen available for photosynthetic proteins.
The efficiency of CO₂ diffusion to the chloroplasts is diminished with thicker cell walls.

Both mechanisms play crucial roles in reducing photosynthetic efficiency in long-lived leaves.

A Curious Latitudinal Pattern in Leaf Lifespan: Shorter in Deciduous Trees, Longer in Evergreens

In humid tropical regions, evergreen trees dominate, while in cool temperate zones, deciduous trees that shed their leaves in winter are more common. Interestingly, in subarctic or boreal regions, evergreen species—especially conifers—once again become dominant. When examining the relationship between leaf lifespan and temperature across various plant species, we observe a peculiar pattern: as mean annual temperature decreases, deciduous leaves tend to have shorter lifespans, whereas evergreen leaves live longer (see figure on the right, adapted from Wright et al. 2005).

A lower mean annual temperature generally means longer winters and shorter growing seasons (excluding tropical highlands). Since deciduous trees only retain their leaves during the growing season, it is not surprising that their leaf lifespan shortens with declining temperature. The puzzling question is: why do evergreen trees, in contrast, produce longer-lived leaves in colder climates?

If we consider the cost of leaf construction and the time needed to recover that cost through photosynthesis, we realize that in areas with very short growing seasons, leaves may not pay back their construction cost within a single year. Therefore, evergreen trees growing at higher latitudes, where the growing season is brief, need to retain their leaves for longer to achieve a return on investment.

This idea can be tested using a relatively simple model that estimates the optimal leaf lifespan for maximizing carbon gain (Kikuzawa 1991). Interestingly, the predicted leaf lifespans from such models align well with global patterns observed in real-world data. This suggests that the global variation in leaf lifespan may be shaped by plants adapting to maximize carbon acquisition under their local environmental conditions.

Moreover, leaf lifespan is tightly linked with other leaf traits, such as thickness and nutrient concentration. As a result, large-scale global patterns in leaf traits can, to a significant extent, be explained by models based on carbon economics.

Thin but Tough Leaves—Sophisticated Leaf Mechanical Structure
Thin, flat leaves are ideal for capturing light. Compared by dry mass per unit area, leaves are hundred times lighter than a 1 cent coin. One might expect such thin leaves to be fragile, but in reality, they withstand wind and rain and can live sometimes over 10 years. Leaves are thus “thin yet durable.” From a materials engineering perspective, leaf structure resembles a sandwich structure used in lightweight, stiff constructions such as airplane wings, ski or carton box. Using a novel method we developed to assess tissue stiffness, we demonstrated that leaves possess a highly sophisticated sandwich structure that achieves “thin yet durable” (Onoda et al., 2015).

Height Competition in Plants

The primary reason why plants grow taller is to compete for light. A plant that grows taller than its neighbors can pre-empt sunlight and suppress the growth of individuals beneath it. However, growing too tall comes at a cost—it increases the demands for water transport, structural support, and investment in non-photosynthetic tissues. Therefore, plants cannot grow indefinitely tall; instead, their height is determined by a balance between the benefits of enhanced light acquisition and the costs associated with vertical growth.

Even within a single forest, a wide variety of plant species coexist. Some species dominate the upper canopy, while others are confined to the middle or lower layers. It remains unclear how these species of different heights manage to coexist despite engaging in intense competition for light. In particular, little was known about how much light individuals of different sizes actually acquire and how efficiently they convert it into biomass in natural forests.

Onoda et al. (2014) addressed this question by using advanced techniques such as 3D mapping of leaf and light distribution with LiDAR. They found that while taller trees have higher light interception efficiency, their light-use efficiency (biomass gained per unit light intercepted) tends to decline. As a result, relative growth rate—defined as the product of these two efficiencies—remains largely independent of tree height. This suggests that a trade-off between light interception and light-use efficiency plays a key role in enabling the coexistence of multiple species under size-asymmetric light competition.

This study was conducted in Yakushima, Japan, but similar findings have been observed in mature forests in other places, indicating the generality of these mechanisms across different forest types.

Leaf distribution and crown section (side view)

Light distribution and crown section (side view)

Crown Shyness: A Gentle-Sounding Term with a Competitive Edge

When looking up into the forest canopy, one often notices small gaps between the crowns of neighboring trees. In English, this phenomenon is called crown shyness, implying “trees yielding space to one another.” Despite this gentle-sounding name, these gaps can be more accurately described as “military frontiers” created by the physical collisions of crowns swaying in the wind.

Crown movement is influenced not only by wind speed, but also by traits such as tree height, stem diameter, and wood stiffness—all of which vary among species and individuals. These collisions can suppress the growth of weaker individuals while allowing dominant trees to expand their crown area. Therefore, we hypothesize that traits influencing crown movement and collision damage may be subject to natural selection.

Taking this unique perspective of "canopy conflict," Onoda & Bando (2021) investigated how species traits determine the extent of crown movement and damage from collisions, shedding new light on species competition in the canopy.

More may come in the near future...

Any questions: yusuke.onoda*gmail.com (please replace* with @)

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