Discovering Leaf Patterns that Minimize Shade in Certain Trees

David Palenski
29th January, 2024

Discovering Leaf Patterns that Minimize Shade in Certain Trees

A sapling of Dendropanax trifidus (from study) showing deflection angle (angle between stem and leaf petiole, Φ) for shade.

Image adapted from: Aoyagi et al. / CC BY (Source)
Trees in forests face a fundamental challenge: maximizing light capture for photosynthesis while minimizing self-shading from their own leaves. This is particularly acute for trees growing in the understorey – the shaded environment beneath the forest canopy. Researchers at Hiroshima University[1] have recently discovered a previously unknown leaf arrangement strategy employed by several tree species to overcome this challenge, adding to our understanding of how plants adapt to low-light conditions. The study focused on four species of trees from the Araliaceae family, commonly found in Japanese warm-temperate forests: Gamblea innovans, Chengiopanax sciadophylloides, Dendropanax trifidus, and Fatsia japonica. These trees all share a similar basic structure – they grow with leaves directly attached to a single central stem, a characteristic known as being ‘monoaxial’. The research investigated how these trees arranged their leaves to best capture light in the dimly lit understorey. Traditionally, it was thought that reducing self-shading involved leaves becoming progressively longer and angled further away from the stem as you move down the branch[2]. This arrangement allows lower leaves to receive more light by avoiding being blocked by those above. However, the Hiroshima University team found something surprising: two of the species (G. innovans and C. sciadophylloides) exhibited the opposite pattern. Their leaves became shorter and angled closer to the stem as you moved down the branch. Despite this seemingly counterintuitive arrangement, both groups of species – those with progressively longer petioles and those with progressively shorter ones – achieved crowns with minimal self-shading. The ‘petiole’ is the stalk that attaches a leaf to the stem, and the ‘deflection angle’ is the angle between the petiole and the stem. The study meticulously examined the crown structure of young trees growing in the understorey, measuring petiole length and deflection angles for each leaf. This detailed analysis revealed the two distinct leaf arrangement strategies. This discovery is significant because it demonstrates that there isn’t just one way for trees to solve the problem of self-shading. While previous research highlighted the importance of leaf angle and size in influencing light capture and carbon gain[2][3], this study shows there’s flexibility in how plants achieve this. It suggests that the selective pressure to reduce self-shading – the need to capture as much light as possible – can be met through different morphological pathways. Interestingly, the observed leaf movements aren’t random. Research has shown that plants can actively reorient their leaves to reduce self-shading, a process triggered by detecting changes in light quality[3]. Specifically, a reduction in the ratio of red to far-red light (R:FR) – a signal indicating a leaf is being shaded – prompts the lower leaves to adjust their position. This reorientation can significantly increase carbon gain, particularly for lower leaves[3]. The newly discovered arrangement in G. innovans and C. sciadophylloides may represent a different mechanism for achieving a similar outcome, potentially reducing the need for active leaf movements. Furthermore, the arrangement of leaves, or ‘phyllotaxy’, can influence how much light a plant intercepts[4]. However, plants can compensate for less-than-ideal phyllotactic patterns by adjusting other features like leaf shape and orientation[4]. The findings from Hiroshima University build on this idea, demonstrating that even within a specific growth habit (monoaxial trees), there’s considerable scope for morphological adaptation. The study highlights that while trees in similar environments may face the same selective pressures, they can evolve diverse solutions. This flexibility in morphological realisation was previously underestimated, and this research provides valuable insight into the evolutionary processes shaping plant architecture.

EcologyPlant Science

References

Main Study

1) Newly found leaf arrangement to reduce self-shading within a crown in Japanese monoaxial tree species.

Published 28th January, 2024

https://doi.org/10.1007/s10265-024-01524-5

Related Studies

2) Leaf size and angle vary widely across species: what consequences for light interception?

https://doi.org/10.1046/j.1469-8137.2003.00765.x

3) Petiole twisting in the crowns of Psychotria liminesis: implications for light interception and daily carbon gain.

Journal: Oecologia, Issue: Vol 135, Issue 1, Mar 2003

4) THE ROLE OF PHYLLOTACTIC PATTERN AS A "DEVELOPMENTAL CONSTRAINT" ON THE INTERCEPTION OF LIGHT BY LEAF SURFACES.

https://doi.org/10.1111/j.1558-5646.1988.tb04103.x


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