Tree crown architecture reveals how shade tolerance shapes forest competition and coexistence

Step into any forest, and you will see trees of strikingly different shapes: some shoot up like arrows with tiny crowns, others spread out like umbrellas with broad, leafy canopies. For decades, ecologists have recognized that trees compete intensely for light, but how a tree's physical crown structure influences these competitive interactions has remained unclear. Most trait-based studies have relied on simple measures like tree height, overlooking the multidimensional complexity of crown architecture. 

Now, a team led by Dr. Yunquan Wang at Zhejiang Normal University, together with colleagues from the Zhuji Natural Resources and Planning Bureau, Zhejiang University, and the Jiulongshan National Nature Reserve, has addressed this gap. In a study published in Plant Diversity, the team tagged and measured 3,589 individual trees representing 31 species in a subtropical forest plot. The study provides a mechanistic framework to integrate multidimensional crown architecture and shade tolerance into forest dynamics models.

"We measured total tree height, crown base height (where the live crown begins), and crown width, and then calculated six traits presented for crown architecture," shares lead author Qi Wu. "We tracked individual growth over five years and quantified how crown-trait dissimilarity among neighbors affected growth rates, analyzing light-demanding and shade-tolerant species separately."

Light-demanding species typically have a strategy of rapid vertical growth to overtop their neighbors. "For these trees, having neighbors with very different apical dominance or overall crown shape actually slowed their growth," explains Wu. "This pattern is consistent with environmental filtering, meaning trees with similar crown architectures tend to coexist better because they compete for light in similar ways."

However, when neighboring trees had very different crown projection areas, growth of light-demanding species improved. This suggests that if trees do not overlap much horizontally, they can share light resources more efficiently.

For shade-tolerant species, the story was entirely different. "These trees showed more flexible crown forms, which likely helps them survive under low light. Surprisingly, crown trait dissimilarity had no significant effect on their growth," says Wu. "Instead, their growth was mainly limited by the density of other trees of the same species growing nearby."

This result aligns with the Janzen Connell hypothesis, which proposes that host specific pests and pathogens build up around adults of the same species, and then suppress the growth of nearby individuals. In other words, shade-tolerant species are more shaped by natural enemies than by crown-based competition for light.

"Our findings demonstrate that shade tolerance fundamentally structures how crown traits mediate neighbourhood interactions," says Wu. "gnoring shade tolerance would obscure these individual-level performance patterns, which is critical for predicting species coexistence and forest responses to global change."

The authors note that future work should combine crown traits with organ-level traits (e.g., leaf economics, hydraulic traits) across broader climatic gradients to test the generality of these patterns.

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Contact the author: Yunquan Wang, yqwang@vip.126.com

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