Climate shapes how oaks evolve leaf traits to survive

A new study published in Forest Ecosystems has found that two evergreen oak species in the Himalayan-Hengduan Mountains region adapt to their environments in very different ways by changing how their leaf traits are connected and coordinated.

Researchers from Beijing Forestry University studied 908 trees across 72 populations, focusing on Quercus aquifolioides, which grows in colder high-altitude areas, and Quercus spinosa, which lives in warmer, lower elevations. They looked at leaf trait integration (how closely traits like shape, size, petiole length, and lamina width are linked) and modularity (how traits are grouped and vary together) to understand how the trees cope with different climates.

They found that Q. aquifolioides has more loosely linked leaf traits. This means the traits, like leaf size and shape, can change more independently, which helps the species adjust to harsh, unpredictable conditions like cold and drought. On the other hand, Q. spinosa has tightly linked leaf traits, which work well together in stable environments and make the plant more efficient at using sunlight and water, but less flexible when conditions change.

The team measured leaf morphological traits (including traditional leaf traits and geometric leaf traits) and linked the data to local climate conditions such as temperature and rainfall. They also looked at genetic information to understand how much the differences were influenced by heredity versus the environment.

Their results showed that climate plays a major role. In drier and colder places, Q. aquifolioides had thicker leaves with lower specific leaf area, which helps reduce water loss. In contrast, Q. spinosa had thinner, larger leaves better suited to warm, wet areas with steady sunlight.

Importantly, the study found no signs of "character displacement," a process where species living in the same area evolve to be more different to avoid competition like recently reported in other deciduous oaks. Instead, the main driver of their differences was climate, not competition with each other.

The study offers critical insights for conservation. Oaks are keystone species across Northern Hemisphere, and understanding how their traits evolve in response to climate can help predict which populations are resilient and which are vulnerable as global conditions shift.

“It’s not about one species being better than the other,” says Prof. Du, the corresponding author. “It’s about having the right strategy for the harsh environment in high altitude.”

By studying how oak leaves have evolved, this study provides a rare window into how trees adapt across ecological gradients, offering lessons for managing forests in a warming world.