Leaf Toughness, Silicon and Insect Damage in Chinese Forests

Leaf toughness is among ecology’s most readily understood concepts. The logic is that leathery, fibrous foliage takes more effort for insects to chew, meaning trees that put resources into tougher leaves ought to suffer less damage.

However, fresh measurements spanning dozens of forest species in China directly challenged that assumption.

Instead, insects appeared to favour the tougher leaves, while the most effective protection came from a much less apparent source.

Measuring leaf damage

The research was headed by Longxin Lu at the South China Botanical Garden (SCBG), which is part of the Chinese Academy of Sciences.

Lu’s team assessed leaf damage in 61 tree and shrub species across five forests with distinct climates.

For every species, researchers calculated the proportion of leaf area removed by insects - known as leaf herbivory - and compared it with 11 measurable leaf characteristics.

Mean leaf loss across the forests was approximately six and a half percent. Before any individual trait was identified, one finding was already clear.

When all variables were considered together, leaf traits themselves explained damage more effectively than either climate or the local insect community.

Why leaf toughness can increase herbivory

Leaf strength delivered the most unexpected result. Species with tough, leathery foliage - precisely the type expected to frustrate a hungry caterpillar - experienced greater rather than lower damage. This ran counter to textbook expectations.

“The herbivory patterns were far from what we expected. Species with tougher leaves actually suffered more herbivory,” said Lu.

He interprets this finding as part of a continuing evolutionary arms race, in which insects have gradually developed stronger mouthparts capable of overcoming mechanical defences.

This pattern is not confined to these forests. Other studies have observed the same relationship: damage by chewing insects increased as leaves became thicker in one investigation of plantation and natural forests.

Silicon as leaf armour

If toughness was not the protective factor, another trait was. Leaves containing more silicon - a mineral absorbed by plants from the soil - consistently lost less surface area to insects.

Silicon is believed to make leaf tissue more difficult to digest, apparently acting independently of a plant’s other defences.

In an experiment involving tropical tree seedlings, extra silicon reduced the amount of damage caterpillars could inflict.

This points to a defensive mechanism that conventional models often overlook. The authors say explanations of plant defence should include silicon alongside the tougher tissues and bitter chemicals that researchers usually assess.

The heat tolerance trade-off

A further surprise concerned a plant’s ability to cope with heat. Species with higher heat tolerance - those better able to keep leaves functioning as temperatures rise - sustained more feeding damage rather than less.

“Surprisingly, species with higher heat tolerance experienced greater herbivory,” said study co-author Dr. Hui Liu.

The most probable reason is that productive, vigorous plants present a more appealing food source.

A flourishing plant may provide a more nutritious meal, attracting insects instead of deterring them.

Heat tolerance, generally regarded as evidence of resilience, therefore comes with a cost that had not been included in researchers’ models.

The evergreen cost

Whether a tree retains leaves throughout the year or sheds them each autumn also affected damage levels.

Evergreen species experienced more damage than deciduous trees, which lose their leaves in autumn and produce new ones each spring.

An evergreen leaf can remain on a branch for years, allowing insects to encounter it season after season. By contrast, a deciduous leaf is shed in autumn and replaced in spring, giving the tree a fresh start.

That extended availability accumulates over time. A leaf present for several years supplies more potential meals than one that survives for only a single growing season: small bites repeated year after year.

Climate has a more limited influence

Hotter, wetter forests with a wider variety of insects recorded greater overall leaf loss than cooler, drier forests. Climate and insect communities plainly still mattered.

This matches observations from other forests. Damage generally rises when the local insect community becomes more diverse, as a study in subtropical China also found.

Yet after the researchers assessed every factor together, leaf characteristics remained better predictors of damage than weather conditions or the surrounding insect mix.

The environment can shift the figures, but a plant’s own traits do most of the determining.

Wider implications of the study

Before this research, ideas about why insects select certain leaves over others were largely shaped by leaf toughness and growth strategy. The two findings alter that understanding.

Silicon appears to provide a subtle but genuine protective barrier. Heat tolerance, long viewed solely as resilience, also acts as an unseen disadvantage. Neither had been considered by ecologists as a leading driver.

As the climate becomes warmer and rainfall less predictable, this difference may help forecasters identify forests likely to experience the greatest insect pressure and the tree species at highest risk.

It also offers plant breeders and conservationists an additional factor - silicon - to consider when choosing what to plant.