Leaf Toughness, Silicon and Insect Herbivory in Chinese Forests

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

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

Insects instead favoured the tougher leaves, while the most effective defence proved to be something much less apparent.

Assessing leaf herbivory

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

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

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

Mean leaf loss across the sites was roughly six and a half percent. Before any individual trait stood out, though, a clear finding had already appeared.

When every factor was considered together, leaf traits themselves were better at predicting damage than either climate or the local insect community.

Why leaf toughness can backfire

Leaf strength delivered the most unexpected result. Plants with hard, leathery foliage - leaves expected to blunt the appetite of a hungry caterpillar - experienced greater, rather than lower, damage. This was the reverse of standard 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 evidence of a continuing evolutionary arms race, in which insects gradually develop stronger mouthparts capable of overcoming mechanical defences.

These Chinese forests are not alone in showing this pattern. One study of plantation and natural forests likewise reported that chewing-insect damage increased as leaves became thicker.

Silicon as plant armour

If toughness did not offer protection, another factor did. Leaves containing more silicon - a mineral absorbed by plants from the soil - repeatedly lost less surface area to insects.

Silicon is believed to make leaf tissue more difficult to digest, apparently acting separately from a plant’s other defensive mechanisms.

In an experiment involving tropical tree seedlings, supplementary silicon reduced the damage caterpillars were able to inflict.

This points to a defensive layer often omitted from conventional models. The authors say explanations of plant defence should include silicon alongside the tougher tissues and bitter chemical compounds that researchers usually assess.

The heat-tolerance trade-off

A further surprise concerned plants’ ability to withstand heat. Species more capable of maintaining leaf function as temperatures rise suffered heavier feeding damage, rather than less.

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

The most likely reason is that vigorous, highly productive plants are more attractive to insects.

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

Heat tolerance, generally viewed as an indicator of resilience, therefore comes with a cost that researchers had not incorporated into their models.

The cost of being evergreen

Whether a tree retained leaves throughout the year or dropped them each autumn also affected the results.

Evergreen species sustained more damage than deciduous trees, which lose their foliage in autumn and produce new leaves every spring.

An evergreen leaf can remain on a branch for years, leaving it available to insects season after season. A deciduous leaf falls in autumn and is replaced in spring, effectively beginning again.

This extended availability accumulates over time. A leaf that persists for several years provides more feeding opportunities than one present for only a single growing season - small bites repeated year after year.

Climate has a lesser influence

Overall leaf loss was higher in forests that were warmer and wetter and supported a wider range of insect species than in cooler, drier forests. Climate and insect communities plainly had an effect.

This is consistent with observations elsewhere: damage generally increases as the local insect community becomes more diverse, as another study in subtropical China found.

Yet after the researchers assessed all factors together, the leaves’ own characteristics remained stronger predictors of damage than weather conditions or the surrounding insect mix.

The environment shifts the figures, but the plant’s traits make most of the difference.

Wider implications of the research

Prior to this research, ideas about why insects select certain leaves over others were largely shaped by leaf toughness and growth strategy. Two results now reshape that view.

Silicon appears to be an understated yet genuine protective barrier. Heat tolerance, long regarded solely as resilience, also functions as a concealed disadvantage. Ecologists had not considered either to be a principal driver.

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

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