Forest diversity effects on insect herbivores: do leaf traits matter?

Area of Science:

• Ecology
• Forestry
• Entomology

Background:

• Insect herbivore damage is often lower in diverse plant communities.
• The mechanisms behind this phenomenon, particularly the role of host plant traits, are not fully understood.

Purpose of the Study:

• To investigate how tree species diversity influences insect herbivory on silver birch (Betula pendula) through changes in leaf traits.
• To differentiate between trait-mediated and frequency-mediated effects of diversity on herbivory.

Main Methods:

• A long-term forest diversity experiment in Finland with varying tree species mixtures (1, 2, 3, and 5 species).
• Measurement of 16 leaf traits and herbivory by four insect guilds (chewers, gall formers, leaf miners, rollers) on silver birch.
• Analysis of how leaf traits and tree species frequency relate to herbivore damage across the diversity gradient.

Main Results:

• Increased tree species richness led to a lower frequency of birch, reduced its leaf area, and decreased chewing damage.
• Associational resistance to leaf miners was directly linked to birch frequency, not mediated by leaf traits.
• Leaf trait variation contributed to associational resistance, but this was driven by the abundance of other species, not species richness per se.

Conclusions:

• Plant diversity can reduce herbivory through both trait-mediated (e.g., leaf area) and frequency-mediated mechanisms.
• The relative abundance of neighboring species, rather than species richness alone, is a key driver of associational resistance.
• Understanding stand structure and foliar traits is crucial for predicting plant diversity effects on herbivory.