Abstract

There is mounting evidence that top-down control by natural enemies helps maintain plant diversity in natural ecosystems. Previous work has concentrated on either forest trees or grassland herbs. Our knowledge of how natural enemies affect herb diversity in forests is limited.

We used fungicides and insecticide to experimentally examine the effects of fungi and insects on herb abundance and diversity at the community-wide scale and within groups of species with certain traits. We also assessed how the effects of fungi and insects on herb assemblages were modified by neighbouring tree diversity and composition in an old-growth temperate forest.

We found that fungicides increased overall herb abundance by 7% while decreasing herb diversity by 5%. This effect of fungicides on both herb abundance and diversity weakened with the increase of neighbouring tree diversity. Insecticide did not affect either herb abundance or diversity noticeably, except in areas dominated by deciduous trees where insecticide application reduced herb diversity.

Fungicides and insecticide decreased the diversity of relatively less defended species (i.e. thin-leaved and non-clonal) but had no effect on the diversity of herbs with stronger defensive traits (i.e. thick-leaved and clonal). Fungicides increased the abundance of non-mycorrhizal (NM) species but not arbuscular-mycorrhizal (AM) species, although the effect of fungicides on herb diversity was unrelated to species' mycorrhizal association. Insecticide had stronger effects on the abundance and diversity of NM species than AM species.

Synthesis. We conclude that fungi and insect herbivores are critical to regulating herb abundance and diversity in this temperate forest, with their effects dependent on species' traits and neighbouring tree diversity and composition. Our results highlight the importance of top-down control of understory herb communities by natural enemies in temperate forests, advancing our understanding of the processes shaping plant diversity in natural ecosystems.