Moose browsing alters tree diversity effects on birch growth and insect herbivory

Abstract

Summary 1.Producer diversity is known to affect a wide range of ecosystem processes including plant growth and insect pest resistance. Consumers such as mammalian herbivores too have been shown to modify plant growth and insect herbivory by triggering changes in host plants. However, few studies have investigated whether consumer effects interact with plant species diversity effects on a focal plant. 2. To unravel consumer-diversity interactions, we recorded both the presence and intensity of winter browsing by moose (Alces alces) on silver birch (Betula pendula) in a long-term forest diversity experiment in Finland and measured birch tree growth as well as insect chewing damage during the following growing season. 3. Although browsing on birch by moose was not affected by tree species richness, the intensity of moose damage altered tree diversity effects on birch tree growth. At minor browsing intensity, tree height, trunk diameter and canopy projections showed positively humped relationships with tree diversity, peaking at 3-species mixtures. Growth of moderately browsed trees increased with tree species richness, but growth of severely browsed birch trees was unaffected. 4. Moose browsing also altered the direction of tree diversity effects on insect herbivory on birch. Unbrowsed trees experienced lower insect chewing damage in mixed stands (associational resistance) whilst browsed trees suffered more insect chewing damage in diverse stands (associational susceptibility). Increasing browsing intensity also reversed the relationship between tree species richness and insect chewing damage from negative to positive. 5. The observed interactions between moose browsing and tree species richness effects could be explained by lower canopy cover of more diverse stands compared with birch monocultures, leading to increased re-growth capacity and more high-quality foliage of browsed birch trees in more open diverse stands. 6. Our findings demonstrate that both the presence and intensity of mammalian browsing may modify the magnitude and even the direction of tree diversity effects on tree growth and susceptibility to insect herbivory. Differences in consumer impact among studies may thus potentially explain much of the observed variability in plant diversity effects on ecosystem functioning and must therefore be taken into account in future studies.

Do birds see the forest for the trees? Scale-dependent effects of tree diversity on avian predation of artificial larvae

“…As tree species richness increases, the presence of tree species differing in growth rates and foliage structure may result in less horizontal canopy space used and, thus, a more open canopy (Lang et al 2011). This has previously been shown in the Satakunta experiment, with canopy cover around birch trees decreasing with tree species richness (Muiruri et al 2015). As a result, artificial larvae may be more visible to birds when the focal trees are surrounded by heterospecifics, but these effects are likely to be restricted to small spatial scale and unlikely to manifest at plot level.…”

“…In order to examine the role of structural complexity on bird predation, we used tree height measurements from 2011 where ten randomly chosen trees of each species were assessed in each experimental plot (Muiruri et al 2015). For each plot, we calculated a mean and standard deviation of tree heights, using data for all species combined in mixtures.…”

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

“…Growth–defence tradeoffs have often been implicated here as a possible explanation for negative (or positive) diversity effects on antiherbivore defences (Moreira et al ., ), with evidence emerging for reduced herbivore resistance in more diverse and productive stands (McArt & Thaler, ). However, in this study system, birch height growth is known to be consistent across the species richness gradient (Muiruri et al ., ), and thus, even if growth and antiherbivore defences are negatively correlated, we might not expect there to be any associated patterns in defensive traits across the diversity gradient.…”

Plant diversity effects on insect herbivores and their natural enemies: current thinking, recent findings, and future directions

“…Surprisingly, however, this area of research has developed independently of plant diversity studies, despite the fact that diversity effects on herbivory would be expected to influence plant induced responses and thus potentially lead to plant-mediated interactions among herbivores. For example, Muiruri et al [47 ] recently found that moose browsing on birch altered responses to tree species diversity by insect herbivores feeding on birch. Similarly, unpublished work by some of the co-authors of this paper indicates that genotypic diversity in Lima bean (Phaseolus lunatus) influenced damage by early-season herbivores (leaf chewers and aphids) and such effects in turn determined responses to diversity by late-season seed predators (Abdala-Roberts et al, unpublished data).…”