Background losses of woody plant foliage to insects show variable relationships with plant functional traits across the globe

Abstract

Summary Despite a long history of research, we lack a comprehensive understanding of the reasons behind pronounced variation in foliar losses to insects among plant species. Our aim was to test the hypothesis that plant functional traits and/or ecological strategies are good predictors of the background losses of woody plant foliage to insects (i.e. losses occurring when herbivore populations are at their ‘normal’ densities) at the global scale. We conducted a quantitative synthesis of published and original data on foliar losses of 793 species of woody plants belonging to 128 families from ca. 500 localities world‐wide, representing tropical, temperate and polar climate zones. The background foliar losses to insects varied among plant life forms, increasing from dwarf shrubs to large trees, and were higher in inherently fast‐growing species than in slow‐growing species. These patterns supported predictions of both the apparency and growth rate (resource availability) hypotheses for data combined across localities, but only outside the tropics. Foliar losses to insects slightly but significantly increased with specific leaf area (SLA) in all climate zones. No differences were noted in background herbivory between woody plant species with deciduous and evergreen foliage, with different shade tolerance and with different successional status. Synthesis. Factors affecting the distribution of herbivory among species of woody plants differed among the climate zones, and the predictive power of at least some of the theories/hypotheses addressing plant–herbivore interactions at large spatial scales varies among climates and/or biomes. The average background losses of woody plant foliage to insects across localities can be predicted from the inherent growth rate of the plant species, its life form and SLA, although these plant characteristics jointly explain only a minor part of the total variation observed in the primary data.

The association of leaf lifespan and background insect herbivory at the interspecific level

“…This result suggests that latitudinal variation in leaf lifespans also may be an important factor that shapes the latitudinal patterns of herbivory observed in recent studies (Kozlov et al 2015b, Zhang et al 2016. Kozlov et al (2015a) found a positive correlation between herbivory and SLA, but in our study, we detected no such relationship. Within a species, a previous study also detected a similar pattern (Garibaldi et al 2011).…”

“…Based on the species list that included data on herbivory, we searched the data on the leaf traits of each species in an open access database (Kattge et al 2011) and other published literature (Coley 1983, Escudero et al 1992, Reich et al 1998, Wright et al 2004, Tian 2007, Chen et al 2012, Li 2014, Kozlov et al 2015a, Li et al 2016. We focused on traits intimately related to plants' growth and defense strategies: leaf lifespan, specific leaf area (SLA), and nitrogen and phosphorus content (Wright et al 2004).…”

“…Based on data available currently, making population-level matches between leaf traits and herbivory is nearly impossible on a global scale (Kozlov et al 2015b, Lim et al 2015. However, for leaf traits, we used the averaged value of each species over all the analyses, the method used commonly in recent global analyses of leaf traits (Kozlov et al 2015a, Díaz et al 2016. However, for leaf traits, we used the averaged value of each species over all the analyses, the method used commonly in recent global analyses of leaf traits (Kozlov et al 2015a, Díaz et al 2016.…”

“…In all these analyses, the plant growth form (woody or non-woody) was used as a covariate, because of its strong effect on herbivory (Turcotte et al 2014a, Kozlov et al 2015a, b, Zhang et al 2016). The combined effects of different leaf traits on herbivory were analyzed whenever possible.…”

Forest biodiversity, ecosystem functioning and the provision of ecosystem services

“…For instance, bark beetle outbreaks causing large-scale mortality have been observed to shift forests from carbon sinks to carbon sources (Kurz et al 2008). Even minor chronic damage such as background defoliation (Kozlov et al 2015) may result in significant growth loss in trees (Zvereva et al 2012).…”