Global importance of large-diameter trees

Aim: To examine the contribution of large‐diameter trees to biomass, stand structure, and species richness across forest biomes. Location: Global. Time period: Early 21st century. Major taxa studied: Woody plants. Methods: We examined the contribution of large trees to forest density, richness and biomass using a global network of 48 large (from 2 to 60 ha) forest plots representing 5,601,473 stems across 9,298 species and 210 plant families. This contribution was assessed using three metrics: the largest 1% of trees ≥ 1 cm diameter at breast height (DBH), all trees ≥ 60 cm DBH, and those rank‐ordered largest trees that cumulatively comprise 50% of forest biomass. Results: Averaged across these 48 forest plots, the largest 1% of trees ≥ 1 cm DBH comprised 50% of aboveground live biomass, with hectare‐scale standard deviation of 26%. Trees ≥ 60 cm DBH comprised 41% of aboveground live tree biomass. The size of the largest trees correlated with total forest biomass (r2 = .62, p < .001). Large‐diameter trees in high biomass forests represented far fewer species relative to overall forest richness (r2 = .45, p < .001). Forests with more diverse large‐diameter tree communities were comprised of smaller trees (r2 = .33, p < .001). Lower large‐diameter richness was associated with large‐diameter trees being individuals of more common species (r2 = .17, p = .002). The concentration of biomass in the largest 1% of trees declined with increasing absolute latitude (r2 = .46, p < .001), as did forest density (r2 = .31, p < .001). Forest structural complexity increased with increasing absolute latitude (r2 = .26, p < .001). Main conclusions: Because large‐diameter trees constitute roughly half of the mature forest biomass worldwide, their dynamics and sensitivities to environmental change represent potentially large controls on global forest carbon cycling. We recommend managing forests for conservation of existing large‐diameter trees or those that can soon reach large diameters as a simple way to conserve and potentially enhance ecosystem services

A 'Mitey' Influence? Life History, Impacts and Distribution of a Gall-inducing Arthropod in a Temperate Forest Canopy

Mature forest canopies worldwide sustain an enormous diversity of arthropods, many of which are specialist natural enemies. However, with the exception of species that exhibit massive outbreaks, host-specific canopy arthropods are thought to have relatively little influence on tree health and overall forest productivity. My thesis examines the role of one such arthropod in a temperate forest stand in central Ontario. The maple spindle gall mite Vasates aceriscrumena (Riley) (Acari: Eriophyoidae) is a host-specific canopy parasite that induces galls on leaves of sugar maple Acer saccharum Marsh. I examine three diverse topics related to this host-parasite system: 1) the seasonal phenology of this mite in mature sugar maple canopies, 2) the impacts of galling on host physiology and growth and 3) the distribution of this mite across host ontogeny and within the broader context of the local forest community. With respect to phenology, I document a previously unobserved interaction between the gall-inducer and a gall-invading mite (ubiquitous in the canopy but new to science) and consequences of this on the bionomics of galling mite populations. This work is also the first to examine differential physiological responses to galling across two distinct stages in the ontogeny of the host. Infected leaves in mature trees show drastic reductions in gas-exchange processes (photosynthesis, stomatal conductance and water use efficiency) while infected sapling leaves show no such detectible responses. Further, I find a significant negative correlation between radial increment growth in mature trees and levels of mite galling, as well as significantly increased galling frequency in mature trees compared with understory saplings. Finally, I explore the relationship between galling abundance and abiotic and biotic variables within a large mapped forest plot and show that gall densities are most strongly correlated with local species diversity and less so with host densities. Overall, my research provides a new perspective on the influence of host-specific, dispersal-limited canopy arthropods as major drivers of ‘age-dependent’ reductions in physiological performance and growth of older trees and as natural enemies that are strongly associated with local forest community distribution patterns.Ph