Tree canopy arthropods have idiosyncratic responses to plant ecophysiological traits in a warm temperate forest complex

Abstract

Biodiversity studies on forest canopies often have narrow arthropod taxonomic focus, or refer to a single species of tree. In response, and to better understand the wide range of drivers of arthropod diversity in tree canopies, we conducted a large-scale, multi-taxon study which (a) included effect of immediate surroundings of an individual tree on plant physiological features, and (b), how these features affect compositional and functional arthropod diversity, in a warm, southern Afro-temperate forest. We found that tree species differed significantly in plant physiological features and arthropod diversity patterns. Surprisingly, we found negative correlation between surrounding canopy cover, and both foliar carbon and arthropod diversity in host trees, regardless of tree species. Subtle, tree intraspecific variation in physiological features correlated significantly with arthropod diversity measures, but direction and strength of correlations differed among tree species. These findings illustrate great complexity in how canopy arthropods respond to specific tree species, to immediate surroundings of host trees, and to tree physiological features. We conclude that in natural forests, loss of even one tree species, as well as homogenization of the crown layer and/or human-induced environmental change, could lead to profound and unpredictable canopy arthropod biodiversity responses, threatening forest integrity.

Figure 1

Visualization of assemblage composition results for the herbivore, predator, detritivore and tourist guilds using Canonical Analysis of Principal coordinates (CAP) between the eight tree species based on Bray–Curtis dissimilarities.

Figure 2

Study region in Africa, showing the five forests from which arthropods were sampled. OB = Oubos, GVB = Grootvadersbosch, KB = Kleinbos, WV = Woodville, WEB = Witelsbos. Mean annual temperature for southern Afro-temperate forests is 16.7 °C, characterized by cool winters (8–20 °C) and warm summer (13–25 °C) months. The three western study forests ranged in elevation between 370 – 410 m above sea level, and the two eastern forests, closer to the coast, are located ca. 250 m.a.s.l. Map generated using R statistical software (rstudio.com) version 3.6.2 through ggplot2, raster and rgdal packages with shape file imported from the GADM database (GADM.org).