The distribution of a host‐specific canopy parasite is linked with local species diversity in a northern temperate forest

Abstract

Question Is the spatial distribution and density of the maple spindle gall mite Vasates aceriscrumena (MSGM) positively correlated with the distribution and density of its host? Is the distribution of MSGM influenced by non‐host species and abiotic factors? Location Temperate mixed hardwood forest stand, Haliburton Forest and Wildlife Reserve, Ontario, Canada. Methods We used the mapped locations of host and non‐host trees to investigate the cause of spatial variation in the density of MSGM within an 8.8‐ha forest plot in central Ontario, Canada. Gall densities were determined from fallen leaves, collected at 20‐m intervals. We used Mantel and partial Mantel tests to compute the correlation between gall density and several spatially variable biotic and abiotic factors: (1) host density and basal area, (2) density of non‐host stems, (3) overall stem density, (4) stem species diversity and (5) topography. Results The density of leaf galls was weakly correlated with host density and basal area. Although the correlation with host density and basal area was statistically significant, leaf gall density was more strongly correlated with overall tree species richness and overall stem density. Gall densities were highest at the boundaries of neighbourhoods containing high and moderate sugar maple (Acer saccharum) densities. Partial Mantel tests indicated that the observed spatial correlations held when controlling for the potential influence of topography. Conclusions Based on the spatial relationships documented here, we speculate that the mechanism responsible for the correlation between the MSGM and non‐host stems is parasite‐induced host stress. Separate studies have established a strong negative impact of the MSGM parasite on sugar maple stem growth. We suggest that by weakening the competitive ability of its host, the parasite indirectly promotes local species diversity through competitive release. Given the high diversity and prevalence of leaf gall parasites in mixed hardwood stands, depression of host dominance by leaf parasites may represent an unexplored mechanism for the maintenance of species diversity in northern temperate forests.

Age‐related Crown Thinning in Tropical Forest Trees

“…For this purpose we conducted a survey of canopy gap fraction of Acer saccharum trees sampled in upland areas of a 13.5‐ha forest dynamics plot at Haliburton Forest, central Ontario (Patankar et al . ). Diameter measurements utilized the 2007 plot census, and the median of three observations for each tree was determined in the field.…”

“…POWER ANALYSIS AND MEASUREMENT VALIDATION.-Before research was undertaken in the tropical study sites, we obtained a large sample of Acer saccharum (N = 646) to verify that moosehorn densiometer measurements could detect the age-related crownthinning pattern documented by Nock et al (2008), and, if so, to determine the minimum sample size needed to detect this trend. For this purpose we conducted a survey of canopy gap fraction of Acer saccharum trees sampled in upland areas of a 13.5-ha forest dynamics plot at Haliburton Forest, central Ontario (Patankar et al 2014). Diameter measurements utilized the 2007 plot census, and the median of three observations for each tree was determined in the field.…”