Conserving Ash (Fraxinus) Populations and Genetic Variation in Forests Invaded by Emerald Ash Borer Using Large-scale Insecticide Applications

摘要

Emerald ash borer (EAB) has killed millions of trees since its accidental introduction to southeastern Michigan more than 20 years ago. Near the invasion epicenter, nearly all mature ashes have died, reproduction has ceased, and the seed bank depleted, leaving an "orphaned" cohort of established seedlings and saplings. Because of high seedling mortality, it is possible that seedlings that established recently may have lower genetic variation than those that established before the EAB invasion. Insecticides can successfully protect ash trees from EAB and clusters of treated ash trees may slow ash mortality by reducing EAB densities. Therefore, insecticides, in high enough densities, may protect ash trees in order to maintain reproduction, regeneration, and genetic variation. I tested these questions at Five Rivers Metroparks in southwestern Ohio and the Upper Huron River Watershed in southeastern Michigan.;From 2014--2013; 2016, green and white ash mortality differed between parks, with survival higher at Sugarcreek, Englewood, and Germantown Metroparks (low EAB impact) than at Cox Arboretum, Taylorsville, and Twin Creek (high EAB impact). I found that survival of untreated green-white ashes increased with percentage of ash phloem area treated, but only in parks with low EAB impact. Additionally, survival of untreated trees was higher when the nearest treated ash was within 100 m, percentage of ash phloem treated was high, and EAB impact was low. However, this pattern was not observed in parks with high EAB impact. These results suggest that treating ash trees with insecticide may slow the progression of ash mortality if the program is initiated when ash mortality is still low.;There were more flowering green and white ash trees (treated and untreated) in plots with higher percentage ash phloem treated. In parks with high EAB impact, seedling density was low and was not affected by insecticide treatment. In parks with low EAB impact, seedling densities increased with percentage of ash phloem treated. Density of seedlings increased with density of flowering ash trees and decreased with height of herbaceous understory vegetation, which may suppress seedling population through competition for resources. Overall, these results show that protecting trees with insecticide can maintains ash reproduction. Patterns of genetic variation of seedling and sapling populations differed in Michigan and Ohio. In Michigan, established seedlings and saplings had similar allelic richness, but there was also less genotypic variation in the larger seedlings and saplings than in smaller seedlings. In Ohio, newly germinated and small established seedlings had higher allelic richness than larger established seedlings, but each population had similar number of effective alleles (high frequency alleles). Larger seedlings had the least genotypic variation and newly established seedlings had the most. Collectively, these results are consistent with a loss of genetic variation in Michigan, but no loss of genetic variation in Ohio. EAB-induced ash mortality and density of treated ash trees also had no effect on genetic variation in populations of ash seedlings, but the impact of the EAB invasion and the conservation benefits of insecticide treated trees may intensify as ash mortality increases over time.