Author
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DIMITRI, LINDSAY - University Of Nevada |
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Longland, William - Bill |
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Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/21/2017 Publication Date: 7/20/2017 Citation: Dimitri, L.A., Longland, W.S. 2017. Distribution of western juniper seeds across an ecotone and implications for seed dispersal processes. Western North American Naturalist. 77:212-222. DOI: https://doi.org/10.3398/064.077.0209 Interpretive Summary: Western juniper is a native tree that has been expanding into productive rangelands for over a century, resulting in reduced availability of forage plants important to wildlife and livestock. Like many juniper species, western juniper produces a fruit-like “berry” that is eaten by birds, which then digest the pulp and pass the undigested seeds. Although it has long been assumed that this is how western juniper seeds are dispersed, there is often a second step in the process; seed-eating rodents harvest juniper seeds from bird feces and place them in superficially buried surface caches, where their chance of germinating is increased. To determine if western juniper seeds passed by birds in the first step of the process are concentrated near the edge of juniper forests, we counted seeds within a forest and at increasing distances from the forest edge during winter and summer 2016. At each seed counting location, we counted numbers of seeds in open spaces and in places that provide perches for birds –under trees, under shrubs, and on large rocks. More seeds were found around bird perches, especially under trees and shrubs, than in open spaces, and numbers of seeds in all of these locations decreased as one moved further from the forest. Many more seeds were found in winter counts, when birds were actively eating juniper berries, than in summer, after rodents had been given sufficient time to remove the vast majority of them, many of which would have been placed in surface caches. However, the effect of seed-caching rodents on juniper expansion is limited by the short distances that birds move seeds from forest edges in the first step of seed dispersal. Therefore, managers should focus efforts to combat juniper expansion near forest edges. Technical Abstract: Western juniper forests have been the focus of extensive research and management due to range expansion and infilling that began over a century ago. Understanding juniper seed dispersal is vital to identifying processes behind increases in density and range. Dispersal of Juniperus seeds has generally been attributed to consumption of female juniper cones (aka, “berries”) by frugivorous birds and mammals, which then defecate seeds after gut passage. However, recent studies have found that scatter-hoarding rodents harvest and cache juniper seeds, and this may constitute a secondary mode of dispersal that accounts for more seedling recruitment than does primary dispersal by frugivores. We considered implications of juniper seed dispersal by frugivorous birds and scatter-hoarding rodents by examining the distribution of western juniper seeds after dispersal by birds from an intact juniper forest into a burned, open area. We surveyed four microsites (juniper canopy, shrub canopy, rock, open) across the forested and burned habitats for bird dispersed seeds during winter and summer 2016. Western juniper seeds were significantly more abundant in winter compared with summer surveys, in the forested compared with the burned habitat, and under juniper canopies compared with other microsites. There was an inverse relationship between number of bird dispersed seeds in each microsite and distance to the forest habitat. We suggest that scatter-hoarding rodents are important to the dispersal process, as they remove seeds from high density microsites such as tree canopies and redistribute them. We consider the relevance of these findings to western juniper forests experiencing infilling and expansion as well as to those impacted by climate-induced mortality. |
