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Case Study 2 Neighborhood Models of the Allelopathic Effects of an Invasive Tree Species Gómez-Aparicio, L. and C. D. Canham Neighborhood analyses of the allelopathic effects of the invasive tree Ailanthus altissima in North American forests. Journal of Ecology 96:

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Lorena Gómez-Aparicio (Instituto de Recursos Naturales y Agrobiología, Sevilla, Spain) Tree of heaven (Ailanthus altissima) Introduced from China in 1784 The cast of characters…

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Neighborhood Effects of Canopy Trees on Ecosystem Properties Gómez-Aparicio, L. and C. D. Canham Neighborhood models of the effects of invasive tree species on ecosystem processes. Ecol. Monogr. 78:69-86 Estimated footprint of a 30 cm DBH Tree of Heaven… Bottom line: Ailanthus increases soil fertility relative to background effects of the native tree species…

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Allelopathic effects of Ailanthus l Direct effects of the invasive species on nutrient availability are not the whole story…. l Lab studies have isolated an allelopathic exudate from Ailanthus (ailanthone) l Could allelopathy by Ailanthus negate any positive effects of the species on soil N and Ca? l Will the magnitude of the allelopathic effect vary for different species of tree seedlings? Gómez-Aparicio, L. and C. D. Canham Neighborhood analyses of the allelopathic effects of the invasive tree Ailanthus altissima in North American forests. Journal of Ecology 96:

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Basic field methods l Select 20 locations in each of 3 sites with a range of abundance of A. altissima within the immediate neighborhood (and map the exact locations of those trees relative to the sample locations) l Two quadrats at each location, one with activated carbon mixed into the soil l Plant a seedling of each of three native tree species into each quadrat

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Statistical model Response = site j * size * Ailanthus effect (A) where t = treatment (activated carbon or control), and can be positive (facilitation) or negative (inhibition). where DBH and distance are the size and distance to neighboring Ailanthus… NOTE: separate models were fit using either = 0 or = 2

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Alternate Models l Our “null” model: Set Ailanthus effect to 1, and just fit a model for site and plant size effects… l An alternate model: test whether the magnitude of allelopathic effects was site specific: Response = site j * size Response = site j * size * Ailanthus effect (A) But now, varies as a function of both treatment and site…

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The error term and PDF l Error terms varied depending on the response variable - Survival: logistic regression (more later…) - Seed emergence: binomial - Growth: normally distributed, but with variance a power function of the mean Note: estimates of for the 3 seedling species were ~ 1.5

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Model Comparison (as Hypothesis Tests)

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Parameter Estimates

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Ailanthus neighborhood index (ANI) Shapes of the effective allelopathic footprint of Ailanthus for the 3 native seedling species The implications of alpha ( ): models with = 0 had the highest likelihood (and lowest AIC). Thus, the density of Ailanthus (stems > 2 cm DBH) was more important than their cumulative biomass…

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Proportionate Change allelopathy without allelopathy Responses of native tree seedlings to Ailanthus allelopathy… Red oak (Quercus rubra) Allelopathy shifts the neighborhood effect from negative to downright nasty… Solid circles: activated carbon (no allelopathy); Open circles: control

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Proportionate Change Ailanthus Neighborhood Index (ANI) Sugar maple (Acer saccharum) Allelopathy shifts the neighborhood effect from neutral to negative… Solid circles: activated carbon (no allelopathy); Open circles: control allelopathy without allelopathy

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Proportionate Change Ailanthus Neighborhood Index (ANI) Red maple (Acer rubrum) Allelopathy shifts the neighborhood effect from strongly positive to neutral… Without allelopathy

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