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:

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…

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…

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:

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

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

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…

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

Model Comparison (as Hypothesis Tests)

Parameter Estimates

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…

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

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

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