12 research outputs found
Timing of Favorable Conditions, Competition and Fertility Interact to Govern Recruitment of Invasive Chinese Tallow Tree in Stressful Environments
The rate of new exotic recruitment following removal of adult invaders (reinvasion pressure) influences restoration
outcomes and costs but is highly variable and poorly understood. We hypothesize that broad variation in average
reinvasion pressure of Triadica sebifera (Chinese tallow tree, a major invader) arises from differences among habitats in
spatiotemporal availability of realized recruitment windows. These windows are periods of variable duration long enough to
permit establishment given local environmental conditions. We tested this hypothesis via a greenhouse mesocosm
experiment that quantified how the duration of favorable moisture conditions prior to flood or drought stress (window
duration), competition and nutrient availability influenced Triadica success in high stress environments. Window duration
influenced pre-stress seedling abundance and size, growth during stress and final abundance; it interacted with other
factors to affect final biomass and germination during stress. Stress type and competition impacted final size and biomass,
plus germination, mortality and changes in size during stress. Final abundance also depended on competition and the
interaction of window duration, stress type and competition. Fertilization interacted with competition and stress to
influence biomass and changes in height, respectively, but did not affect Triadica abundance. Overall, longer window
durations promoted Triadica establishment, competition and drought (relative to flood) suppressed establishment, and
fertilization had weak effects. Interactions among factors frequently produced different effects in specific contexts. Results
support our âoutgrow the stressâ hypothesis and show that temporal availability of abiotic windows and factors that
influence growth rates govern Triadica recruitment in stressful environments. These findings suggest that native seed
addition can effectively suppress superior competitors in stressful environments. We also describe environmental scenarios
where specific management methods may be more or less effective. Our results enable better niche-based estimates of
local reinvasion pressure, which can improve restoration efficacy and efficiency by informing site selection and optimal
Management
Why pictures look right when viewed from the wrong place
A picture viewed from its center of projection generates the same retinal image as the original scene, so the viewer perceives the scene correctly. When a picture is viewed from other locations, the retinal image specifies a different scene, but we normally do not notice the changes. We investigated the mechanism underlying this perceptual invariance by studying the perceived shapes of pictured objects viewed from various locations. We also manipulated information about the orientation of the picture surface. When binocular information for surface orientation was available, perceived shape was nearly invariant across a wide range of viewing angles. By varying the projection angle and the position of a stimulus in the picture, we found that invariance is achieved through an estimate of local surface orientation, not from geometric information in the picture. We present a model that explains invariance and other phenomena (such as perceived distortions in wide-angle pictures)