Twig abscission in trembling aspen: Ecological causes and responses to shading and pruning.

摘要

Many shade-intolerant tree species abscise twigs and branches, but no study to date has directly examined the causes of abscission or evaluated the adaptive value of the behavior I hypothesize that unproductive twigs are selectively abscised and that this helps trees to maintain high growth rates: by annually abscising unproductive twigs, whole-tree growth rates are maximized because resources are allocated only to productive twigs. I tested whether light availability, the condition of the terminal bud and several factors affecting the relative sink strength of twigs are used as cues for abscission. I also tested the prediction that twig abscission has a positive effect on the growth of the remaining twigs. I studied nine natural clones of trembling aspen (Populus tremuloides Michx.) in southeastern lower Michigan.;In a 2-year study of twigs on branches in 3 clones, I followed the initiation, growth, light environments, bud conditions and fates of twigs to identify likely cues for abscission. Logistic regression analyses showed that twigs were significantly more likely to abscise if they experienced lower light environments, were initially smaller, experienced lower growth rates, or had stunted or damaged terminal buds. These results support the hypothesis that twigs with low productivity are being abscised. Twigs with stunted buds were also from lower light environments, indicating that abscission of twigs in shaded positions is mediated in part by the effects of light on bud development.;I tested directly the roles of light quantity and quality in causing twig abscission in a 1-year shading experiment, in which I shaded branches in 10 clones and observed the growth, buds, and fates of twigs. Treatments included no-shade, clear plastic, neutral shade, and green plastic (to simulate the shade of vegetation). Neutral and green shade allowed 5% transmission of incident light; in addition, the green shade reduced the red:far-red ratio to 0.1. The quantity, but not the quality, of light was important in determining twig fate: abscission rates were doubled under shade, but were similar under the neutral and green shade treatments. However, by affecting the development of buds, light quality may indirectly lead to abscission: stunted buds were 50% more frequent under green shade than under the other treatments, and twigs with stunted buds are more likely to be abscised.;Finally, I tested the hypothesis that removal of some twigs benefits remaining twigs by reducing intrabranch competition for resources. Pruning half of the twigs from branches before bud-expansion began in the spring significantly increased the growth of remaining twigs by the end of the growing season. Pruning in mid-season improved the water status of the remaining twigs in the year of pruning, but did not increase their growth. Thus, natural seasonal abscission has the potential to increase whole-tree growth rates, whereas abscising twigs during periods of stress primarily minimizes losses by maintaining, rather than improving, the growing conditions of the remaining twigs.