Abstract
To elucidate changes in the defensive traits of tree seedlings under global environmental changes, we evaluated foliar defensive traits of the seedlings of successional trees, such as beech, oak, and magnolia grown in a natural-light phytotron. Potted seedlings were grown under the combination of two CO2 concentrations (360 vs. 720 ppm) and two nitrogen (N) treatments (4 vs. 15 kg N ha−1 yr−1) for two growing seasons using quantitative chemical analyses and anatomical method. We hypothesized that the effects of CO2 and N depend on the successional type, with late successional species providing greater defense of their leaves against herbivores, as this species exhibits determinate growth. Beech, a late successional species, responded the most to both elevated CO2 concentration (eCO2) and high N treatment. eCO2 and low N supply enhanced the defensive traits, such as the high leaf mass per area (LMA), high carbon to N ratio (C/N ratio), and increase in the concentrations of total phenolic and condensed tannin in agreement with the carbon–nutrient balance (CNB) hypothesis. High N supply decreased the C/N ratio due to the high N uptake in beech leaves. Oak, a mid–late successional species, exhibited different responses from beech: eCO2 enhanced the LMA, C/N ratio, and concentration of total phenolics of oak leaves, but only condensed tannin increased under high N supply. Magnolia did not respond to all treatments. No interactive effects were observed between CO2 and N supply in all species, except for the concentration of total phenolics in oak. Although the amounts of phenolic compounds in beech and oak varied under eCO2 and high N treatments, the distribution of these compounds did not change. Our results indicate that the changes in the defensive traits of forest tree species under eCO2 with N loading are related to the successional type.
Highlights
The C/N ratio increased under elevated CO2 concentration (eCO2) treatment and decreased under high N treatment in both beech and oak (Figure 2c)
Our results indicated that beech seedlings grown under eCO2 produced low-quality foliage for herbivorous insects due to the increase in the toughness, C/N ratio, and concentrations of total phenolic and condensed tannin irrespective of the decrease in the
Our study revealed that the effects of eCO2 and high N on the defensive traits of successional, deciduous, broad-leaved tree species vary among species and are related to the successional type
Summary
CO2 concentration and increases in nitrogen (N) deposition due to high human activities, have raised concern about changes in plant–insect relationships, as these environmental factors modify the defensive traits of leaves through physiological changes in plants [1,2]. These environmental changes will induce a new relationship between insect and woody plants in the mixed deciduous broad-leaved forest based on the free air elevated CO2 experiment [3]. The responses of plant defense to environmental changes have been frequently explained by the well-known carbon–nutrient balance (CNB) hypothesis [4].
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