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Abstract:
Eucalyptus plantations occupy approximately 10 million ha of land in the tropics and, increasingly, afforestation and reforestation projects are relying on this genus to provide rapid occupation of degraded sites, large quantities of high-quality wood products, and high rates of carbon sequestration. Members of the genus Eucalyptus are also very high emitters of isoprene, the dominant volatile organic compound emitted by trees in tropical ecosystems, which significantly influences the oxidative capacity of the atmosphere. While fertilization growth response of these trees has been intensively studied, little is known about how fertilization and tree age alter isoprene production from plantations of these trees. Here we examined the effects of fertilization and tree age on leaf-level isoprene flux from 2- and 6-year-old trees in a Eucalyptus saligna experimental forest in Hawaii. Leaf-level emission at a given canopy height did not differ between fertilized and unfertilized 6-year-old trees likely because leaf nitrogen content did not vary with fertilization. Across treatments, however, the standardized emission rate of isoprene (emission at a standard light and temperature) followed patterns of leaf N content and declined with canopy depth. Although leaf nitrogen content was similar between 2-year and 6-year fertilized trees, leaf-level emission rates declined with stand age. Surprisingly, despite differences in stand leaf area and leaf area distribution, modeled canopy-level isoprene flux was similar across stands varying in fertilization and tree age. Model results suggest that leaf area index was high enough in all treatments to absorb most of the light penetrating the canopy, leading to similar canopy flux rates despite the very different sized canopies.
