Exploration of complex interactions among atmoshperic CO2, foliar quality, insect herbivores and decomposition processes
Hall, Myra Carmen
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Atmospheric CO2 concentrations have increased exponentially over the last century and continuing increases are expected to have significant effects on ecosystems. We investigated the interactions among atmospheric CO2, foliar quality, litter quality, herbivory and decomposition within a scrub oak community. Sixteen plots of open-top chambers were followed, eight at ambient levels of CO2 and eight at elevated levels of CO2 . To assess the effects of CO2 on foliar quality and herbivory we focused on three oak species, one nitrogen fixing legume and six different insect herbivore feeding guilds. Plant species differed in their relative foliar chemistries over time, however, the only consistent differences were higher nitrogen concentrations and lower C:N ratios in the nitrogen fixer when compared to the oak species. Under elevated CO2, damage by herbivores decreased for four of the six insect groups investigated. Overall declines in both foliar quality and herbivory under elevated CO2 treatments suggest that damage to plants may decline as atmospheric CO2 levels continue to rise. To define links between foliar quality, herbivory and litter quality we focused on three dominant oak species and three herbivore damage categories. We found variation in litterchemistry associated with CO2 and herbivory. However, changes in litter chemistry from year to year were far larger than effects of CO2 or insect damage, suggesting that these may have only minor effects on litter decomposition. To assess the influence of litter quality on decomposition and the influence of herbivory on litter quality along with subsequent decomposition we performed two three-year decomposition experiments. We found that despite variation in litter quality associated with CO2, herbivory and their interaction there was no subsequent effect on rates of decomposition under ambient atmospheric conditions. However, the chamber in which the decomposition took place resulted in significant differences in decomposition rates. Litter decomposing under elevated CO2 decomposed more rapidly than litter under ambient CO2, and exhibited higher rates of mineral nitrogen accumulation suggesting that the atmospheric conditions during the decomposition process have a greater impact on rates of decomposition and nitrogen cycling than do the atmospheric conditions under which the foliage was produced.