Photosynthesis and water-use efficiency of sugar maple (Acer saccharum) in relation to pear thrips defoliation

An experimental introduction of pear thrips (Taeniothrips inconsequens Uzel), a major defoliator in sugar maple (Acer saccharum Marsh.) forests in northeastern North America, was conducted in a field plantation to determine if compensatory, gas exchange occurs in response to feeding damage by this piercing-sucking insect. Sugar maple trees were enclosed in netting (167 micrometer mesh) and pear thrips adults were introduced before leaf expansion in the spring. Pear thrips reduced whole-tree leaf area by approximately 23% and reduced leaf size (both mass and area) by 20% in the upper crown. Measurements of net CO2 assimilation rate (Anet) and stomatal conductance were made on tagged foliage that was later analyzed for stable carbon isotope composition (delta 13C) to provide estimates of short- and long-term leaf water use efficiency (WUE). Pear thrips feeding reduced Anet for fully expanded leaves by approximately 20%, although leaf chlorophyll content and leaf mass per unit area were apparently not affected. Comparison of Anet, gs, instantaneous WUE and leaf delta 13C between damaged and control trees as well as visibly undamaged versus moderately damaged foliage on pear thrips-infested trees indicated that there were no effects of pear thrips feeding damage on WUE or leaf delta 13C. Long-term WUE among sugar maple trees in the field plantation, indicated by leaf delta 13C analysis, was related to shorter-term estimates of leaf gas exchange behavior such as gs and calculated leaf intercellular CO2 concentration (Ci). We conclude that pear thrips feeding has no effect on leaf WUE, but at the defoliation levels in our experiment, it may reduce leaf Anet as a result of direct tissue damage or through reduced gs. Therefore. even small reductions in leaf Anet by pear thrips feeding damage may have an important effect on the seasonal carbon balance of sugar maple when integrated over the entire growing season.