Seven-year-old ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) saplings and one- and two-year-old ponderosa pine seedlings of a Sierra Nevada and a Rocky Mountain seed source, respectively, were exposed to CO(2)-enriched atmospheres in an outdoor open-top chamber facility for 2.5 years. Seedling growth (main stem diameter, height, volume) increased with increasing CO(2) concentration, though the two populations exhibited different patterns of response. By the beginning of the last growth season, however, the trees under the highest CO(2) concentrations showed signs of stress that included accelerated needle abscision, chlorosis, and apparent alteration of tolerance to heat. The stress response is at least partly attributable to elevated foliar temperatures resulting from CO(2)-induced stomatal closure, which in turn lowered transpirational cooling of needles.

The investigation of the effects of atmospheric pollution on metabolism and growth of woody species has long been hampered by the very difficult task of exposing mature individuals to controlled environments of air pollutants. This paper describes the design, construction, and testing of an alternative tool to whole‐tree enclosures for measuring pollution response in mature woody tissue. The chamber is a new design, though not a new concept, and is referred to as a branch exposure chamber. Designed primarily for ozone and acid precipitation exposures (and used additionally for CO2 measurements), the branch exposure chamber incorporates four major parts: support structure, fan‐air supply unit, charcoal filter unit, and exposure chamber. The exposure chamber is a 1.5‐m long by 0.7‐m diam. cylinder. The chamber is constructed of Teflon sheeting stretched over an aluminum frame; the aluminum frame is totally wrapped with nonreactive, aluminum‐backed Teflon tape. Three zones in the chamber affect exposure of the experimental tissue: an initial buffer region for mixing, a main exposure region, and an exhaust frustrum. Aerodynamic testing of the chamber‐mixing characteristics show that mixing is uniform and complete within the main exposure region. Thermal buildup within the chamber was a maximum of 3 °C under a wide range of ambient meteorological conditions. Based on current field trials of the chamber, material deterioration due to environmental variables (e.g., ultraviolet radiation, heat oxidants), is not expected to affect operation of the chamber for 24 mo. The BEC is inexpensive to build and operate, and represents a viable alternative to a whole‐tree chamber.

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