Performing Gas-Exchange Measurements On Excised Branches - Evaluation And Recommendations

In forest canopies, it is common to perform leaf-level gas-exchange measurements on recently excised branches, often without testing for excision-related biases. We conducted a formal test of excision effects using gas-exchange measurements from cut and uncut canopy branches of three deciduous hardwoods - sugar maple (Acer saccharum Marsh.), tulip poplar (Liriodendron tulipifera L.), and white oak (Quercus alba L.). Across all species, excision immediately reduced photosynthesis and stomatal conductance by 27-62% relative to pre-excision rates. In white oak, which had particularly long (> 100 cm) vessels, gas exchange was more impaired for shorter (similar to 30 cm) as compared to longer (similar to 100 cm) branches. Additional hypotheses linking branch height and species water-use strategy to excision bias were tested but not confirmed. A survey of 23 previously published studies confirms that our results are not without precedent. Excision-related biases should be considered when interpreting measurements performed on excised branches.