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 (~ 30 cm) as compared to longer (~ 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.