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 from cut and uncut canopy branches three deciduous hardwoods - sugar maple (Acer saccharum Marsh.), tulip poplar (Liriodendron tulipifera L.), white oak (Quercus alba L.). Across all species, immediately reduced photosynthesis stomatal conductance by 27-62% relative pre-excision rates. oak, which had particularly long (> 100 cm) vessels, gas exchange was more impaired shorter (~ 30 as compared longer branches. Additional hypotheses linking branch height species water-use strategy bias were tested but not confirmed. A survey 23 previously published studies confirms that our results are precedent. Excision-related biases should be considered when interpreting performed