Coping with branch excision when measuring leaf net photosynthetic rates in a lowland tropical forest

Measuring leaf gas exchange from canopy leaves is fundamental for our
understanding of photosynthesis and for a realistic representation of
carbon uptake in vegetation models. Since canopy leaves are often
difficult to reach, especially in tropical forests with emergent trees up
to 60 meters at remote places, canopy access techniques such as canopy
cranes or towers have facilitated photosynthetic measurements. These
structures are expensive and therefore not very common. As an alternative,
branches are often cut to enable leaf gas exchange measurements. The
effect of branch excision on leaf gas exchange rates should be minimised
and quantified to evaluate possible bias. We compared light-saturated leaf
net photosynthetic rates measured on excised and intact branches. We
selected branches positioned at three canopy positions, estimated relative
to the top of the canopy: upper sunlit foliage, middle canopy foliage, and
lower canopy foliage. We studied the variation of the effects of branch
excision and transport amongst branches at these different heights in the
canopy. After excision and transport, light-saturated leaf net
photosynthetic rates were close to zero for most leaves due to stomatal
closure. However, when the branch had acclimated to its new environmental
conditions – which took on average 20 minutes –light-saturated leaf net
photosynthetic rates did not significantly differ between the excised and
intact branches. We therefore conclude that branch excision does not
affect the measurement of light-saturated leaf net photosynthesis,
provided that the branch is recut under water and is allowed sufficient
time to acclimate to its new environmental conditions.