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

Abstract

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 m 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 minimized 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 among 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 min-lightsaturated 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. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Is photosynthetic enhancement sustained through three years of elevated CO2exposure in 175-year oldQuercus robur?

“…Measurement on detached branches were conducted no longer than 45 minutes after collection. Previous studies investigating measurements of gas exchange on severed or attached branches found no significant differences between the two methods (Bader et al, 2016; Verryckt et al, 2020). A–C i curves were measured at a Q of 1800 μmol m -2 s -1 (in 2015 and 2019) or 1200 μmol m -2 s -1 (in 2017) and at a leaf temperature of 25 °C.…”

Non-structural carbohydrate concentrations in woody organs, but not leaves, of temperate and tropical tree angiosperms are independent of the ‘fast-slow’ plant economic spectrum

“…Photosynthetic capacity by mass (Amass, nmol CO2 g -1 s -1 ): Photosynthetic capacity was measured on six leaves from two sun-lit branches in both tropical forest sites using a LI-6400 portable photosynthetic system (LI-COR, Lincoln, NE, USA). Branches for photosynthesis determination were placed in a bucket of water during the measurements to avoid disruption of water transport within the xylem (Verryckt et al, 2020). The photosynthetic capacity under saturating light (Amax) was measured at 2000 µmol m -2 s -1 .…”

Rapid estimation of photosynthetic leaf traits of tropical plants in diverse environmental conditions using reflectance spectroscopy

“…Branches were excised from the trees before dawn, immediately put in buckets filled with water and re-cut under water at least 20 cm up from the original cut, toward the tip of the branch to avoid cavitation as described previously [ 38 , 43 ]. Importantly, measurements made on cut branches show no statistically significant differences from in situ measurements on intact branches when precautions are taken to wait for acclimation and to avoid cavitation [ 44 – 46 ]. Gas exchange measurements and reflectance spectra were measured in a nearby shaded area.…”