Abstract
Abstract. Isoprene (Is) emissions by plants represent a loss of carbon and energy resources leading to the initial hypothesis that fast growing pioneer species in secondary tropical forests allocate carbon primarily to growth at the expense of isoprenoid defenses. In this study, we quantified leaf isoprene and methanol emissions from the abundant pantropical pioneer tree species Vismia guianensis and ambient isoprene concentrations above a diverse secondary forest in the central Amazon. As photosynthetically active radiation (PAR) was varied (0 to 3000 µmol m−2 s−1) under standard leaf temperature (30 °C), isoprene emissions from V. guianensis increased without saturation up to 80 nmol m−2 s−1. A nonlinear increase in isoprene emissions with respect to net photosynthesis (Pn) resulted in the fraction of Pn dedicated to isoprene emissions increasing with light intensity (up to 2 % of Pn). Emission responses to temperature under standard light conditions (PAR of 1000 µmol m−2 s−1) resulted in the classic uncoupling of isoprene emissions (Topt, iso > 40 °C) from net photosynthesis (Topt, Pn = 30.0–32.5 °C) with up to 7 % of Pn emitted as isoprene at 40 °C. Under standard environmental conditions of PAR and leaf temperature, young V. guianensis leaves showed high methanol emissions, low Pn, and low isoprene emissions. In contrast, mature leaves showed high Pn, high isoprene emissions, and low methanol emissions, highlighting the differential control of leaf phenology over methanol and isoprene emissions. High daytime ambient isoprene concentrations (11 ppbv) were observed above a secondary Amazon rainforest, suggesting that isoprene emissions are common among neotropical pioneer species. The results are not consistent with the initial hypothesis and support a functional role of methanol during leaf expansion and the establishment of photosynthetic machinery and a protective role of isoprene for photosynthesis during high temperature extremes regularly experienced in secondary rainforest ecosystems.
Highlights
Due to its vast territorial expansion, high species diversity, and long growing season, the Amazon forest in South America is responsible for an estimated 15 % of global terrestrial photosynthesis (Malhi et al, 2008)
In order to investigate the possibility that the highly abundant pantropical pioneer species V. guianensis dedicates a significant fraction of Pn to volatile isoprenoid emissions to the atmosphere, we first conducted controlled light experiments on intact branches in the field using a new portable photosynthesis and volatile organic compound emission system based on thermal desorption gas chromatograph/mass spectrometer system (GC-MS) (Jardine et al, 2015)
The results show that during the 2014 rainy season in the central Amazon (13 May 2014), light stimulation of Pn up to 15 μmol m−2 s−1 in young mature leaves was associated with Is emissions, which continued to increase with light up to the maximum photosynthetically active radiation (PAR) intensity (2000 μmol m−2 s−1) where emission rates were 30 nmol m−2 s−1
Summary
Due to its vast territorial expansion, high species diversity, and long growing season, the Amazon forest in South America is responsible for an estimated 15 % of global terrestrial photosynthesis (Malhi et al, 2008). Jardine et al.: Methanol and isoprene emissions from Vismia guianensis a long-term trend of increased mortality-driven shortening of carbon residence times in the Amazon forest (Brienen et al, 2015). This effect has been attributed to increased climate variability, as recurrent drought episodes occurred in the region (Phillips et al, 2009; Lewis et al, 2011). If tropical forests are becoming more dynamic, gap-phase processes can play a more central role in determining carbon residence times, which have been described as the largest uncertainty in terrestrial vegetation responses to climate and elevated CO2 (Friend et al, 2014)
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