Spruce resins constitute a strong sink for methane (CH4)
Authors: Machacova Katerina , Schindler Thomas , Mander Ülo , Soosaar Kaido 
Published: March 2020

&lt;p&gt;Woody plants are known to emit methane (CH&lt;sub&gt;4&lt;/sub&gt;) as an important greenhouse gas into the atmosphere. Recent studies show that tree stems might be also sinks for CH&lt;sub&gt;4&lt;/sub&gt;; however, the mechanisms of CH&lt;sub&gt;4&lt;/sub&gt; uptake and its fate are unknown. Norway spruce (&lt;em&gt;Picea abies)&lt;/em&gt; is characterised as negligible CH&lt;sub&gt;4 &lt;/sub&gt;source in boreal forests. Even though spruce trees have been widely planted for its wood in large-scale monocultures in European temperate forests, no studies have focused on their CH&lt;sub&gt;4&lt;/sub&gt; exchange potential in the temperate zone.&lt;/p&gt;&lt;p&gt;We determined stems of Norway spruce growing in a temperate zone aiming to find out whether the tree stems exchange CH&lt;sub&gt;4&lt;/sub&gt; with the atmosphere and how they contribute to the forest trace gas exchange.&lt;/p&gt;&lt;p&gt;The measurements were performed at the experimental station of the &amp;#8216;Kranzberg Forest Roof Experiment&amp;#8217; near Freising, Germany, in June 2019. Fluxes of CH&lt;sub&gt;4&lt;/sub&gt; in mature tree stems were measured using non-steady-state stem chamber systems (n=32) installed in stem vertical profile approx. two weeks prior to measurements using a portable greenhouse gas analyser. Moreover, resins sampled from spruce stems were investigated for their CH&lt;sub&gt;4&lt;/sub&gt; exchange potential. Control measurements were performed to ensure that the fluxes do not originate from used chamber materials, in particular silicones used for chamber installation.&lt;/p&gt;&lt;p&gt;Our preliminary results show that the spruce stems can be a strong sink for CH&lt;sub&gt;4&lt;/sub&gt; (-0.288 &amp;#177; 0.053 mg CH&lt;sub&gt;4&lt;/sub&gt; m&lt;sup&gt;-2&lt;/sup&gt; stem area h&lt;sup&gt;-1&lt;/sup&gt;, mean &amp;#177; s.e.), even if a small amount of resin is present on the bark. The stems exuded resins to different extent (covering 4.8 &amp;#177; 1.3% of the stem surface area in chambers), partly as a result of smoothening of rough surface layers of dead bark for chamber installation. However, even spruce stems without obvious &amp;#8220;injuries&amp;#8221; released small amounts of resins for unknown reasons (response to drought, bark-beetle attack, etc.?). The incubated resin samples consistently consumed CH&lt;sub&gt;4&lt;/sub&gt; (-12.0 &amp;#177; 1.7 mg CH&lt;sub&gt;4&lt;/sub&gt; m&lt;sup&gt;-2&lt;/sup&gt; resin area h&lt;sup&gt;-1&lt;/sup&gt;). Moreover, the detected stem CH&lt;sub&gt;4&lt;/sub&gt; uptake negatively correlated with the resin occurrence in the stem chambers (R&amp;#178; = 0.884). After re-calculation of the stem fluxes to resin area, the CH&lt;sub&gt;4&lt;/sub&gt; consumption rates of stems and resin samples were in the same order of magnitude at median level (-13.2 and -12.0 mg CH&lt;sub&gt;4&lt;/sub&gt; m&lt;sup&gt;-2&lt;/sup&gt; resin area h&lt;sup&gt;-1&lt;/sup&gt;, resp.).&lt;/p&gt;&lt;p&gt;Concluded, the spruce resins appear to be a very strong and until now undiscovered sink for CH&lt;sub&gt;4&lt;/sub&gt;. Even one small droplet of resins on bark can turn the known negligible CH&lt;sub&gt;4&lt;/sub&gt; exchange of intact spruce stems into strong CH&lt;sub&gt;4&lt;/sub&gt; sinks, having thus severe impact on the overall forest CH&lt;sub&gt;4&lt;/sub&gt; balance. This consumption potential of fresh resins should be considered by estimation of forest ecosystem CH&lt;sub&gt;4&lt;/sub&gt; balance especially in areas, where resin bleeding is widely spread or is to be expected (bark-beetle areas, drought events, tree harvest, clear-cutting).&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;em&gt;Acknowledgement&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&lt;em&gt;This research was supported by the Czech Science Foundation (17-18112Y) and National Sustainability Program I (LO1415). We thank Prof. Thorsten Grams for all his kind support, and Jan Hrdli&amp;#269;ka and Thomas Feuerbach for their technical support.&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;