Het dagboek van de oudste bomen ter wereld

Janne

De Vreese

In het jaar 536 na Christus verduistert een vulkaanuitbarsting de hemel boven Europa en Azië. De oogsten mislukken en enkele jaren later slaat de pest toe. Meer dan een eeuw lang blijft het in Eurazië kouder dan normaal. De oudste bomen ter wereld, hoog in de bergen van Californië, vertellen een ander verhaal. Ik las hun jaarringen met röntgenstralen en laserlicht. In tegenstelling tot bij ons, bleek de kou in Californië kort en hevig te zijn, maar niet langdurig.

Een zon die verdween

In 536 na Christus wordt het ineens schemerig. Een reusachtige vulkaanuitbarsting heeft zoveel zwavelhoudend stof de lucht in geblazen dat het zonlicht de aarde nauwelijks nog bereikt. Respectievelijk 4 en 11 jaar later volgen nog twee uitbarstingen.

Historische bronnen en klimaatonderzoek koppelen die jaren aan koude zomers, mislukte oogsten en honger in Europa en Azië. Onderzoekers noemen die periode de Late Antieke Kleine IJstijd. Deze duurde een dikke 100 jaar.

Maar hoe zat het aan de andere kant van de oceaan? Dat weten we nauwelijks. Voor Noord-Amerika bestaan er uit die tijd weinig lange, nauwkeurige klimaatarchieven. Zonder die archieven blijft het gissen of een uitbarsting overal even hard toesloeg.

Bomen met een dagboek

Daar komen mijn bomen om de hoek kijken. Bristlecone pine, het weinig gebruikte “borstelden” in het Nederlands, is een dennensoort die hoog in de White Mountains van Californië groeit. Ze worden duizenden jaren oud, en zelfs hun dode hout blijft eeuwenlang bewaard. Een boom die in 536 groeide, kunnen we dus nog steeds lezen.

Zoals algemeen geweten groeit een boom elk jaar één ring bij. Echter, hoe warmer de zomer, hoe dichter en harder het laatste hout van die ring wordt. Een koude zomer laat dus een zachtere ring achter. Je kan een stam op die manier lezen als een jaarlijks weerbericht.

Om die dichtheid te meten, stopte ik staaltjes hout van de bristlecone pine in een speciaal daarvoor ontwikkelde CT-scanner. Voor deze streek bestond er nog geen dichtheidsreeks die de hele periode dekte. Daar mocht ik aan meewerken.

Kort en hevig

Na lange analyses van de scans zijn de drie uitbarstingen duidelijk zichtbaar. Na 536, 540 en 547 duikt de temperatuur telkens kort naar beneden. Daarna herstelt ze snel.

Gemiddeld was de periode zelfs relatief warm, ook vergeleken met de koude periode van de vroege middeleeuwen en met de Romeinse warme periode. De jaren schommelden wel sterk: het ene jaar koud, het volgende jaar weer warm. Zulke wisselingen kunnen voor mensen en ecosystemen bijna even zwaar zijn als aanhoudende kou.

Een klimaatramp ligt niet als een deken van gelijke dikte over de wereld.

Mijn conclusie: een klimaatramp ligt niet als een deken van gelijke dikte over de wereld. In het westen van Noord-Amerika kwam de kou in korte, scherpe vlagen, niet als de aanhoudende vrieskou die Eurazië teisterde.

Blauwe ringen en laserlicht

Soms vriest het vroeg in de herfst, terwijl de laatste cellen van een ring nog aan het verharden zijn. Dat laat een litteken achter: een zogeheten “blue ring”, die onder de microscoop letterlijk blauwig oogt. Die ring bewijst dat de kou laat in het seizoen toesloeg.

Zo herken je een "blue ring"

Rechtsboven een gewone ring, linksonder een "blue ring". De laatste cellen van de blauwe ring kregen door de vroege kou te weinig lignine mee en kleuren daardoor blauwachtig onder de microscoop.

Dat verharden gebeurt dankzij lignine. Stel je cellulose, het materiaal waaruit hout voornamelijk is opgebouwd, voor als een ijzeren wapening en lignine als het beton errond. Zonder voldoende beton blijft de wand zwak.

Tot nu toe beoordeelden onderzoekers blauwe ringen voornamelijk op het oog: wel of niet. Daarom ging ik na of dat ook meetbaar kan. Dat kan met Raman-spectroscopie: je schiet een laser op een celwand, en het teruggekaatste licht verraadt welke moleculen er aanwezig zijn, als een soort chemische vingerafdruk. Die techniek bestaat al langer, maar was nog niet gebruikt om jaar na jaar de lignine in bristlecone pines te meten.

Ik testte het op de jaren 1860 tot 1884, waarvan bekend is dat er blauwe ringen in zitten. Het verschil tussen lignine en cellulose scheidde blauwe jaren duidelijk van gewone jaren. Voor zover we weten, is dit de grootste reeks Raman-metingen op jaarringen tot nu toe.

Wordt vervolgd

Ik wil niet overdrijven. De lasermetingen komen van een beperkt aantal stalen, en op sommige plekken verbrandde de laser het hout, waardoor metingen ontbreken. De volgende stap is hout uit de afgelopen eeuw met de laser te meten. Die metingen kunnen we dan vergelijken met echte thermometermetingen, en dat op meer bomen en meer plaatsen.

Toch is de richting duidelijk. Een boom onthoudt meer dan alleen hoe dik zijn ringen zijn. Met laserlicht kunnen we leren hoe hij zich tegen een late vorstnacht verzette.

De bomen in Californië overleefden de uitbarstingen met een korte huivering. Bijna vijftien eeuwen later fluisteren ze ons nog altijd hoe die jaren voelden.

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