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Tropische gebergtebossen en klimaatsverandering: Een gemiste kans?
Stijn
Bruneel
Voedseldriehoek van het bos
Tropische bossen worden misleidend bestempeld als de “longen” van de planeet. Naamgeving is immers een privilege weggelegd voor de dominante soort van de Aarde, die zich maar wat graag als de referentie beschouwd. Bossen leggen namelijk net koolstofdioxide (CO₂) vast en produceren daarbij zuurstof (O₂), terwijl longen juist het tegenovergestelde doen. En toch biedt de vergelijking met vitale organen, de menselijke arrogantie even terzijde, een kader om bossen beter te begrijpen. Net zoals mensen afhankelijk zijn van voedingsstoffen, water en klimaat voor hun groei en ontwikkeling, zo zijn ook bossen afhankelijk van hun omgeving. Om in te schatten hoe bossen zullen omgaan met klimaatsverandering en de verwachte verhoogde CO₂ concentraties is er dringend nood aan een soort “voedseldriehoek” voor bossen. Zullen bomen een sterkere groei vertonen door de grotere atmosferische CO₂-concentraties of zullen ze gelimiteerd worden door andere factoren in hun omgeving? Om hierop een antwoord te vinden trok ik samen met een team studenten en vrijwilligers tot diep in het “hart” van de tropische bossen van het Andesgebergte van Ecuador.
Tropische bossen staan in voor 55% van de boskoolstofopslag (bos-C-opslag), maar worden wereldwijd bedreigd. Ecuador bijvoorbeeld heeft de tweede hoogste graad van ontbossing binnen Zuid-Amerika met een jaarlijks verlies van 0.6% bos. Het alarmerende tempo waarin tropisch bos verdwijnt in combinatie met het beperkte inzicht in het functioneren van deze ecosystemen dwingen ons tot meer onderzoek. Binnen het beperkte onderzoek rond tropische bossen ligt de focus voornamelijk op laaglandbossen zoals het Amazonewoud, terwijl gebergtebossen zowel letterlijk als figuurlijk in nevel verhuld blijven. Een even belangrijke motivatie om te focussen op het Andesgebergte zijn de uitgesproken hoogteverschillen van de bossen. Een hoogtegradiënt is immers een ideaal openlucht-laboratorium om op natuurlijke wijze de invloed van de omgeving te achterhalen. Meer specifiek om de rol van voedingstoffen (“nutriënten” in de plantenwereld) zoals stikstof (N) en fosfor (P) in tropische bossen te bepalen.
Hoogtegradiënten als openlucht-laboratoria
Wanneer je een berg beklimt word je geconfronteerd met graduele veranderingen in de vegetatie. Niet alleen worden de bomen kleiner, meer gedrongen en al maar meer overdekt door mossen, maar ook de soortensamenstelling past zich aan. Het is de shift in omgevingscondities die een gelijkaardige shift in plantengemeenschappen triggert met soorten die zijn aangepast aan de condities op een bepaalde hoogteligging. De belangrijkste verandering op grotere hoogte is de lagere temperatuur, die ervoor zorgt dat afbraakprocessen vertragen waardoor de nutriënten in afgevallen bladeren en ander organisch materiaal langzamer worden vrijgesteld. Hierdoor ontstaat er een opstapeling van de nutriënten op de bosbodem, die onbeschikbaar zijn voor plantengroei. Behalve de temperatuur zijn er natuurlijk ook andere factoren die de nutriëntenvoorraad kunnen beïnvloeden zoals de inkomende UV-straling, nevel, etc.
We verwachtten dus dat met stijgende hoogte de hoeveelheid aan planten-beschikbare nutriënten daalt en dat bomen als respons meer gaan investeren in de wortels ten koste van de bovengrondse delen zoals de stam, takken en bladeren. Bomen moeten immers een groter volume bodem doorzoeken om de nodige hoeveelheid nutriënten te verzamelen die de “voedseldriehoek” ze voorschrijft. Bomen worden echter niet enkel gelimiteerd door nutriënten, ook water, licht, etc. kunnen limiterend werken. Onze hypothese was dat er een daling is in de bovengrondse koolstofopslag met stijgende hoogte en dat deze daling te wijten is aan een shift van licht- naar nutriëntenlimitatie met stijgende hoogte. Als bomen weinig licht krijgen, investeren ze sterk in stam, takken en bladeren om het licht optimaal te benutten voor fotosynthese terwijl bij nutriëntenlimitatie bomen eerder zullen investeren in wortelontwikkeling.
Onze resultaten stonden echter lijnrecht tegenover onze vooropgestelde hypothese: De hoogste en laagste bovengrondse koolstofopslag waren immers te vinden op de respectievelijk hoogste en laagste hoogteliggingen. In een eerste poging om dit onverwachte resultaat te verklaren, bogen we ons over de experimentele setup van ons onderzoek. Enerzijds werd de bovengrondse koolstofopslag ingeschat met behulp van bestaande empirische relaties die de biomassa van bomen linken aan diameter-, hoogte- en houtdensiteitmetingen. Op die manier konden we op een relatief eenvoudige manier een inschatting maken van de bovengrondse biomassa, maar blijven we onwetend over de fout op onze inschattingen. Anderzijds worden deze gebergtebossen gekenmerkt door een uitgesproken dynamiek te wijten aan erosie en windval die binnen een relatief klein gebied aanleiding kan geven tot aanzienlijke verschillen in bosontwikkeling en koolstofopslag.
Nutriënten als hoofdrolspelers
Deze onzekerheden zijn echter onvoldoende om de relatief grote verschillen in bovengrondse koolstofopslag te verklaren. Daarvoor moesten we op zoek naar een dieperliggende oorzaak, zoals de nutriëntenconcentraties van de bodem. De belangrijkste nutriënten binnenin het bosecosysteem zijn stikstof en fosfor, die samen met koolstof in vaste verhoudingen worden opgenomen door bomen. Een belangrijke vaststelling was de shift in nutriëntenlimitatie van fosfor-limitatie naar stikstof-limitatie met stijgende hoogte, te wijten aan de verschillende oorsprong en gedrag van stikstof en fosfor. De hoge temperaturen in het laaglandbos promoten stikstofmineralisatie, waardoor meer stikstof wordt vrijgesteld voor plantengroei. Daarenboven zijn de bodems typisch ouder in laaglandbos waardoor de accumulatie van stikstof via atmosferische depositie verder gevorderd is. Daartegenover staan de jongere bodems van het gebergtebos, die hun beperkte ouderdom te danken hebben aan landverschuivingen en vulkanisme. Doordat fosfor almaar minder beschikbaar wordt voor planten doorheen de tijd, vinden we in deze relatieve jonge bodems grotere hoeveelheden aan planten-beschikbare fosfor terug. Samengevat, lijkt de hoge planten-beschikbaarheid van fosfor op grotere hoogte een belangrijke rol te spelen in de bovengrondse koolstofopslag van deze tropische bossen.
De verwachte temperatuurstijging zal de afbraak van organisch materiaal versnellen en daarenboven zullen ontbossingen en veeteelt aanleiding geven tot verhoogde CO₂-uitstoot en verhoogde atmosferische depositie. Inzicht in de nutriëntendynamiek van tropische bossen is daarom onmisbaar om de toekomst van deze fragiele ecosystemen in te schatten. Daarnaast geven onze resultaten aan dat er een mogelijke onderschatting is van de koolstofopslag in tropische gebergtebossen. Herbebossingen van verwaarloosde gebergtegraslanden kunnen daarom een belangrijke bijdrage leveren in de mitigatie van klimaatsverandering via koolstofopslag en tegelijkertijd lokale boeren van een duurzaam inkomen voorzien. Voldoende argumenten om in te zetten op een integrale bescherming van natuurlijke bossen, oprichting van diverse herbebossingsprojecten en onderzoek dat beiden combineert.
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