Climate Change

How will climate change affect carbon sequestered in tropical forests?

New study emphasises not just species diversity but also geodiversity

By Kapil Subramanian
Published: Thursday 19 March 2020

Trees have an iconic status in the fight against climate change and in the environmental movement in general. As sacked United Nations Conference of Parties (UN CoP) president Claire Perry O’Neill put it in New Delhi: “Don’t we all love to plant a tree”.

Nature-based solutions are key to the 26th Conference of Parties to the United Nations Framework Convention on Climate Change (Glasgow action plan), as well as its strategy to manage public expectations by shifting the focus from rapid decarbonisation.

But how much carbon can forests suck up? What is the mitigation potential of forests?

In 2019’s special report on climate change and land, the Intergovernmental Panel on Climate Change (IPCC) estimated that the mitigation potential from reducing deforestation and land degradation — while highly uncertain — was at best about a sixth of annual energy sector emissions alone.

The IPPC further concluded that forests “do not continue to sequester carbon indefinitely”, noting that any sequestration gains are “at risk from future loss (or sink reversal) triggered by disturbances such as flood, drought, fire, or pest outbreaks, or future poor management.”

The world has seen a spectacular example of these risks in Brazil and Australia in recent months.

Evidence of the uncertainty of estimates of carbon stored in forests is mounting.

A study last year by the University of Queensland found greenhouse gas emissions from damage to tropical rainforests are being underestimated by a factor of six.

Even more worryingly, a study by the University of Leeds published earlier in March concluded that tropical forests were losing their ability to sequester carbon and may soon become net emitters, rather than net sinks.

How will climate change itself impact tropical forests and the carbon stored in them?

Traditional large-scale projections are limited, as remote sensing techniques integrate over large areas and ignore spatial heterogeneity.

Hence, they fail to accurately represent ecosystem processes crucial to carbon sequestration.

A new study by the Institute for Applied Systems Analysis (IAASA) published on March 19, 2020 in journal Scientific Reports attempts to shed light on this heterogeneity and its impact on ecosystems functioning.

This includes carbon sequestration by focussing on fifteen undisturbed, lowland tropical forests located at the Pacific slope of southwestern Costa Rica.

It is often emphasised that diverse forests store more carbon. The new study, however, confirms previous findings that this approaches a plateau in tropical forest with high species diversity, though species diversity may be crucial for other environmental reasons.

The actual composition of the mix of species was found equally important.

Species diversity and species composition are governed by climatic factors — affecting not just temperatures but also water availability — and soil type.

Soils may themselves evolve with climate change as a result of weathering due to rainfall patterns.

The study found ecosystem functioning was affected not just by biological diversity but also by a diversity of abiotic factors, which the paper terms geodiversity.

The study found that plant species diversity was indirectly controlled by climatic factors through their effect on water availability.

Plant species richness decreased with mean annual temperature and more so with increasing temperature variation.

Carbon storage in vegetation increased mean wood density and soil phosphorus content, but decreased with increasing climatic water deficit.

The findings are sobering for the prospects of mitigating climate change, given they indicate ecosystem functioning including carbon sequestration could be more adversely affected by climate change than previously thought.

A decrease in forest carbon sinks could further contribute to climate change which in turn could further decrease carbon sinks and so on.

Florian Hofhansl — one of the authors of the study — emphasised, however, that more research was needed since this took into account a small region.

Research needed the involvement of multiple scientific disciplines, including not just botany and plant ecology — but also geology — for a fuller picture.

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