Climate change - the past as a guide to the future - plants cannot quickly adapt to warmer temperatures

56 million years ago, the Earth suddenly heated up – and many plants stopped working properly

Vera Korasidis, The University of Melbourne and Julian Rogger, University of Bristol

Around 56 million years ago, Earth suddenly got much hotter. Over about 5,000 years, the amount of carbon in the atmosphere drastically increased and global temperatures shot up by some 6°C.

As we show in new research published in Nature Communications, one consequence was that many of the world’s plants could no longer thrive. As a result, they soaked up less carbon from the atmosphere, which may have contributed to another interesting thing about this prehistoric planetary heatwave: it lasted more than 100,000 years.

Today Earth is warming around ten times faster than it did 56 million years ago, which may make it even harder for modern plants to adapt.

Rewinding 56 million years

Plants can help regulate the climate through a process known as carbon sequestration. This involves capturing carbon dioxide from the atmosphere via photosynthesis and storing it in their leaves, wood and roots.

However, abrupt global warming may temporarily impact this regulating function.

Investigating how Earth’s vegetation responded to the rapid global warming event around 56 million years ago – known formally as the Paleocene-Eocene Thermal Maximum (or PETM) – isn’t easy.

To do so, we developed a computer model simulating plant evolution, dispersal, and carbon cycling. We compared model outputs to fossil pollen and plant trait data from three sites to reconstruct vegetation changes such as height, leaf mass, and deciduousness across the warming event.

The three sites include: the Bighorn Basin in the United States, the North Sea and the Arctic Circle.

We focused our research on fossil pollen due to many unique properties.

First, pollen is produced in copious amounts. Second, it travels extensively via air and water currents. Third, it possesses a resilient structure that withstands decay, allowing for its excellent preservation in ancient geological formations.

A shift in vegetation

In the mid-latitude sites, including the Bighorn Basin – a deep and wide valley amidst the northern Rocky Mountains – evidence indicates vegetation had a reduced ability to regulate the climate.

Pollen data shows a shift to smaller plants such as palms and ferns. Leaf mass per area (a measure of leaf density and thickness) also increased as deciduous trees declined. Fossil soils indicate reduced soil organic carbon levels.

The data suggest smaller, drought-resistant plants including palms thrived in the landscape because they could keep pace with warming. They were, however, associated with a reduced capacity to store carbon in biomass and soils.

In contrast, the high-latitude Arctic site showed increased vegetation height and biomass following warming. The pollen data show replacement of conifer forests by broad-leaved swamp taxa and the persistence of some subtropical plants such as palms.

The model and data indicate high-latitude regions could adapt and even increase productivity (that is, capture and store carbon dioxide) under the warmer climate.

A glimpse into the future

The vegetation disruption during the PETM may have reduced terrestrial carbon sequestration for 70,000-100,000 years due to the reduced ability of vegetation and soils to capture and store carbon.

Our research suggests vegetation that is more able to regulate the climate took a long time to regrow, and this contributed to the length of the warming event.

Global warming of more than 4°C exceeded mid-latitude vegetation’s ability to adapt during the PETM. Human-made warming is occurring ten times faster, further limiting the time for adaptation.

What happened on Earth 56 million years ago highlights the need to understand biological systems’ capacity to keep pace with rapid climate changes and maintain efficient carbon sequestration.

Vera Korasidis, Lecturer in Environmental Geoscience, The University of Melbourne and Julian Rogger, Senior Research Associate, School of Geographical Sciences, University of Bristol

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate change - when rainforests are no longer carbon sinks - the Australian example

A crucial store of carbon in Australia’s tropical forests has switched from carbon sink to carbon source

Hannah Jayne Carle, Australian National University; Adrienne Nicotra, Australian National University; David Bauman, Institut de recherche pour le développement (IRD); Michael N Evans, University of Maryland, and Patrick Meir, Australian National University; University of Edinburgh

One approach to help fight climate change is to protect natural forests, as they absorb some atmospheric carbon released by burning fossil fuels and store large volumes of carbon.

Our new research on Australia’s tropical rainforests challenges the assumption that they will keep absorbing more carbon than they release.

We found that as climate change has intensified over the past half-century, less and less carbon has been taken up and converted to wood in the stems and branches of the trees in these forests. Woody biomass is a large and relatively stable store of carbon in forests, and acts as an important indicator of overall forest health.

The effect has been so pronounced that the woody biomass of these forests has gone from being a carbon sink to a carbon source. This means carbon is being lost to the atmosphere due to trees dying faster than it is being replaced by tree growth.

This is the first time woody biomass in tropical forests has been shown to switch from sink to source. Our research indicates the shift likely happened about 25 years ago.

It remains to be seen whether Australian tropical forests are a harbinger for other tropical forests globally.

What did we find?

Since 1971, scientists have tracked around 11,000 trees in 20 tracts of tropical rainforest in Australia’s far northeast, now part of the Queensland Permanent Rainforest Plots Network. This 49-year research effort is one of the world’s longest and most comprehensive of its kind.

We analysed this long-term data and found a clear signal: woody biomass switched from being a carbon sink to a carbon source about 25 years ago.

Why? One reason: trees are dying twice as fast as they used to.

Tropical rainforest tree species are adapted to generally warm, wet conditions. As the climate changes, they are subjected to increasingly extreme temperatures and drier conditions.These kinds of extreme climate events can damage wood and leaves, limiting future growth and leading to higher rates of tree death.

We also found tree deaths from cyclones reduced how much carbon these forests could absorb. Cyclones in far north Queensland are projected to become increasingly severe under climate change. They are also likely to push further south, potentially affecting new areas of forest.

Isn’t carbon dioxide plant food?

Burning fossil fuels and other human activities have increased carbon dioxide levels in the atmosphere. This should make it easier for plants to absorb CO₂ from the air, photosynthesise and grow. Given this, Earth system models predict higher atmospheric CO₂ levels will stimulate plant growth and increase how much carbon tropical forests can take up.

Also, remote sensing shows the canopies of tropical forests on Australia’s east coast are about 20% greener than they were in the 1980s. This suggests forest canopy growth has increased due to higher levels of CO₂ in the atmosphere. But this isn’t the whole picture.

Our data shows any potential increase in photosynthesis resulting in greener forest canopies has not translated to greater carbon storage in stems and branches.

The reason may be that tree growth can be limited by water, nutrients and heat. Our work suggest that warmer and drier conditions have limited tree growth even as CO₂ concentration has increased.

In a separate study, scientists artificially increased CO₂ and found the extra carbon taken up by leaves wasn’t being stored as extra woody growth. Rather, it was quickly released through roots and soil microbes.

What about other forest carbon stocks?

It will be challenging to find out whether these forests as a whole (including wood, roots, leaves and soils) have declined in carbon sink capacity.

The use of a specialised research tool known as eddy covariance towers could help, as these measure overall CO₂ movement into and out of ecosystems.

As of yet, only 15 years of this kind of data from three tropical Australian sites is available, which currently limits our ability to describe the fuller impact of climate change.

In any case, we know carbon stored in forest canopies and soils is often broken down and released back to the atmosphere faster than carbon in woody biomass.

So while Australia’s tropical rainforest carbon stores remain large, they may be less secure and reliable than in decades past.

Long term datasets are vital

When people visit Australia’s tropical rainforests, they can see intact stretches of biodiverse forest and large, carbon-rich trees. It’s hard to directly see the changes we have detected – for now, they’re only visible in the data.

Without high-quality long-term datasets, this signal would have been almost impossible to detect. Unfortunately, persistent funding shortages for long-term ecological monitoring threaten the continuity of these hugely valuable datasets.

Australia has the potential to assume a globally leading role in tropical ecosystem science. In light of state and national biodiversity and emission reduction commitments, Australian governments should support continued monitoring of vital ecological research sites.

Tropical forests may not be saviours

The fact that woody biomass in Australia’s tropical rainforests is now a net source of carbon has major implications.

These findings challenge our future reliance on forests as natural absorbers of extra atmospheric carbon.

We don’t know yet whether all tropical forests will respond similarly. Evidence on carbon sink capacity is mixed. Rainforests in South America are showing a decline while African rainforests are generally not.

Overall, the world’s tropical forests remain very significant stores of carbon and biodiversity. Their protection remains essential despite the climate risks they face.

Hannah Jayne Carle, Postdoctoral Researcher in Tropical Forest Ecology, Hawkesbury Institute for the Environment, WSU, Australian National University; Adrienne Nicotra, Professor of Ecology and Evolution, Research School of Biology, the Australian National University, Australian National University; David Bauman, Research Scientist in Plant Ecology, Institut de recherche pour le développement (IRD); Michael N Evans, Professor in Earths Systems Science, University of Maryland, and Patrick Meir, Honorary Professor of Forest Ecosystems, Australian National University; University of Edinburgh

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Loss of carbon absorption another impact of drought

The latest issue of Science carries some concerning research showing a reduction in the level of trees/plants to provide carbon sinks for the planet. Drought and changed rainfall patterns have apparently led to this reduction of plant coverage but this in turn places an urgent emphasis on taking steps to replace plant loss as well as develop other alternative methods to capture carbon emissions.

Science 20 August 2010:

Vol. 329. no. 5994, pp. 940 - 943
DOI: 10.1126/science.1192666

Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009
Maosheng Zhao* and Steven W. Running

Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.

Numerical Terradynamic Simulation Group, Department of Ecosystem and Conservation Sciences, the University of Montana, Missoula, MT 59812, USA.