Europe’s conifers have been warming the planet, say scientists – But other scientists predict a widespread loss of conifers due to climate change
Feb 24th 2016
Two research projects into conifer trees and their relationship to climate change have produced two apparently contradictory results. One research project looked at forest management in Europe since 1750 and concluded that new conifer plantations have created a 0.1°C rise in temperature across the region; in other words, these conifers were contributing to climate change. However, a second research project has predicted a widespread loss of conifer woodlands in South-West USA and in the Northern Hemisphere precisely as a result of climate change.
Research into Europe’s Forests and the Impact of Afforestation
This research was carried out by scientists at the Laboratory of Climate Science and Environment in Gif-sur-Yvette, France, and received coverage on BBC News earlier this month (6th February). The research project set out to investigate the last 250 years of forestry in Europe, looking at historical data, forest management practice, and its potential impact on the climate through the use of computer models and simulations.
Historical data shows that the area covered by Europe’s forests decreased by about 190,000 square kilometres between 1750 and 1850, when woodlands were commonly used to supply fuel and for industrial use such as ship building.  The Industrial Revolution saw coal replace wood as the major fuel, and from 1850 to the present day the area covered by Europe’s forests has grown by around 386,000 square kilometres, roughly 10% more land than before 1850. The increase is largely due to afforestation programmes, and an estimated 85% of Europe’s woodlands is now managed by humans. 
Trees absorb carbon dioxide from the atmosphere naturally through the process of photosynthesis, and the commonly-held view is that planting more trees will generally mitigate the impact of climate change. However, the common forestry practice, driven by economics, is to plant fast growing, needle-leaved evergreens such as pine and spruce, thought to be more commercially valuable than the slower growing broadleaved trees such as oak. Lead researcher Dr Kim Naudts said this replacement of broadleaved trees with conifers has not had the positive impact one might expect, but rather was making a small contribution to climate change.
She explained: “By changing the forest, we also make changes to the amount of radiation, water, and energy that the forest releases.” Compared to broadleaved trees, conifers are darker and absorb more light, reflecting less solar radiation back into space. They also release less water into the atmosphere through evaporation, and consequently have a lesser cooling effect on the atmosphere. In addition, the management practice of fast growth and fast removal tends to release carbon that would otherwise be stored in forest debris, dead wood and soil. The researchers say this practice means far less carbon is being stored than would have been the case if nature was left to its own devices. Dr Kim Naudts said: “Even well managed forests today store less carbon than their natural counterparts in 1750.” 
To measure the impact on climate change, the researchers built computer models that incorporated 250 years of forestry data, including the distribution of tree species and the methods used to harvest wood. Whereas previous models have focused on changes in land types and their impact (such as farmland, forests, heath, and so forth), the model created by Dr Kim Naudts and her team examines how the forests were actually used. A three-dimensional representation of the forest canopy, and its changes in the last 250 years, allows the researchers to see differences in how various tree species interact with the atmosphere. The model also includes the removal of trees for wood products or fuel, and looks at how forestry practice, such as thinning the forest but not removing it entirely, might affect the climate.
The result of these simulations has led the researchers to conclude that the changes in Europe’s forests have created a 0.1°C rise in temperature in the region, with 0.08°C being attributed to the twin factors of solar radiation and evaporation, as explained above, and 0.02°C being attributed to the practice of removing trees for commercial use. The researchers calculate that this rise in temperature equates to 6% of the global warming attributed to the burning of fossil fuels. They say 6% is a significant amount and believe that similar impacts are likely in regions where the same type of afforestation has taken place.
“We shouldn’t put our hopes on forests to mitigate what is an emission problem,” Dr Naudts told BBC News. “Our results indicate that in large parts of Europe, a tree planting programme would offset the emissions but it would not cool the planet, especially not if the afforestation is done with conifers.” The researchers argue that a programme of replacement should be considered. This would mean replacing the conifers as they are harvested with broadleaved species.
Research into the Risk of Tree Mortality as a Result of Climate Change
The second research project was carried out by an international team of scientists led by forest ecologist Nate McDowell at the Los Alamos National Laboratory in New Mexico. The scientists set out to investigate the risk of tree mortality as a result of climate change, looking in particular at the impact of drought and rising temperatures on conifers in the pine and juniper woodlands of the South-West USA. In a five-year study, the research team developed and evaluated computational models and simulations which were validated by field experiments, the aim of which was to understand tree mortality at three levels: at the individual plant level; at a regional level; and at a global level.
A news release from the Laboratory explains the impact of drought on a needle-leaf tree as follows: During a prolonged drought, it says, “the very mechanism that a tree uses to preserve its water stores can be its undoing. The tree closes the stomata on its needles to prevent water loss, but this prevents the tree’s food source, CO2, from entering, halting photosynthesis. As the air becomes hotter and drier, subsequent pressure change pulls more water from the roots than can be supplied and the water tension in the plant’s vascular system (xylem) can become so great that the straw-like columns no longer support water flow. The hydraulic system can collapse or the tree undergoes the starvation process, and it subsequently becomes defenceless against bark beetles and disease since it can no longer secrete the thick resin that protects it. As the tree decays after death, the carbon stored in its tissues is released into the atmosphere as carbon dioxide.”
The researchers found that a key predictive element of a tree’s mortality is its ‘pre-dawn water potential.’ A plant’s pre-dawn water potential is a measure of water stress and indicates its water status, resulting in part from soil water availability and atmospheric water demands on the plant’s water use. Experimentally, the team found that dominant evergreens in the South-West died when the tree’s pre-dawn water potential fell to levels that impaired the transport and storage of water and carbon.
The news release explains that the team generated predictions using multiple process-based and empirical models which included data from two of the world’s largest drought studies, both based in New Mexico and developed by Los Alamos National Laboratory. These models were backed up and validated using observations and field experiments. In the field experiments, a large drought plot was installed at one site which manipulates precipitation to test the impacts of drought, and the researchers then monitored a tree’s reaction to these changes. Their field experiments restricted precipitation by 50% to mimic drought conditions, and this resulted in an 80% mortality rate in the mature pines.
The news release continues: “In parallel, the scientists developed cutting edge representations of tree mortality within their models and subsequently evaluated them against the drought-manipulation results as well as against an independent set of data from another site in Los Alamos where pre-dawn water potential was monitored monthly for more than two decades. This resulted in the generation, and subsequent confidence in, state-of-the-art models of forest stress and mortality during drought.” The news release says that the regional models “accurately predicted the pre-dawn water potential of evergreens and 91% of the predictions exceeded mortality thresholds this century due to rising temperatures.”
Moving on from a regional understanding, the scientists compared their regional models with results from global vegetation models to examine independent simulations. They discovered that the global models simulated mortality throughout the Northern Hemisphere that was of similar magnitude, but on a much broader spatial scale, as the evaluated ecosystem models predicted for the South-West USA. The press release says that the conclusion of widespread conifer loss in the Northern Hemisphere is consistent with widespread observations of accelerating forest mortality in North America. Lead researcher Nate McDowell said: “We have been uncertain about how big the risk of tree mortality was, but our ensemble of analyses – including experimental results, mechanistic regional models and more general global models – all show alarming rates of forest loss in coming decades.”
In their conclusion, the authors of the research state that the atmospheric demand for water (known as the vapour pressure deficit) is potentially the largest climate threat to survival because increasing temperatures are driving a chronic increase in evaporative demand despite increases in humidity. The press release says: “In other words, according to Nate McDowell, trees may suffer in many places around the world, even in humid climates, due to global warming.”
Comparing the conclusions of these two research projects, one might well be baffled by what appear to be contradictory findings: on the one hand, conifers are contributing to climate change; on the other hand, conifers will be wiped out by climate change. The comparison suggests that conifers are pursuing a course of self-destruction, and the driver of the course is the plant’s very structure, together with the biochemical processes that, in other species, ensure a plant’s survival.
However, there are a number of differences in the scope of the research projects, and in their conclusions. The most obvious is that the first project is concerned with the past whilst the second is concerned with the future. The first project looks at the historical record and the impact that forestry practice might have had on the climate. The second project makes use of figures that are predicted for rising temperatures over the next century to make predictions about tree mortality. Given that the impact of climate change (in terms of rising temperature) has been more dramatic in recent decades, what has happened over the last 250 years is not a reliable guide to what may happen in the next 250. In addition, conditions vary from one country to another, and even within smaller regions: in the UK in recent times, for instance, whilst parts of the country were suffering from continued rainfall and flooding, other parts were suffering from drought.
On the research into Europe’s forests, Patrick Monahan highlights the temptation to extend the results to other regions. Writing in Science Magazine, he says: “But Europe’s temperature increase was in large part due to the continent’s specific history of forestry, its location, and the kind of tree species that are present there. The tropics, especially, play by different rules – there, slowing deforestation is almost certain to contribute to cooling, because trees in the tropics release comparatively more water into the atmosphere, seeding clouds that reflect light.” He also quotes a scientist who wasn’t engaged on the Europe forestry project, who points out that their model is only one of many possible models: “If a different model were to use the same parameters, it might find different results.”
On the second project, the authors of the research note that there are uncertainties and assumptions that could make their models underestimate or overestimate potential tree mortality. The predictions are concerned with temperature rise and the impact of drought, but exclude other factors such as wildfire and ‘islands of survival.’ Nate McDowell says: “Resolving these uncertainties is a critical next step for the international community because we need forests now more than ever to absorb carbon dioxide, even as that carbon dioxide and associated warming is threatening their survival. Based on the outcomes of the recent climate talks in Paris, we need to protect our forest to reduce the warming, but we simultaneously need to know how the warming can take out forests.”
If we assume that the management of Europe’s forests does not change, and its conifer plantations add their contribution to a global rise in temperature, then we have the scenario whereby Europe’s conifers could be aiding the demise of their distant relations in South-West USA. Which leads us to conclude with that question on predictability framed by Edward Lorenz in 1979 in an address to the American Association for the Advancement of Science (AAAS): “Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?” 
 A typical ship built in the 1700s incorporated the wood of 3,000 large oak trees, according to figures in Forests and sea power: The timber problem of the Royal Navy, 1652-1862, Robert G Albion, 1926. Cited in Trade and Dominion, J H Parry, 1971.
 “Overall, human activity has removed roughly half of the world’s natural forests, with the greatest losses in densely populated countries. With the exception of Russia, less than 1% of Europe’s ‘old-growth’ forests remain, while some 95% of the continental United States’ forests have been logged since European settlement began.” AAAS Atlas of population & environment, University of California Press, 2000.
 Figures from the Carbon Dioxide Information Analysis Center at the Oak Ridge National Laboratory, USA, estimate that a conifer ecosystem complex stores 13 kilos of carbon per square metre, whereas a mid-latitude temperate broad-leaved forest complex stores 9 kilos of carbon per square metre. Cited in AAAS Atlas of population & environment, University of California Press, 2000.
 Cited in ‘The Butterfly Effect’, in Chaos – Making a New Science, James Gleick, 1979.
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