2020-02-21
Forests play a key role in determining climate, locally and globally – and their importance is even more critical now as global temperatures continue to rise.
By Chris Johansen and Jess Beckerling
There are many interactions and feedback loops between forests and climate, which we need to appreciate if we are to effectively address climate change. Of course, reducing greenhouse gas emissions is a priority. But recognising the role of forests, and other permanent vegetation with significant biomass, in sequestering carbon needs to be integral to climate mitigation strategy.
How forests affect climate
The legacy of ancient forests
The first point to note is that ancient forests shaped the atmosphere and biosphere within which life as we currently know it evolved. The oldest known fossil forest has been dated at 386 million years old. These ancient forests, through photosynthesis, absorbed atmospheric carbon dioxide (CO2) and released oxygen (O2), creating the atmospheric conditions conducive to the evolution of higher forms of plant and animal life with which we are familiar today.
Over the past million years or so atmospheric CO2 levels remained within the range of 200-300 ppm (parts per million), creating relatively stable climatic conditions – considering the heat trapping properties of CO2 – for contemporary life forms to prosper and evolve. Relatively suddenly, at an increasing rate over the last century, atmospheric CO2 levels have now shot up to 415 ppm and continue to relentlessly rise.
Although we are appreciative of these ancient forests for giving us our very existence, they still have the power to ruin that existence – through the extensive burning of their fossilised remains that humans have accelerated over the past two centuries. The decaying remains of ancient forests became fossilised, with the sequestered carbon therein primarily turning into coal.
Today’s forests
The removal of existing forests, to harvest wood and/or expand agricultural land, has also significantly contributed to raising atmospheric CO2 levels. For example, native forests of south-western WA in near pristine conditions would contain 100-200 tonnes of carbon per hectare (t C/ha). If cleared, and assuming only 15% ends up as wood products (i.e. C still stored), 312-624 t CO2/ha would be released into the atmosphere.
On the other hand, if south-western WA forests are left intact they can extract and store considerable quantities of atmospheric CO2. It has been estimated that dry forests in Australia can sequester 0.5-2 t C/ha/year (2-7.3 t CO2/ha/year), depending on local conditions. Thus forest removal has a two-fold negative consequence: pumping more CO2 into the atmosphere and curbing its sequestration.
It has been argued that reforestation quickly recovers CO2 lost in deforestation as young growing trees have a faster growth rate – and hence faster CO2 sequestration rate – than old trees. However, because of the lesser total biomass of young vs old forests the CO2 sequestration per ha per year of old forests is greater.
Calculations on regenerating forests in New South Wales indicate that it would take about 50 years for a regenerating forest to reach 75% of its potential carbon carrying capacity (carbon content of an undisturbed native forest) and about 150 years to reach 90%. Regenerated forests are usually logged within 100 years. So, maximum CO2 sequestration is better assured by leaving old growth forests rather than their logging followed by reforestation.
Local effects
Extensive canopies of trees – forests – act as biotic pumps that attract rain. Evapotranspiration from forests enhances the flow of water vapour into the atmosphere directly above the forest. This provides nuclei for water vapour condensation and, due to lowered atmospheric pressure, attracts moisture-bearing winds from external sources (e.g. off the ocean).
Further, eucalypt forests in particular emit aerosols of volatile organic compounds which act as additional nuclei for water vapour condensation. Thus it would be expected that forest removal by itself would reduce rainfall over that area.
Rainfall in south-western WA has declined on average by about 20% over the past half century. It has been generally thought that this is a consequence of global-level climate change, causing the westerly cold fronts bearing winter rains to move further south over time.
However, Andrich and Imberger (2013) have shown that a large proportion of this rainfall decline can be attributed to removal of native forests. They calculated that the extent of decline increased with increasing distance from the west coast. The rainfall decline near the coast represented the global-level climate change effect but moving inland there was an additional effect, attributable to native forest removal. They estimated that around 62% of the 15-25% rainfall decline in the wheatbelt may be attributed to land clearing, and 55% of the 15-25% rainfall decline in the escarpment, which runs at 30-100 km inland along the west coast.
In hot weather, forests also reduce ground temperatures on a local basis, through shading of the ground surface and the evaporative cooling effect of water evaporation via transpiration. In cold weather, forests have an insulating effect from cold winds as compared to relatively bare ground.
How a changing climate affects forests
As CO2 is the feedstock of photosynthesis, increasing atmospheric concentrations should enhance plant growth rates and hence biomass formation. Further, moderate increases in temperature would also be beneficial to growth rates in forests other than dry sclerophyll types. Advantages yes – but usually outweighed by the disadvantages.
Reduced rainfall exacerbated by increased evaporation and transpiration at higher temperatures induces water stress, resulting in slow growth and reduced CO2 uptake. Drying to the point of desiccation of foliage greatly enhances fire risk. In a relatively stable climate, forest species would have evolved to be in equilibrium with potential pests and diseases. Climate induced changes in temperature and moisture conditions can adversely shift that balance.
What if?
Although hindsight – backed by contemporary scientific understanding – can dissect historical errors, it is of interest to examine what could have been. If our predecessors had left native forests on, say, 50% of south-western WA, and particularly on the coastal plain and escarpment, how much better off would we now be?
For a start, the current and ever increasing fresh water crisis would be much less than it is due to higher rainfall in reservoir catchments and their lesser salinisation (e.g. as in Wellington Dam). Although there would be less available land suitable for cropping (the Wheatbelt), its outer limit would not have shrunk to the extent that it now has and land within that limit would be receiving higher and more reliable rainfall – resulting in higher and more stable production even though from a reduced area. And, not to mention the reduced biodiversity loss, soil salinisation and general soil degradation that is associated with deforestation.
However, our predecessors did not understand ecology, and simply viewed forests as a resource to be harvested for wood products or cut down for agriculture. Even if they did understand, would there have been the political will to preserve forest areas for a sustainable future? Probably not, as even today’s politicians ignore climate and forest science in order to pursue their short term political goals.
Into the future
Can we to any extent alleviate the problem our forebears, and now we, have created? A first step is to preserve remaining old growth and regenerating native forests, with no excuses for their continued logging.
Secondly, reafforestation on as large a scale as possible, certainly on marginal land and particularly on the coastal plain and adjacent escarpment areas. But reafforestation on a permanent basis with native, adapted species – not plantations that are regularly harvested. Tree plantations are best considered as a component of the agricultural system.
However, forgetting the political will to take these steps – which in any case is just not there at this stage – global level climate change would complicate this approach. Existing forests would need to adapt to, and regenerating or newly planted forests would need to evolve in, temperature and moisture environments different to and more harsh than the environmental conditions in which our unique native forests have evolved. Of particular concern is the fire threat, with enhanced desiccation conditions increasing frequency and intensity of fires. Another concern is the threat of once subliminal pests and diseases, invigorated by changed climatic conditions.
Certainly, global climate change will continue to adversely impact south-western WA, but we should be trying our best to minimise the superimposed effect of deforestation. And, simply, the more perennial vegetation that is allowed to grow will indeed contribute to reducing the global climate change effect through sequestration of CO2.
This article originally appeared in Green Issue.
Chris Johansen is the co-editor of the Greens WA’s Green Issue. Jess Beckerling is the Campaign Director at Forests For Life and the Convenor of the WA Forest Alliance.
Image credit: Kevin Utting via Flickr and published under the Creative Commons Attribution-Share Alike 2.0 Generic license.