Updated: Oct 18, 2021
Using trees to cool both cities and non-urban areas was presented as a tool for climate adaptation in part one of this blog. While trees have been known for their cooling effects for some time, forests have been protected in parts of Europe for hundreds of years due to the effect they were also observed to have on local climates. It's not just temperature that trees effect, however, it's also rainfall.
Trees change the way in which both liquid and gaseous water moves through landscapes, transporting water quickly into deeper parts of the soil profile and back out again. Infiltration beneath trees is in fact much higher than under surrounding grasslands, with infiltration under eucalypts in excess of 140 mm per hour on fine-grained (clay) soils. This is compared to coarse textured soils under crop lands, where infiltration only reaches 2 to 23 mm per hour, and fine textured soils under crop where infiltration is zero to 8 mm per hour.
Despite the lower infiltration rate, agricultural soils tend to have higher moisture compared to forested areas because they need more water than crops.
It is counter-intuitive to think of trees as useful in agricultural areas if they both move water below the depths which crops can use and use more soil moisture. However, evidence suggests that in some parts of the globe the climate and land-use are tightly coupled―that is the climate is closely linked to land-use and land-use is closely linked to the climate. In these areas, including both south-eastern Australia and Western Australia, trees are important drivers of local climate conditions including rainfall. Trees drive the climate by transporting groundwater inaccessible to grasses and crops back into the atmosphere in a process called evapotranspiration. This groundwater could otherwise be lost to the biosphere for hundreds of thousands of years, only becoming available to plants and animals again once it is discharged again at the ground surface.
There's also evidence to suggest trees effect the climate by drawing moist air from coastal areas inland in a phenomenon called a biotic pump. This is through the formation of low-pressure zones above forests when they photosynthesise. These low-pressure zones then draw moist air. Biotic pumps differ from evapotranspiration as it means moisture is transported to, rather than recycled within environments. Analysis of this phenomenon across continents indicates that in the absence of forests, precipitation decreases from coastal areas inland. Where forest is present precipitation remains constant during periods of photosynthesis.
Since colonisation, 1.2 million km2 (13%) of Australia has been deforested, with this land clearance more intense in some regions than in others. Decreases in rainfall have already been attributed to this deforestation. For example, around 90% of the forest in Western Australia was cleared between 1950 and 1980. The scientists who investigated this decrease attributed between 55 and 62% of rainfall reduction in Western Australia to this land clearance.
It is well known that land-clearance in Australia affected biodiversity decline and caused widespread erosion. Yet, it is less well known that the very trees cleared to allow agricultural production also previously brought the rain on which the agriculture depends. How tree replanting may reverse these effects and potentially draw water inland from the oceans or generate clouds from groundwater drawn from deep within soils is not yet known.
We at Murrang Earth Sciences are working to understand whether revegetating the landscape will alter both rainfall and temperatures in local climates, and if so, how much would we have to plant for this to happen. We are investigating whether there are areas in which trees would cause more cooling or rainfall than others and whether we can even understand this with the science available. Whatever the answers, our aim is to develop solutions for people to act for themselves to mitigate the effects of the global climate change on their environment, agriculture, livelihood, and backyard.