Deforestation is prevalent in recent news, as well it should be with areas the size of a football pitch being lost every second in 2017. This dramatic rate of deforestation stems from increased housing demands, demand for timber-related products and agriculture. As a result, deforestation rates are higher than they have been for most of human history. However, the Earth’s history is riddled with periods with marked decline of forested ecosystems; one of the earliest is known as the Carboniferous Rainforest Collapse (CRC) which occurred approximately 305 million years ago. I think it is necessary to look at the CRC to understand potential long-term effects of deforestation and how we might prevent a similar collapse of forested ecosystems today.
During the Carboniferous (359-299 Ma) the landscape was dominated by extensive tropical rainforests consisting of club mosses, ferns, horse-tails and lycophytes. The climate during the early Carboniferous was warm and humid, supporting an array of large arthropods, amphibians and basal. The fauna characteristic of the Carboniferous was largely a result of the unusual atmospheric composition of the time; atmospheric oxygen levels were as high as 35%, compared to a modern day 15%. This allowed animals that relied mostly on cutaneous respiration, such as insects and amphibians, to reach phenomenal sizes, (see video). Interestingly, the high oxygen levels of may have aided the aquatic-terrestrial transition of tetrapods, which is perhaps one of the single most important biological events in history.
However, throughout the Carboniferous global temperatures began to cool - the increased burial of carbon matter, paired with the northwards movement of Gondwana, initiated glaciations and aridification which led to the collapse of the Carboniferous rainforests and reduced the once continuous ecosystems into isolated patches, similarities can be seen in modern forests as a result of human activity.
The cooling climate led to the extinction of numerous species adapted to humid climates, such as many lycopsid flora and amphibians (modern amphibians are also in decline as a result of deforestation and habitat fragmentation, but disease has also played a large part). The dramatic loss of photosynthetic organisms had a profound impact on the atmospheric oxygen, (which fell by around 15% in the following Permian) resulting in the extinction of giant arthropods. The CRC marked the end of the Carboniferous period and the dominance of amphibious species. Reptiles quickly diversified to occupy the niches left empty, as they thrived in the arid climates which followed.
It is however important to take caution when using the Carboniferous as an analogue for modern deforestation. For example, the drop in oxygen resulting from the dramatic loss of photosynthetic organisms occurred at a time where oxygen levels were already higher than they are today (i.e. the 15% drop in atmospheric oxygen during the Permian may seem disastrous, but it actually resulted in levels similar to today). An important difference to modern climates (which are warming) is that the Carboniferous world was cooling, with continents stretching from pole to pole preventing mixing of polar and tropical waters as well as providing a platform for ice sheet growth in the Southern hemisphere. These factors should be taken into account when using the CRC to investigate the effects of modern deforestation.
Whilst the CRC may have been triggered by natural processes rather than human activity, the event shows us how increased global deforestation can impact the ecosystem, resulting in biodiversity loss, reduced oxygen levels and increased CO2 levels. Perhaps we should regard the CRC as warning of what our future may resemble if we continue to ignore the impacts of deforestation.
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