Do past fires explain current carbon dynamics of Amazonian forests?

Amazon forests process and store large quantities of carbon in vegetation and soils. These forests, assumed to be mostly 'old-growth' and fire-free are exhibiting a remarkable feature-they are gaining the equivalent mass of a small car every year in aboveground biomass per hectare of forest (0.89 Mg/ha/yr). These gains are attributed to increasing atmospheric CO2, which has a fertilization effect on tree growth. However, fire in these 'old-growth' forests may be more recent than expected (in the last few centuries), and regrowth from fire together with soil charcoal-which has a fertility effect-may be contributing together with CO2 fertilisation to the observed increases in biomass in these apparently mature forests. Current understanding of drivers of these increases is limited by, (i) an unknown fire history of plots used to monitor change, and (ii) lack of information of how resource change affects these forests. The effects of pre-modern fire on forest regrowth and the gain have not been evaluated.

Our pilot analysis of radiocarbon dated fire from soil charcoal indicates that even the wettest Amazon rainforest has burned, with forests considered to be 'old-growth' having burned within the last few centuries, and 70% of plots (n=70) containing visible soil charcoal fragments. Periodic drier climate and fire use by Native Americans before their populations collapsed ~450 years ago following Europeans colonisation may have resulted in a higher fire frequency than currently observed. Forest regrowth from these and more recent fires may still be occurring in forests considered to be 'undisturbed', e.g., some trees may grow to be 980 years old in central Amazonia, so that forest considered 'old-growth' may still be approaching equilibrium as long-lived trees mature following fire.

Fire also produces soil pyrogenic carbon (PyC) as charcoal that is found in archaeological sites in terra preta soils and in upland soils across Amazonia far from evidence of human settlement. Soil PyC increases soil fertility on otherwise nutrient poor soils, and being resistant to decomposition, may have increased soil fertility across the Amazon. Pre-modern fire and soil PyC are therefore two important ingredients in understanding how Amazonian forests currently function.

We will determine whether regrowth following past fire and soil PyC fertility effects in 'old-growth' permanent forest plots across Amazonia contributes to the observed carbon sink. We have developed a basin-wide network of on-the-ground sample plots, and because methods of measuring the forest with these are fully standardised even across nations they represent an excellent opportunity to measure the response of Amazon forest to historic fire and soil PyC.

In permanent forest plots across the Amazon Basin our Objectives are:
1) determine spatial patterns in 'time since last fire';
2) determine soil PyC stocks, and how these are affected by climatic, edaphic conditions, and fire intensity;
3) using results from (1) and (2), determine whether spatial patterns of productivity and carbon gains in aboveground tropical forest trees in Amazonia are consistent with regrowth from historical fire disturbance and by soil PyC acting as a soil fertility enhancer

Our research will improve understanding of fire patterns across the Amazon for long-term forest plots (the same plots used to estimate the current carbon sink). We will provide a first quantification of: soil PyC stocks, basin-wide environmental drivers of soil PyC stocks, and whether soil fertility is greater where soil PyC levels are higher. This will be a first large-scale test of whether forest productivity, structure, and increases in carbon can be attributed to regrowth from historic fire and soil PyC fertility effects. The results are vital for conservation planning, to estimate the longevity of the carbon sink, and for policy such as Reducing Emissions from Deforestation and Degradation (REDD+).

The main beneficiaries of this research include:
* Policy-makers, because there is special concern about the loss of forest and climate change risks to the Amazon region. Better understanding of the threats to potentially one of the most important 'hot spots' in the earth system will benefit public and policy-makers alike. For example, these include organisations such as the UN Development program, the IPCC, and local conservation managers.
*NGOs such as the Forest Stewardship Council, and agribusiness in transitional forest areas.
* Climate modellers working at the Met Office.
* The general public, in the UK, in South America, and worldwide, because the historic, contemporary and future response of forests to fire has important implications for soil-vegetation-climate interactions. Changing climate, its impacts on nature, and devising effective mitigation and adaptation is at the forefront of public consciousness as one of the defining challenges of the century.
* School children and university students in South America, who will be most affected by climate change and will benefit from additional awareness and training to help them to contribute to how their communities adapt to, and mitigate against, changing climate.

How?
* The project team has a strong record with demonstrated public, private sector, and governmental impact. For example, governmental agencies in Latin America and the UN Development Program in Brasilia will benefit by learning more about the impacts of fire and long-term change of Amazonian forests, building on previous presentations by the PI to these organisations. Project collaborators interact with the Brazilian 'King of Soya', the largest soya farmer in the world, where we have long-term forest plots located for this study. We will continue with these avenues and with others such as through colleagues of the team at Conservation International and the World Bank, to facilitate application of our results to conservation management plans, REDD+, and national-level policy decisions.
* To further reach the public and private sectors, and the wider scientific community we will use the well-visited websites to which we currently contribute for disseminating outputs, protocols & metadata (www.rainfor.org) and the data (www.forestplots.net) where visitors also access data management and analysis tools. We will also develop multilingual material to be hosted on South American collaborator's websites. We will develop multilingual press releases of research findings through the University of Exeter news office, and request that collaborators publicise the material through local university networks.
* Key messages will be disseminated via scientific briefs made available through the RAINFOR and Exeter websites, especially targeting decision-makers who rely on estimates of forest response to fire and long-term estimates of above-ground biomass gain. We have also hosted Earth Day projects for local school children and will construct interactive displays to use as educational tools during school visits about forest conservation and the effects of climate change on tropical forests.
* In addition to supervising UK and South American PhD students for the project, we will run an analysis workshop in Brazil to train students and facilitate interaction with international researchers from diverse fields. Based on past experience, we will invite local TV stations to the final talks in order to convey our results to the Latin American public.
* We will work with Brazilian PhD students to pursue Brazilian funding through the "Science without Borders" program, allowing the students to spend one year working with the project team at the Univ of Exeter and Leeds.
*We propose constructing a user-friendly Amazon fire web portal to make data easily available to a wider public, as well as public engagement events through public lectures as we have done in Brazil in the past.