Do past fires explain current carbon dynamics of Amazonian forests?

亚马逊森林在植被和土壤中加工和储存大量的碳。这些被认为大多是“古老的”和无火的森林表现出一个显著的特征——它们每年每公顷森林的地上生物量（0.89毫克/公顷/年）相当于一辆小型汽车的质量。这些收益归因于大气中二氧化碳的增加，这对树木的生长有施肥作用。然而，在这些“原始”森林中，火灾发生的时间可能比预期的要晚（在过去的几个世纪里），而在这些明显成熟的森林中，火灾和土壤炭的再生（具有肥力效应）可能与二氧化碳施肥一起促成了已观察到的生物量的增加。目前对这些增加的驱动因素的了解受到以下因素的限制：(i)用于监测变化的样地的火灾历史未知，以及（ii）缺乏关于资源变化如何影响这些森林的信息。前现代火灾对森林再生和收益的影响尚未得到评估。我们对土壤木炭的放射性碳年代测定的初步分析表明，即使是最潮湿的亚马逊雨林也被烧毁了，被认为是“古老生长”的森林在过去的几个世纪里被烧毁了，70%的地块（n=70）含有可见的土壤木炭碎片。大约450年前，随着欧洲人的殖民统治，美洲原住民在人口崩溃之前，周期性的干燥气候和火的使用可能导致了比目前观察到的更高的火灾频率。在那些被认为是“未受干扰”的森林中，由于这些火灾和最近的火灾而重新生长的森林可能仍在发生，例如，在亚马逊中部，一些树木可能长到980岁，因此，被认为是“古老生长”的森林可能仍在接近平衡，因为长寿的树木在火灾后成熟。火也会产生土壤热原碳（PyC）作为木炭，在亚马逊地区的陆地土壤和高地土壤的考古遗址中发现，远离人类定居的证据。土壤PyC增加了营养贫乏土壤的土壤肥力，并且抵抗分解，可能增加了整个亚马逊地区的土壤肥力。因此，前现代火灾和土壤PyC是了解亚马逊森林当前功能的两个重要因素。我们将确定亚马逊地区“原始”永久森林地块在过去的火灾和土壤PyC肥力效应后的再生长是否有助于观察到的碳汇。我们已经建立了一个覆盖整个流域的地面样地网络，因为测量森林的方法是完全标准化的，即使在不同的国家，它们也代表了一个很好的机会来测量亚马逊森林对历史火灾和土壤PyC的反应。在整个亚马逊盆地的永久森林地块中，我们的目标是：1)确定“自上次火灾以来”的空间格局；2)确定土壤PyC储量及其受气候、土壤条件和火灾强度的影响；3)利用(1)和(2)的结果，确定亚马逊地区地上热带森林树木的生产力和碳收益的空间格局是否与历史火灾干扰和土壤PyC作为土壤肥力增强剂的再生相一致。我们的研究将提高对亚马逊地区长期森林样地（用于估计当前碳汇的相同样地）火灾模式的理解。我们将提供第一个量化：土壤PyC储量，全流域土壤PyC储量的环境驱动因素，以及土壤PyC水平较高的土壤肥力是否更高。这将是第一次大规模测试森林生产力、结构和碳的增加是否可以归因于历史火灾和土壤PyC肥力效应的再生。这些结果对于保护规划、估计碳汇的寿命以及减少森林砍伐和退化造成的排放（REDD+）等政策至关重要。

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 enhancerOur 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+).