Smoldering Shifts to Flame as Climate Forces Forest Change
By Dorothy Peteet — January 2017

    Biomass burning is a major global influence, affecting biogeochemical cycles, atmospheric chemistry and the carbon cycle. Emissions from biomass burning can change the composition of the atmosphere, which in turn can affect climate, both regionally and globally. Modern fire studies have shown that the types of emissions produced by the fires, including organic (OC) and black carbon (BC) vary with the type of combustion.

    Two kinds of BC are produced in combustion processes through different formation pathways: char is an impure form of graphitic carbon from combustion residue formed directly by pyrolysis in smoldering fires, while soot is a combustion condensate produced by gas-to-particle conversion at relatively high temperatures (> 600°C) in flame. Due to the different ways in which they form, the relative proportions of char and soot vary with fire type and thus have their own distinct relationships to climate. However, the separation of char from soot has rarely been applied in paleoclimate studies using sediment analysis, much less in investigations of long-term records of paleo-fires.

    Our record from Linsley Pond in Connecticut — afamous pond in ecological history studied by Yale biologist G.E. Hutchinson — lake sediments documents the first paleorecord discrimination between smoldering and flaming fires in the past and indicates the clear relationship to climate change. With a well-dated accelerator mass spectrometry (AMS) radiocarbon chronology (Fig. 1), the Linsley Pond sediments have been used to investigate shifts in paleoevegetation and paleoclimate during the last glacial-interglacial transition in previous publications.

    This paleoclimate research facilitates our investigations into the interactions among climate, ecology, and selected fire indicators. In our study, we determined BC, char, soot, and charcoal concentrations using a thermal/optical method along with the mass accumulation rates (MARs) for these species in the pond sediments.

During the warm Bølling-Allerød (BA) interstadial (a warmer time period) about 15,000 to 13,000 years ago, the climate was temperate and mesic, as evidenced by the appearance of pollen of temperate deciduous oak (Quercus spp.) and pollen and macrofossils of the moisture-loving balsam fir (Abies balsamea). The char MARs during this time showed a slow increase and peaked at the end of the interstadial when temperate oak species became more abundant and boreal spruce (Picea spp.) declined. The colder, wet Younger Dryas (YD) period that followed was also characterized by high char MARs, but with some occasional troughs, and the char MARs showed a peak at the end of the YD when boreal trees, including larch (Larix laricinaPicea, and Abies balsamea abruptly disappeared.

The early Holocene immediately post-YD shows a sharp decrease in char MARs, while warmth-loving plants, including Quercus spp. and white pine (Pinus strobus) thrived, indicating a warmer, drier climate. Indeed, the atmospheric temperature in the early Holocene increased 3-6°C in only 50 yr. The decline in char MARs was followed almost a millennium later by an abrupt increase at about 10,800 years ago when the temperature was warm and eastern hemlock (Tsuga canadensis) reached a maximum abundance. Similar pollen results indicating a warm, mesic climate are typical regionally. Overall, the trends in the soot MAR profile were opposite those for char (Fig. 1, rows D and E) — for example, stable and low soot MARs were found for the cold and wet Younger Dryas. An abrupt increase in soot MARs occurred in the early Holocene when an increase in P. strobus indicated a warmer, drier climate, and a decrease in soot MARs subsequently occurred when P. strobus declined and warmer, more mesic conditions prevailed.

In contrast to the char MARs, which show a clear relationship with local pollen data as described above, the soot MARs exhibited a stronger relationship with regional climate. That is, the soot MAR time-series showed a clear “V” shaped pattern with lower values evident during the Younger Dryas period.

Most notably, dramatic changes in the MARs of BC, char, and soot occurred when the climate shifted abruptly. These findings indicate that wildfires were more prevalent in the early warm, dry Holocene. They also suggest that a greater production of soot occurs in dry climate zones and more char in wetter areas. This means that the types of fires and possibly even their potential environmental effects may be predicted from models using climate variables.