The thick black line (CDL) connecting SWO Loci on each temperature isotherm (solid black dots) separates ecosystems with different fire probability responses to precipitation in the CCd diagram.

<p>The CDL (thick black line) separates ecosystems with two dominate precipitation influences on fire probability. Example ecosystems 14 and 15, left of the CDL, may increase in fire probability with more fuel generated by increased plant growth while other wetter ecosystem will decline in fire probability with more humidity and fuel moisture. The SWO Loci (black dots) represent the maximum probability of fire occurrence at a given temperature and precipitation. Circles with numbers are selected ecosystems from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0180956#pone.0180956.t002" target="_blank">Table 2</a> for illustrative purposes in the diagram’s climate space. These diagramed probability estimates do not match full PC2FM equation predictions, especially at elevations above 2000 m where reduced O₂ concentrations may become more (or less) significant in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0180956#pone.0180956.e002" target="_blank">Eq 1</a>.</p

Example of precipitation effects interacting with temperature in a higher resolution CCd diagram of fire probability in cooler (< 285 K) climates.

<p>Example of changes in a wet-cool location, the in Lower Elwha River, Olympic Peninsula, Washington [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0180956#pone.0180956.ref014" target="_blank">14</a>] where fire probability will be affected by changing temperature and precipitation. Using a 3° change in temperature, from 282 to 285, if annual precipitation increases from 225 to 250 cm (line a) with a 3°C increase in temperature the fire probability with increase to 0.011 (mean interval of 91 years) but less than without precipitation increase. The same temperature increase without a change in precipitation will increase the fire probability from 0.0065 (MFI = 153) to 0.0125 (mean interval = 80 years) (line b). If on the other hand annual precipitation decreases from 225 to 200 cm (line c) with a 3°C increase in temperature the fire probability increases to 0.016 (mean interval of 62 years). The black line (d) illustrates how fire probability can be the same under different annual precipitation along the same temperature isotherm (285 K).</p

A Combustion-Climate diagram (CCd) of climate influences on fire probability.

<p>Climate simulated fire probabilities for ‘natural’ ecosystems using mean maximum temperature and annual precipitation in the PC2FM [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0180956#pone.0180956.ref010" target="_blank">10</a>]. This rate diagram explains two temporal differences related to the combustion of ecosystems. Temperature and precipitation affect the reaction rate at the time the reaction occurs while the rate of fuel production determines the fuel concentration and its combustion rate. These two timing conditions differentially determine the rates of the two components of the PC2FM model: ARterm and the PTrc3 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0180956#pone.0180956.e002" target="_blank">Eq 1</a>). Most ecosystems with adequate carbon bond production fall within the precipitation and temperature limits of this diagram. The red vertical-diagonal isotherms of simulated temperatures are 3°C (3 K) apart. Thus, the y-axis distances between points on the vertical temperature isotherms at the same or different precipitation, estimate fire probability change. For example, with an increase of 30 cm in precipitation (from 135 to 165 cm) and a 3°C in temperature (from 297 to 300 K) results in a change in fire probability from 0.2 to 0.30 or from MFI values of 5 to 3.3 years.</p

Descriptive fire data, including predicted and actual fire probability and mean fire intervals for 16 locations used in the <i>Combustion-Climate diagram</i> (CCd) (Fig 3).

<p>Fire interval data represent pre fire suppression periods.</p

Temperature and fire frequency paired sites with comparable fire probability but different annual precipitation.

<p>Temperature and fire frequency paired sites with comparable fire probability but different annual precipitation.</p