3 Results

3.1 Global and biome distributions of ecoregion flammability thresholds

Ecoregion flammability thresholds (EFTs) were identified for 772 of the 867 ecoregions classified in the Terrestrial Ecosystems of the World dataset (Olson et al., 2001), representing a total area of 128.5 M km2 – about 87% – of the Earth’s terrestrial surface (Figures 2a and 3). The global mean EFT identified was 12.2%, with a median of 11.5%, and an interquartile range of 6.92%. Only 17 ecoregions representing 0.8 M km2 reported thresholds over the expected maximum of 30%; most of these, however, are likely due to the limitation of fuel moisture playing a smaller role in the importance of fire for that ecoregion, or artifacts associated with insufficient fire data. The inflection point slope estimates (IPSEs) extracted from the P(BA < DFFMC) models shows the strongest relationship between DFFMC and burnt area appears to be primarily in different types of desert, tropical and subtropical savannas, higher-latitude forests, and some select tundra environments (Figure 2b).
Across biomes, the lowest identified EFTs were associated with types of shrublands, woodlands, and savanna biomes (Figure 3). The highest identified EFTs, on the other hand, tended to represent higher-latitude forests, as well as tundra. The average wetter temperate and tropical forested biomes closely followed these high-latitude biomes, but also tended to hold the widest range of EFTs, suggesting more complex geospatial relationships between fire, fuel moisture, and anthropogenic stressors. The Kruskal-Wallis rank sum test for differences in EFTs between biome types was strongly significant (p < 0.001), while the pairwise multiple comparison statistics calculated using Dunn’s test highlight like and unlike biome types. The statistical grouping of biomes with similar EFTs shows a clear gradient across climate, productivity, and vegetation types, albeit with a wide intra-biome variability associated with the complexity of ecoregions within biomes (Figure 3). See supporting information for pairwise multiple comparison statistics, as well as EFT summary statistics across both biome type and all existing hierarchical realm and biome classifications.

3.2 Climatic controls on ecoregion flammability thresholds

The two-dimensional nonmetric multidimensional scaling (NMDS) ordination reveals the role of eco-climatological factors in effecting the identified EFT across the different biome types (Figures 4a-b). The first dimension is primarily driven by precipitation and associated proxies: Higher annual precipitation, net primary productivity (NPP), and more tree cover are interlinked, influencing fuel-fire relationships in moisture-limited or -driven ecoregions. The second scaled dimension is driven primarily and negatively by temperature and is strongly linked to moisture-limited ecoregions with lower EFTs. Precipitation seasonality has a negative effect across both the first and second scaled dimensions, with higher values of precipitation seasonality representing stronger seasonal dryness (e.g., deserts and savannas with arid summer conditions). Similarly, higher herbaceous vegetation cover is associated with cooler, drier ecoregions. Outside of higher-latitude tundra and forest types, Earth’s biome types on average hold lower EFTs driven by temperature and precipitation on the second NMDS axis (Figure 4b).
The average EFT is well below the expected 30% limit for carbon fibre saturation across the full range of the productivity-fire activity index gradient (Figure 5a), noting that the majority of the Earth’s ecoregions by areas have a NPP below 10 t C ha-1year-1 (Figure 5b). Within the intermediate NPP range (i.e., roughly 1.2 – 5.4 t C ha-1year-1 per year for the evaluated ecoregions) where fire activity indices are maximal, EFTs are moderately higher than on either side of this productivity range or in those ecoregions with less fire activity. The lowest EFTs occur within the least productive environments across the entire range of fire activity. Low EFTs also occur to the right of the intermediate NPP range within those ecoregions that are frequently burnt. Both these areas of extremely low EFTs also tend to have the sharpest associated IPSEs (Figure 5c). There ultimately exists strong relationships between DFFMC and burnt area across most of the Earth’s ecoregions, apart from those highly productive, wet tropical forests, (Figure5b).
Our application of generalized additive modelling of EFT against environmental predictors showed a strong effect of the three bioclimatic indicators, with less explanatory power for biogeographic and ecological variables. This final model reported a correlation coefficient of determination (R2 ) of 0.682 and a root-mean-square error (RMSE ) of 0.038 for the complete distribution of EFTs. See supporting information for comparable statistics for training, testing, and cross-validation datasets. Our analysis of variable importance shows that RMSE dropout loss was sensible between predictor variables (supporting information). The three nonlinear bioclimatic indicators representing mean annual precipitation, mean annual temperature, and mean precipitation seasonality report the greatest influence on the model’s accuracy, followed distantly by the terrestrial realm and biome classifications as random effects. Median percent herbaceous cover reported a consistent, negligible effect on the model RMSE loss. The effects statistics confirm this, showing that precipitation is the strongest predictor of the global EFT distribution. In contrast to the variable importance analysis, the effect of precipitation seasonality on EFTs is larger and more significant than temperature (Table 1). See supporting information for model evaluation steps including conditional effects and both the predicted and expected posterior distributions.