Table 4. Model fits for the global response of tree mortality to drought.
Data sets varied with tree group (angiosperm and gymnosperm), tree mortality metric (annual mortality rate (% y-1) and mortality rate during droughts (%)), and drought metric (MAP and SPEI). Best-fit models are highlighted in bold and are displayed graphically in Figs 3, 4 and 5. For polynomial models, we fitted all possible two- and three-factor models and only selected a model with cubic terms when it had an AIC lower than all other models and an R2 higher than all other models. MAP, mean annual precipitation; SPEI, standardized precipitation-evapotranspiration index; AIC, Akaike’s information criterion.
| All data (MAP) | R2 | AIC | Angiosperms (MAP) | R2 | AIC | Gymnosperms (MAP) | R2 | AIC |
| Mortality rate during droughts (%) | Mortality rate during droughts (%) | Mortality rate during droughts (%) | ||||||
| Linear | 0.385 | 2498 | Linear | 0.362 | 1191 | Linear | 0.197 | 960 |
| Log | 0.389 | 2496 | Log | 0.385 | 1186 | Log | 0.153 | 968 |
| Exponential (Fig 3A) | 0.514 | 1156 | Exponential (Fig 4A) | 0.432 | 540 | Exponential (Fig 5A) | 0.328 | 339 |
| Polynomial | 0.410 | 2487 | Polynomial | 0.392 | 1185 | Polynomial | 0.192 | 963 |
| Annual mortality rate (% y-1) | Annual mortality rate (% y-1) | Annual mortality rate (% y-1) | ||||||
| Linear | 0.314 | 2018 | Linear | 0.321 | 899 | Linear | 0.279 | 787 |
| Log | 0.392 | 1988 | Log | 0.362 | 891 | Log | 0.312 | 782 |
| Exponential | 0.397 | 1150 | Exponential | 0.352 | 502 | Exponential (Fig 5B) | 0.427 | 382 |
| Polynomial (Fig 3B) | 0.401 | 1984 | Polynomial (Fig 4B) | 0.372 | 889 | Polynomial | 0.315 | 779 |
| All data (SPEI) | R2 | AIC | Angiosperms (SPEI) | R2 | AIC | Gymnosperms (SPEI) | R2 | AIC |
| Mortality rate during droughts (%) | Mortality rate during droughts (%) | Mortality rate during droughts (%) | ||||||
| Linear | 0.044 | 2607 | Linear | 0.013 | 1247 | Linear | 0.107 | 974 |
| Log | — | — | Log | — | — | Log | — | — |
| Exponential | 0.006 | 1332 | Exponential | 0.003 | 612 | Exponential | 0.050 | 374 |
| Polynomial (Fig 3C) | 0.048 | 2606 | Polynomial (Fig 4C) | 0.025 | 1246 | Polynomial (Fig 5C) | 0.111 | 973 |
| Annual mortality rate (% y-1) | Annual mortality rate (% y-1) | Annual mortality rate (% y-1) | ||||||
| Linear | 0.116 | 2080 | Linear | 0.034 | 944 | Linear | 0.297 | 785 |
| Log | — | — | Log | — | — | Log | — | — |
| Exponential | 0.065 | 1259 | Exponential | 0.026 | 557 | Exponential | 0.153 | 424 |
| Polynomial (Fig 3D) | 0.147 | 2071 | Polynomial (Fig 4D) | 0.038 | 944 | Polynomial (Fig 5D) | 0.322 | 781 |