Table 1. Essential structural features of old growth in fire-adapted forests. Note that whether or not a feature is essential may depend on scale, e.g., patch, stand, or landscape. For example, age variability is likely at a landscape scale, and snags and large dead and downed fuels may not exist in some patches. Structural feature Essential Structural Feature? Comment Large trees No Tree size depends on individual site characteristics (species, precipitation, soils, etc.) and competition. Young trees may be large, and old trees may be small. Old trees Yes Trees develop structural characteristics that are relatively unique when old. Examples are dead tree tops, flattened crowns, different branch characteristics, diversity in crown form, altered bark color and texture. Age variability No This is an additional feature in some old-growth forest types. Some forests regenerate episodically (even aged) with most trees establishing in a few years to a decade, probably in conjunction with wet years and large seed crops, and in concurrence with relatively long intervals between fires. Others may regenerate over decades (uneven aged). Snags and large dead and downed fuels Yes Snags and large downed wood are essential elements of old forests, although frequent historical fires may have limited the accumulation of dead wood. The density and sizes of these features vary depending on forest type, precipitation, and other factors. Snags and large dead and downed fuels may be unevenly distributed across the landscape. Between-patch structural variability Yes High variability is a critical feature of these forests. Within-patch variability may be low, but variation among patches is large. Proportions of patches with different developmental stages varies depending on forest type, climate, etc.