Changes in a landscape usually involve many intertwined factors. These factors include species-specific properties (like the life span of birches), the environment (e.g., drought or soil type), natural disturbances like fire and wind, and human influences like logging. All these factors act at various scales across the landscape and through time. Changes occur over months, years, decades, centuries, and thousands of years. Drought during a summer month, for example, may act as a short-term trigger for changes in the birch forest, whereas soils and slopes generally remain the same for millennia. Environments also vary across space, reflecting local site factors and differences in history. These differences may be sharp and local, as when they reflect different glacial formations or soils, or more gradually distributed through our region, as with differences in climate.