Interactions across scales - forest fires

From RAWorkbook

Forest fires don’t happen every year in the same place. While fires may burn different areas during the same year, the same patch of ground doesn’t generally re-burn, hence ecologists define fire frequency or return intervals of fires. That interval is related to different processes operating at different time scales.

The complexities of forest fire dynamics can seem overwhelming, but can be simplified into a few factors. One factor is the amount of fuel on the ground. This is usually equivalent to the amount of standing vegetation or biomass. Another factor is the spatial distribution of that fuel. In order for fire to spread, burnable material (fuel) must be in close proximity. A third factor involves how easily the fuel can be ignited. Dry spells with little or no rain allow fires to burn more readily because the fuel is drier and easier to ignite. The final key factor is ignition, which provides the spark to start a fire. Ignition usually comes from lightning or from the hands of humans. Each of these factors changes over different time intervals. Perhaps the quickest is ignition (milliseconds for lightning). Fuels accumulate over years. Many grasslands require one to three years for sufficient plant growth to carry a fire; forests an order of magnitude more time. Droughts can occur on at least two time scales; an annual one (such as monsoonal precipitation with wet seasons and dry seasons) and a multi-decadal cycle.

Fires occur when the following set of conditions prevail: sufficient fuel loads, fuels that are connected across an area, dry conditions that foster combustion, and an ignition source. This convergence of conditions can be described as a cross-scale interaction. Ignitions operate on a short-time scale, the process of plant growth occurs over years, fuel loads and drought cycles occur on longer-time scales, on the order of decades. Similarly, ignitions are local, plant growth is local, fuel loads can spread fires across large areas, and droughts cover large areas. The panarchy model at left represents the dynamic interaction among hierarchically arranged levels of a system.

Plants grow and fuel loads accumulate over time. Yet the speed at which they move from an r to K phase in the adaptive cycle differ. It is the accumulation of individual plant growth over a larger area that determines the fuel load for a fire. In other words, the aggregation of smaller-scale entities (plants that burn) generates the release or ‘creative destruction’ at the scale of a patch or forest. This cross-scale interaction, as the smaller, faster variables in a system coalesce to create a disturbance (fire) at the larger scale is referred to as a revolt.

Processes and structures at even larger scales than the fire influence post-fire recovery. Many fire-adapted plants have seeds that are stored in cones for years and then released following a fire. Those seeds reflect years of plant growth, not to mention years of evolutionary pressure. At larger spatial scales, seeds from unburned areas colonize the burned areas during the back loop phase. Dispersed by wind, birds, and other organisms, this influx of resources is critical for post- disturbance re-colonization. Since the seeds are developed prior to the disturbance, they are considered part of the system’s memory. Memory (e.g., resources) at larger scales is critical to the back loop of the focal scale. Infusions of capital in the form of seeds and nutrients in a forest are crucial for post-fire recovery. In the social and economic domains, property insurance (a form of memory), low interest loans, and recovery funds are critical to the recovery from natural disasters such as hurricane Katrina.