Factors Regulating Moisture Content of Live Fuels

Living plants and dead fuels respond quite differently to weather changes. The moisture content of a living plant is closely related to its physiology. The major variations in moisture are seasonal in nature, although shorter term variations are also brought about by extreme heat and drought. The Wildland Fire Assessment System produces daily maps of live fuel moisture across the U.S.

Internal factors

Fuel moisture fluctuates diurnally, with fluctuations greatest in fine living fuels and least in large living tissues. Fuel moisture also fluctuates seasonally, with young foliage having water contents two to three times the content in senescent foliage.

Climatic variation

Climate not only influences the length of the growing season, but can also cause periods of cold-induced dormancy or heat or drought induced quiescence. Long-term climate patterns (particularly extended droughts) can override weather events, creating a lag in changes in living plants’ fuel moisture contents.

Site factors and fuel environment

Site conditions such as soil and canopy cover can cause differences in moisture content within the same species. Elevation and aspect affect local microclimate and produce local differences in seasonal development of many plant species (Schroeder and Buck 1970). In mountain topography, for example, lower elevations and southern exposures favor the earliest start of the growing season.

Species groups

Seasonal moisture patterns vary among deciduous and evergreen species and woody and herbaceous species. Deciduous species generally have higher moisture content than evergreen species. Moisture content in deciduous species also varies more throughout the season, reaching a peak soon after bud break and decreasing after seasonal growth has finished. Because they retain old leaves for several years, evergreen shrubs have a more complex pattern of seasonal moisture content. As with evergreen shrubs, conifers have complex annual patterns of moisture content because they retain needles for several years. In general, old needles reach their lowest moisture content when new needles are being formed. In the southeastern U.S., conifers may flush more than once during the growing season. Moisture content for herbaceous species may be more variable throughout the year than any other species group. With annual species, all of the plant is new tissue at the beginning of the growing season, and all of the plant can become cured at the end. Once cured, herbaceous species respond to atmospheric conditions as a dead fuel. This is particularly true for grasses in areas with hot, dry summers (Byram 1959, Blackmarr and Flanner 1975).

Plant part

The major live fuel categories are foliage, twigs, branches, stems, cones and fruits, and roots. Foliage and other non-woody fuel moisture content are generally very high (>100 – 300 percent), varying among and within species. Foliage moisture is greatest in new foliage, decreasing as the foliage ages. Southeastern pocosin shrub species foliage emerges with 200 to 300% moisture contents and declines rapidly to 100-150% at the end of the growing season. Retained foliage on evergreen shrubs has 100% moisture content, varying little during the year (Blackmarr and Flanner 1975). Cones and other fruiting structure fuels have moisture contents that vary tremendously by season and developmental stage. Small woody fuels (twigs and branches) have lower fuel moisture contents, but these values are considered stable. Stems of pocosin shrub species maintain moisture contents at 100% throughout the year (Blackmarr and Flanner 1975). Still larger living woody fuels (small stems and boles) have lower, albeit more stable moisture contents.