FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

y species morphology and physiology. The amount of water in plant
tissue, and thus its moisture content, relates closely to events during a
plant's seasonal growth cycle (plant phenology). For a given species,
the maximum and minimum moisture content values and the average
values during different parts of the growing season are controlled more
closely by the plant structure and its adaptations to the general climate
of the area, than by daily weather. Seasonal timing of drying for specific
deciduous shrub, forb, and grass species were found to be similar
between wet and dry growing seasons, although moisture levels were
generally higher in the wet season (Brown et al. 1989).
(2) Site factors. Site conditions can cause differences in moisture
content within the same species, possibly because of physiological
conditioning or even a genetic adaptation to the site (Reifsnyder 1961).
Differences in foliar moisture content within a single species were
related both to differences in substrate and the amount of shading
provided by a forest canopy (Blackmarr and Flanner 1968).
(3) Climatic variation. Climate affects such factors as the timing and
length of the growing season, the length of the green-up period, and the
existence of seasonal periods of cold- induced dormancy or drought or
heat-induced quiescence.
c. Differences among species groups. There are characteristic
differences in seasonal moisture patterns for groups of species.
Deciduous leaved woody plants tend to have higher moisture content
values than evergreen leaved plants, and the seasonal pattern of
moisture changes tends to vary more. Coniferous trees have entirely
different foliar moisture patterns than deciduous trees. Herbaceous
species moisture levels can be higher or lower than that of associated
shrubs, depending on the species present and the time of year. There
are differences in average maximum and minimum moisture values
among species within any group, depending upon the morphology of
the species, and the relative amount of new and old growth on the
plant.
d. Deciduous leaved shrubs. The general pattern for deciduous
leaved shrubs is for moisture to rapidly increase to a peak level soon
after bud break and begin to decrease after all new seasonal growth
has occurred. Moisture then slowly declines for the remaining part of
the growing season until leaves cure.