The human ecology of climate change - Climate Change: Biological and Human Aspects - page 400

fire record in low-altitude pine forests of the western USA. These forests are prone
to drying. There are also open forests that lie above the steppe and grasslands of the
valleys and basins. At higher altitudes there is more moisture and the forest becomes
enclosed and subalpine. The low-altitude forests have been affected by human land
use (hence fires there have a potential health impact) and there is some controversy as
to how best to return some areas to their 'natural' state. One question is whether the
recent severe fires are atypical. Another concerns the role that climate change may
(or may not) be playing. Conventional wisdom has also been challenged, in that fires
have affected both open and closed forests: open forests were thought to be more
fire-resistant.
Our current understanding of past low-altitude forest fires is based largely on two
records. The first record is the fire scars of living and dead trees. As such, this only
provides a record within about 500 years, and a record that is increasingly biased
with age towards fires that scar but do not kill. Second is charcoal records from lake
sediments. These can often provide a record going back a few thousand years but
only in the main relating to montane and subalpine forests: only when a forest is
sufficiently higher than a lake is any charcoal transferred by watercourses.
Conversely, Pierce and colleagues (2004) looked at fire-related sediments at low
altitudes. Severe fires can remove surface litter and small plants, so enabling soil
movement (especially following storms or snow melt) that result in debris-flow
deposits. These accumulate in alluvial fans, and other places in valley floors, and
can be carbon-dated. Pierce et al. found that over the past 8000 years in xeric (that
is, characterised by dry conditions) ponderosa pine ( Pinus ponderosa ) forests, fire-
related debris flows were associated with warm, dry periods such as the medieval
climatic anomaly (MCA) around 1100-1300 (see Chapter 4). This contrasts with

cooler, wetter periods such as the Little Ice Age (

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1550-1850) that saw fires
from the dense grass cover, but only minor episodes of sedimentation. Adding this
knowledge to the previous picture it becomes clear that strategies of forest manage-
ment need to recognise that the frequency and nature of fires varies with location
(hydrology and geology) and climate regimen. So, one-size-fits-all forest manage-
ment may not be appropriate in that we may not be currently minimising risks to
human settlements that are mainly associated with lower-, rather than higher-, altitude
forests.
Forest fires also affect whole ecosystems and the humans associated with them.
Conversely, the physiological effects of extreme heat (and cold) are a manifestation of
effects on the whole organism, be it plant or animal (including humans), as discussed
above. However, there are also effects of extremely hot weather at the cellular level.
Hot weather events tend to be associated with sunnier days, which combined with the
human behavioural response of spending more time outdoors during such events can
result in an increased exposure to ultraviolet (UV) radiation. Sunburn (erythema) and
skin cancer are largely due to UV-B radiation, which has a spectral range centring
around 300 nm. But there are other cellular degenerative effects more related to UV-A
radiation, which has a spectral range centring around 340 nm.
The risk of skin cancer is greatest among fair-skinned populations that evolved in
northern latitudes that are exposed to lower levels of intense sunshine. It is thought
that the evolutionary driver for pale skin was to enable the skin to continue to produce

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