Geology Reference
In-Depth Information
creating a mysterious kind of 'suction' that draws in new water all the way down at the
roots. On warm days, water entering a leaf from the soil is heated up by the sun's rays,
and passes out of the leaf pores as water vapour. The summed effect of evaporation of
water from leaf surfaces and transpiration of water from within the plant is considered
to be a single process known as evapotranspiration, which is vitally important for Gaia's
climate. Because of it, a huge amount of solar energy is stored as latent heat in water
vapour that can travel long distances before condensing to release its energy as heat,
sometimes thousands of kilometres away. But evapotranspiration also has local effects.
In the deciduous forests of the north-eastern United States, temperature rises steeply in
the early spring when, unimpeded by leaves, the sun's rays warm the ground. But as the
leaves unfurl and swell out to their full size, the rate of temperature increase drops dra-
matically because evapotranspiration cools and moistens the air.
Foliage is thus very important in regulating the surface climate. In general, the more
leafy a forest, the more evapotranspiration and so the more cloud production, local rain-
fall, local cooling and plant matter production by photosynthesis. A more diverse flora
almost certainly improves transpiration by providing a bigger and more varied mat of
below-ground root structures with better water-trapping abilities, and it could also en-
hance evaporation by providing a larger and more complex total leaf surface area from
which rainwater can evaporate. Both of these effects send more water vapour into the
air for cloud-making. Some plants evapotranspire more than others. Because they have
far fewer leaf pores, needleleaf trees pass less water into the air than their broadleaved
cousins, and as a consequence needle leaves seed fewer clouds, thereby keeping them-
selves warmer—an advantage in the high latitudes.
Another climatically important characteristic of vegetation is its roughness, a meas-
ure of how much resistance plants give to the wind. When wind blowing over the land
surface encounters plants such as trees, grasses and shrubs it transfers some of its energy
to the leaves, making them dance about. This sometimes frenzied leafy dance mixes
the air, making both evapotranspiration and the transfer of sensible heat from leaf to air
much more effective than on a perfectly still day. The higher up the canopy you go, the
more efficient are these transfers of energy from wind and sun to leaf. A dense rainforest
canopy, with its high roughness, will transfer much more energy to the air than the far
less leafy, low roughness grasses in a savannah. The intricate leaf surfaces of a more
diverse flora create a rougher land surface that increases air turbulence, and this could
well increase the transfers of heat and moisture to the air, influencing weather patterns
on both local and global scales.
These impacts of biodiversity on local and global climates in turn feed back to influ-
ence biodiversity itself. Clouds seeded by the Amazon forests keep the forest cool and
recycle its water, thereby allowing the forest to persist and preventing the encroachment
