Plants respond and adapt (evolve) to the climatic and environmental conditions under which they
grow. The most important of these are: temperature, precipitation, light, land factors, and air
quality. We are not talking about "weather" here. Weather is defined as the constantly changing
combinations of temperature, rain, light, wind, and relative humidity which assume a particular
pattern. Climate is the accumulated hour-to-hour, day-to-day, week-to-week and
season-to-season patterns of weather. The accumulated variations in weather for a particular
location, area or region defines its climate.
Climate determines which plants can be grown at a particular location and where and when they should be planted. This is also true for microclimates that may be found or created in the urban landscape. This climatic dependence doesn't mean that plants from one location can't grow in another. Plants are adapted to and grow under a range of climatic conditions. For example, plants can be grown in many areas of California that come from all parts of the world with all kinds of climates. But, there are limitations to what climates a particular plant will tolerate and thrive in. For any plant, it's important to know what climate it's native to (for which it had adapted) so that it can be grown in a location in which it will thrive.
Check out this location on the World-Wide Web (http://www.fs.fed.us/land/ecosysmgmt/pages/ecoreg1_home.html) and point on the west coast of the Provinces map to find the 'California Coastal Chapparal Forest Shrub Province' (or find another province of interest to you). Note that there is an interrelationship between: climate, vegetation, land surface form, soils, and fauna.
Plants respond primarily to the availability of water and to
variations (highs, lows) in temperature. Some plants are more
tolerant of high temperatures and some plants are more tolerant
of low temperatures. These temperature tolerances are based
on many factors including the climate to which they are native
and their particular anatomical and morphological
characteristics. Some plants (e.g., apples, cherries, many lilacs)
require cold temperatures to flower and fruit, thus completing
their life cycle. Plants also show great diversity in their water
requirements and tolerances. Those plants native to areas with
high rainfall (rain forests) require constant, high quantities of
water, whereas those plants native to deserts require very low
quantities or infrequent applications of water. Mismatching the
water requirements of plants (e.g., providing too little water or
too much water) generally leads to decreased plant growth,
declining health, and death.
We have very little control over the weather and climate in
which we live. However, by understanding what the generally
expected environmental extremes are, we maximize our chances
of successfully growing plants. For example, information on the
local weather and climate is available in many newspapers
agricultural and weather services,
the Farmer's Almanac
Data in these weather & climate sources often includes: high,
low and average temperatures for each day and month, record
high and low temperatures and precipitation, average last spring
and first fall frost dates, long-term-average daily and monthly
precipitation, average hours of sunlight, and wind velocities.
Temperature is the most important, single factor affecting climate in a locale. It determines:
From a global perspective, climatic temperature characteristics and changes are influenced by the greenhouse effect (http://www.iinet.net.au/%7Eecwa/greenhouse.html). The greenhouse effect describes how short-wavelength (high energy) radiation penetrates the Earth's atmosphere and is either absorbed or reflected (re-radiated). The radiant energy is re-radiated as longer wavelengths (lower energy) that can't penetrate or escape the Earth's atmosphere. Whenever radiation (e.g., light) is reflected or re-radiated it loses energy.
The result is a net gain in energy that is trapped inside the Earth's atmospheric "pane." This is quite analogous to what happens in a greenhouse on a bright, sunny day, such as depicted below.
The "greenhouse effect" of CO2 and other trace gases. the sun's radiation is dominated by short wavelengths, which are reflected or absorbed by the earth's surface. The radiation absorbed is reradiated at longer wavelengths, which can be absorbed by atmospheric gases including CO2. Higher concentrations of these gases in the atmosphere reduce the net emission of longwave radiation to space, warming the earth. (In W. H. Schlesinger, 'Biogeochemistry: An Analysis of Global Change' from MacCracken (1985).
Rainfall or precipitation is a second important climatic factor
and is usually characterized in terms of the quantity of water and
its distribution. Precipitation is usually measured and reported in
linear units, such as, inches or centimeters. Some places on
Earth average less than a few inches of rain per year and some
average more than 400 inches per year (more than 1 inch per
day!). The yearly distribution of rainfall is also important,
especially for plants growing in soils of relatively low
water-holding capacities (e.g., sand). Generally speaking, plant growth is greatest in locations
where rainfall is evenly distributed over the year as compared to locations that experience high
rainfall periods followed by drought.
Light is important to plants as a source of energy for
photosynthesis, but it also is important to plants as an
environmental signal associated with changing seasons
(changing temperatures and daylengths). Light intensity is
measured in a variety of units, including footcandles, lumens,
lux or (the modern) µmoles. m-2. sec-1 (read as micromoles
per square meter per second.)
Terrestrial (Land) Factors
Terrestrial factors that influence climate include:
The latitude, or distance from the Earth's equator, affects the amount of solar radiation impinging on the surface depending on the angle of the sun. This also affects the daylength.
As depicted below, tropical plants grow in latitudes near the equator and alpine plants grow in latitudes far north and south with sub-tropical and temperate plants growing in between. Plant types are distributed in response to temperature and rainfall.
(Note: If you have difficulty reading the figure above, refer to your class handout. We'll improve this one as soon as possible.)
On average, there is a 1o F decrease for every 300 foot rise in altitude. This is due primarily due to the greenhouse effect (http://www.iinet.net.au/%7Eecwa/greenhouse.html.) Thinner air and high altitudes absorb and retain less heat, making it hot and bright in direct sun and cool in the shade. (add OH graphic)
Water has a tempering (moderating) effect. Low air
temperatures are raised near bodies of water and high
temperatures are lowered.
Review the diagram below to see how a body of water moderates air temperatures during cold and warm weather.
As moisture-containing clouds encounter mountains and rise up into cooler temperatures, water condenses and is released as rain. On the western coast of the United States, it is common to see the western slopes of mountains with relatively high rainfall whereas the eastern slopes have relatively low rainfall. This phenomenon is commonly referred to as a rain shadow. In the diagram below, note the differences in annual precipitation as east-bound (left to right in the diagram ) storm systems rise up the mountain ranges, cool, and release their moisture.
Microenvironments can significantly affect temperature extremes and thus plant growth and survival.
For example, low areas where cold air tends to settle are not good areas to plant frost-sensitive
plants. Likewise, plants that do not tolerate high temperatures or high light intensities should not be
planted on the south or west walls of most California homes.
Pollutants in the air (e.g., gases, particulates, and dusts) can adversely affect plant growth. Some plants are more sensitive to air pollution than others. Ozone (O3), Sulfur dioxide (SO2) and nitrous oxides (NOX) are among the most damaging to plants and are also contributors to the incidence of acid rain in the eastern United States.
The United States Department of Agriculture (USDA) has developed the Hardiness Zone Map that depicts the normally expected high and low temperatures in all locations in the U.S. (http://www.pathfinder.com/cgi-bin/VG/vg-zonefinder.)
The Sunset 'Western Garden Book' also contains hardiness zone maps that are more detailed and
focused on areas in the western United States.
Steps can be taken to protect plants from temperatures to which they are not adapted, but the best
prevention for high or low temperature damage is to avoid using plants in the landscape that are not
adapted to the typical high/low temperature range. However, abnormal temperatures can occur
anywhere and plants may need protection.
For cold protection:
The water may freeze and cause a heavy load; but the temperature around the plant won't go much
below freezing (32 F).
For heat protection:
Congratulations! You've completed this chapter. You may want to take a break or go on to the Practice or 'In Your Neighborhood' Activities.