Module 9: Light and Temperature

The Determiners of Climate: Sunlight, Moisture, Temperature, and Wind

By Dr. James A. Danoff-Burg, Columbia University

Thus far we have discussed only the biotic factors that structure local ecosystems. Today we begin the Third Section of the SEE-U program and will discuss the abiotic factors that determine what local ecological features we will encounter. Climate (Module 9), soils (Module 10), chemical cycles (Module 11), and natural disturbances (Module 12) often have a greater effect on determining local ecosystem structure than do the biotic factors.

We begin this Section by discussing climate. What are the factors that determine climate? How do these forces differ daily and annually? How does changing them affect the local ecology and organismal biology?

As you may be aware, sunlight, temperature, water, and wind are the four forces that combine to determine the local climate. Climate in turn sets the stage and determines the outcome of many biotic interactions. All of these climatic components predictably change both temporally and geographically.

Sunlight. Solar intensity increases as we go toward the Equator, toward mid-day, and as we increase in elevation.

These are changes that occur at a larger geographic scale, but light intensity also changes locally. Light increases in response to decreased forest canopy complexity and increasing proximity to patch edges. It also changes in more complicated ways in response to the myriad of soil types (via fertility) and to slope and because of the response of plants to these variables.

Light changes affect the plants that can locally exist. Insufficient light will kill many species of plants. If many plants do not get sufficient solar energy to power photosynthesize, they stop growing. Lack of growth is essentially a death knell for plants. Herbivory, competition, and normal mechanical disturbances (wind, being trod upon, etc.) combine to ensure that tissue loss is constant. Therefore, light levels determine which plants can exist locally.

Interestingly, low light is required for many plant species that specialize on life in under-canopy environments, such as in the Atlantic Rainforest in Brazil. These plants have a greater diversity of photosynthetic pigments that enable the plant to better exploit all of the available radiant energy. If these shade plants are exposed to high light, they will have inhibited photosynthesis and will eventually die in a few days.

Moisture. Water largely determines environments by dictating what organisms can live where and how they will grow. Moisture levels tend to increase with proximity to the ocean and other terrestrial water bodies. Local humidity is also set by temperature, wind, and light levels. As each of these other climatic determinants increase, humidity generally declines.

In extremely arid environments, such as those surrounding Biosphere Two, the tightly clustered nature of rainfall patterns has produced two types of plant root systems. To capture the rapid runoff of the infrequent and torrential spring and summer rains, many of these desert plants have evolved a shallow, but extensive network of roots. Other desert plant lineages have evolved to have a central taproot that grows down until it reaches the water table, often extending down several meters. Frequently, the plants in desert environments have at least as much root biomass as they do shoot biomass.

In addition to the type of root systems present, water availability also determines the type and rate of photosynthesis. Most plants have their leaf pores (stomata) open during the day when they do the bulk of their photosynthetic activity so that they can absorb CO₂. This would not be feasible for many desert plants. Water loss during the day through their open stomata would frequently lead to a faster rate of water loss than they could absorb through their roots, particularly during dry periods.

Succulent desert plants instead rely on a slightly different photosynthetic pathway and are collectively referred to as CAM plants. CAM plants do part of the photosynthetic reactions during the day (the "light reactions") and then another part at night (the "dark reactions"). The stomata (pores in the leaves for gas exchange) of these plants are closed during the day and open at night. They typically have thick, waxy leaves to provide for the storage of the products of the light reactions. These are a set of rather elegant solutions to an otherwise intractable problem.

Temperature. The third of our four important climatic factors is temperature and it generally decreases with increasing latitude (as you go north) and altitude (as you go up). As you know if you have ever hiked up a mountain, it gets colder as you go up. In fact, scientists have determined that for every 1000 meters that we go up in altitude, there is a decrease in temperature of about 6 ° C that is equivalent to increase our latitude a linear distance of 500 to 750 km at the same elevation.

This rule, often called Humboldt's Rule for the 1807 paper of its first describer, predicts that the plant communities will transform similarly with changes in temperature and altitude.

The rule states that generally as you go up in altitude 1000 m, the plant community will also change as though you had moved about 500 km towards the poles at the same altitude. What we see is actually more complicated, as slope, soil types, drainage, and other locally varying features do not make this rule so clear. Nonetheless the general trend holds.

Of the four determinants of climate, the effect of changing temperature on organisms is often the most intuitive for many people. The metabolic costs to organisms living at a relatively stable temperature are lower than those living in an environment with highly variable temperature. Those living in a less stable environment need to have evolved features that can cope with the extremes, which is energetically costly. Those that have evolved in a stable climate can focus their metabolic efforts on the base requirements of life. Some researchers suggest that the long-term climatic stability of the Amazon basin helps to explain the overwhelming species diversity present there.

Species living in areas with temperatures below freezing must cope with this phenomenon or die. There are few options available to species living in Northern Temperate Forests like at the Black Rock Forest. Plants usually completely die (annuals), die back only to roots (many perennials), or go into a period of greatly reduced growth (perennials and trees) during cold or excessive heat.

During inclement times of the year, many animals go into a resting period during which their metabolism and respiration are significantly slowed. Depending on whether this resting period occurs during winter (due to cold) or summer (due to heat or aridity) and some of the specifics of their responses, the rest period is called either hibernation in the winter or estivation in the summer. (In insects, a similar resting period during embryological development is called diapause.) Alternatively, many other animal species stay active during freezing periods. To do this, they must consume extra energy to meet their elevated metabolic requirements that come about as a consequence of activity at these times.

Wind, the fourth of our four climate producers, has the effect of amplifying the effects of temperature on organisms. Winds increase heat loss by transpiration (the loss of water through membranes or pores) in plants and through heat and water loss in animals. Strong winds can also uproot and blow down trees. All of these wind-related influences can change the ecosystem locally.

Wind also distributes weather patterns around the Earth and frequently causes disastrous weather phenomena such as hurricanes and tornadoes or Nor'easters. These disasters may cause irrevocable changes to ecosystems and completely restructure them as a consequence.

Wind is usually formed due to a meeting of weather fronts, which are themselves formed as a consequence of one simple observation from physics: warm gasses rise and colder gasses sink. Air around the Equator is warmed by the great quantities of light and heat. As a consequence it rises and starts to move toward the poles where areas of cooler air are located. This cycle, plus the rotation of the Earth, is responsible for most large-scale wind patterns. Locally, wind can be created by differences in land temperature, which would trigger the same process by heating air, which then rises and moves.

There are many beneficial affects posed by wind for plants. Wind helps to pollinate many species of plants, spread seeds, remove harmful gasses, bring in many species of animals that are wind-dispersed, and many other forces. Wind is also necessary for creating hardy and strong trees. When it was first created, there was no wind inside of Biosphere 2. Plants grew relatively quickly, but they frequently fell over before they were of reproductive age. After some intensive observations and experimentation, it was determined that the lack of wind created trees with much softer wood than that species would normally make in the wild. They grew more quickly than they did in the wild, but they were harmed in the long run as a consequence.

These four abiotic influences largely determine climate and they thus contribute to weather. Weather and climate set the table for all other biotic interactions. In some senses, no study of ecology is complete, unless it takes into account these forces. At a minimum, field ecologists must try to control for these influences when designing their experiment.

Additional Relevant Online Resources

The Worldbook Encyclopedia has a thorough discussion of the factors determining Climate.

The Department of Geography of the University of Oregon has a set of informative Global Climate Animations illustrating annual changes in each of the climate variables. It is from this page that the animations on this page were borrowed.

The International Research Institute (IRI) for Climate Prediction has a page explaining the basis and patterns of the natural phenomenon of El Niņo/Southern Oscillation (ENSO) phenomenon.

Consult this page from NOAA, OGP, and UCAR discussing El Niņo and Climate Prediction.

A paper by Alan L. McNab and Thomas R. Karl of the National Oceanic and Atmospheric Administration discusses the relationship between Climate and Droughts in the Southwestern US.

A plain text page that has ample information on the relationship between Sun and Climate is available from Judith Lean and David Rind and published in CONESQUENCES: Volume 2, Number 1, during 1996.

The Center for the Study of Carbon Dioxide and Global Change has several pages that discuss CAM plants and the affect of elevated CO₂.

Dr. Raymond Russo of Indiana University Purdue University Indianapolis has an informative page of graphs illustrating the impacts of temperature on organismal activity.

Diapause in insects from the website to accompany Developmental Biology, Sixth Edition by Scott F. Gilbert.

An interesting paper is available from Dr. Ronald L. Hanson of the USGS on evapotranspiration and droughts.

Knowledge Centre of New Zealand has an interesting discussion of the impact of winds in determining Anticyclones and New Zealand weather patterns

The Weather Channel has a page discussing the importance of wind in producing Nor'easters, which frequently afflict the BRF.

A paper by Dr. Scott Mori and John L. Brown of the New York Botanical Garden entitled a Report on Wind Dispersal in a Lowland Moist Forest in Central French Guiana presents interesting reading about the beneficial effects of wind on plants.

All Materials Copyright © 2000 by James Danoff-Burg

All Rights Reserved.