Plants and Plant Communities

While an animal can choose its environment to best suit its energetics, plants are pretty well stuck with whatever environment happens to exist at the location and time of their growth. Over generations, selection and adaptation result in leaf morphologies, canopy structures, etc. which give the plants native to a particular environment a competitive advantage for that location. Desert plants that experience frequent shortages of water, for example, tend to have narrow leaves, while leaves of plants from more moist environments may be much larger. We might ask ourselves what environmental limitations there are to leaf size and other leaf characteristics related to energy exchange, or whether there is an optimum leaf form for a particular leaf environment. Answers to questions like these have obvious application in managed ecosystems such as agriculture. The answers are likely to be found both in studying the physics of energy and mass exchange, and in observing the characteristics of natural plants and plant communities in different environments.

Three factors must be favorable for a leaf to remain alive. Average net photosynthesis must be positive and the leaf water potential and temperature must remain within nonlethal bounds. Mature leaves apparently have no mechanism for importing sugars, so a leaf which is not able to maintain a positive net photosynthesis abscises. Net photosynthetic rate is determined by environmental factors and by the water balance of the leaf. To get a clear picture of plant responses to environment the environmental effects on leaf temperature, leaf water balance, and photosynthesis need to be considered.

In this chapter we also consider these processes in plant communities, but only in a simple sense. So-called big leaf models are often used to model temperature, transpiration, and photosynthesis in plant communities. The equations for such models are similar to those for individual leaves, but with conductances adjusted appropriately. We first consider the effects of environment on transpiration and leaf or canopy temperature, and then present several models that relate photosynthesis to light, temperature, and transpiration. Finally, we combine the photosynthesis and energy balance equations to predict response of photosynthesis to plant and environmental variables, and attempt to specify optimum leaf form for a particular environment.

Define the following words

Energetics, morphology, conductance, appropriately, canopy, ecosystem.

Complete the sentences

1. While an animal can choose its environment…

2. We might ask ourselves…

3. Answers to questions like these…

4. The answers are likely to be found…

5. To get a clear picture of plant responses…

6. The equations for such models are similar to…

7. We first consider…

8. Finally, we combine…

Put the following words and word combinations into the gaps

selection / adaptation / application / remains / mature / transpiration / adjusts / predict

The loss of water from the leaf surface, called ______________, literally pulls water up the stem from the roots which have the greater water potential.

Darwin was the first to propose natural ______________ as an explanation for the mechanism of evolution that produced the diversity of life on earth.

The more complex the ______________ of an individual organism, the less likely that recombination will improve it, and the more likely that recombination will disrupt it.

Focusing is accomplished by contraction and relaxation of the ciliary muscle, which ______________ the curvature of the lens.

The origin of bipedalism, the key event in the evolution of hominids, ______________ a mystery.

The first and perhaps most obvious commercial ______________ of genetic engineering was the introduction of genes that encode clinically important proteins into bacteria.

Pleiotropic effects are difficult to ______________, because the genes that affect a trait often perform other functions we may know nothing about.

Courtship songs are sung by ______________ males and are species-specific.