CH12 Ecological Role of Solar

Radiation

阮宏华

By Dr. Honghua Ruan

南京林业大学

Nanjing Forestry University

4.3 The compensation point of light and the saturation point of light

4.4 The shade tolerant of trees

树种耐荫性 : 在林业上 , 是指树种在林冠的庇荫条件下能否正常生长并完成其更新的能力 .

1) 阳性树种 : 只能在全光照条件下正常生长发育 ,在树冠下一般不能正常完成下种更新的树种 , 如 : 松 , 桦 , 杨等

2) 耐荫树种 : 能忍受庇荫 , 在林冠下能正常生长并完成更新的树种 , 如 : 云杉 , 冷杉 , 铁杉 , 杜英 ,红豆杉等

3) 中性树种 : 介于以上两树种之间 . 红松 , 杉木 ,木荷

Shade Tolerance:

One thing that any gardener and anyone who has lived in the world long enough will eventually realize is that some plants like to grow in sun while others like shade. The conditions in sun and shade are very different.

What is a full sun environment like?

• This picture shows cedar and black cherry, among other small plants, growing in a recently cut field near Cambridge, Maryland. The conditions in direct sun are quite harsh. While the sun provides the energy that plants need to grow, it also provides destructive UV (ultraviolet) rays and causes high rates of evaporation, causing things to dry out. Areas in full sun tend to get a lot hotter during the day than at night, so temperature variation is large. Think of how you get sunburned and dehydrated when you sit out in the sun for a few hours, and then realize that many plants have to sit in the sun all day long. Plants need special adaptations to tolerate full-sun conditions. They need to conserve water, and they need protection from UV rays. On the other hand, plants in full sun have ample energy available to them, and can afford to use that energy to conserve water or protect from UV rays; energy is often not a limiting factor in full sun.

What is a shady environment like?

• This picture shows a forest growing along Tuquan Glen in Lancaster county, Pennsylvania. The environment in shade is very different from sun. There is much less available sunlight for plants to use for energy. At the same time, the environment is milder. The temperature is more moderate, there is little threat from UV rays, and evaporation is less of a problem. As shade increases, the environment gets more and more mild, but less and less energy is available to plants. Thus, in order to adapt to shade, plants need to conserve energy, but don't need to worry as much about things like water conservation.

• All large trees will have their tops in full sun when they reach the top of the forest canopy. But many trees begin their lives in shade, and some medium and small trees never reach the top of the canopy. How well trees grow in shade varies from one type of tree to another, and sometimes, it depends on the conditions as well. (For example, some species become more shade tolerant as the soil gets richer, or in warmer climates.)

How do shade-tolerant trees and shade-intolerant trees compare (如何鉴别树种的耐荫性 )?

Shade Tolerant Shade Intolerant (Pioneer)

Growth Rate slow fast

Seeds sprout Under a forest canopy In open areas, on exposed ground

Needs to conserve Energy Water , nutrients

Has abundant Water, nutrients Energy

Blossom out late early

Life Long short

CP and SP lower higher

others 枝叶密 , 枝下高低 , 自然整枝弱 , 树冠透光度小 , 林相密

相反

• This table is an oversimplification. Most trees do not fit well into one category. There are a few cases in which it is easy to classify a tree: for example, Eastern Hemlock in the most shade-tolerant of all large trees in the U.S. It has all the characteristics listed in the table above. But most trees fall somewhere in between.

• So why is this important?

• Shade-tolerant species tend to replace intolerant species over time, once the forest canopy becomes dense. If a species is not shade-tolerant, it will not be able to reproduce in a forest. Shade tolerant trees will sprout under the forest canopy, and grow to small or moderate sizes. When a tree dies and a gap appears, the shade tolerant trees will already have grown to considerable size, and they will be able to respond by growing quickly to fill in the gap. Shade-intolerant trees, although faster growing, usually do not have the ability to compete in small gaps. When the trees in an area become well established and change little over time, the ecosystem is called a "climax community".

• If shade-tolerant species win out in the end, why are there so many intolerant species still left?

• First, even in many mature forests, there are enough moderate and large disturbances to open up enough gaps for less tolerant species to survive.

• Second, there are some areas where, due to conditions, trees in a forest never get big enough to fully close the gaps in the canopy. This is true both in cold climates, dry areas, areas with extremely low nutrients or poor soil, areas prone to fire, along rivers, or in areas polluted by humans. One of the reasons that we see fewer shade-tolerant species these days, however, is that humans have altered the environment to favor intolerant species; there are fewer undisturbed areas where shade-tolerant species thrive.

The factors influencing on shade tolerant of species

1) Age

2) Climate

3) Soil

4.5 Light-energy and forest productivity

The efficiency of light-energy using

For forest

comm

unity

, is 0.5-

1.5%

For single leaf , is 5-15%

For c

rop

field

, 1-5

%

How to improve the efficiency of light-energy using

Net primary productivity (NPP):

NPP=f ( LAI*PE*T )

Where LAI is photosynthetic Area, T is Photosynthetic Time, PE is Photosynthetic Efficiency.

1) Increasing LAI ( 增加光合作用面积 ):

Agri-forestry,

Multi-layer structure of canopy,

mixture forest

Planting with a available density

The ways to improve the efficiency of light-energy using

2) Improve photosynthetic efficiency:

Tree species selecting

Forest management

Increasing photosynthetic time:

It is hard to increase photosynthetic time in a forest.

1. Definitions: Ecology; Ecosystem; The compensation point of light and the saturation

point of light; Ecological Pyramid; Food Chain; Ecological Efficiency

2. Ozone and Greenhouse Gases• Name three greenhouse gases?

• What are the impacts of ozone in troposphere and stratosphere to the earth’s biosphere?

3. List three ecosystem processes affecting the following: • Nutrient inputs

• Nutrient outputs

4 What basic components do a ecosystem include?

Quiz-1