The fundamental factor that creates forest stratification is the light environment. Plants adopt different strategies toward light and are divided into sun trees (sun plants) and shade trees (shade plants).
Basic Knowledge
Plants absorb carbon dioxide and produce organic matter through photosynthesis, while consuming oxygen through respiration. When light is weak, photosynthesis amount
Sun plants (sun trees): Both the light compensation point and light saturation point are high. They grow rapidly in strong light but cannot grow in dark places. Examples: akamatsu, kuromatsu, konara, karamatsu, and shirakamba.
Shade plants (shade trees): Both the light compensation point and light saturation point are low. They can grow in weak light, but grow relatively slowly in strong light. Examples: sudajii, tabunoki, arakashi, buna, momi, and tsuga.
During forest succession, sun trees first invade bright bare land. When the forest floor becomes dark, sun-tree seedlings cannot grow, while shade trees that can grow in darkness develop in the lower layers and eventually replace the forest with a shade-tree-dominated forest.
Light compensation point (the light intensity at which photosynthesis and respiration are equal)
Light saturation point (the light intensity above which photosynthesis no longer increases)
Sun plants (sun trees) (high light compensation and saturation points; prefer bright places)
Shade plants (shade trees) (low light compensation and saturation points; can grow in dark places)
Apparent photosynthetic rate (CO2 absorption = gross photosynthesis − respiration)
Gross photosynthetic rate (the amount of organic matter actually produced by photosynthesis)
Further Exploration
A graph of photosynthetic rate has CO2 absorption on the vertical axis and light intensity on the horizontal axis, and always begins at a negative value equal to respiration in darkness. Be careful: it does not pass through the origin. The point where the graph crosses the horizontal axis is the light compensation point, and the point where the increase stops is the light saturation point. The difference between the origin and the graph's intercept represents the respiration rate.
Sun trees require direct sunlight, but their respiration rate per leaf is also high, so they cannot survive in weak light. Shade trees use light efficiently because their respiration rate is low and can survive on the small amount of light reaching the forest floor. This produces the replacement of dominant species during succession. Their leaf structures also differ: sun trees have a thick palisade tissue and small, thick leaves, whereas shade trees have thin, broad leaves that make maximum use of limited light.
- Light compensation point = light intensity where photosynthesis = respiration
- Light saturation point = light intensity beyond which stronger light is useless
- Sun trees have high compensation and saturation points (such as akamatsu)
- Shade trees have low compensation and saturation points (such as buna and sudajii)
- Apparent photosynthetic rate = gross photosynthetic rate − respiration rate
- In darkness, the graph is negative because only respiration occurs
- A shade-tree-dominated forest moves toward the final stage of succession, the climax forest
Points to Note
① The light compensation point and light saturation point are different concepts. Above the compensation point, photosynthesis produces a net gain. ② Sun trees are not only “fast in strong light”; they also “cannot survive in weak light.” ③ The relationship between apparent photosynthesis and gross (true) photosynthesis is: gross = apparent + respiration. ④ Do not confuse examples: akamatsu is a sun tree, while buna is a shade tree.
Practice
- What is the light intensity at which photosynthesis and respiration are equal called?
- Which has a lower light compensation point, sun trees or shade trees?
- Apparent photosynthetic rate = gross photosynthetic rate − 〇〇〇. What term belongs in 〇〇〇?