森林林床上方因土壤呼吸釋出較高濃度的CO2，可顯著提高苗木的淨光合作用率，有利於幼齡樹苗在低光環境長期存活。然而，陽性樹苗在森林內受限於光資源不足，縱使土壤呼吸提高大氣CO2濃度，也無法令其淨光合作用率由負值轉變為正值。為瞭解此生態現象，本研究在屏科大森林系苗圃，選定不同耐陰等級的天然更新樹苗，測定光合作用CO2利用效率(CUE)及光補償點隨CO2濃度提高之變化，探討陽性樹苗為何無法存活於CO2濃度較高的低光環境。
本研究第一項試驗測定試驗地林床上方大氣CO2濃度的基本資料，結果發現林床上方隨著高度增加，CO2濃度會逐漸降低。林床表面於7:00之年平均大氣CO2濃度為500 μl L-1，在高度100 cm處則降至456 μl L-1，且由清晨至下午，CO2濃度會逐漸降低。在季節方面以夏季CO2濃度最高，冬季顯著最低。第二項試驗測定五類耐陰等級各3種樹苗的光合作用CO2反應，得知極耐陰樹苗的CO2補償點顯著最高，但CO2飽和點及CO2飽和淨光合作用率均以先驅樹種顯著最高。第三項試驗測定上述15樹種苗木的CUE，結果發現光量≦60 μmol photon m-2 s-1的條件下，中等耐陰、耐陰與極耐陰三類樹苗的CUE會顯著高於先驅及陽性樹種苗木。在光量≧90 μmol photon m-2 s-1的條件下，先驅及陽性樹種苗木的CUE則會顯著高於另三類耐陰等級者。本研究第四項試驗探討陽性樹苗要在什麼光量及CO2濃度組合條件下，其淨光合作用率才會由負值轉變為正值。結果發現極耐陰樹種苗木，在光量10 μmol photon m-2 s-1的條件下，於CO2濃度420 μl L-1時即可令其淨光合作用率由負值轉變為正值。相反的，先驅及陽性樹種苗木則要當光量提高至20 μmol photon m-2 s-1，於CO2濃度≧480 μl L-1時，其淨光合作用率才會呈現正值。
綜上所述，在森林冠層鬱閉的低光林下環境，較耐陰的樹苗具有較高的CO2利用效率，因此可有效利用土壤呼吸自然釋出的較高濃度CO2資源。相反的，先驅及陽性樹種苗木則需要有更高的光資源或更高的CO2資源，才能使其淨光合作用率由負值轉為正值。

Higher CO2 concentrations released by soil respiration atop the forest floor could significantly increase the net photosynthetic rates of seedlings, which benefit the long-term survival of young seedlings under low light environment. Yet for shade-intolerant seedlings, their net photosynthetic rates could not be turned from negative to positive under limited light resources even with the CO2 elevated by soil respiration. To investigate the reason why shade-intolerant species being unable to survive in high CO2 concentrations but low light environment, this study measured the variations of CO2 use efficiency (CUE) and light compensation point as CO2 concentrations enriched for natural recruits selected from various shade-tolerant classes. The experiment was conducted in the nursery of Department of Forestry, National Pingtung University of Science and Technology.
The first experiment was to establish the vertical profile of CO2 concentrations atop the forest floor. Results showed that CO2 concentrations would gradually decrease with the heights ascended. The average ambient CO2 concentration at 7 AM was 500 μl L-1 at the forest floor, and decreased to 456 μl L-1 at 100 cm above the forest floor. The CO2 concentration would also gradually decrease from dawn to evening, higher in summer and lower in winter. The second experiment was to measure the photosynthetic CO2 responses of 3 species each from the 5 shade-tolerant classes. Results showed that seedlings of very shade-tolerant species had the highest CO2 compensation points, while pioneer species showed the highest CO2 saturation point and net photosynthetic rate at CO2 saturation point. The third experiment measured the CUE of the 15 species. Results showed that, when light intensity was ≦ 60 μmol photon m-2 s-1, the CUE of mid shade-tolerant seedlings, shade-tolerant seedlings, and very shade-tolerant seedlings were significantly higher than that of pioneer species and shade-intolerant species; when light intensity was ≧ 90 μmol photon m-2 s-1, the CUE of pioneer species and shade-intolerant species were significantly higher than that of the other 3 classes. The fourth experiment measured various combination of light intensity and CO2 concentration, investigated combination that would turned the net photosynthetic rate of shade-intolerant species from negative to positive. Results showed that under the condition of 10 μmol photon m-2 s-1 light intensity, the net photosynthetic rate would be turned to positive while ambient CO2 concentration enriched to 420 μl L-1. However, the net photosynthetic rates of pioneer species and shade-intolerant species could turn positive only when the light intensity reached to 20 μmol photon m-2 s-1 and CO2 concentration ≧ 480 μl L-1.
In conclusion, under dense forest low-light environments, those seedlings with higher shade-tolerant ability would show higher CUE, and could use the higher ambient CO2 concentration elevated by soil respiration more efficiently. On the other hand, seedlings of pioneer and shade-intolerant species required higher light and CO2 resources to turn their net photosynthetic rates from negative to positive.

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