斑光對林下四種樹苗一日光合作用總量的貢獻

森林樹冠層會阻擋大部分的陽光，導致林內入射光量甚低，但林下樹苗因為斑光提供較高的光能，可增加光合作用碳收穫。本研究於屏東科技大學校園的人工林，選擇林下香楠、樹杞、毛柿及蘭嶼肉豆蔻共4種樹苗各10株，探討林內斑光對各樹種苗木一日光合作用CO2固定量的貢獻。本研究於7:30～16:30期間，以兩套攜帶式光合作用系統，測定葉片照到的瞬間光量及淨光合作用率動態變化，分析各苗木白天照到的斑光之性狀(次數、光量、持續時間等)，以及斑光對淨光合作用率的影響。該林分內白天背景光量甚低，相對光量低至1%以下。在斑光性狀方面，斑光光量在10~25 μmol photon m-2 s-1的個數為斑光總數的61%；有54%的斑光其持續時間短於30秒；單株樹苗一日可照到15~23個斑光。在斑光對淨光合作用率影響方面，林下樹苗於白天9小時期間照到斑光的時間平均僅38分鐘，但照到斑光時累積的光量可高達白天總光量的39%，有斑光時每分鐘的CO2固定量為無斑光時的5倍。在斑光對苗木一日CO2固定量的貢獻方面，香楠為30%、樹杞為25%、毛柿高達51%、蘭嶼肉豆蔻則為28%，平均為34%。然而，香楠苗木照到斑光時，淨光合作用率隨光量而提高的反應，較另3種樹苗明顯。本研究也發現大氣CO2濃度提高可降低此4種樹苗的光補償點及暗呼吸率，可能有利於樹苗在此低光環境下增加碳收穫。本研究結論為斑光對林下樹苗的光合作用具有重要性，可提高一日碳固定量，有利於樹苗在光資源極低的森林下層存活及生長。

關鍵字

斑光性狀；一日CO2固定量；低光環境；淨光合作用率；林下樹苗

並列摘要

Forest canopy intercept most of the sunlight, thus reducing irradiance in the undestory. However, sunflecks inside the forest could provide understory seedlings higher light energy to increase photosynthetic carbon gain. This study, conducting in a plantation in the campus of National Pingtung University of Science and Technology, chose 10 seedlings each of 4 species, namely Machilus zuihoensis var. zuihoensis, Ardisia sieboldii, Diospyros blancoi, and Myristica ceylanica var. cagayanensis to investigate the contributions of sunflecks to the daily photosynthetic CO2 fixation in each species. Two portable photosynthesis systems were employed to measure the dynamics of instantaneous light intensity and net photosynthetic rate of leaves during 7:30 to 16:30 each measuring day. We analyzed the characteristics of sunflecks (frequency, light intensity, duration, etc.) and the impacts of sunflecks on net photosynthetic rates. The background light intensity was very low with relative light intensity < 1% in the stand. In the aspects of characteristics of sunflecks, the number of sunflecks with light intensity 10~25 μmol photon m-2 s-1 accounted for 61% of the total number of sunflecks, 54% of the sunflecks lasted < 30 seconds, and a single seedling received in average 15 to 23 sunflecks daily. In the aspects of impacts on net photosynthetic rates, the understory seedlings received only 38 minutes during 9 hours of daytime, yet the accumulated light intensity by sunflecks could reach as high as 39% of total daylight. The amount of CO2 fixation per minute of understory seedlings when receiving sunflecks was 5 times more than no sunflecks received. In the aspects of contribution of su nflecks to daily CO2 fixation in each species, the contribution was 30%, 25%, 51%, and 28% for Mac. zuihoensis var. zuihoensis, A. sieboldii, D. blancoi, and Myr. ceylanica var. cagayanensis, respectively. The average contribution was 34%. However, comparing to the other 3 species, seedlings of Mac. zuihoensis var. zuihoensis showed more obvious increasing responses of net photosynthetic rates when received sunflecks. This study also found that elevated CO2 concentration could lower the light compensation point and decrease dark respiration rate in seedlings of all 4 species, which may increase carbon gain for seedlings in the low light environments. In conclusion, sunflecks were critical for photosynthesis of understory seedlings and could increase their daily CO2 fixation, thereby benefited the survival and growth of seedlings in forests with low light resources.