光環境對臺灣棲蘭山區亞熱帶雲霧林內兩種檜木小苗生長與建立之影響

Abstract

台灣扁柏(Chamaecyparis obtusa Sieb. & Zucc. var. formosana (Hayata) Rehder，以下稱扁柏)和紅檜(C. formosensis Matsum.)是台灣特有的兩種檜木樹種，它們只分佈在台灣的山地雲霧林範圍內。觀察發現扁柏小苗主要分佈在扁柏林下或森林孔隙內，而紅檜小苗則在較大的干擾地生長，因此推測扁柏小苗比紅檜小苗耐陰，此耐陰性的差異可能是造成兩種小苗在不同環境更新的原因之一。為了驗證此推論，本研究在台灣東北部棲蘭山區，比較不同類型生育地中自然生長的紅檜與扁柏小苗，量測其在各生育地的存活率、生長量、葉綠素含量、形態以及光合作用特性等，並記錄棲蘭山區包括雲霧動態之氣象因子，及調查不同生育地之光環境與生長基質特性。此外，分別在溫室和生長箱中，將兩種檜木小苗種植在模擬不同生育地之光量下，檢測小苗在獲得充足的水分和養分之生長條件下的表現。結果發現在棲蘭山區之氣候有明顯的季節性、非常潮濕和光量低等特徵。此區域一年有平均約40％以上的時間受雲霧籠罩，霧發生時林冠上光量平均降低約88％，以年平均而言，林上光量因雲霧之發生衰減約70％。扁柏天然林下的斑光發生頻率和照光時間也受雲霧遮蔽而大幅減少。調查發現，在棲蘭山區，扁柏小苗在野外的適存光量較紅檜小苗低。不論是在自然生育地或是人工控制環境，兩種植物小苗在形態和生理上的特性都展現了對不同光環境的可塑性：在低光下，紅檜和扁柏小苗在光合作用特性上沒有明顯差異，且在被保護並供給充分水分、養分情況下皆可存活。雖然如此，在低光下，扁柏小苗在環境養分和水分充足時提高分配在葉部的生物量，而當養分受限時，則降低每單位葉面積分配到的生物量，兩種形態上的適應都使葉部面積增加以適應陰暗環境；相反的，紅檜小苗在低光環境下，主要以增加莖的長度來因應，而不是調整葉部。在較高光量環境下，紅檜小苗比扁柏小苗具有較高的存活率、光合作用率、葉相對生長率，而使生物量明顯比扁柏小苗高，因此比扁柏具有競爭力。在持續的高光下，兩種檜木小苗的光合作用效率和葉相對生長率都被明顯的抑制，顯示紅檜和扁柏小苗都非極端陽性樹種。由本研究結果來看，此區域雲霧頻繁所造成之光量降低可能對扁柏小苗的存活和生長比紅檜小苗有利。本研究證明紅檜和扁柏兩種小苗早期的生長受光環境影響有差異的表現，可能進一步影響兩種檜木小苗在自然環境下之存活、更新及分佈。

Chamaecyparis obtusa Sieb. & Zucc. var. formosana (Hayata) Rehder (CO) and C. formosensis Matsum. (CF), are two endemic tree species distributed exclusively in the mountainous cloud forests of Taiwan. The majority of CO seedlings is found under the canopy of mature forests or in small gaps, while those of CF predominantly under more open conditions in disturbed field. Such a differential distribution pattern was considered to be attributable to their difference in shade-tolerance. To test the hypothesis, the survival, growth, chlorophyll content, morphological and photosynthetic traits of young seedlings of either species were compared in Chi-lan-shan area of northeastern Taiwan. Data such as canopy openness, the micrometeorological conditions including fog occurrence and light intensity above and under the canopy of Chamaecyparis forest were collected and then were simulated in the controlled environment conducted in the growth chamber as well as in the green house to test the performance of seedlings under optimal water and nutrient conditions. The results showed that distinct seasonality, perhumid and dim light conditions were the most distinguished characteristics of the climate in Chi-lan-shan area. Fog occurrence reduced 88% of the light intensity reached the canopy and also reduced the frequency and duration of sunflecks under the canopy of Chamaecyparis forest. The frequency of fog occurrence has been as high as 40% of annual time and has reduced ca. 70% of the accumulated photosynthetically active radiation. The distribution pattern of the seedlings in Chi-lan-shan area shows that CO established in habitats with lower light regime than CF. Both species display a plasticity to the various light environments, either in the natural or controlled environments, but in different ways. Under low light condition, there was no significant difference between these two species in photosynthetic traits. Nevertheless, to acclimate to shade condition, seedlings of CO increase the leaf mass ratio under sufficient supplies of nutrients or reduce the leaf mass per area under poor nutrient supply. In contrast to CO, seedlings of CF increase the stem length. Under higher light condition such as in forest gaps, CF displays higher survival rate, photosynthetic capacity, relative growth rate of leaves, and, as a result, higher plant biomass. Hence, CF is considered to be superior to CO under higher light environment. However, continued high light reduces the photosynthesis and leaf growth of either species significantly. The seedlings of either two species are not extremely sun-acclimated. Results from this study suggest that the reduction of light intensity due to fog occurrence in the studied area provides a more favorable condition for CO than for CF. This study also provides evidence that the light environment has played an important role not only in affecting the growth of Chamaecyparis seedlings at early stage, but also in the subsequent survival, regeneration, and distribution of either species in the natural environment.