透過模型解析孟宗竹林產量周期及擴張機制

Abstract

孟宗竹（Phyllostachys pubescens）是東亞地區重要的經濟物種，可供食用或作為結構材料，並能固碳。近二十年發現孟宗竹有擴張並取代周圍植被的現象。透過生態模擬了解孟宗竹生產及擴張的影響因子，或有助於為野外研究及管理政策提供新的見解。 長期記錄顯示管理孟宗竹林的新筍產量有豐欠年循環。現有假說認為兩年換葉周期與林齡結構失衡是豐欠循環的維繫機制。然並非各地管理孟宗竹林都有豐欠年，或同一竹林僅部分時期有明顯豐欠年，而既有假說於此無法提供解釋。本研究第一部分即透過年齡結構族群成長模型(Age-structured population growth model)分析影響新筍產量周期的潛在因素。模型結果顯示(1)來自老葉、一年生新葉的碳水化合物與 (2)來自儲藏於竹根、地下莖的碳水化合物，皆會讓孟宗竹林的新筍產量趨向長期穩定，而非豐欠循環。 根據前人研究，地下莖長度、資源量與空間分布是影響孟宗竹擴張的潛在因子，本研究第二部分即透過個體基礎模型(Individual-Based Model)探討導致孟宗竹易於擴張的影響因子。結果顯示，在資源量適中、地下部低成本的情況下，延長地下莖可以增加孟宗竹擴張成功的機會，並且讓擴張面積更大；而在資源量豐富、地下部高成本的情況下，延長地下莖則對族群存續並無好處。此結果或能進一步提供線索，解釋何以單桿散生竹多分布於溫帶、合軸叢生竹分布於熱帶。

Moso bamboo (Phyllostachys pubescens) is useful for food, materials, and carbon fixation. In East Asia countries, the Moso bamboo forest has expanded in these decades. The understanding of the factors that influence productivity and expansion phenomenon based on ecological modeling approaches might help to propose new insights for field measurements and development of management policy in the Moso bamboo forests. Long-term records in managed Moso bamboo forests have shown the two-year cycles of new shoots production. One previous hypothesis said the two-year leaf lifespan and uneven composition of newer and older leaves every year is the causes of the two-year production cycles. However, why the cycles were not obvious in some cases if the leaf lifespan is the cue for the two-year cycles? The first part of this study aimed to understand the potential determinants of inter-annual cycles of new shoots production in Moso bamboo stand through an age-structured population growth model. The model analysis demonstrated that the two-year cycles could easily disappear due to the considerable contribution of carbohydrates originating from (1) photosynthesis of old leaves and first-year culms’ leaves and (2) carbon storage of roots and rhizomes. The results suggested that only the two-year leaf lifespan could not be a complete explanation for the biennial production cycles, and further study about the allocation of carbon from photosynthesis of different-age leaves could contribute. In the second part, this study explored expansion of Moso bamboos. According to previous studies, the rhizome length and spatial distribution of resource might be the key factors for expansion. The second part of this study aimed to find out the possible factors promising Moso bamboo expansion based on an individual-based model developed in this study. The results indicated the rhizome length changed success frequency of expansion and territory of bamboo population. In general, elongating rhizome could improve the frequency of expansion when the cost for elongations was low and the amount of resource was middle-level. On the contrary, bamboo received little benefit or even negative impacts from elongation in high-cost and high-level resource amount condition. The former and latter simulations might provide reasonable interpretations for difference in habitat of bamboos with monopodial and sympodial rhizome systems, respectively.