應用拉格朗日模式於種子飄散之研究

Abstract

植物族群的遷徙及擴張，主要是由其種子之長距離飄散(Long-Distance Dispersion)決定。尤其由風力承載的種子，需要較為複雜的模式才能準確描述其飄散行為。本研究目的為利用拉格朗日隨機飄散模式(Lagrangian Stochastic Dispersion Model)，模擬種子於森林流場中飄散軌跡，結合紊流動能消散率間歇性，探討種子在不同大氣狀況下之飄散情形；並改變各項模式參數 (地表摩擦速度、釋放高度以及種子終端速度)，檢視其對種子長距離飄散之影響，評估各項因素之相對重要性。 研究結果顯示，不穩定大氣狀況及消散率間歇性皆會增加種子長距離飄散能力，且消散率間歇性增幅較大。不穩定大氣更可以加強間歇性造成的垂直飄散速度驟增，使更多粒子被抬升至冠層以外，傳輸至較遠地區；同時在極不穩定大氣(h/L = -1)時會使近距離密集落地位置往前靠近釋放來源。在加入紊流間歇性下，穩定大氣長距離飄散能力和中性大氣時相近，並會使近距離密集落地位置往後遠離種子釋放來源。另外，無論在何種大氣狀況下，增加地表摩擦速度、釋放高度以及較小的終端速度皆可使種子長距離飄散能力增加 (其中以釋放高度的改變對加入紊流間歇性之模式結果影響最大)；且在不穩定狀態下，這些長距離飄散能力的增加程度會被放大。

The migration and expansion of plant species are determined by the Long-Distance Dispersion (LDD). The more sophisticated mechanistic dispersal model is needed especially for the LDD of the wind-driven seeds. This study simulated the seed dispersion trajectories in the canopy turbulence by using the Lagrangian Stochastic Dispersion Model under different atmospheric stabilities in conjunction with the effect of the intermittency of the turbulent kinetic energy dissipation rate. Also, the effects of friction velocity, seed release height and seed terminal velocity are studied. The results showed that both the unstable atmosphere and the inclusion of the dissipation rate intermittency in the model could increase seeds’ LDD. The number of seeds which escape the canopy volume by the dissipation intermittency is increased under unstable atmosphere, hence more seeds can be transported to the further distance. Under the strong unstable atmosphere, the peak location of dispersal kernel tends to be closer to the source when the dissipation intermittency is included. The ability of LDD is similar under neutral and stable atmospheric condiotions, and the peak location will be further away from the source under stable condiotion. Also, no matter which atmospheric condition, higher friction velocity, higher seed release height and lower seed terminal velocity all increase the LDD of seeds. The change of LDD due to the change of the friction velocity, seed release height, and the seed terminal velocity, would be enhanced under the unstable condition.