臺灣椪柑之供給反應－－卡門濾波處理法

Supply Response of Citrus-Ponkans in Taiwan-- A Kalman Filter Approach

椪柑長久以來為本省重要水果之一。然近年來在受到嚴重經濟衝擊下，其生產結構卻沒有明顯地調整，究其原因，應是受到椪柑之多年生特性使然。由於椪柑之多年生作物待性，故其生產計劃涉及對現存面積存量之估算，並受樹齡分配結搆影響甚大。因而柑農之生產決策應包含屬長期投資的新植與短期調整的砍除，二者必須個別分析，方能洞悉柑農之供給反應。本文即由多年生之觀點來探討柑農之新植與砍除二決策行為。為解決歷史資料不足之限制，乃採用狀態──空間模型以及卡門濾波方法，並將椪柑依其產量分成五個樹齡層來探討。本研究之重要實證結果為：1.椪柑長、短期供給皆屬價格無彈性，彈性值分別為0.1426及0.3555，顯示柑農不論長期或短期之生產調整能力皆差。可見椪柑受其多年生特性影響，故生產決策受限於樹齡層的結構，以致目前椪柑預期收益雖然不好，但仍不見生產結構有明顯調整，因此近年來椪柑之種植面積與產量並未有顯著的變化。2.目前椪柑以第二樹齡層面積為最多，因其為幼齡層，故未來本省椪柑仍將繼續增產中。3.歷年椪柑砍除面積中以第二與第四樹齡層的變化較少，而以第三與第五樹齡層變動得較劇烈，其主要是因為老化淘汰及果樹病變。4.各樹齡層面積對新植面積的影響程度漸減。面新植面積受椪柑價格與成本的影響都很顯著但程度皆有限。因此柑農之生產決策乃以產出量多寡為依據，較不重視經濟因素（如價格與成本）之變化，故未來應輔導柑農以需求為導向來調整供給。

Citrus-Ponkans have been an important traditional fruit crop in Taiwan. Recently, the Ponkans industry has arisen a deficit under the production costs soaring. Meanwhile, the fast changing economic environment has placed tremendous impacts on the Ponkans industry. However, significant adjustment in planting acreages and production of Ponkans have not been observed which might be attributed to the perennial crop characteristics of Ponkans. Because the bearing tree productions by age are not the same, the supply behavior of Ponkans includes a new planting decision involving long term investment plans and removal decisions of the short run adjustment. Both of the decision behavior thus need to be analyzed separately. In order to econometrically estimate these structural equations separately sufficient data are required. But such data are not available indeed. This study intends to solve the data deficiency by adopting the state-space model in conjunction with an optimal estimates generating iterative algorithm, the Kalman fitter, to analyze the dynamics of the Ponkans supply response. Major empirical results are summarized in the following: (1) The short-run and long-run price elasticities of Ponkans supply generated from the Kalman filter iterative algorithm are inelastic. It implies that the supply decisions of Ponkan growers are not so easy to be adjusted due to the fact that perennial crops cannot be adjusted suddenly. (2) Currently, the Ponkans acreage of second age category (2-6 years old) is the highest among the five age groups of Ponkans. Because it is the younger age category, the per tree yield of Ponkans will continue to increase in the near future. (3) Over the years, the removal acreage of the second age category and the fourth category exhibit a slightly lower range of variation. However, the third and the fifth age categories exhibit somewhat greater variability due to old tree retirement and diseases.(4) Existing acreage in older age categories has less influence on new plantings than does existing acreage in younger age tree groups. Ponkan grower decisions mostly depend on the production of existing trees, and the effects of economic factors such as expected market returns are neglected.