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The Nutrient Dynamics of the Suhu Coastal Casuarina Stands
四湖木麻黃林隨著齡級生長，地表枯枝落葉堆積量逐漸增多，地表有機物分解過程中所產生腐質酸及黃酸等，會導致表層土壤pH值降低，而下層土壤則受含鹽量高之地下水影響而呈鹼性反應，且pH(H2O)與pH(KCl)的差異極小。礦質土層之可交換性陽離子，以鈣的鹽基飽和度最高，且由於過低的K+/Ca2+ 比，恐會造成土壤K+, Ca2+, Mg2+間的不平衡，影響幼齡木對鉀離子的吸收利用。
本地區木麻黃林木主根分布主要在30-40 cm深以內，根系的生長因受地下水位較高的影響，而呈水平狀伸展。木麻黃林分葉（木麻黃真正的葉退化成小鱗片狀，圍繞於小枝節上。本文所稱之葉，嚴格應稱為接合小枝，branchlets，以下皆同）生物量隨著林齡增大而明顯增多，9年生林分葉量可達7.4 ton/ha。葉量空間分布方面，3年生從1 m高處起即有分布，而9年生在6 m高以下則幾無葉分布，可見9年生林分冠層受鬱閉影響，林冠下層枝葉因光照不足而逐漸枯死。
木麻黃林分葉、枝條、幹、根之合計生物量乾重，3年生1號及23號種源林分各為30.9及38.7 ton/ha，而9年生林分則為119.3 ton/ha，其年平均生物量生產則為10-13 ton/ha。3年生林分葉量占總生物量的比例各為10.5％及12.4％，而9年生則僅為6.2％，致使9年生林分因行光合作用之葉量比例減低，而降低其生長速度。3年生林分的年生物量增長量為24.4及18.5 ton/ha，而9年生則因下層劣勢木枯死太多，致生物量呈現負成長，不同齡級木麻黃林分葉量分布及其生物量聚積速率已明顯不同。
四湖地區乾季之月平均降水量為38.7 ㎜，而濕季則高達489 ㎜。四湖木麻黃林分全年幹流水及穿落水各為林外雨量之4.1%及78.2%，穿落水對木麻黃林養分輸入貢獻極大。降水的pH值隨著降水量增加而降低，幹流水因淋洗較多樹體所堆積的有機酸，其pH值明顯較穿落水低。降水之懸浮質、電導度及各種陰陽離子濃度，乾季亦較濕季高出極多，顯示四湖木麻黃林內降水所淋洗的落塵量極高；同時，9年生林木由於樹體高大，藉由降水輸入林內的鈉及氯量亦較3年生林分高。
木麻黃林分幹流水及穿落水合計之鉀、鈣、鎂及鈉量分別為27.7-39.9, 25.2-49.4, 20.9-40.9及128.4-231.5 ㎏/ha/yr，銨態氮及硝酸態氮各為28.2-58.0, 33.6-67.2 ㎏ ha-1，氯及硫酸根各為215.7-407.3, 57.8-113.1 ㎏/ha/yr，此為貧瘠砂地林木生長的重要養分來源。
3年生木麻黃林之淨初生長量為20.2及37.5 ton/ha，而9年生林分則僅6.5 ton/ha，各林分之淨初生長量中皆以枯落葉所佔的比重最高。由於3年生林分之生物量增長量較枯落物量高，而9年生林分則正好相反，顯示3年生林分尚在幼齡快速生長期，而9年生林分則已達壯齡期，整個林分的增長量與枯死量逐漸相等，林分的成長已趨衰退。
3年生木麻黃林分之淨生產效率（net production efficiency）為6.22及7.81 kg/kg，遠高於9年生林分之0.91 kg/kg，顯示二齡級之林分受到干擾後，所需恢復時間差異很大；相對地，3年生林分之生物量聚積比為1.03及1.53，遠低於9年生林分之17.81；不同齡級林分有機物的輪迴時間也大不相同，3年生僅需1.4-2.1年而已，而9年生則需19.2年，本研究之9年生木麻黃林之有機物循環形態已和3年生林分不同。|
Summary The objectives of this study were to investigate the soil properties, microbial biomass, nutrient accumulation, standing biomass, net primary production, and nutrient return by litterfall and precipitation of Casuarina stands in the Suhu coastal area, Yunlin County, southwestern Taiwan. The mechanism of nutrient dynamics of these stands, between the 3 and 9-yr-old stage, and the influence of the growth rate by nutrient cycling in each stand were also investigated. Acidity of the surface soil in the 9-yr-old stand was lower than in the 3-yr-old stands. This is because of the decomposition of the litterfall. However, soil in deeper layers influenced by salty ground water is more alkaline than surface soil. The C/N ratio of the soil in these 3 stands is less than 15, indicating that the decomposition rate of organic matter in the Casuarina stands is rapid. Microbial biomass N was mainly distributed in surface soil and decreased with increasing soil depth. It was abundant in the 9-yr-old stand, which was due to the large amount of organic material. Heterotrophic bacteria and nitrifying bacteria were also abundant in the 9-yr-old stand, indicating that vigorous nitrification was progressing. Branchlet biomass of the 9-yr-old stand was 7.4 ton/ha, and it increased with stand growth. The space distribution of Casuarina stands in 2 age classes was quite different; there were branchlet amounts at all tree height levels in the 3-yr-old stands, but there were branchlets below 6 m in height in the 9-yr-old stand. Biomass values of the 3-yr-old stands were 30.9 and 38.7 ton/ha, whereas that of the 9-yr-old stand was 119.3 ton/ha. The percentages of branchlet biomass to total biomass in the 3 stands were 10.5%, 12.4%, and 6.2%, which decreased with stand growth. There was obvious vigorous growth in the 3-yr-old stands, whereas the 9-yr-old stand was experiencing intense competition; the growth increment was offset by the wither amount, with the result that there was almost no additional increment in standing biomass or nutrient content. Dry and wet seasons were divided according to the monthly rainfall during June 1997 to May 1998 on the Suhu coast. In the dry season (October to May), average monthly rainfall was only 38.7 ㎜, while in the wet season (June to September), it was 489 ㎜. Acidity (pH) of stemflow and throughfall solutions decreased as total rainfall increased, and obviously lower pH values in stemflow solution indicated the leaching of greater amounts of hydrogen ions. Suspended solids and conductivity of these 2 solutions in the dry season were about 10-20 times greater than those in the wet season. Moreover, values of these 2 solutions were 5 to 16 times in the dry season and 4 to 11 times in the wet season each greater than that of rainfall collected outside of stands. Concentrations of ions of sodium, potassium, calcium, magnesium, ammonia, or from nitrate nitrogen, chloride, or sulphate acid were greater in the dry than wet season. These ion concentrations of stemflow solution were greater than those of the throughfall solution in the dry season, but opposite results were observed in the wet season. Most ion amounts were similar between the two ages of stands during the study period. Only sodium and chloride were significantly greater in the 9-yr-old stand than in the 3-yr-old stands, indicating that more salt blown from the sea had adhered to the larger surface area of older trees, while concentrations of other ions produced from both ages of stands were not much different. The amount of nutrient input from throughfall within a year was much greater than that from either stemflow or rainfall outside of stands. The average inputs of different ions through precipitation from the 3 stands were 128.4-231.5 kg/ha/yr for sodium, 27.7-39.9 kg for potassium, 25.2-49.4 kg for calcium, 20.9-40.9 kg for magnesium, 28.2-58.0 kg for ammonium-nitrogen, 33.6-67.2 kg for nitrate-nitrogen, 215.7-407.3 kg for chloride, and 57.8-113.1 kg/ha/yr for sulphate. These high nutrient inputs from precipitation are obviously important to Casuarina stands growing on coastal sandy soils. Annual amounts of litterfall in the 3 Casuarina plantations ranged from 8.4 to 11.5 ton/ha, which could be broken down into 87.5-93.6% branchlet litter, 4.6-6.2% branch litter, and 1.8-6.3% organic litter. High litterfall occurred in the months of April, May, and July, and the falling pattern of litter was the same as the spring falling type of evergreen deciduous forests. Seasonal variation in nitrogen and phosphorus concentrations of litterfall was the lowest during September to November, with bimodal peaks in about June or July and the following February. But the nutrient content curves for potassium, magnesium, and sodium of branch litter showed opposite patterns, with the highest values during November and December, and the lowest about April or May. Nutrient return amounts in the Casuarina stands were 123.5-198.0 for nitrogen, 5.0-8.3 for phosphorus, 19.8-44.2 for potassium, 64.5-85.1 for calcium, 54-10.6 for magnesium, and 17.6-33.0 kg ha-1 yr-1 for sodium. The amounts of nitrogen and sodium return by litterfall in the Suhu Casuarina stands were quite remarkable. The net primary productions of the 3-yr-old Casuarina stands were 20.2 and 37.5 ton/ha, but the 9-yr-old stand yielded only 6.5 ton/ha. Branchlet litter made up the highest proportion in the net primary production of each stand. Biomass increments were higher than the corresponding litterfall in the 3-yr-old stands, whereas the 9-yr-old stand showed the opposite. These data suggest that the 3-yr-old stands were still at a fast-growing stage, while growth in the 9-yr-old stand had slowed considerably, so that increments of the entire stand appeared to almost equal the wither amounts.
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