Silvicultural Growth Performances of Thirteen Endemic Broadleaf Trees of Taiwan

We investigated the growth potential of some endemic broadleaf species used in restorative reforestation. In March 1993, mix-plot planting of the 13 test species native to mid-elevation forests of Taiwan was carried out in a tract situated at 800 m elevation with a southwest facing slope at the Taimali Research Center Taiwan Forestry Research Institute, Taitung, southeastern Taiwan. Each species was planted with 3~5 plot repetitions, and in each plot, 50 trees of the same species were planted with a 2 x 2-m spacing. A survey of sapling growth was carried out periodically, and tree forms were also investigated in later years. Results showed that common elaeocarpus (Elaeocarpus sylvestris) and incense machilus (Machilus zuihoensis var. zuihoensis) saplings had better growth performances and survival rates, and the respective mean height growth had reached 750 and 730 cm 9 yr after out-planting. Their survival rates all reached 60%. With the exception of Pasania kawakamii and Tricalysia dubia which did not reach 5 m in height, the mean height growth of the other indigenous broadleaf trees including Pasania ternaticupula, Schima superba, Schefflera octophylla, Lithocarpus castanopsisifolius, and Cinnamomum micranthum, all exceeded 5m, although their growth was not as fast as some general plantation species such as Fraxinus formosana, and their mean diameter at breast height (dbh) growth all reached 7 cm. The net height growth of the studied species in the first 3 yr, was 30~50 cm yr-1, with the exception of Zelkova serrata which reached 80 cm yr-1. However, all species had net height growth exceeding 80 cm yr-1 during 3~5 yr, with E. sylvistris reaching 140 cm yr-1. The fast-growing period of the saplings among tree species lasted to 88 mo after out-planting. These results suggest that endemic broadleaf species have tremendous afforestation potential in mid- and low-elevation plantation forests of Taiwan. When coupled with good tending and management of the forests, even greater silvics potential can be realized.
Even in the early stage of growth, saplings often had forking tendencies with 2 m or more of crown width of planted trees, which may cause retardation of subsequent growth performance and may also influence the tree quality for timber utilization. In particular, as the average forking rate reached 40%, we deem that pruning and thinning of small-diameter logs should be scheduled to promote tree form quality. A correlated regression analysis of tree height growth and tree age indicated that 2-degree polynomial regression equations (R2 ≥ 0.90) by tree age of tree species can be used to predict the early growth performance of planted trees at the site.

本研究针对若干应用于修复造林的乡土阔叶树种的生长潜力展开调研。1993年3月，研究团队于中国台湾东南部台东地区的台湾林业试验所太麻里研究中心，于海拔800米、西南向坡地的地块中，针对13种原产于台湾中山带森林的受试树种开展混区种植试验。每个树种设置3~5个区组重复，每个区组内以2×2米的株行距种植50株同树种苗木。研究团队定期开展苗木生长调查，并于后续年份对林木形质进行观测。
研究结果显示，杜英（Elaeocarpus sylvestris）和台湾雅楠（Machilus zuihoensis var. zuihoensis）的苗木生长表现与存活率均较为优异，造林定植9年后的平均树高生长量分别达到750厘米与730厘米，二者存活率均达到60%。除小西氏石栎（Pasania kawakamii）和香茜（Tricalysia dubia）的平均树高未达5米外，其余受试乡土阔叶树种包括三斗石栎（Pasania ternaticupula）、木荷（Schima superba）、鹅掌柴（Schefflera octophylla）、锥叶柯（Lithocarpus castanopsisifolius）以及少花桂（Cinnamomum micranthum）的平均树高生长量均超过5米；不过其生长速率不及台湾白蜡（Fraxinus formosana）等常规造林树种，但其平均胸径（diameter at breast height, dbh）生长量均达到7厘米。受试树种在前3年的净树高生长量为30~50厘米/年，仅榉树（Zelkova serrata）达到80厘米/年。但在造林后3~5年期间，所有树种的净树高生长量均超过80厘米/年，其中杜英可达140厘米/年。各树种苗木的速生期可持续至造林后88个月。上述结果表明，乡土阔叶树种在中国台湾中低海拔人工林造林中具备极大潜力；若配合优良的抚育经营措施，可进一步发挥其营林开发潜力。
即便在生长早期，栽植苗木即常出现分叉倾向，其冠幅可达2米以上，这可能抑制后续生长表现，同时影响林木的用材品质。尤其当平均分叉率达到40%时，需规划开展修枝与小径木疏伐作业，以提升林木形质。针对树高生长与林龄的相关回归分析表明，基于树种林龄构建的二次多项式回归方程（决定系数R²≥0.90）可用于预测本试验点栽植苗木的早期生长表现。