Landscape-Scale Forest Dynamics in the Luquillo Experimental Forest in Puerto Rico 1936-1989

This study examined landscape-scale forest dynamics in the Luquillo Experimental Forest (Puerto Rico). The analysis was based on vegetation maps created from aerial photographs taken in 1936 and 1989. For details on methods and results, please see the published paper (Foster, D. R., M. Fluet and E. R. Boose. 1999. Human or natural disturbance: landscape-scale dynamics of the tropical forests of Puerto Rico. Ecological Applications 9: 555-572). The Abstract from the paper is reproduced below. "Increasingly ecologists are recognizing that human disturbance has played an important role in tropical forest history and that many assumptions concerning the relative importance of natural processes warrant re-examination. To assess the historical role of broad-scale human versus natural disturbance on an intensively studied tropical forest we undertook a landscape-level analysis of forest dynamics in the Luquillo Experimental Forest (LEF; 10,871 ha) in eastern Puerto Rico. Using aerial photographs (1936 and 1989), GIS, a model of topographic exposure to hurricane winds, and historical data, we sought to: (1) document historical changes in extent, cover and type of forest vegetation, (2) evaluate the distribution of land-use and hurricane impacts, (3) assess the contributions of these processes in controlling current vegetation patterns, and (4) relate these results to ongoing ecological, conservation and natural resource discussions. "With over 1000 m of relief in the LEF, the broad vegetation zones of Tabonuco (below 600 m a.s.l.), Colorado (600-900 m), Dwarf (above 900 m), and Palm forest are determined by environmental gradients. However, over the past 60-100 years forest extent, cover, and type have been transformed: in 1936, 40% of the LEF was unforested or secondary forest and less than 50% had continuous canopy (more than 80% cover); in 1989, less than 97% was continuous forest. Secondary forest and agricultural lands in 1936 were replaced largely by Tabonuco and Colorado forest, which increased from 8% and 28% (1936) to 26% and 45% (1989). "These broad-scale vegetation dynamics are best explained by a gradient of human land use, intense at low elevations and decreasing on steep, high terrain, which peaked historically around 1900 followed by a gradual decline in agriculture. GIS analysis and historical sources suggest that essentially all of the LEF was affected by human activity and that Tabonuco forest, which is the focus of LTER research, has been most substantially altered and is largely of secondary origin. Rapid reforestation following agricultural decline has obscured much of the past land use and confirms the resiliency of some tropical forests to intensive human disturbance. Impacts of earlier hurricanes (e.g., in 1928 and 1932), though not evident in the broad forest pattern in 1936, may be significant in explaining the distributions of Colorado and Palm forest. Damage from Hurricane Hugo in 1989 indicates that natural disturbance is increasingly important as land use declines and forest cover and height increase. However, this study and post-Hugo studies emphasize that land-use legacies are long-lasting and need to be considered in modern ecological studies and natural resource management. The subtle though persistent effects of historical human activities may have profound consequences for modern forest ecosystems in the tropics."

本研究聚焦波多黎各卢奎约实验森林（Luquillo Experimental Forest）的景观尺度森林动态。分析基于1936年与1989年航空影像绘制的植被图。有关研究方法与结果的详细内容，请参阅已发表论文（Foster, D. R., M. Fluet 与 E. R. Boose. 1999. 人为扰动抑或自然扰动：波多黎各热带森林的景观尺度动态. 《生态应用》9卷: 555-572）。以下为该论文的摘要复刻："生态学家日益认识到，人为扰动在热带森林历史中发挥了重要作用，而诸多关于自然过程相对重要性的假设亟待重新审视。为评估大范围人为与自然扰动在受深入研究的卢奎约实验森林（Luquillo Experimental Forest，简称LEF，面积10871公顷，位于波多黎各东部）的历史作用，我们开展了景观尺度的森林动态分析。本研究借助航空影像（1936年与1989年）、地理信息系统（GIS）、面向飓风风害的地形暴露模型与历史数据，旨在达成以下目标：(1) 记录森林植被的分布范围、覆盖度与类型的历史变化；(2) 评估土地利用与飓风影响的分布格局；(3) 解析这两类过程对当前植被格局的调控作用；(4) 将研究结果与当前生态学、保护生物学与自然资源相关讨论相结合。""卢奎约实验森林的海拔落差超过1000米，其主要植被带——塔博努科林（海拔低于600米）、科罗拉多林（600-900米）、矮林（海拔高于900米）与棕榈林——由环境梯度决定。然而在过去60至100年间，森林的分布范围、覆盖度与类型均发生了显著转变：1936年，该实验森林40%的区域为无林地或次生林，仅不足50%的区域拥有连续冠层（盖度超过80%）；至1989年，不足97%的区域为连续森林。1936年的次生林与农用地大多被塔博努科林与科罗拉多林取代，二者占比分别从1936年的8%与28%提升至1989年的26%与45%。""上述景观尺度的植被动态，可通过人类土地利用梯度得到最佳解释：低海拔区域的土地利用强度最高，随着地形陡峭程度提升、海拔升高，强度逐渐降低。历史上，土地利用强度在1900年左右达到峰值，随后农业活动逐渐减少。地理信息系统分析与历史资料显示，卢奎约实验森林几乎全域均受人类活动影响；作为长期生态研究（Long-Term Ecological Research，简称LTER）核心研究对象的塔博努科林，受干扰程度最为显著，且大多为次生起源。农业活动衰退后的快速造林掩盖了多数既往土地利用痕迹，这也印证了部分热带森林对高强度人为扰动的恢复力。早期飓风（如1928年与1932年飓风）的影响虽未在1936年的宏观森林格局中体现，但或许可有效解释科罗拉多林与棕榈林的分布格局。1989年雨果飓风造成的破坏表明，随着土地利用强度下降、森林覆盖度与林分高度提升，自然扰动的影响日益凸显。然而，本研究与雨果飓风后的后续研究均强调，土地利用遗留效应持久存在，在现代生态学研究与自然资源管理中必须予以考量。历史人类活动产生的细微却持续的影响，可能对热带地区的现代森林生态系统造成深远影响。"