Impacts of Hemlock Harvesting in Central Massachusetts 2003-2009

The recent unimpeded infestation of the hemlock woolly adelgid (HWA; Adelges tsugae) across the northeastern U.S. is driving large-scale hemlock decline and mortality. HWA has already infested over 40% of the towns in Massachusetts and, as a result, many landowners are considering pre-emptively harvesting their eastern hemlock (Tsuga canadensis) stands. To examine the local impacts of this cutting activity, we examined stand and ecosystem dynamics in 10 hemlock-dominated sites on public and private lands where hemlocks were harvested between 1 to12 years ago with nearby uncut forests. Forty-two to ninety-seven percent of hemlock stems and basal area were removed from the harvested sites. Sapling densities of 10,500 to 24,000 stems ha-1 dominated the vegetation at older cuts and consisted primarily of black birch (Betula lenta), red maple (Acer rubrum), and white pine (Pinus strobus). Seedling densities averaged 5 m-2 across sites, peaked at 20 m-2 at 6 to 9 year old sites, and consisted of hemlock, white pine, black birch, and red maple. Additional understory species that were common in cuts sites included various raspberry (Rubus) species, often averaging 30% cover, hay-scented fern (Dennstaedtia punctilobula), and various sedge (Carex) species. Harvesting resulted in soils that were 3 to 5 (deg C) warmer and tended to be drier in recent vs. older cuts. Net nitrogen mineralization rates in organic soils averaged just over 10 kg N ha-1yr-1 in recent cuts and approx. 5 kg N ha-1yr-1 in older cuts. Mineral soil net mineralization rates were lower among all cuts, ranging from 2.1 to 3.5 kg N ha-1yr-1. Nitrification rates were low in both soil horizons at all harvest ages. Recent cuts had much higher nitrogen capture (NH4- + NO3+) on resin bags averaging around 1200 μg N g resin-1 vs. 300 to 600 μg N g resin-1 captured in older cuts. Nitrification rates and nitrate capture on resin bags were lower in Massachusetts sites than those reported on sites with a long history of HWA that were then subsequently logged. Findings from this study corroborate past work, as more recent cuts also exhibited increased birch and maple establishment and higher N availability than older cuts. In contrast to prior studies, cutting in Massachusetts sites also led to abundant conifer regeneration of white pine and hemlock, suggesting that seed tree availability, seedbed characteristics, and lack of HWA at several of the MA sites contributed to the higher conifer seedling densities found in this study.

近期，美国东北部地区的铁杉球蚜（hemlock woolly adelgid, HWA; Adelges tsugae）无阻碍扩散侵染，正引发大规模铁杉衰退与死亡。目前，马萨诸塞州已有超40%的城镇遭受HWA侵染，因此诸多土地所有者正考虑对其所属的东部铁杉（Tsuga canadensis）林分开展预防性采伐。为探究此类采伐活动对林分与生态系统动态的本地影响，我们在公共及私有土地上的10个以铁杉为主导的林分中开展研究：这些林分的铁杉分别在1至12年前被采伐，并设置邻近未采伐林分作为对照样地。采伐样地中，铁杉植株及胸高断面积的移除比例为42%至97%。采伐年限较久的样地中，幼树密度达10500至24000株/公顷，植被组成以黑桦（Betula lenta）、红枫（Acer rubrum）与白松（Pinus strobus）为主。各样地的幼苗密度平均为5株/平方米，在采伐后6至9年的样地中达到峰值20株/平方米，幼苗组成涵盖铁杉、白松、黑桦与红枫。采伐样地中常见的其他林下植物还包括多种悬钩子属（Rubus）物种（平均盖度约30%）、香碗蕨（Dennstaedtia punctilobula）以及多种莎草属（Carex）物种。采伐操作导致土壤温度升高3至5摄氏度；相较于采伐年限较久的样地，近期采伐的样地土壤更为干燥。有机土层的净氮矿化速率方面，近期采伐样地平均略高于10千克氮/公顷·年，而采伐年限较久的样地约为5千克氮/公顷·年。所有采伐样地的矿质土层净氮矿化速率均较低，区间为2.1至3.5千克氮/公顷·年。所有采伐年限的样地中，两层土壤的硝化速率均处于较低水平。近期采伐样地的树脂袋氮捕获量（NH4+ + NO3-）显著更高，平均约为1200微克氮/克树脂，而采伐年限较久的样地仅为300至600微克氮/克树脂。马萨诸塞州样地的硝化速率及树脂袋硝态氮捕获量，均低于已有研究报道的长期遭受HWA侵染后再进行采伐的样地水平。本研究结果与过往研究相符：相较于采伐年限较久的样地，近期采伐样地的桦树与枫树更新更为旺盛，且氮素可利用性更高。与过往研究不同的是，马萨诸塞州的采伐样地还实现了白松与铁杉的大量针叶树更新，这表明本研究中多个样地具备种源树可利用性、苗床特性优势，且未受HWA侵染，这些因素共同促成了本研究中观测到的较高针叶树幼苗密度。