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，铁杉球蚜（hemlock woolly adelgid, HWA；Adelges tsugae）在美国东北部持续蔓延，导致铁杉大规模衰退和死亡。HWA已侵扰马萨诸塞州超过40%的城镇，因此许多土地所有者正考虑对其东部铁杉（Tsuga canadensis）林分进行预防性采伐。为探究此类采伐活动的局部影响，我们研究了10个以铁杉为主的样地（位于公共和私人土地上，铁杉于1至12年前被采伐）的林分和生态系统动态，并以附近未砍伐的森林作为对照。

采伐样地中移除了42%至97%的铁杉树干和基面积。砍伐年限较长的样地中，植被以幼苗密度为10500至24000株/公顷的植株为主，主要包括黑桦（Betula lenta）、红枫（Acer rubrum）和白松（Pinus strobus）。各站点幼苗密度平均为5株/平方米，在砍伐后6至9年的站点达到峰值20株/平方米，主要包括铁杉、白松、黑桦和红枫。采伐样地中常见的其他林下物种包括多种悬钩子属（Rubus）植物（盖度通常平均为30%）、香蕨（Dennstaedtia punctilobula）以及多种薹草属（Carex）植物。

采伐导致土壤温度升高3至5摄氏度，且近期采伐样地的土壤相比砍伐年限较长的样地更干燥。近期采伐样地有机层土壤的净氮矿化速率平均略高于10千克氮/公顷/年，而砍伐年限较长的样地约为5千克氮/公顷/年。所有采伐样地矿质层土壤的净氮矿化速率均较低，范围在2.1至3.5千克氮/公顷/年之间。所有采伐年限样地的两个土壤层中硝化速率均较低。近期采伐样地树脂袋的氮捕获量（NH4- + NO3+）显著更高，平均约为1200微克氮/克树脂，而砍伐年限较长的样地为300至600微克氮/克树脂。

马萨诸塞州样地的硝化速率和树脂袋硝酸盐捕获量低于此前报道的、具有长期HWA侵扰史后被采伐的样地。本研究结果与以往研究一致，即近期采伐样地的桦树和枫树定植量增加，且氮有效性高于砍伐年限较长的样地。与先前研究不同的是，马萨诸塞州样地的采伐还促进了白松和铁杉等针叶树的大量更新，这表明母树可用性、苗床特征以及部分马萨诸塞州样地中HWA的缺失，是本研究中针叶树幼苗密度较高的原因。