Ecosystem and Vegetation Response to Hemlock Logging in Southern New England 1999-2005

This study compares the magnitude and trajectory of vegetation and ecosystem function dynamics associated with the direct impact of hemlock woolly adelgid (Adelges tsugae; HWA) infestation versus the indirect consequences of HWA-induced damage in the form of salvage and pre-emptive logging of hemlock (Tsuga canadensis (L.) Carriere) forests. The study was conducted within an area extending from southern Connecticut up to and including the Connecticut River lowlands west to the Berkshire Plateauin central Massaschusetts, USA. Overstory and understory vegetation and ecosystem function parameters such as decomposition and nitrogen cycling were examined in logged and unlogged portions of 10 hemlock stands varying in HWA damage intensity. Intensive hemlock logging generated more rapid and pronounced microenvironment and vegetation changes than chronic HWA damage. Black birch (Betula lenta L.) seedling densities and percent cover of brambles (Rubus L. spp.), sedges (Carex L. spp.) and hay-scented fern (Dennstaedia punctilobula Michx.) were significantly higher in recent harvests versus HWA-damaged and undamaged sites. High black birch sapling densities (greater than 7000 ha –1) were common in the older harvests but not in adjacent, HWA-damaged portions of these sites. Undamaged sites had 20% more forest floor mass than HWA-damaged sites and double the mass of older cuts. Mass loss rates of cellulose paper suggest that conditions were more favorable for decomposition in the damaged and older logged sites. Recently cut sites had significantly larger inorganic N pools than undamaged forests, although total net nitrogen (N) mineralization rates were not significantly different among treatments. Nitrification rates of 0.2 kg ha-1 day-1 measured in the oldest cuts were three times greater than in HWA-damaged sites and over 200 times greater than in undamaged hemlock sites. However, resin bag capture in the older cuts was similar to amounts captured in undamaged and damaged forests, suggesting that excess nitrogen was being utilized in vegetative uptake. In contrast, relatively large amounts of ammonium and nitrate captured in recent harvests indicate higher N availability, less vegetative uptake, and a greater potential for N leaching. Results suggest that both the decline associated with HWA infestation and the indirect effects of HWA in the form of logging are generating profound changes in structure, composition, and ecosystem function in these forests, although at different spatial and temporal scales. Hemlock harvesting imposed more abrupt microenvironmental changes, and rapidly reduced vegetative cover while chronic HWA infestation led to gradually thinning canopies. Both disturbances led to black birch dominated forests, although logging resulted in greater amounts of shade-intolerant regeneration, higher soil pH and nitrification rates, and reduced forest floor mass. Preemptive cutting of undamaged forests may lead to greater N losses than those associated with HWA infestation or logging of deteriorated hemlock forests, due to reduced vegetative uptake. Silvicultural methods that allow for vegetation establishment prior to harvesting will likely lessen the ecological impacts of hemlock removal.本研究对比了铁杉球蚜（Adelges tsugae; HWA）侵染的直接影响，与铁杉球蚜引发损伤后采取的补救性采伐及预防性采伐（针对加拿大铁杉Tsuga canadensis (L.) Carriere林）所带来的间接后果，二者在植被与生态系统功能动态的幅度及轨迹上的差异。本研究的研究区域涵盖美国马萨诸塞州中部从康涅狄格州南部向北延伸至康涅狄格河低地、西至伯克希尔高原的范围。研究对10个受铁杉球蚜损伤程度不同的铁杉林分的采伐与未采伐区域，开展了上层林冠、下层植被以及分解作用、氮循环等生态系统功能参数的调查分析。高强度的铁杉采伐比长期受铁杉球蚜侵染的损伤更能快速且显著地改变微环境与植被状况。近期采伐区域的黑桦（Betula lenta L.）幼苗密度，以及悬钩子属（Rubus L. spp.）、薹草属（Carex L. spp.）和细叶碗蕨（Dennstaedtia punctilobula Michx.）的盖度，均显著高于受铁杉球蚜损伤的样地与未受损样地。老龄采伐迹地的黑桦幼树密度普遍较高（超过7000株·公顷⁻¹），但相邻的受铁杉球蚜损伤的样地则未出现该现象。未受损样地的枯落物层生物量比受损伤样地高20%，是老龄采伐迹地的两倍。纤维素纸的质量损失速率表明，在受损伤样地与老龄采伐迹地中，分解作用的环境条件更为适宜。近期采伐样地的无机氮库显著大于未受损森林，尽管各处理组的总净氮矿化速率并无显著差异。老龄采伐迹地的硝化速率达0.2 kg·ha⁻¹·d⁻¹，是受铁杉球蚜损伤样地的3倍，较未受损铁杉林高出200倍以上。不过，老龄采伐迹地的树脂袋氮捕获量与未受损及受损伤样地相近，这表明过量的氮被植被吸收利用。与之形成对比的是，近期采伐样地捕获的铵态氮与硝态氮含量较高，这意味着氮素可利用性更高、植被吸收更少，且氮素淋溶风险更大。研究结果表明，无论是铁杉球蚜侵染引发的林分衰退，还是以采伐为代表的铁杉球蚜间接影响，均会对这些森林的结构、组成与生态系统功能产生深刻改变，只是二者的时空尺度有所不同。铁杉采伐会引发更剧烈的微环境变化，并快速降低植被盖度；而长期的铁杉球蚜侵染则会导致林冠逐渐稀疏。两种干扰均会导致林分以黑桦为主，但采伐会带来更多的喜光物种更新、更高的土壤pH值与硝化速率，并降低枯落物层生物量。由于植被吸收作用减弱，对未受损林分开展预防性采伐，可能比铁杉球蚜侵染或受损伤铁杉林的采伐带来更严重的氮素流失风险。在采伐前预先促进植被建立的营林措施，或可减轻铁杉移除带来的生态影响。