Ecosystem Response to Hemlock Woolly Adelgid in Southern New England 1998-2019

In 1998 we began examining the response of ecosystem processes to the stress and mortality caused by the introduced hemlock woolly adelgid (HWA) in southern New England. Healthy hemlock forests typically have slow decomposition and N cycling rates due to their low foliar N content and cool microclimate. However, thinning canopies associated with HWA infestations are starting to reverse this trend, due to dramatic increases in light levels and soil temperature. Within 8 study sites varying in HWA infestion level, we continue to investigate the magnitude and duration of N dynamics associated with HWA infestations by measuring nitrogen (N) mineralization rates using close-topped soil cores during the last five years. In addition, ion-exchange resin bags are used to estimate the spatial availability of N within sites and the extent to which NO3 is being lost. Measurements of gravimetric moisture content and soil temperature were used with hemispherical photographs to assess microenvironmental changes. During the first five years of this study, thinning canopies from heavy HWA damage resulted in increased light, soil temperature, and mineral soil moisture, and decreased forest floor moisture content. Heavily infested sites continue to have larger extractable NH4 and NO3 – N pools, and significantly higher net nitrification rates than healthy hemlock forests. In addition, resin bags captured more ammonium and nitrate in infested versus uninfested stands. Results indicate that introduced pests and selective tree decline can rapidly and dramatically alter ecosystem processes, even prior to the onset of extensive tree mortality. In 2001, we began examining 2 additional stands that contain high overstory hemlock mortality and a dense black birch understory. We will continue to sample these stands as they deteriorate to determine the extent to which changes in overstory composition, microenvironment, and soil conditions produce fundamental changes in the cycling of nitrogen. Several decomposition studies have also been undertaken to examine how decomposition may be driving N-cycling changes in infested stands. Our study has focused on three key drivers of decomposition: (i) changes in foliar quality due to HWA herbivory, (ii) changes in forest floor microclimate that occur as the canopy thins, and (iii) the effect of species composition change. All three have had an impact on decomposition. Furthermore, our data suggest that these changes are coupled with N-cycling dynamics and may helped to elucidate the mechanisms driving increased N availability in these forests. We sampled hemlock foliar N, C, and lignin along an extensive gradient of hemlock forests. These stands range from uninfested to forests with complete hemlock mortality and currently dominated by black birch (Betula lenta). Foliar % C and lignin were not affected by HWA infestation however, foliar N was higher in infested stands. Higher initial foliar N was found to increase the rate of N immobilization in decomposing foliage. Additionally, our data suggest that in many forests infested foliage may switch from a net sink to a net source of N more rapidly compared to uninfested foliage. This may be an important contribution to increased N availability in infested forests. Altered microclimate has had an important effect on foliar decomposition. Surface litter decomposition was slowed in many infested forests due to poor conditions for microbial establishment on litter. However, our work suggests this effect may be limited only to litter at the forest floor surface. Cellulose paper buried at the forest floor mineral soil interface had significantly greater mass loss in infested stands. We also observed that as surface litter became buried, its rate of mass loss increased in infested stands. In 2001 we began a comparative study of hemlock, black birch, and mixed litter decomposition. During 2002 we collected litter bags after six and twelve months of decomposition. Results after six months show that black birch litter decomposed more rapidly than hemlock and mixed litter. The higher rates of litter decomposition that occur after the switch from hemlock to black birch litter-fall are likely a major mechanism of forest floor mass loss after hemlock mortality and logging. Taken as a whole, our studies illustrate the dynamic effects of hemlock woolly adelgid infestation on decomposition in eastern hemlock forests. All stages of infestation studied had significant effects on decomposition by altering the chemical changes in litter, altering litter inputs (hemlock to black birch), and soil microclimate conditions.

1998年，我们启动研究，旨在探究新英格兰南部地区外来入侵物种铁杉球蚜（hemlock woolly adelgid, HWA）引发的胁迫与树木死亡对生态系统过程的影响。健康铁杉林通常因叶片氮含量较低且微气候凉爽，表现出较慢的分解速率与氮循环速率。然而，伴随HWA侵染出现的林冠疏伐，正开始逆转这一趋势——这是因为光照水平与土壤温度出现显著升高。我们在8个HWA侵染程度各异的研究样地中，于过去五年间采用封顶式土芯法测定氮（nitrogen, N）矿化速率，以此持续探究与HWA侵染相关的氮动态变化幅度与持续时长。此外，我们还使用离子交换树脂袋，估算样地内氮的空间有效性，以及硝态氮（NO₃⁻）的流失程度。我们通过测定重量含水率与土壤温度，并结合半球摄影法，评估微环境变化。在本研究的前五年中，受HWA重度为害的林冠疏伐，导致光照、土壤温度与矿质土壤含水率升高，而枯落物层含水率降低。重度侵染样地的可提取态铵态氮与硝态氮池显著更大，净硝化速率也远高于健康铁杉林。此外，相较于未侵染林分，侵染样地的树脂袋捕获到更多铵态氮与硝态氮。研究结果表明，外来害虫与选择性树木衰退，甚至在出现大规模树木死亡之前，就能快速且显著地改变生态系统过程。2001年，我们启动了对另外2个林分的研究：这些林分上层铁杉死亡率较高，林下植被以浓密的黑桦为主。我们将持续对这些林分进行采样以追踪其退化过程，明确上层林分组成、微环境与土壤条件的变化，在多大程度上会引发氮循环的根本性改变。我们还开展了多项分解研究，以探究分解过程如何驱动侵染样地的氮循环变化。本研究聚焦于分解过程的三个关键驱动因素：(i) HWA取食引发的叶片品质变化；(ii) 林冠疏伐导致的枯落物层微环境变化；(iii) 物种组成变化的影响。这三个因素均对分解过程产生了显著影响。此外，我们的数据表明，这些变化与氮循环动态存在耦合关系，或有助于阐明推动这些森林氮有效性提升的机制。我们沿铁杉林侵染程度的完整梯度（从未侵染到完全丧失铁杉、目前以黑桦[Betula lenta]为主的林分），采集铁杉叶片样品并测定其氮、碳与木质素含量。研究发现，叶片碳百分比与木质素含量不受HWA侵染的影响，但侵染样地的叶片氮含量更高。初始叶片氮含量较高，会提升分解过程中叶片的氮固持速率。此外，我们的数据表明，在许多侵染林分中，侵染叶片相较于未侵染叶片，能更快地从氮的净汇转变为氮的净源——这或许是侵染林分氮有效性提升的重要贡献因素。微环境改变也对叶片分解产生了重要影响。在许多侵染林分中，由于枯落物表面微生物定殖条件不佳，地表枯落物的分解速率有所减缓。但我们的研究表明，这一效应可能仅局限于森林地表的枯落物。埋置于枯落物层与矿质土壤界面的纤维素滤纸，在侵染样地中的质量损失率显著更高。我们还观察到，随着地表枯落物被掩埋，其在侵染样地中的质量损失速率会有所提升。2001年，我们启动了一项针对铁杉、黑桦以及混合枯落物分解的对比研究。2002年，我们在分解进行6个月与12个月后回收了枯落物袋。6个月的分解结果显示，黑桦枯落物的分解速率快于铁杉枯落物与混合枯落物。从铁杉枯落物向黑桦枯落物的输入转变后，枯落物分解速率升高，这可能是铁杉死亡与采伐后森林地表质量损失的主要机制。综合来看，我们的研究阐明了铁杉球蚜侵染对东部铁杉林分解过程的动态影响。我们所研究的各侵染阶段，均通过改变枯落物的化学特性、枯落物输入模式（从铁杉转为黑桦）以及土壤微环境条件，对分解过程产生了显著影响。