Impact of Hemlock Woolly Adelgid on Canopy Throughfall in Southern New England 2002

Non-native insect pests may strongly disrupt forest ecosystems and trigger major shifts on nutrient cycling, structure and composition. Although the immediate impact of these pests are frequently examined as physical disturbances (i.e., defoliation, decline in leaf area, and tree mortality) that initiate changes in ecosystem function, the insects often generate fundamental biochemical and trophic changes in tree canopies that may be equally important in altering ecosystem dynamics. Consequently, investigation of the linkages between canopy-level, ecosystem and environmental impacts may be critical for a thorough understanding of functional, structural, and compositional changes resulting from pest infestation. We sought to establish a better understanding of these linkages for the hemlock woolly adelgid (HWA), which is devastating hemlock forests in an expanding region across eastern North American and has the potential to eliminate this long-lived and extremely shade-tolerant species across much of its range. We examined the impact of the adelgid on hemlock needle chemistry and epiphytic microorganisms, litter production, and shoot growth in stands differing in their levels of infestation and linked these to shifts in canopy nutrient cycling and stand and landscape dynamics. HWA initiated major changes in canopy biomass and distribution. Whereas uninfested trees exhibit a decline in canopy biomass from the center to the periphery and a positive correlation between total needle litter and estimated biomass, infested trees support predominantly woody biomass, have significantly less total canopy biomass, produce less new foliage and exhibit no correlation between litter and canopy biomass. Foliar %N was strongly influenced by needle age and the level of infestation and was highest in young foliage supporting the highest densities of HWA. Foliar %C was unaffected by HWA or foliar age. Epiphytic microorganisms on hemlock needles exhibited little variation in abundance within canopies, but bacteria, yeasts and filamentous fungi were significantly more abundant on medium and heavily infested than uninfested trees. Throughfall chemistry, quantity, and spatial pattern were strongly altered by HWA. Beneath uninfested trees throughfall exhibits a strong gradient, decreasing in volumes from the canopy periphery to the trunk. Beneath infested trees the amount of throughfall is greatly increased, spatially unpatterned and characterized by higher concentrations of nitrogen compounds, carbon and cations. Across the southern New England landscape there is a strong south to north gradient of decreasing tree and sapling mortality and understory compositional change that correspond to the duration of infestation. Regionally, black birch, a nitrogen demanding species that is competitively enhanced by increasing nitrogen availability is profiting most from hemlock decline.非本土昆虫害虫可严重扰乱森林生态系统，并引发养分循环、群落结构与物种组成的重大转变。尽管这类害虫的直接影响常被视作引发生态系统功能改变的物理干扰（即落叶、叶面积下降与树木死亡），但这些昆虫往往会在树木冠层中引发根本性的生化与营养级联变化，这类变化对改变生态系统动态同样至关重要。因此，探究冠层尺度、生态系统与环境影响之间的关联，对于全面理解害虫侵染引发的功能、结构与组成变化至关重要。本研究旨在针对铁杉球蚜（hemlock woolly adelgid, HWA）厘清上述关联：该害虫正在北美东部不断扩张的区域内肆虐铁杉林，并有潜力在其分布的大部分区域彻底消除这种长寿且极耐阴的树种。我们针对侵染程度各异的林分，探究了该球蚜对铁杉针叶化学、附生微生物、枯落物产生以及枝条生长的影响，并将这些指标与冠层养分循环、林分与景观动态的变化建立关联。铁杉球蚜引发了冠层生物量与分布的重大改变。未受侵染的树木，其冠层生物量由中心向边缘递减，且总针叶枯落物量与估算生物量呈正相关；而受侵染的树木则以木质生物量为主，总冠层生物量显著更低，新生叶片产量更少，且枯落物量与冠层生物量之间无相关性。叶片的氮（%N）含量受针叶树龄与侵染程度的显著影响，在承载铁杉球蚜最高密度的新生叶片中，氮含量最高。叶片的碳（%C）含量则不受铁杉球蚜或叶片年龄的影响。铁杉针叶上的附生微生物在冠层内的丰度变化不大，但在中度至重度侵染的树木上，细菌、酵母菌与丝状真菌的丰度显著高于未受侵染的树木。穿透雨的化学组成、水量与空间格局均因铁杉球蚜的侵染发生显著改变。未受侵染树木下方的穿透雨存在显著梯度，水量由冠层边缘向树干逐渐减少；而受侵染树木下方的穿透雨量大幅提升，空间分布无规律，且氮化合物、碳与阳离子的浓度更高。在新英格兰南部的景观中，树木与幼树死亡率、林下植被组成变化均呈现从南到北逐渐降低的梯度，且这一梯度与侵染持续时长相对应。区域层面来看，黑桦（black birch）是一种需氮物种，其竞争力会随氮素可利用性提升而增强，因此在铁杉林衰退的过程中获益最多。