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、结构与组成的重大转变。尽管这类害虫的直接影响常被视为引发生态系统功能改变的物理干扰（即落叶、叶面积下降以及树木死亡），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具有重要意义。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所导致的功能、结构与组成变化或至关重要。本研究旨在加深对铁杉球蚜（hemlock woolly adelgid, HWA）相关关联的认知，该害虫正在横跨北美东部的不断扩张区域内摧毁铁杉林，且有可能在其大部分分布范围内消灭这一长寿且极耐阴的树种。我们针对不同侵染程度的林分，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关联起来。铁杉球蚜引发了冠层生物量及其分布的重大改变：未受侵染的树木，其冠层生物量从中心向边缘递减，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，冠层总生物量显著更低，新叶产生量更少，且枯落物量与冠层生物量之间无相关性。叶片氮含量百分比受针叶龄级与侵染程度的强烈影响，在承载铁杉球蚜最高密度的年轻针叶中含量最高。叶片碳含量百分比则不受铁杉球蚜侵染或针叶龄级的影响。铁杉针叶上的附生微生物丰度在冠层内变化较小，但中重度侵染树木上的细菌、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发生了显著改变。未受侵染树木下方的穿透雨呈现显著梯度，水量从冠层边缘向树干递减；而受侵染树木下方的穿透雨量大幅增加，空间分布无规律，且氮化合物、碳与阳离子浓度更高。在新英格兰南部区域景观中，树木与幼树死亡率、林下植被组成变化呈现显著的由南向北递减梯度，这与侵染持续时长相对应。从区域尺度来看，黑桦作为一种需氮物种，其竞争力随氮有效性提升而增强，因此从铁杉林衰退中获益最多。