Harvard University, Harvard Forest , Petersham, MA , USA.
PeerJ. 2013 Feb 19;1:e41. doi: 10.7717/peerj.41. Print 2013.
Loss of foundation tree species rapidly alters ecological processes in forested ecosystems. Tsuga canadensis, an hypothesized foundation species of eastern North American forests, is declining throughout much of its range due to infestation by the nonnative insect Adelges tsugae and by removal through pre-emptive salvage logging. In replicate 0.81-ha plots, T. canadensis was cut and removed, or killed in place by girdling to simulate adelgid damage. Control plots included undisturbed hemlock and mid-successional hardwood stands that represent expected forest composition in 50-100 years. Vegetation richness, understory vegetation cover, soil carbon flux, and nitrogen cycling were measured for two years prior to, and five years following, application of experimental treatments. Litterfall and coarse woody debris (CWD), including snags, stumps, and fallen logs and branches, have been measured since treatments were applied. Overstory basal area was reduced 60%-70% in girdled and logged plots. Mean cover and richness did not change in hardwood or hemlock control plots but increased rapidly in girdled and logged plots. Following logging, litterfall immediately decreased then slowly increased, whereas in girdled plots, there was a short pulse of hemlock litterfall as trees died. CWD volume remained relatively constant throughout but was 3-4× higher in logged plots. Logging and girdling resulted in small, short-term changes in ecosystem dynamics due to rapid regrowth of vegetation but in general, interannual variability exceeded differences among treatments. Soil carbon flux in girdled plots showed the strongest response: 35% lower than controls after three years and slowly increasing thereafter. Ammonium availability increased immediately after logging and two years after girdling, due to increased light and soil temperatures and nutrient pulses from leaf-fall and reduced uptake following tree death. The results from this study illuminate ecological processes underlying patterns observed consistently in region-wide studies of adelgid-infested hemlock stands. Mechanisms of T. canadensis loss determine rates, magnitudes, and trajectories of ecological changes in hemlock forests. Logging causes abrupt, large changes in vegetation structure whereas girdling (and by inference, A. tsugae) causes sustained, smaller changes. Ecosystem processes depend more on vegetation cover per se than on species composition. We conclude that the loss of this late-successional foundation species will have long-lasting impacts on forest structure but subtle impacts on ecosystem function.
加拿大铁杉是北美东部森林的一个假设的基础物种，由于非本地昆虫白杨枝瘿蚜的侵袭以及通过预防性的择伐采伐而减少，在其大部分范围内正在减少。在重复的 0.81 公顷样地中，通过砍伐和移除或通过环割来模拟白杨枝瘿蚜的损害来杀死加拿大铁杉。对照样地包括未受干扰的铁杉和中龄硬木林，这些林分代表了 50-100 年后的预期森林组成。在应用实验处理措施之前和之后的两年内，测量了植被丰富度、林下植被覆盖度、土壤碳通量和氮循环。自处理措施实施以来，一直在测量凋落物和粗木质残体（CWD），包括枯立木、树桩和倒下的原木和树枝。在环割和采伐样地中，上层林冠的基面积减少了 60%-70%。在硬木或铁杉对照样地中，平均覆盖度和丰富度没有变化，但在环割和采伐样地中迅速增加。在采伐后，凋落物立即减少，然后缓慢增加，而在环割样地中，随着树木死亡，铁杉凋落物会出现短暂的脉冲。在整个过程中，CWD 体积保持相对稳定，但在采伐样地中高出 3-4 倍。由于植被的快速再生，采伐和环割导致了生态系统动态的短期变化，但总的来说，年际变异性超过了处理措施之间的差异。在环割样地中，土壤碳通量的反应最为强烈：三年后比对照样地低 35%，此后缓慢增加。在采伐后两年和环割后两年，由于光照和土壤温度升高以及落叶带来的养分脉冲和树木死亡后吸收减少，铵态氮的可用性立即增加。本研究的结果阐明了在白杨枝瘿蚜感染的铁杉林的全区域研究中一致观察到的模式背后的生态过程。加拿大铁杉的丧失机制决定了铁杉林生态变化的速度、幅度和轨迹。采伐导致植被结构的突然和大规模变化，而环割（以及由此推断，白杨枝瘿蚜）则导致持续的较小变化。生态系统过程更多地取决于植被覆盖本身，而不是物种组成。我们得出的结论是，这种晚生基础物种的丧失将对森林结构产生持久的影响，但对生态系统功能的影响则较为微妙。
