Kristensen Jeppe A, Michelsen Anders, Metcalfe Daniel B
Department of Physical Geography and Ecosystem Science Lund University Lund Sweden.
Geological Survey of Denmark and Greenland Copenhagen Denmark.
Ecol Evol. 2020 Sep 21;10(20):11684-11698. doi: 10.1002/ece3.6803. eCollection 2020 Oct.
Herbivores can exert major controls over biogeochemical cycling. As invertebrates are highly sensitive to temperature shifts (ectothermal), the abundances of insects in high-latitude systems, where climate warming is rapid, is expected to increase. In subarctic mountain birch forests, research has focussed on geometrid moth outbreaks, while the contribution of background insect herbivory (BIH) to elemental cycling is poorly constrained. In northern Sweden, we estimated BIH along 9 elevational gradients distributed across a gradient in regional elevation, temperature, and precipitation to allow evaluation of consistency in local versus regional variation. We converted foliar loss via BIH to fluxes of C, nitrogen (N), and phosphorus (P) from the birch canopy to the soil to compare with other relevant soil inputs of the same elements and assessed different abiotic and biotic drivers of the observed variability. We found that leaf area loss due to BIH was ~1.6% on average. This is comparable to estimates from tundra, but considerably lower than ecosystems at lower latitudes. The C, N, and P fluxes from canopy to soil associated with BIH were 1-2 orders of magnitude lower than the soil input from senesced litter and external nutrient sources such as biological N fixation, atmospheric deposition of N, and P weathering estimated from the literature. Despite the minor contribution to overall elemental cycling in subarctic birch forests, the higher quality and earlier timing of the input of herbivore deposits to soils compared to senesced litter may make this contribution disproportionally important for various ecosystem functions. BIH increased significantly with leaf N content as well as local elevation along each transect, yet showed no significant relationship with temperature or humidity, nor the commonly used temperature proxy, absolute elevation. The lack of consistency between the local and regional elevational trends calls for caution when using elevation gradients as climate proxies.
食草动物能够对生物地球化学循环施加主要控制。由于无脊椎动物对温度变化高度敏感（变温动物），在气候变暖迅速的高纬度系统中，昆虫的数量预计会增加。在亚北极山地桦树林中，研究主要集中在尺蛾爆发上，而背景昆虫食草作用（BIH）对元素循环的贡献却知之甚少。在瑞典北部，我们沿着9个海拔梯度估算了BIH，这些梯度分布在区域海拔、温度和降水量的梯度范围内，以便评估局部与区域变化的一致性。我们将BIH造成的叶片损失转化为从桦树冠层到土壤的碳（C）、氮（N）和磷（P）通量，以便与相同元素的其他相关土壤输入进行比较，并评估观察到的变异性的不同非生物和生物驱动因素。我们发现，BIH造成的叶面积损失平均约为1.6%。这与苔原的估计值相当，但远低于低纬度地区的生态系统。与BIH相关的从冠层到土壤的C、N和P通量比从衰老凋落物和外部营养源（如生物固氮、大气N沉降和P风化，根据文献估算）输入土壤的量低1 - 2个数量级。尽管对亚北极桦树林的整体元素循环贡献较小，但与衰老凋落物相比，食草动物排泄物输入土壤的质量更高且时间更早，这可能使这种贡献对各种生态系统功能来说格外重要。BIH随着叶片N含量以及每个样带的局部海拔显著增加，但与温度、湿度或常用的温度替代指标绝对海拔没有显著关系。当使用海拔梯度作为气候替代指标时，局部和区域海拔趋势之间缺乏一致性，这需要谨慎对待。
