Département de biologie, Université Laval, Québec, Qc, Canada.
PLoS One. 2012;7(12):e51940. doi: 10.1371/journal.pone.0051940. Epub 2012 Dec 13.
Densification of the shrub layer has been reported in many subarctic regions, raising questions about the implication for large herbivores and their resources. Shrubs can tolerate browsing and their level of tolerance could be affected by browsing and soils productivity, eventually modifying resource availability for the caribou. Our objective was to assess the compensatory growth potential of a subarctic shrub, Betula glandulosa Michx., in relation with caribou browsing and nutriment availability for the plants. We used a simulated browsing (0, 25 and 75% of available shoots) and nitrogen-fertilisation (0 and 10 g m(-2)) experiment to test two main hypotheses linking tolerance to resource availability, the Compensatory Continuum Hypothesis and the Growth Rate Hypothesis as well as the predictions from the Limiting Resource Model. We seek to explicitly integrate the relative browsing pressure in our predictions since the amount of tissues removed could affect the capacity of long-lived plants to compensate. Birches fully compensated for moderate browsing with an overall leaf biomass similar to unbrowsed birches but undercompensated under heavy browsing pressure. The main mechanism explaining compensation appears to be the conversion of short shoots into long shoots. The leaf area increased under heavy browsing pressure but only led to undercompensation. Fertilisation for two consecutive years did not influence the response of birch, thus we conclude that our results support the LRM hypothesis of equal tolerance under both high and low nitrogen availability. Our results highlight that the potential for compensatory growth in dwarf birch is surpassed under heavy browsing pressure independently of the fertilisation regime. In the context of the worldwide decline in caribou herds, the reduction in browsing pressure could act synergistically with global climate change to promote the current shrub expansion reported in subarctic regions.
灌木层的密集化在许多亚北极地区都有报道，这引发了人们对大型食草动物及其资源的影响的疑问。灌木可以耐受啃食，其耐受程度可能受到啃食和土壤生产力的影响，最终改变驯鹿的资源可利用性。我们的目的是评估一种亚北极灌木，即胶桦（Betula glandulosa Michx.）的补偿生长潜力，以及其与驯鹿啃食和植物养分供应的关系。我们使用模拟啃食（0、25 和 75%的可利用枝条）和氮施肥（0 和 10 g m(-2))实验来测试两个主要假设，即与资源可利用性相关的耐受性，补偿连续体假说和生长率假说，以及限制资源模型的预测。我们试图在预测中明确纳入相对啃食压力，因为去除的组织量可能会影响长寿植物补偿的能力。胶桦可以完全补偿适度的啃食，其总叶生物量与未啃食的胶桦相似，但在重度啃食压力下则补偿不足。解释补偿的主要机制似乎是将短枝转化成长枝。在重度啃食压力下，叶面积增加，但仅导致补偿不足。连续两年的施肥对胶桦的反应没有影响，因此我们得出结论，我们的结果支持在高氮和低氮供应下均具有同等耐受性的 LRM 假说。我们的结果表明，在重度啃食压力下，矮桦补偿生长的潜力会超过，而与施肥制度无关。在全球驯鹿种群减少的背景下，啃食压力的减少可能与全球气候变化协同作用，促进亚北极地区目前报道的灌木扩张。
