Department of Forestry and Natural Resources, Purdue University, 715 W. State Street, West Lafayette, Indiana, 47907, USA.
Ecol Appl. 2019 Oct;29(7):e01958. doi: 10.1002/eap.1958. Epub 2019 Jul 18.
In eastern North America, oak (Quercus) regeneration failure has spurred management using silvicultural approaches better aligned with the autecology of oaks. In particular, shelterwood harvests can create favorable intermediate light conditions for oak establishment and prescribed fire is predicted (by the oak-fire hypothesis) to favor oak regeneration. These approaches substantially modify forest structure and may affect crucial trophic interactions including the conditional mutualism between oaks and granivorous rodents that scatterhoard acorns, which shifts along a continuum from antagonistic to mutualistic depending on external factors. We investigated how overwinter survival and dispersal of northern red oak (Quercus rubra) acorns were influenced by location within or outside of group shelterwood harvests (small canopy gaps created throughout an intact forest stand) with and without prescribed fire. We conducted two concurrent experiments to test (1) dispersal and survival of acorns presented on the forest floor and (2) acorn pilferage rates from caches that mimic squirrel scatterhoards in shelterwood gap/group interiors, edges, and the uncut forest matrix in burned and unburned forest stands. In both experiments, acorn survival was generally higher in burned than unburned stands. Acorn survival from forest floor presentations was higher in the unharvested forest matrix than harvest gap interiors; however, there was no effect of proximity to harvest gaps on survival of cached acorns. Survival of cached acorns was associated with understory vegetative cover (-), coarse woody debris cover (-), and distance to nearest tree (+), but uncorrelated with canopy cover above the cache. Our results suggest that reduced understory cover following prescribed fire may increase perceived habitat riskiness for granivores resulting in higher acorn survival up to 2 yr post-fire. These findings unify the oak-fire and oak-granivore conditional mutualism hypotheses, and suggest that the environmental conditions following prescribed fire and group shelterwood harvests may shift the oak-granivore conditional mutualism in a direction beneficial for oak regeneration.
在北美东部，橡树（Quercus）的更新失败促使人们采用更符合橡树特性的造林方法进行管理。特别是，间伐可以为橡树的建立创造有利的中等光照条件，而计划火烧被预测（根据橡树火假说）有利于橡树的更新。这些方法极大地改变了森林结构，可能会影响到关键的营养相互作用，包括橡树和散布橡果的食果啮齿动物之间有条件的互利共生关系，这种关系沿着从敌对到互利的连续体变化，具体取决于外部因素。我们研究了北红橡树（Quercus rubra）橡果的越冬存活率和散布率如何受到群集疏伐（在整个完整林分中创建小树冠空隙）内或外的位置以及是否进行计划火烧的影响。我们进行了两个同时进行的实验，以测试（1）在林地上展示的橡果的散布和存活率，以及（2）从模仿松鼠散布行为的贮藏库中窃取橡果的比率，这些贮藏库位于疏伐缺口/群集的内部、边缘和未砍伐的森林基质中，在火烧和未火烧的林分中。在这两个实验中，橡果在火烧林分中的存活率通常高于未火烧林分。从林地上展示的橡果存活率在未采伐的森林基质中高于采伐缺口内部；然而，在贮藏库附近对存活橡果的生存没有影响。贮藏库中贮藏的橡果的存活率与林下植被覆盖度（-）、粗木质残体覆盖度（-）和距最近树木的距离（+）有关，但与贮藏库上方的树冠覆盖度无关。我们的结果表明，计划火烧后林下覆盖度的降低可能会增加食果动物对栖息地的感知风险，从而导致火灾后 2 年内橡果的存活率更高。这些发现统一了橡树火和橡树食果动物有条件互利共生假说，并表明计划火烧和群集疏伐后环境条件可能会使橡树食果动物的有条件互利共生关系朝着有利于橡树更新的方向转变。
