Assessing the use of bison for savanna restoration at Cedar Creek Ecosystem Science Reserve: Soil Carbon and Nitrogen

Oak savanna is the most threatened ecosystem in Minnesota and fire, alone, is not restoring and preserving it. Our savanna restoration research started more than a half century ago in what had once been native savanna at Cedar Creek. It has shown that burning about 4 to 7 times per decade eliminates shrubs and non-savanna tree species and restores prairie grassland species. However, our 50 years of research is also showing that these frequent and intense fires are preventing oaks from regenerating. Bison are now known to be a keystone species for restoring and preserving grasslands, but their roles in savanna ecosystems remain unknown. In grasslands, bison preferentially graze the dominant warm season grasses that would otherwise outcompete wildflowers, thereby promoting plant coexistence and enhancing plant diversity. Here we propose to test whether bison grazing might promote the growth and survivorship of oak seedlings in burned savannas by reducing grass fuel for fires and by knocking back dominant grass competitors. We will maintain the existing fire frequencies and the design of the long-term burning experiment, while adding bison grazing as an additional factor in part of several burn units on the southeast side of the property. Bison will graze during the summer and early fall seasons. Grazing exclosures will be established, and oak seedlings will be planted, to test effects of bison grazing on early oak growth and survivorship. The outcomes we plan to achieve are to: (1) discover better restoration and preservation practices for savanna ecosystems; (2) determine how these practices impact savanna biodiversity; and (3) educate Minnesotans about the ecological heritage of their state, including the roles that bison, fire and biodiversity play in the functioning of savannas and other Minnesota ecosystems. We will achieve these goals and outcomes by: (1) restoring bison grazing to 200 acres of oak savanna; (2) experimentally testing whether bison grazing promotes savanna biodiversity; and (3) disseminating results to K-12 students and visitors.

橡树稀树草原（oak savanna）是明尼苏达州受威胁程度最高的生态系统，仅依靠火烧无法完成其修复与保护工作。我们的稀树草原修复研究始于半个多世纪前，实验地点位于雪松溪曾为原生稀树草原的区域。研究结果表明，每十年进行4至7次火烧，可清除灌木与非稀树草原树种，恢复草原草本物种群落。然而，长达50年的研究同时揭示，这类高频高强度的火烧会阻碍橡树的自然更新。目前学界已证实，美洲野牛（bison）是修复与保护草原的关键物种（keystone species），但其在稀树草原生态系统中的功能仍有待探明。在草原生态系统中，美洲野牛优先取食优势暖季草本植物；这类植物若不受干扰，会通过竞争压制野生花卉的生长，因此野牛的取食可促进植物群落共存，提升植物多样性。本研究拟验证：在经火烧处理的稀树草原中，美洲野牛的啃食是否可通过降低火灾的草本燃料负荷、抑制优势草本竞争者，从而促进橡树幼苗的生长与存活。我们将保留现有火烧频率与长期火烧实验的设计方案，在该地块东南部的多个火烧单元中，选取部分区域增设美洲野牛放牧作为额外实验变量。野牛的放牧时段设定为夏季与早秋。我们将设置放牧排除区（grazing exclosures）并栽植橡树幼苗，以此测试美洲野牛啃食对橡树早期生长与存活的影响。本研究预期达成以下目标与成果：（1）探索更适配稀树草原生态系统的修复与保护方案；（2）明确此类方案对稀树草原生物多样性的影响机制；（3）向明尼苏达州民众科普本州的生态遗产，包括美洲野牛、火烧干预与生物多样性在稀树草原及明尼苏达州其他生态系统的运行中所发挥的作用。我们将通过以下路径达成上述目标与成果：（1）在200英亩的橡树稀树草原中恢复美洲野牛放牧；（2）通过实验验证美洲野牛啃食是否可提升稀树草原生物多样性；（3）向K-12阶段学生及公众访客分享研究成果。