Table_2_Temporal change in abundance of potential nesting cavities for wood ducks (Aix sponsa) in Northern Minnesota, United States.docx

Wood duck females often nest in tree cavities located in large-diameter deciduous trees. Temporal changes in forest age and composition during recent decades may have influenced the abundance of stems with suitable nesting cavities in the forested portion of Northern MN, United States. Thus, we ascertained whether temporal changes in the abundance of such stems occurred in this area during six Forest Inventory and Analysis (FIA) evaluation cycles (i.e., 1977 to 2015–2019). We used FIA data and independently estimated proportions of stems with suitable cavities in different tree-species, health-status, and diameter classes to estimate change in the abundance of such stems at three spatial scales. These spatial scales increased in areal extent from a study area to three ecological subsections to the Laurentian Mixed Forest ecological province. We used a Bayesian analytical approach to accommodate changes in FIA protocols among cycles. Both the abundance of stems with suitable cavities and an attribute associated with cavity occurrence changed during the analysis period, but findings were not entirely consistent among spatial scales. The estimated abundance of both suitable stems overall and those of late-successional tree species generally increased at all scales during the analysis period. Since 1990, increases in the abundance of health-impacted stems that were suitable for nesting occurred at all spatial scales. Our estimated densities of suitable stems during the 2005–2009 (1.20 [85% CrI: 1.04–1.39] suitable stems / ha) and 2015–2019 cycles (1.41 [85% CrI: 1.21–1.63] suitable stems / ha) were substantially greater than the <0.25 and 0.25–0.49 suitable stems / ha another study predicted would occur in Northern Minnesota during 2008 and 2018, respectively. Our results can inform forest management decisions and research directions for follow-up studies of nesting wood ducks.

雌性林鸳鸯（Wood duck）通常在大直径落叶树的树洞中筑巢。近数十年来森林年龄结构与组成的动态变化，或已对美国明尼苏达州北部林区内具备适宜筑巢树洞的林木株数丰度造成影响。为此，本研究针对该区域在6次森林资源清查与分析（Forest Inventory and Analysis, FIA）评估周期（1977年至2015–2019年）内，这类适宜林木的丰度是否发生动态变化展开了探究。本研究依托FIA公开数据，并结合不同树种、健康状况及径级下具备适宜树洞林木占比的独立估算结果，在三个空间尺度上对这类适宜林木的丰度变化进行了估算。这三个空间尺度的面积范围依次从研究区域拓展至3个生态亚区，最终覆盖劳伦琴混交林生态省（Laurentian Mixed Forest ecological province）。考虑到不同评估周期内FIA调查规程的调整，本研究采用贝叶斯分析方法以适配数据差异。研究期内，具备适宜筑巢树洞的林木丰度及与树洞形成相关的林木属性均发生了改变，但不同空间尺度下的结果并不完全一致。整体而言，所有尺度上的适宜林木总株数及晚演替树种的适宜林木株数在研究期内均呈普遍上升趋势。自1990年起，所有空间尺度下受健康胁迫且适宜筑巢的林木株数均有所增加。本研究估算得到的2005–2009年与2015–2019年周期内的适宜林木密度分别为1.20株/公顷（85%可信区间：1.04–1.39）与1.41株/公顷（85%可信区间：1.21–1.63），远高于另一项研究分别预测的2008年明尼苏达州北部<0.25株/公顷、2018年0.25–0.49株/公顷的结果。本研究结果可为林鸳鸯筑巢相关的森林经营决策及后续相关研究方向提供参考依据。