Data from: A three decade assessment of climate-associated changes in forest composition across the north-eastern USA

1. Climate-associated changes in forest composition have been widely reported, particularly where changes in abiotic conditions have resulted in high mortality of sensitive species and have disproportionately favored certain species better adapted to these newer conditions. In the northeastern USA and southeastern Canada, few studies have examined climate-related influences associated on forest composition, and none have considered broad-scale changes over a long temporal (>25 years) period. 2. We used US Forest Service Forest Inventory and Analysis data from 1983-2014 across four northeastern states (Maine, New Hampshire, New York, and Vermont) to assess temporal and spatial changes in the occurrence and abundance of American beech, sugar maple (Acer sacharum L.), red maple (A. rubrum L.), and birch (Betula spp.) saplings. We also tested the effects of biotic and abiotic factors on the distribution of the four studied deciduous species over the entire studied period. 3. Occurrence and abundance of American beech have increased substantially over the past three decades, whereas the occurrence and abundance of three-other deciduous species have decreased in all ecological provinces of the northeastern USA, except the Midwest Broadleaf ecological province. Consequently, a clear shift in species composition is currently underway in the beech-maple-birch (BMB) forests of the northeastern USA, with uncertain consequences for future ecosystem structure and function. 4. In the studied region and over the entire studied period, the distribution of increased occurrence and abundance of beech relative to three-other deciduous species were associated with the higher temperature and precipitation as well as higher conspecific basal area and dead tree basal area. 5. Synthesis and applications: The change from BMB forests to beech-dominated forests and beech encroachment to new forest areas across the northeast may expand in areas where higher intensity harvesting/disturbances (i.e., large-scale canopy openings) do not occur, which would be a management concern as the beech is associated with the beech-bark disease. Our results emphasize the need for management strategies such as higher intensity harvesting methods, vegetation management, and controlling browsing pressure to reduce the beech dominance.

1. 气候相关的森林群落组成变化已被广泛报道，尤其在非生物（abiotic）条件改变导致敏感树种大量死亡，且不成比例地青睐更适应新环境的特定树种的区域。在美国东北部与加拿大东南部地区，鲜有研究探讨气候相关因子对森林群落组成的影响，且尚无研究覆盖超过25年的长期大范围时空变化。
2. 本研究使用美国林务局（US Forest Service）森林资源清查与分析（Forest Inventory and Analysis）数据，涵盖美国东北部4个州（缅因州、新罕布什尔州、纽约州与佛蒙特州）1983-2014年的观测数据，以评估美洲山毛榉（American beech）、糖枫（Acer sacharum L.）、红枫（A. rubrum L.）以及桦树（Betula spp.）幼树的出现频率与多度的时空变化。同时，本研究还检验了整个研究周期内，生物（biotic）与非生物（abiotic）因子对4种研究落叶树种分布的影响。
3. 过去三十年间，美洲山毛榉的出现频率与多度显著上升；而除中西部阔叶生态省（Midwest Broadleaf ecological province）外，美国东北部所有生态省中的其余3种落叶树种的出现频率与多度均有所下降。因此，美国东北部山毛榉-枫-桦（beech-maple-birch, BMB）林的群落组成正发生显著转变，其对未来生态系统结构与功能的影响尚不明确。
4. 在整个研究周期内，研究区域中山毛榉相较于其余3种落叶树种出现频率与多度上升的分布格局，与更高的气温、降水量，以及更高的同种胸高断面积（conspecific basal area）和枯木胸高断面积显著相关。
5. 总结与应用：若东北部地区未实施高强度采伐/干扰（即大规模林冠开窗），则BMB林向山毛榉主导林转变，以及山毛榉向新林区入侵的趋势可能进一步扩张。由于山毛榉与山毛榉树皮病（beech-bark disease）相关联，该现象将成为森林管理的潜在隐患。本研究结果表明，需采取高强度采伐、植被管理以及控制啃食压力等管理策略，以削弱山毛榉的优势地位。