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. 气候驱动的森林群落组成变化已被广泛报道，尤其是在非生物条件改变导致敏感树种高死亡率，并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，针对森林群落组成的气候相关影响研究寥寥无几，且尚无研究关注时长超过25年的大范围长期变化。
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）数据集，覆盖美国东北部4个州（缅因州、新罕布什尔州、纽约州与佛蒙特州），时间跨度为1983-2014年，旨在评估美洲山毛榉、糖枫（*Acer sacharum* L.）、红枫（*A. rubrum* L.）以及桦木属（*Betula* spp.）幼树的出现率与丰度的时空变化。同时，本研究还检验了整个研究周期内，生物与非生物因子对4种研究落叶树种分布的影响。
3. 在过去三十年中，美洲山毛榉的出现率与丰度显著上升；而除中西部阔叶生态省之外，美国东北部所有生态省内的其余3种落叶树种的出现率与丰度均有所下降。因此，美国东北部的山毛榉-枫木-桦木（BMB）林正经历明确的群落组成转变，其对未来生态系统结构与功能的影响尚不明朗。
4. 在研究区域与整个研究周期内，相较于其余3种落叶树种，山毛榉出现率与丰度上升的分布格局，与更高的气温、降水量，以及更高的同种个体断面积、枯木断面积显著相关。
5. 综合与应用：在美国东北部，从山毛榉-枫木-桦木林向山毛榉主导林的转变，以及山毛榉向新林区的侵入，可能在未发生高强度采伐/干扰（即大规模林冠开窗）的区域进一步扩张。由于山毛榉与山毛榉树皮病（beech-bark disease）存在关联，这一现象将成为森林管理的关注点。本研究结果强调，需制定高强度采伐、植被管理、控制取食压力等管理策略，以削弱山毛榉的优势地位。