Data from: Century-long tree population dynamics in a deciduous forest stand in central Sweden

Question: We quantify tree dynamics over a century of free development in a small broadleaved forest dominated by Fraxinus excelsior and Ulmus glabra. What are the internal and external factors driving the changes, and how predictable are they? What were the time scale and effects of the spread of Dutch elm disease (DED)?
Location: Vårdsätra, eastern central Sweden.
Methods: The survival, growth and recruitment of all trees (≥ 12 cm in girth) were monitored in 1912, 1967, 1988 and 2013 (more often for a part of the forest). Woody species in the field and shrub layers were surveyed in permanent plots in 1976 and 2012. We used transition matrix models to project changes in population sizes and species composition within the century and for 2050.
Results: The results indicate that the forest was in a successional development during the first period. The species composition had stabilised by 1967, except for an expansion of Acer platanoides and the drastic effect of DED that struck the forest around 2000. It took only a decade to kill virtually all large elms in the forest, leading to strong decrease in stem density and basal area. The evidence for effects of DED is still weak, but there has been an increase in saplings, notably of Fraxinus, Prunus padus, Ulmus, and of shoots of Corylus avellana. Several species that are abundant in the vicinity and as seeds fail to establish (Picea abies, Betula spp., Quercus robur, Populus tremula). Projections for 2050 based on the third period (1988-2013) are probably unrealistic since also Fraxinus may disappear because of the recent arrival of the ash dieback.
Conclusions: Slow dynamics in forests that could follow from climate change will locally probably be overruled by unforeseen catastrophes, such as invasions by forest pathogens. These initiate changes with long lag phases difficult to quantify. Still, a dense deciduous forest can resist invasion of colonist species and of regionally dominant conifers; the reason being unfavourable conditions for establishment rather than dispersal limitation

### 问题
本研究量化了以欧洲白蜡树（Fraxinus excelsior）和欧洲榆（Ulmus glabra）为优势种的小型阔叶林长达一个世纪的自然发育动态。旨在厘清三个核心问题：驱动林分变化的内外因子有哪些？其变化的可预测性如何？荷兰榆树病（Dutch Elm Disease, DED）的传播时间尺度与影响效应是什么？

### 研究地点
瑞典中东部的Vårdsätra。

### 研究方法
于1912年、1967年、1988年及2013年对所有胸径≥12cm的林木的存活、生长与补充更新情况进行监测（部分林分的监测频率更高）。1976年与2012年通过永久样地对林分草本层与灌木层的木本植物物种进行调查。本研究采用转移矩阵模型，对百年尺度内的种群规模与物种组成变化进行推演，并预测至2050年的林分动态。

### 研究结果
结果显示，该林分在初始阶段处于演替发育进程中。至1967年，物种组成已趋于稳定，仅挪威槭（Acer platanoides）出现种群扩张，且约2000年前后侵袭林分的荷兰榆树病（DED）产生了剧烈影响。该病害仅耗时十年便几乎灭杀了林分内所有大型榆树，导致林分茎秆密度与胸高断面积大幅下降。尽管目前关于荷兰榆树病影响的实证证据仍较为有限，但林分内幼树数量有所增加，其中尤以欧洲白蜡树、稠李（Prunus padus）、榆属植物以及榛树（Corylus avellana）的萌条增长最为显著。多种在周边区域常见且可通过种子抵达该区域的物种未能成功定植，包括欧洲云杉（Picea abies）、桦木属（Betula spp.）、夏栎（Quercus robur）以及欧洲山杨（Populus tremula）。基于1988-2013年时段数据对2050年的预测或不切实际，因为近期白蜡枯病（ash dieback）的入侵，欧洲白蜡树也可能面临消亡风险。

### 研究结论
气候变化可能引发的森林动态缓慢变化，在局部尺度上或被森林病原入侵等不可预见的灾害所抵消。这类灾害会触发具有长滞后阶段的生态变化，且此类变化难以量化。尽管如此，茂密的落叶阔叶林仍可抵御外来定居物种与区域优势针叶树种的入侵，其核心原因在于定植条件不利，而非传播限制。