1. To comprehensively understand the impact of anthropogenic activities on biodiversity, we must understand how biodiversity has changed over time and what are the underlying processes. A growing body of evidence has shown that beta diversity reveals more about temporal changes in biodiversity compared with alpha diversity. Temporal beta diversity indicates, for example, degrees of change in species composition at single locations through time.
2. We examined whether freshwater plant communities showed different patterns in temporal beta diversity in relation to concomitant changes in environmental conditions across decades. To do this, we used presence–absence data of lake plants for five decades (1940s–2010s) from southern Finland and calculated temporal beta diversity indices (TBI) for each lake between pairs of decades to the whole community, hydrophytes and helophytes. To get insights into possible processes behind the observed trends, we decomposed TBIs into beta diversity contributed by either temporal losses or temporal gains of species. We related TBIs and their loss and gain components to lake landscape position and changes in environmental variables.
3. Based on comparisons of TBIs between the survey decade pairs, the temporal change in aquatic plant communities was modest through decades. Hydrophyte assemblages have changed more than helophyte assemblages. The main changes in temporal beta diversity occurred from the 1940s to the 1970s, when the gain of new species was the dominant process in the lakes throughout the landscape. Following that period, there was only modest changes, but from the 2000s to the 2010s, the dominant process was the loss of species. Temporal changes in environmental conditions played a key role in explaining the TBI.
4. Our results showed that relying on only two survey points in time can result in limited knowledge of the ecological phenomenon under study and, for example, an exceptional year in terms of weather conditions can hinder detecting overall long‐term trends in compositional changes. Therefore, future studies should try to combine data from several decades to overcome the typical limitations of temporal information.

中文翻译：

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数十年来环境因素的变化决定了湖泊植物的时间β多样性

1. 为了全面了解人为活动对生物多样性的影响，我们必须了解生物多样性如何随着时间变化以及潜在的过程是什么。越来越多的证据表明，与α多样性相比，β多样性揭示了更多关于生物多样性的时间变化。时间β多样性表明，例如，单个位置上物种组成随时间的变化程度。
2. 我们研究了数十年来，淡水植物群落相对于环境条件的变化是否表现出不同的时间β多样性模式。为此，我们使用了芬兰南部五十年（1940年代至2010年代）湖泊植物的存在与缺失数据，并计算了数十年来成对的整个社区，水生植物和双生植物之间每个湖泊的时间β多样性指数（TBI）。为了深入了解所观察到的趋势背后的可能过程，我们将TBIs分解为由于物种的时间损失或时间收益而造成的β多样性。我们将TBI及其损失和收益成分与湖泊景观位置和环境变量的变化相关联。
3. 根据调查十年对之间的TBI的比较，水生植物群落的时间变化在几十年内都是适度的。水生植物组合的变化超过了水生植物组合。β暂时性多样性的主要变化发生在1940年代至1970年代，当时新物种的获得是整个景观湖中的主要过程。在此之后，变化很小，但是从2000年代到2010年代，主要的过程是物种的丧失。环境条件的时间变化在解释贸易壁垒方面起着关键作用。
4. 我们的结果表明，仅依靠两个调查时间点可能会导致对所研究的生态现象的了解有限，例如，天气状况异常的一年可能会阻碍检测组成变化的总体长期趋势。因此，未来的研究应尝试合并几十年来的数据，以克服时间信息的典型局限性。