Snags (standing dead trees) are important biological legacies in forest systems, providing numerous resources as well as a record of recent tree mortality. From 1997 to 2017, we monitored snag populations in drought-influenced mixed-conifer and ponderosa pine (Pinus ponderosa) forests in northern Arizona. Snag density increased significantly in both forest types. This increase was driven largely by a pulse in snag recruitment that occurred between 2002 and 2007, following an extreme drought year in 2002, with snag recruitment returning to pre-pulse levels in subsequent time periods. Some later years during the study also were warmer and/or drier than average, but these years were not as extreme as 2002 and did not trigger the same level of snag recruitment. Snag recruitment was not equal across tree species and size classes, resulting in significant changes in species composition and size-class distributions of snag populations in both forest types. Because trees were far more abundant than snags in these forests, the effect of this mortality pulse on tree populations was far smaller than its effect on snag populations. Snag loss rates increased over time during the study, even though many snags were newly recruited. This may reflect the increasing prevalence of white fir snags and/or snags in the smaller size classes, which generally decay faster than snags of other species or larger snags. Thus, although total numbers of snags increased, many of the newly recruited snags may not persist long enough to be valuable as nesting substrates for native wildlife. Increases in snag abundance appeared to be due to a short-term tree mortality “event” rather than a longer-term pattern of elevated tree mortality. This mortality event followed a dry and extremely warm year (2002) embedded within a longer-term megadrought. Climate models suggest that years like 2002 may occur with increasing frequency in the southwestern U.S. Such years may result in additional mortality pulses, which in turn may strongly affect trajectories in abundance, structure, and composition of snag populations. Relative effects on tree populations likely will be smaller, but, over time, also could be significant.

中文翻译：

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亚利桑那州北部针叶和黄松混交林断枝种群干旱造成的二十年变化

断枝（死木）是森林系统中重要的生物遗产，提供大量资源以及近期树木死亡的记录。从1997年到2017年，我们监测了亚利桑那州北部受干旱影响的针叶树和美国黄松（Pinus muderosa）混交林中的粗枝种群。两种森林类型的断枝密度均显着增加。增长的主要原因是2002年至2002年极端干旱之后2002年至2007年之间出现了新的障碍物募集活动，随后的这段时间中，障碍物的征募活动恢复到了脉冲前的水平。研究期间的后几年也比平均温度更高和/或更干燥，但是这些年份并不像2002年那样极端，并且没有触发相同水平的障碍招募。不同树种和大小等级的障碍物招募不平等，导致这两种森林类型中粗枝种群的物种组成和大小类别分布发生重大变化。因为在这些森林中树木比粗枝木丰富得多，所以这种死亡脉冲对树木种群的影响远小于其对粗枝种群的影响。在研究期间，即使新招募了许多障碍，障碍损失率也会随着时间的推移而增加。这可能反映了较小尺寸类别的白色冷杉枝条和/或枝条的流行增加，它们通常比其他物种的枝条或较大枝条的枝条腐烂更快。因此，尽管障碍物的总数增加了，但是许多新招募的障碍物可能不会持续足够长的时间以至于不能作为天然野生动物的筑巢底物而有价值。断枝丰度的增加似乎是由于短期树木死亡率“事件”而不是长期树木死亡率升高的模式所致。这次死亡事件是在长期特大干旱中发生的干旱且极其温暖的一年（2002年）之后发生的。气候模型表明，像2002年这样的年份可能在美国西南部发生的频率越来越高。此类年份可能导致额外的死亡脉冲，进而可能严重影响断枝种群的数量，结构和组成轨迹。对树木种群的相对影响可能较小，但随着时间的流逝，影响也可能很大。气候模型表明，像2002年这样的年份可能在美国西南部发生的频率越来越高。此类年份可能导致额外的死亡脉冲，进而可能严重影响断枝种群的数量，结构和组成轨迹。对树木种群的相对影响可能较小，但随着时间的流逝，影响也可能很大。气候模型表明，像2002年这样的年份可能在美国西南部发生的频率越来越高。此类年份可能导致更多的死亡脉冲，进而可能严重影响断枝种群的数量，结构和组成轨迹。对树木种群的相对影响可能较小，但随着时间的流逝，影响也可能很大。