Environment and Natural Resources Institute, University of Alaska Anchorage, Anchorage, Alaska, 99508, USA.
Pacific Northwest Research Station, USDA Forest Service, Anchorage, Alaska, 99501, USA.
Ecology. 2018 Jun;99(6):1284-1295. doi: 10.1002/ecy.2223. Epub 2018 Apr 30.
The boreal biome represents approximately one third of the world's forested area and plays an important role in global biogeochemical and energy cycles. Numerous studies in boreal Alaska have concluded that growth of black and white spruce is declining as a result of temperature-induced drought stress. The combined evidence of declining spruce growth and changes in the fire regime that favor establishment of deciduous tree species has led some investigators to suggest the region may be transitioning from dominance by spruce to dominance by deciduous forests and/or grasslands. Although spruce growth trends have been extensively investigated, few studies have evaluated long-term radial growth trends of the dominant deciduous species (Alaska paper birch and trembling aspen) and their sensitivity to moisture availability. We used a large and spatially extensive sample of tree cores from interior Alaska to compare long-term growth trends among contrasting tree species (white and black spruce vs. birch and aspen). All species showed a growth peak in the mid-1940s, although growth following the peak varied strongly across species. Following an initial decline from the peak, growth of white spruce showed little evidence of a trend, while black spruce and birch growth showed slight growth declines from ~1970 to present. Aspen growth was much more variable than the other species and showed a steep decline from ~1970 to present. Growth of birch, black and white spruce was sensitive to moisture availability throughout most of the tree-ring chronologies, as evidenced by negative correlations with air temperature and positive correlations with precipitation. However, a positive correlation between previous July precipitation and aspen growth disappeared in recent decades, corresponding with a rise in the population of the aspen leaf miner (Phyllocnistis populiella), an herbivorous moth, which may have driven growth to a level not seen since the early 20th century. Our results provide important historical context for recent growth and raise questions regarding competitive interactions among the dominant tree species and exchanges of carbon and energy in the warming climate of interior Alaska.
北方生物群落约占世界森林面积的三分之一，在全球生物地球化学和能量循环中发挥着重要作用。在阿拉斯加北部进行的众多研究表明，由于温度引起的干旱胁迫，黑云杉和白云杉的生长正在下降。云杉生长下降和有利于落叶树种建立的火灾发生频率变化的综合证据，导致一些研究人员提出该地区可能正在从云杉主导转变为落叶林和/或草原主导。尽管已经广泛研究了云杉的生长趋势，但很少有研究评估主要落叶树种（阿拉斯加纸皮桦和颤杨）的长期径向生长趋势及其对水分供应的敏感性。我们使用来自阿拉斯加内陆的大量和广泛分布的树木样本，来比较不同树种（白云杉和黑云杉与桦树和颤杨）之间的长期生长趋势。所有树种都在 20 世纪 40 年代中期表现出生长高峰，尽管在不同树种中，峰值后的生长情况差异很大。在峰值后最初下降之后，白云杉的生长几乎没有表现出趋势，而黑云杉和桦树的生长从 20 世纪 70 年代到现在略有下降。与其他树种相比，颤杨的生长变化更大，从 20 世纪 70 年代到现在呈急剧下降趋势。桦树、黑云杉和白云杉的生长对水分供应的变化非常敏感，这一点从与空气温度的负相关关系和与降水的正相关关系中可以得到证明。然而，在过去几十年中，与之前 7 月降水的正相关关系与白杨叶甲（Phyllocnistis populiella）的种群增加之间的相关性消失了，白杨叶甲是一种食草蛾，它可能使白杨的生长水平达到了自 20 世纪初以来从未见过的水平。我们的研究结果为最近的生长情况提供了重要的历史背景，并提出了关于主要树种之间竞争相互作用以及在阿拉斯加内陆变暖气候下碳和能量交换的问题。
