Temperate forests in Central Europe suffer from climate change-induced productivity and vitality reductions and increased tree mortality. Most field work assessing climate change effects in forests refers to mature trees and does not cover interaction between climate change and nitrogen deposition. Here we show in a study of 54 forest sites representing different combinations of climatic conditions and atmospheric nitrogen deposition across Germany that nitrogen significantly affects the drought tolerance of tree regeneration in the field. We compared shoot length increment and regeneration density of Central Europe’s naturally most dominant tree species, European beech () with three species, which are discussed as potentially more drought-tolerant replacement tree species for climate change adaptation of forestry. Growth of beech was reduced with increasing nitrogen deposition identifying high reactive nitrogen loads as an additional threat for this species, but between-site variation of beech growth was not dependent on climatic parameters despite growth reductions after the drought summer 2018 across all study sites. Remarkably, the only species in the between-site comparisons where low growth was attributable to dry climate was Douglas fir (), as shoot length increment was reduced at sites with dry spring climate. Silver fir () showed increasing growth with increasing nitrogen deposition in combination with increasing drought, probably due to reduced ammonium uptake. Shoot length increment in sessile oak () was not affected by the between-site variation of either drought or nitrogen. Our results (combined with results of regeneration density) indicate that high atmospheric nitrogen deposition, which is primarily found in regions with intensive livestock farming in Central Europe, modifies the climate change response of temperate tree species. Interaction of drought and nitrogen deposition should thus be addressed in the debate on climate change adaptation of forestry.

中文翻译：

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不同氮沉降水平下干旱对温带树木再生生长和密度的影响

中欧的温带森林遭受气候变化导致的生产力和活力下降以及树木死亡率增加的影响。大多数评估气候变化对森林影响的实地工作都是针对成熟的树木，并不涵盖气候变化与氮沉降之间的相互作用。在这里，我们对 54 个代表德国不同气候条件和大气氮沉降组合的森林地点进行了一项研究，结果表明，氮显着影响田间树木再生的耐旱性。我们将中欧天然最主要的树种欧洲山毛榉 () 的枝条长度增量和更新密度与三个树种进行了比较，这三个树种被认为是适应林业气候变化的潜在更耐旱的替代树种。山毛榉的生长随着氮沉降的增加而减少，这表明高活性氮负荷是对该物种的额外威胁，但尽管所有研究地点的山毛榉生长在 2018 年夏季干旱后都出现了减少，但山毛榉生长的不同地点变化并不依赖于气候参数。值得注意的是，在地点间比较中，唯一因气候干燥而导致低生长的物种是花旗松 ()，因为在春季气候干燥的地点，枝条长度增量减少。随着氮沉降的增加以及干旱的加剧，银杉的生长速度加快，这可能是由于铵吸收的减少。无柄橡树的枝条长度增量不受干旱或氮的位点间变化的影响。我们的结果（与再生密度的结果相结合）表明，主要存在于中欧畜牧业集约化地区的高大气氮沉降改变了温带树种的气候变化响应。因此，在关于林业适应气候变化的辩论中应解决干旱和氮沉降的相互作用。