Hupperts Stefan F, Berninger Frank, Chen Han Y H, Fenton Nicole, Jean Mélanie, Köster Kajar, Larjavaara Markku, Mack Michelle C, Nilsson Marie-Charlotte, Palviainen Marjo, Prokushkin Anatoly, Pumpanen Jukka, Seedre Meelis, Simard Martin, Gundale Michael J
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
Department of Environmental and Biological Sciences, University of Eastern Finland, Joensuu, Finland.
Glob Chang Biol. 2025 Aug;31(8):e70398. doi: 10.1111/gcb.70398.
Climate change has increased the size and frequency of wildfires across the boreal biome. Severe wildfires in boreal forests have been found to trigger shifts from evergreen to deciduous canopies, which has cascading effects on carbon and nitrogen cycling. Ecosystem productivity and carbon uptake in boreal forests are strongly linked with nitrogen, and Earth system models increasingly depend on our understanding of the nitrogen balance to predict post-fire carbon uptake. To investigate the post-fire boreal nitrogen balance, we combined a mass balance approach and literature synthesis to estimate rates of nitrogen accumulation and nitrogen inputs across a network of 18 boreal wildfire chronosequences that varied in both wildfire regime and post-fire canopy type, comprising 527 forest stands. We found that deciduous- or mixed-dominance boreal forests establishing after severe, stand-replacing fires had the highest nitrogen accumulation rates (15.7 ± 3.8 kg ha year), while evergreen-dominated forests establishing after surface- or mixed-severity fires had the lowest nitrogen accumulation rates (1.4 ± 1.1 kg ha year). Annual known inputs from nitrogen deposition and biological nitrogen fixation combined, estimated from published data, largely failed to explain the rate of nitrogen accumulation, particularly in deciduous or mixed-dominance forests establishing after stand-replacing fires, suggesting that the origins of most nitrogen in these forest types remain poorly understood. As the frequency of severe wildfires increases across the boreal biome and shifts toward deciduous canopies become more common, our study reveals a large knowledge gap in the resulting nitrogen balance that needs to be resolved in order to improve predictions of forest carbon uptake.
气候变化增加了北方生物群落野火的规模和发生频率。研究发现，北方森林的严重野火会引发树冠从常绿向落叶的转变，这对碳和氮的循环产生连锁反应。北方森林的生态系统生产力和碳吸收与氮密切相关，地球系统模型越来越依赖我们对氮平衡的理解来预测火灾后的碳吸收。为了研究火灾后北方的氮平衡，我们结合了质量平衡方法和文献综述，以估算18个北方野火时间序列网络中的氮积累速率和氮输入速率，这些时间序列在野火状况和火灾后树冠类型方面各不相同，涵盖527个林分。我们发现，在严重的、造成林分更替的火灾后形成的落叶或混合优势北方森林的氮积累速率最高（15.7±3.8千克·公顷·年），而在地表或混合强度火灾后形成的常绿优势森林的氮积累速率最低（1.4±1.1千克·公顷·年）。根据已发表数据估算的氮沉降和生物固氮的年度已知输入量，在很大程度上无法解释氮积累速率，特别是在林分更替火灾后形成的落叶或混合优势森林中，这表明这些森林类型中大部分氮的来源仍知之甚少。随着北方生物群落中严重野火的频率增加，向落叶树冠的转变变得更加普遍，我们的研究揭示了由此产生的氮平衡方面存在很大的知识空白，为了改进对森林碳吸收的预测，这一空白需要得到解决。
