Department of Integrative Biology, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA.
Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado, 80523, USA.
Ecology. 2019 Mar;100(3):e02626. doi: 10.1002/ecy.2626. Epub 2019 Feb 25.
The extent of young postfire conifer forests is growing throughout western North America as the frequency and size of high-severity fires increase, making it important to understand ecosystem structure and function in early seral forests. Understanding nitrogen (N) dynamics during postfire stand development is especially important because northern conifers are often N limited. We resampled lodgepole pine (Pinus contorta var. latifolia) stands that regenerated naturally after the 1988 fires in Yellowstone National Park (Wyoming, USA) to ask (1) How have N pools and fluxes changed over a decade (15 to 25 yr postfire) of very rapid forest growth? (2) At postfire year 25, how do N pools and fluxes vary with lodgepole pine density and productivity? Lodgepole pine foliage, litter (annual litterfall, forest-floor litter), and mineral soils were sampled in 14 plots (0.25 ha) that varied in postfire lodgepole pine density (1,500 to 344,000 stems/ha) and aboveground net primary production (ANPP; 1.4 to 16.1 Mg·ha ·yr ). Counter to expectation, foliar N concentrations in lodgepole pine current-year and composite needles (1.33 and 1.11% N, respectively) had not changed over time. Further, all measured ecosystem N pools increased substantially: foliar N increased to 89 kg N/ha (+93%), O-horizon N increased to 39 kg N/ha (+38%), and mineral soil percent total N (0-15 cm) increased to 0.08% (+33%). Inorganic N availability also increased to 0.69 μg N·[g resin] ·d (+165%). Thus, soil N did not decline as live biomass N pools increased. Among stands, biomass N pools at postfire year 25 remained strongly influenced by early postfire tree density: foliar and litterfall N concentrations declined with lodgepole pine density and ANPP, but the foliar N pool increased. Lodgepole pine ANPP correlated negatively with annual resin-sorbed N, and we found no indication of widespread N limitation. The large increases in N pools cannot be explained by atmospheric N deposition or presence of known N fixers. These results suggest an unmeasured N source and are consistent with recent reports of N fixation in young lodgepole pine.
年轻的针叶林火灾后在北美西部的范围正在扩大，因为高严重性火灾的频率和规模在增加，因此了解早期演替林分的生态系统结构和功能非常重要。了解火灾后林分发育过程中的氮（N）动态尤其重要，因为北方针叶树通常受到 N 限制。我们对 1988 年黄石国家公园（美国怀俄明州）火灾后自然再生的黑云杉（Pinus contorta var. latifolia）林分进行了重新采样，以回答以下问题：(1) 在快速森林生长的 10 年（火灾后 15 至 25 年）中，N 库和通量如何变化？(2) 在火灾后 25 年，N 库和通量如何随黑云杉密度和生产力的变化而变化？在 14 个面积为 0.25 公顷的样地（0.25 公顷）中，我们采样了黑云杉的叶片、凋落物（年凋落物、林下凋落物）和矿质土壤，这些样地的火灾后黑云杉密度（1,500 至 344,000 株/公顷）和地上净初级生产力（ANPP；1.4 至 16.1 Mg·ha ·yr）各不相同。与预期相反，黑云杉当年生叶片和复合针叶中的叶片 N 浓度（分别为 1.33%和 1.11%的 N）并没有随时间而变化。此外，所有测量的生态系统 N 库都大幅增加：叶片 N 增加到 89 kg N/ha（增加 93%），O 层 N 增加到 39 kg N/ha（增加 38%），矿质土壤总 N 百分比（0-15 cm）增加到 0.08%（增加 33%）。无机 N 可利用性也增加到 0.69μg N·[g 树脂]·d（增加 165%）。因此，随着生物量 N 库的增加，土壤 N 并没有减少。在各林分中，火灾后 25 年的生物量 N 库仍然受到早期火灾后树木密度的强烈影响：叶片和凋落物 N 浓度随黑云杉密度和 ANPP 而下降，但叶片 N 库增加。黑云杉 ANPP 与每年树脂吸附的 N 呈负相关，并且我们没有发现广泛存在 N 限制的迹象。N 库的大量增加不能用大气 N 沉降或已知固氮生物的存在来解释。这些结果表明存在未测量的 N 源，并且与最近关于年轻黑云杉固氮的报道一致。
