Feast not famine: Nitrogen pools recover rapidly in 25-yr-old postfire lodgepole pine.
Feast not famine: Nitrogen pools recover rapidly in 25-yr-old postfire lodgepole pine.
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盛宴而非饥荒：25 年树龄的火灾后黑松中的氮池迅速恢复。
作者：
M. Turner;T. G. Whitby;W. H. Romme
随着严重火灾的频率和规模的增加，整个北美西部的火灾后针叶林的范围正在扩大，这使得了解早期森林的生态系统结构和功能变得非常重要。了解氮（N）的动态在火灾后的林分发展是特别重要的，因为北方针叶树往往是N有限的。我们重新采样lodgepole松（松contorta变种。1988年美国怀俄明州黄石国家公园国家公园火灾后自然更新的阔叶林，研究（1）在森林快速生长的10年（火灾后15 ~ 25年）内，N库和N通量是如何变化的？(2)在火灾后25年，如何N池和通量变化与火炬松密度和生产力？火炬松树叶，凋落物（每年凋落物，森林地面凋落物），和矿质土壤进行了采样，在14个地块（0.25公顷），不同的火灾后火炬松密度（1,500至344,000茎/公顷）和地上净初级生产力（ANPP; 1.4至16.1毫克·公顷-1 ·年-1）。与预期相反，叶氮浓度lodgepole松本年度和复合针叶（1.33和1.11%的N，分别）并没有随着时间的推移而改变。此外，所有测量的生态系统氮库都大幅增加：叶面氮增加到89 kg N/ha（+93%），O层氮增加到39 kg N/ha（+38%），矿质土壤总氮百分比（0-15 cm）增加到0.08%（+33%）。无机氮有效性也增加到0.69 μ gN·[g树脂]-1.d-1（+165%）。因此，土壤氮并没有下降，活生物量氮库增加。在林分中，生物量N池在火灾后25年仍然强烈影响火灾后早期的树木密度：叶和凋落物N浓度下降，黑松密度和ANPP，但叶N池增加。火炬松ANPP呈负相关，每年树脂吸附N，我们没有发现广泛的N限制的迹象。氮库的大量增加不能用大气氮沉降或已知固氮菌的存在来解释。这些结果表明存在一个不可测量的氮源，并且与最近关于幼年黑松氮固定的报道一致。
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-1 ·yr-1 ). 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]-1 ·d-1 (+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.