Functional attributes of conifers expanding into temperate semi-arid grasslands modulate carbon and nitrogen fluxes in response to prescribed fire

Fire exclusion is a key factor driving conifer expansion into temperate
semi-arid grasslands. However, it remains unclear how reintroducing fire
affects the aboveground storage of carbon (C) and nitrogen (N) in the
encroaching tree species and belowground in soils. To assess the impacts
of fire reintroduction C and N pools and fluxes in areas of conifer
expansion we targeted a region of the Northern Great Plains that has
experienced extensive woody plant expansion (WPE) of two species:
ponderosa pine (Pinus ponderosa) and juniper (Juniperus spp). We
quantified tree mortality of both species to estimate the amount of dead
biomass C and N produced by a recent prescribed fire, in addition to
changes in soil C, pyrogenic C (PyC), and N concentrations across a
woody-cover gradient using a before/after/control experimental design.
Post-fire soil chemical analysis revealed a 2-year increase in mineral
soil C, PyC and N, suggesting the return of fire led to the transfer of
partially combusted plant organic matter back to the soil.
 Further, we found that functional trait differences between the
two species influenced the distribution of living conifer biomass-N prior
to fire. Despite junipers having 41% less total aboveground biomass than
ponderosa, they contained two times more aboveground N. Prescribed fire
resulted in 88% mortality of all mature juniper stems and increased fire
severity correlated with greater pre-fire juniper cover. Ponderosa
mortality varied by size class, with >40 cm stem diameter class
having only 28% mortality. High mortality and greater aboveground N
storage in juniper biomass, compared to ponderosa, led to 77% of the total
conifer biomass N lost. Consequently, the functional attributes of
expanding trees differentially contribute to fluxes of C and N after the
return of fire, with junipers acting as conduits for N movement due to
their relatively higher N content in less fire-resistant tissues and
ponderosa serving as important and more stable storage pools for C.
Together, these findings highlight the importance of considering
species-specific traits when planning WPE management strategies at
landscape-scales, particularly when goals include C storage or soil
nutrient status.

火抑制是驱动针叶林向温带半干旱草原扩张的关键因素。然而，目前仍不明确重新引入火会对入侵树种的地上碳（C）、氮（N）储量以及土壤地下碳氮库产生何种影响。
为评估针叶林扩张区域内火的重新引入对碳氮库及碳氮通量的影响，本研究选取北美大平原北部区域作为研究对象——该区域已发生两种木本植物的大规模扩张（woody plant expansion, WPE）：美国黄松（Pinus ponderosa）与桧属植物（Juniperus spp.）。本研究采用前后对照（before/after/control, BACI）实验设计，定量了两种目标树种的死亡率，以估算近期计划性火烧（prescribed fire）产生的死亡生物量碳氮总量；同时测定了木本覆盖梯度下的土壤碳、热解碳（pyrogenic C, PyC）与氮浓度变化。
火烧后土壤化学分析结果显示，矿质土壤的碳、热解碳与氮含量在两年内均有所提升，表明火的重新引入促使部分燃烧后的植物有机质重新进入土壤库。
此外，本研究发现两种目标树种的功能性状差异，会影响火烧前活体针叶树生物量氮的分布格局。
尽管桧属植物的总地上生物量较美国黄松低41%，但其地上氮储量却是美国黄松的两倍。
计划性火烧导致所有成熟桧属植株的死亡率达88%，且火烧烈度与火烧前桧属植物的盖度呈正相关关系。
美国黄松的死亡率则随径级变化：茎径大于40厘米的个体死亡率仅为28%。
相较于美国黄松，桧属植物死亡率更高且地上氮储量更大，这使得针叶树总生物量氮中有77%得以释放。
因此，扩张型木本植物的功能属性会在火重新引入后，对碳氮通量产生差异化影响：桧属植物因其耐火性较弱的组织中氮含量相对更高，成为氮迁移的载体；而美国黄松则是更为稳定的重要碳储存库。
综上，本研究结果表明，在景观尺度下制定木本植物扩张管理策略时，需充分考虑物种特异性性状，尤其是当管理目标涉及碳储存或土壤养分状况时。