Increasing tree density accelerates stand‐level nitrogen cycling at the taiga–tundra ecotone in northeastern Siberia
Increasing tree density accelerates stand‐level nitrogen cycling at the taiga–tundra ecotone in northeastern Siberia
复制标题
DOI：
10.1002/ecs2.4175
复制
发表时间：
2022-07
期刊：
影响因子：
2.7
通讯作者：
R. Hewitt;H. Alexander;Brian Izbicki;M. Loranty;S. Natali;X. Walker;M. Mack
中科院分区：
文献类型：
--
作者：
R. Hewitt;H. Alexander;Brian Izbicki;M. Loranty;S. Natali;X. Walker;M. Mack
随着气候变暖，针叶林-苔原交错带 (TTE) 的树木密度预计会增加，这可能会加剧这种氮 (N) 有限环境中对地下资源的竞争。为了确定树木密度增加对氮循环和生产力的影响，我们研究了指示土壤氮可用性的土壤特性，以及与西伯利亚东北部 TTE 的单优势落叶松 (Larix cajanderi) 树木密度梯度上的碳 (C) 和氮循环相关的地上和地下树级特征以及林分特征。我们没有从土壤、树木或林分水平的氮循环特征中发现较低的氮利用率或随着密度的增加对氮的种内竞争加剧的一致证据。活性层厚度下降，但树脂吸附的氮和土壤有机层厚度并不随着树木密度的增加而变化。然而，随着树木密度的增加，地下有更多的分配给林分粗根和细根，这种分配模式表明土壤资源有限。与 C 相关的叶面性状（%C、δ13C 和吸收）对密度敏感，表明光等非营养资源对叶面化学计量的重要性。随着树木密度的增加和个体树木生产力的降低，地上和地下的树级氮和生物量库下降，氮吸收、氮再吸收、氮利用效率和分配给缓慢循环组织（如木材）的减少。在林分水平上，我们的研究结果表明，氮周转率较高，氮获取量增加，分配给氮含量相对较高的短寿命组织，氮停留时间减少，林分生产力随着树木密度的增加而提高。然而，这些积极的关系在树木密度最高时受到限制。我们对生物量、碳和氮分配以及地上损失的变化的观察，以及随着树木密度的增加而增加的根密度，可能对碳和氮循环产生强烈影响，应该在 TTE 动态和气候反馈模型中得到体现。
As climate warms, tree density at the taiga–tundra ecotone (TTE) is expected to increase, which may intensify competition for belowground resources in this nitrogen (N)‐limited environment. To determine the impacts of increased tree density on N cycling and productivity, we examined edaphic properties indicative of soil N availability along with aboveground and belowground tree‐level traits and stand characteristics related to carbon (C) and N cycling across a tree density gradient of monodominant larch (Larix cajanderi) at the TTE in far northeastern Siberia. We found no consistent evidence from soil, tree, or stand‐level N cycling characteristics of lower N availability or greater intraspecific competition for N with increased density. Active layer thickness declined, but resin‐sorbed N and soil organic layer thickness did not covary with increased tree density. There was, however, greater allocation belowground to stand‐level coarse and fine roots with increased tree density, an allocation pattern suggestive of limited soil resources. Foliar traits related to C (%C, δ13C, and resorption) were responsive to density indicating the importance of non‐nutrient resources, like light, to foliar stoichiometry. As tree density increased and individual trees had lower productivity, tree‐level N and biomass pools aboveground and belowground declined tracking decreases in N uptake, N resorption, N use efficiency, and allocation to slow cycling tissues like wood. At the stand level, our findings show high N turnover with increased N acquisition, allocation to short‐lived tissues with relatively high N content and reduced N residence time, and greater stand productivity as tree density increased. Yet, these positive relationships were curtailed at the highest tree densities. Our observations of shifts in biomass, C and N allocation, and loss aboveground, along with greater root density with increased tree density, could have strong impacts on C and N cycling and should be represented in models of TTE dynamics and feedbacks to climate.