Appalachian Laboratory, University of Maryland Center for Environmental Science, Frostburg, Maryland, 21532, USA.
Jonah Ventures, Manhattan, Kansas 66502, USA.
Nat Plants. 2016 Sep 12;2:16133. doi: 10.1038/nplants.2016.133.
There is wide agreement that anthropogenic climate warming has influenced the phenology of forests during the late twentieth and early twenty-first centuries(1,2). Longer growing seasons can lead to increased photosynthesis and productivity(3), which would represent a negative feedback to rising CO2 and consequently warming(4,5). Alternatively, increased demand for soil resources because of a longer photosynthetically active period in conjunction with other global change factors might exacerbate resource limitation(6,7), restricting forest productivity response to a longer growing season(8,9). In this case, increased springtime productivity has the potential to increase plant nitrogen limitation by increasing plant demand for nitrogen more than nitrogen supplies, or increasing early-season ecosystem nitrogen losses(10,11). Here we show that for 222 trees representing three species in eastern North America earlier spring phenology during the past 30 years has caused declines in nitrogen availability to trees by increasing demand for nitrogen relative to supply. The observed decline in nitrogen availability is not associated with reduced wood production, suggesting that other environmental changes such as increased atmospheric CO2 and water availability are likely to have overwhelmed reduced nitrogen availability. Given current trajectories of environmental changes, nitrogen limitation is likely to continue to increase for these forests, possibly further limiting carbon sequestration potential.
人们普遍认为，人为气候变暖影响了 20 世纪后期和 21 世纪初期森林的物候期(1,2)。较长的生长季节会导致光合作用和生产力的增加(3)，这将对 CO2 上升和随之而来的变暖产生负反馈(4,5)。或者，由于更长的光合活跃期以及其他全球变化因素导致对土壤资源的需求增加，可能会加剧资源限制(6,7)，限制森林对更长生长季节的生产力响应(8,9)。在这种情况下，春季生产力的增加有可能通过增加植物对氮的需求超过氮的供应，或者增加早期生态系统氮损失，从而增加植物氮限制(10,11)。在这里，我们表明，对于代表北美东部三种物种的 222 棵树，在过去 30 年中，春季物候期的提前导致树木对氮的需求相对于供应增加，从而导致氮的可利用性下降。观察到的氮可用性下降与木材产量减少无关，这表明其他环境变化，如大气 CO2 增加和水分供应增加，可能已经超过了氮可用性的减少。考虑到环境变化的当前轨迹，这些森林的氮限制可能会继续增加，这可能会进一步限制碳封存潜力。
