Richardson Andrew D, Hollinger David Y, Dail D Bryan, Lee John T, Munger J William, O'keefe John
Complex Systems Research Center, University of New Hampshire, Morse Hall, 8 College Road, Durham, NH 03824, USA.
Tree Physiol. 2009 Mar;29(3):321-31. doi: 10.1093/treephys/tpn040. Epub 2009 Jan 19.
Spring phenology is thought to exert a major influence on the carbon (C) balance of temperate and boreal ecosystems. We investigated this hypothesis using four spring onset phenological indicators in conjunction with surface-atmosphere CO(2) exchange data from the conifer-dominated Howland Forest and deciduous-dominated Harvard Forest AmeriFlux sites. All phenological measures, including CO(2) source-sink transition dates, could be well predicted on the basis of a simple two-parameter spring warming model, indicating good potential for improving the representation of phenological transitions and their dynamic responsiveness to climate variability in land surface models. The date at which canopy-scale photosynthetic capacity reached a threshold value of 12 micromol m(-2) s(-1) was better correlated with spring and annual flux integrals than were either deciduous or coniferous bud burst dates. For all phenological indicators, earlier spring onset consistently, but not always significantly, resulted in higher gross primary productivity (GPP) and ecosystem respiration (RE) for both seasonal (spring months, April-June) and annual flux integrals. The increase in RE was less than that in GPP; depending on the phenological indicator used, a one-day advance in spring onset increased springtime net ecosystem productivity (NEP) by 2-4 g C m(-2) day(-1). In general, we could not detect significant differences between the two forest types in response to earlier spring, although the response to earlier spring was generally more pronounced for Harvard Forest than for Howland Forest, suggesting that future climate warming may favor deciduous species over coniferous species, at least in this region. The effect of earlier spring tended to be about twice as large when annual rather than springtime flux integrals were considered. This result is suggestive of both immediate and lagged effects of earlier spring onset on ecosystem C cycling, perhaps as a result of accelerated N cycling rates and cascading effects on N uptake, foliar N concentrations and photosynthetic capacity.
春季物候被认为对温带和北方生态系统的碳（C）平衡具有重大影响。我们结合以针叶树为主的霍兰德森林和以落叶树为主的哈佛森林美国通量站点的地表-大气CO₂交换数据，使用四个春季开始物候指标对这一假设进行了研究。所有物候指标，包括CO₂源汇转换日期，都可以基于一个简单的双参数春季变暖模型得到很好的预测，这表明在陆地表面模型中改善物候转变的表示及其对气候变化的动态响应具有很大潜力。冠层尺度光合能力达到12微摩尔·米⁻²·秒⁻¹阈值的日期与春季和年度通量积分的相关性比落叶或针叶芽萌发日期更好。对于所有物候指标，春季提前开始始终（但并非总是显著）导致季节性（春季月份，4月至6月）和年度通量积分的总初级生产力（GPP）和生态系统呼吸（RE）更高。RE的增加小于GPP；根据所使用的物候指标，春季开始提前一天会使春季净生态系统生产力（NEP）增加2 - 4克C·米⁻²·天⁻¹。总体而言，我们未检测到两种森林类型对春季提前的响应存在显著差异，尽管哈佛森林对春季提前的响应通常比霍兰德森林更明显，这表明未来气候变暖可能至少在该地区更有利于落叶树种而非针叶树种。当考虑年度而非春季通量积分时，春季提前的影响往往约为两倍。这一结果表明春季提前开始对生态系统碳循环既有即时影响也有滞后影响，这可能是由于氮循环速率加快以及对氮吸收、叶片氮浓度和光合能力的级联效应所致。
