Soil Respiration at Harvard Forest HEM and LPH Towers 1996-2007

Soil respiration, including both CO2 produced during decomposition of organic matter in soil and surface litter, and CO2 produced during respiration of living roots, is a major component of forest-atmosphere carbon exchange, typically comprising 60 to 80% of whole-forest respiration. Soil respiration data can explain a major part of a forest ecosystem’s response to changing climate, and help in evaluating the accuracy of nighttime eddy flux data as estimates of whole-ecosystem respiration measurements. Soil respiration in the footprint areas of the Harvard Forest Hemlock and Little Prospect Hill eddy flux towers has shown generally similar responses to soil temperature and precipitation as soil respiration near the EMS flux tower (see data set HF006), but also some differences. Soil respiration at the Hemlock tower site has been lower than at the other tower sites during very wet summer weather, but higher during dry summer periods. Average estimated annual total soil respiration has been close to the same for all three tower footprint areas, however. There has been some evidence of lower soil respiration in the parts of the Little Prospect Hill tower footprint with lowest soil moisture (those with a high slope position, 100 m from the tower), as compared with other sites during very dry summer periods, but such periods have been very rare during the period of data collection. Soil respiration measured near the Little Prospect Hill tower in summer has also been higher than ecosystem respiration as estimated from nighttime carbon flux measured by an eddy covariance system. This indicates that during some nighttime periods, CO2 is being removed from the volume of air below the eddy flux system by a route other than through the top of the forest canopy near the flux tower, possibly by downslope advection, even under conditions when it initially appeared that atmospheric turbulence was high enough to prevent removal of CO2 by these other mechanisms.

土壤呼吸，涵盖土壤与地表枯落物中有机质分解生成的二氧化碳，以及活根呼吸产生的二氧化碳，是森林-大气碳交换的核心组成部分，通常占全森林呼吸总量的60%至80%。土壤呼吸数据可阐释森林生态系统对气候变化响应的主要部分，同时有助于评估以夜间涡度通量数据作为全生态系统呼吸量估算值的准确性。哈佛森林铁杉通量塔与小展望山通量塔足迹区域内的土壤呼吸，与EMS通量塔附近的土壤呼吸（参见数据集HF006）对土壤温度与降水的响应整体相似，但亦存在部分差异。夏季极端湿润时段，铁杉塔站点的土壤呼吸水平低于其他塔站点；而在夏季干旱时段，该站点的土壤呼吸则高于其他站点。不过，三个通量塔足迹区域的年均土壤呼吸估算总量基本持平。有证据显示，在极端干旱夏季时段，小展望山塔足迹内土壤湿度最低的区域（即距塔100米的高坡位置）的土壤呼吸较其他站点更低，但这类干旱时段在数据采集周期内极为罕见。夏季在小展望山塔附近测得的土壤呼吸，也高于通过涡度协方差系统获取的夜间碳通量所估算的生态系统呼吸量。这表明，在部分夜间时段，尽管最初认为大气湍流强度足以阻断其他机制对二氧化碳的移除，但仍存在其他途径（大概率为下坡平流）在涡度通量系统下方的空气体积中移除了二氧化碳，而非仅通过通量塔附近的林冠顶部。