Soil Water Content at Harvard Forest HEM and LPH Towers 1998-2007

Water content of the top 20 cm of soil was measured using 1.5 inch diameter soil cores. Soil water content is of particular interest and importance in explaining patterns of soil respiration (including root respiration) and ecosystem respiration, 60 to 75% of which occurs below the ground surface at Harvard Forest. Soil water content in deciduous forest near the Little Prospect Hill Forest was in general found to be more variable than near the Hemlock tower. One influence contributing to this is the presence of a water table within 1 m of the soil surface near the hemlock tower, compared to a water table at unknown depth in the deeply drained soils in most parts of Little Prospect Hill. Relatively low evapotranspiration at the Hemlock tower site during the period when deciduous trees are foliated also contributes to higher water content in soil there, and a relatively thick surface organic layer. These influences tend to maintain soil respiration at higher levels in the hemlock forest during dry summers, but excessive moisture in the soil at the Hemlock site during very wet summers appears to suppress soil respiration.采用直径1.5英寸的土芯采样法，测定了0~20cm表层土壤的含水量。土壤含水量对于阐释土壤呼吸（包括根系呼吸）与生态系统呼吸的变化规律具有重要意义，而哈佛森林中有60%~75%的生态系统呼吸发生于地下。研究发现，小展望山森林周边落叶林的土壤含水量总体上较铁杉塔监测点的变异性更强。造成这一差异的影响因素之一是：铁杉塔监测点附近的地下水位距地表不足1米，而小展望山森林大部分区域的强排水性土壤的地下水位深度未知。落叶乔木展叶期内，铁杉塔监测点的蒸散发量相对较低，这同样使得该区域土壤含水量更高，同时地表有机层也相对更厚。上述影响因素往往使得夏季干旱期铁杉林的土壤呼吸维持在较高水平，但在极端湿润的夏季，铁杉塔监测点的土壤含水量过高会抑制土壤呼吸活动。