Herbaceous Stratum Sunfleck Regimes in CRUI Land Use Project at Harvard Forest 1998

Numerous variables related to land use disturbance and recovery processes can influence forest understory irradiance regimes. In addition to season-long measurements of herbaceous stratum microclimates (data archived separately), we explored the degree to which differences in forest community composition and structure generate differences in near-ground sunfleck regimes in our six land use sites (2 plowed, 2 pastured, 2 permanent woodlot). Considerable research has shown that photosynthetic responses to fluctuating irradiance cannot be predicted accurately from steady-state gas-exchange measurements, and that sunflecks may account for both a large fraction of total daily photon fluence and net carbon assimilation by plants in the herbaceous stratum of moist forests. We measured clear-day irradiance patterns to test predictions about how sunfleck frequency, duration, and brightness should vary across the three land-use legacies. We also wished to document sunfleck regimes as part of a companion study on photosynthetic responses by herb and tree species to sunflecks. Twenty-three quantum sensors (Li-190, Li-Cor, Inc., Lincoln, NE) were placed at regularly-spaced interior intersection points across the 30 m x 50 m permanent plot grid in each land use site. The sensors were mounted on stakes 50 cm above ground, leveled, and attached to a Campbell Scientific CR23X datalogger. Photosynthetic photon flux (PPF) values were recorded at all 23 microsites every 6 seconds for 3.5-4.5 hours in the morning (8:00-11:30+ a.m. EST) on one clear day per site in July and August 1998. Using custom software and a sunfleck definition threshold of 100 umol m-2 s-1, sunfleck events were extracted for each microsite across the sampling period and the following were calculated for each fleck event: number of fleck events per hour, sunfleck duration (sec), duration of the shade period prior to the sunfleck (sec), mean PPF across the sunfleck event, and peak PPF during the sunfleck. Descrtiptive statistics calculated across the 23 sample locations per site are reported in this archived data set.诸多与土地利用干扰及恢复过程相关的变量，均可影响森林林下的辐照度格局（irradiance regimes）。除对草本层微气候进行全季观测（数据已单独归档）外，本研究针对6个土地利用样地（2个翻耕样地、2个放牧样地、2个永久林地样地），探究了森林群落组成与结构的差异如何导致近地表光斑光照格局（sunfleck regimes）的差异。已有大量研究证实，无法通过稳态气体交换测定准确预测植物对波动辐照度的光合响应，且在湿润森林的草本层中，光斑可占每日总光子通量的较大比例，同时也是植物净碳同化的重要来源。本研究通过测定晴天光照格局，验证了关于光斑频率、持续时长与亮度如何随三类土地利用遗留效应发生变化的假说；同时，本研究亦旨在记录光斑光照格局，作为针对草本与木本物种对光斑的光合响应开展的配套研究的一部分。本研究在每个土地利用样地的30 m × 50 m永久样地网格中，于规则布设的内部交叉点安装了23台量子传感器（quantum sensor），型号为Li-190（Li-Cor公司，美国内布拉斯加州林肯市）。传感器被固定在距地面50 cm的测杆上，经调平后连接至Campbell Scientific CR23X型数据记录仪。1998年7月至8月，在每个样地的一个晴天，于上午（美国东部标准时间8:00~11:30及之后）对全部23个微生境的光合光子通量（photosynthetic photon flux，PPF）值进行记录，采样时长为3.5~4.5小时，采样间隔为6秒。本研究采用定制软件，以100 μmol·m⁻²·s⁻¹作为光斑定义阈值，从采样时段内各微生境的监测数据中提取光斑事件，并针对每个光斑事件计算以下指标：每小时光斑事件数、光斑持续时长（秒）、光斑出现前的遮阴时段时长（秒）、光斑事件期间的平均PPF，以及光斑期间的峰值PPF。本归档数据集报告了每个样地23个采样点位的描述性统计量。