Photosynthetic Light Response Curves in CRUI Land Use Project at Harvard Forest 1998

Ambient CO2 concentrations in terrestrial ecosystems vary substantially on several spatial and temporal scales as numerous soil, plant, and atmospheric processes respond to irradiance, temperature, moisture and wind. There is one widespread microhabitat in terrestrial ecosystems, the nearground zone, in which CO2 is naturally enhanced above average background levels. CO2 produced by soil respiration diffuses through the litter and boundary layers and dissipates fairly rapidly into the overlying bulk air. However, a marked vertical profile of nearground enriched CO2 (hereafter NEC) is usually present in the first 0-50 cm above ground. The degree of enrichment varies primarily with soil respiration rate and turbulent mixing, secondarily with photosynthesis by plants in the herbaceous stratum, and usually shows marked diel and seasonal variation. References to this CO2 "subsidy" and its effects on plants have occurred occasionally in the literature since 1939, but there have been few detailed studies of either the nearground profile or plant responses in the field, particularly for species that consistently occupy the nearground stratum. Considerable research over the last twenty years in both controlled and field environments has shown that co-occurring plant species may respond differently to artificially elevated CO2. But in contrast to light, temperature, water, and nutrients, plant community ecologists have generally not considered CO2 among the factors that regulate species’ distribution and abundance, except indirectly as it may affect water balance. We have documented differences in forest composition (woody and herbaceous), soil characteristics, microclimates, and nearground CO2 levels among six sites that were formerly plowed, pastured, or continuously forested woodlots in Prospect Hill. We selected three perennial herbaceous species (Aralia nudicaulis, wild sarsaparilla; Clintonia borealis, blue-bead lily; Medeola virginiana, Indian cucumber root) and two dominant tree species in the Harvard Forest system (Acer rubrum, red maple; Quercus rubra (northern red oak) and measured their photosynthetic light responses to ambient CO2 variation within the range commonly encountered in the field (350-450 ppm) to address five questions: (1) What is the overall effect of NEC on net carbon assimilation? (2) Do species differ overall (land use sites combined) in their responses to NEC? (3) Do the land use sites differ overall (species combined) in plant responses to NEC? (4) Are there site x CO2 or species x CO2 interactions in response to NEC? Light response curves were measured at three CO2 levels (350, 400, and 450 ppm inside the cuvette) on 3 randomly-selected, healthy replicates of each species in each of the three sites, generating a total of 135 curves. Gas-exchange measurements were made with a LI-6400 infrared gas analyzer (Li-Cor Inc., Lincoln, NE, USA) during ~7:30-12:30 a.m. solar time in late July and early August 1998. The analyzer was calibrated daily just prior to measurements. Air temperature in the cuvette was maintained at 23 deg C (mean morning air temperature in the sites), and relative humidity was maintained at either constant or slowly rising levels (typically less than 5% increase overall) during the 20-25 minutes required for each curve. Rectangular hyperbolic curves were fitted to the scatterplots and curve parameters (daytime respiration rate, Rday; apparent quantum efficiency, AQE; maximum assimilation rate, Amax; curve convexity; light compensation point, LCP; and light saturation point, Lsat) were estimated using Photosyn Assistant software v. 1.1 (Dundee Scientific, Dundee, Scotland, UK). Six of the curves produced questionable parameters in the quantum yield region and were excluded from further analyses, leaving a total sample size of 129.陆地生态系统中的环境二氧化碳（CO₂）浓度在多种空间与时间尺度上存在显著差异，因众多土壤、植物与大气过程会响应光照强度、温度、湿度及风速变化。陆地生态系统中广泛分布一类微生境——近地面层，其中CO₂浓度天然高于平均背景水平。土壤呼吸产生的CO₂通过枯落物层与边界层扩散，并快速消散至上方的主体大气中。然而，在地面上方0~50 cm范围内，通常存在显著的近地面富集CO₂（nearground enriched CO₂，下文简称NEC）垂直剖面。NEC的富集程度主要取决于土壤呼吸速率与湍流混合强度，其次受草本层植物光合作用的调控，且通常呈现显著的昼夜与季节波动。自1939年以来，学界虽偶有提及这类CO₂“补贴”及其对植物的影响，但针对近地面剖面或野外植物响应的详细研究仍较为匮乏，尤其是针对持续栖息于近地面层的物种而言。 近二十年来，受控环境与野外环境中的大量研究表明，共存植物物种对人工升高的CO₂浓度可能表现出不同的响应。但与光照、温度、水分及养分不同的是，植物群落生态学家通常未将CO₂视为调控物种分布与多度的核心因子之一，仅间接将其视作可能影响植物水分平衡的因素。 我们对美国普罗斯佩克特山（Prospect Hill）6处曾经历耕作、放牧或持续作为林地的样地展开调查，记录了其森林群落组成（木本与草本）、土壤特性、微气候及近地面CO₂水平。我们选取了3种多年生草本植物：裸茎楤木（Aralia nudicaulis，野山萝卜（wild sarsaparilla））、蓝珠百合（Clintonia borealis，blue-bead lily）、印度黄瓜根（Medeola virginiana，Indian cucumber root），以及哈佛森林系统中的2种优势乔木：红枫（Acer rubrum，red maple）与北方红栎（Quercus rubra，northern red oak），并测量了它们在野外常见的350~450 ppm CO₂浓度范围内，对环境CO₂变化的光合光响应，以解答5个科学问题：（1）NEC对净碳同化的整体效应如何？（2）不同物种（合并所有土地利用样地）对NEC的响应是否存在整体差异？（3）不同土地利用样地（合并所有物种）的植物对NEC的响应是否存在整体差异？（4）针对NEC的响应是否存在样地×CO₂或物种×CO₂交互效应？ 我们在3个样地的每个物种的3株随机选取的健康重复样本上，于叶室内设置3个CO₂浓度水平（350、400、450 ppm），测量了光合光响应曲线，共计生成135条曲线。 1998年7月下旬至8月初，于当地太阳时7:30~12:30期间，使用LI-6400型红外气体分析仪（美国内布拉斯加州林肯市Li-Cor公司）开展气体交换测量。仪器于每日测量前进行校准。叶室内空气温度维持在23℃（对应样地晨间平均气温），相对湿度在每条曲线测量所需的20~25分钟内保持恒定或缓慢上升（整体增幅通常小于5%）。 采用矩形双曲线模型对散点图进行拟合，并使用Photosyn Assistant软件v1.1（英国苏格兰邓迪市邓迪科学公司）估算曲线参数：日间呼吸速率（R_day）、表观量子效率（AQE）、最大同化速率（A_max）、曲线凸度、光补偿点（LCP）及光饱和点（L_sat）。其中6条曲线在量子产率区间出现可疑参数，被排除于后续分析之外，最终有效样本量为129条曲线。