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，因诸多土壤、植物及大气过程会对光照、温度、湿度与风速产生响应。陆地生态系统中广泛分布一类微生境——近地层，其中CO₂浓度天然高于平均背景水平。土壤呼吸产生的CO₂会通过枯落物层与边界层扩散，并快速消散至上方的整体大气中。然而，在地面上方0~50厘米范围内，通常存在显著的近地面富集二氧化碳（以下简称NEC）垂直剖面。 该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影响，且通常呈现显著的昼夜与季节变化。自1939年起，学界虽偶尔有提及这类CO₂“补贴”及其对植物的影响，但针对近地面CO₂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层的物种。 近二十年来，受控环境与野外环境中开展的大量研究表明，共存植物物种对人工升高的CO₂浓度可能表现出差异化响应。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并未将CO₂视为调控物种分布与多度的核心因子之一，仅在其可能间接影响水分平衡时才会予以考量。 我们对位于展望山（Prospect Hill）的6处样地展开了调查，这些样地曾分别经历翻耕、放牧或长期作为林地，其森林组成（木本与草本）、土壤特性、微气候及近地面CO₂水平均存在显著差异。我们选取了3种多年生草本植物：裸茎楤木（Aralia nudicaulis，野薜苈）、蓝珠百合（Clintonia borealis）、印第安黄瓜根（Medeola virginiana），以及哈佛森林（Harvard Forest）系统中的2种优势乔木：红枫（Acer rubrum）与北方红栎（Quercus rubra）。我们测定了它们在野外常见CO₂浓度范围（350~450 ppm）内，针对环境CO₂变化的光合光响应曲线，以解答5个科学问题：（1）NEC对净碳同化的整体效应如何？（2）合并所有土地利用样地后，不同物种对NEC的响应是否存在整体差异？（3）合并所有物种后，不同土地利用样地的植物对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CO₂浓度（同化箱内分别为350、400与450 ppm）下测定了光响应曲线，每个样地内的每个物种选取3株随机选择的健康植株作为重复，总计生成135条光响应曲线。气体交换测定使用LI-6400型红外气体分析仪（美国Li-Cor公司，内布拉斯加州林肯市）完成，测定时间为1998年7月下旬至8月初的当地太阳时7:30~12:30。分析仪于每次测定前每日进行校准。同化箱内的空气温度维持在23℃（对应样地早间平均气温），相对湿度在每条曲线测定所需的20~25分钟内保持恒定或缓慢上升（整体增幅通常低于5%）。 我们采用矩形双曲线模型对光合响应散点图进行拟合，并使用Photosyn Assistant软件v1.1（英国苏格兰邓迪市Dundee Scientific公司）估算了曲线参数：日间呼吸速率（Rday）、表观量子效率（AQE）、最大同化速率（Amax）、曲线凸度、光补偿点（LCP）及光饱和点（Lsat）。其中6条曲线在量子产率区域得到了存疑的参数，因此被排除在后续分析之外，最终有效样本量为129。