Seasonal aboveground plant biomass estimates at 15 net primary production (NPP) study sites at Jornada Basin LTER from 1989-ongoing

This data package contains aboveground vegetation cover, volume, and calculated biomass values at the 15 Net Primary Production (NPP) study sites on Jornada Experimental Range (JER) and Chihuahuan Desert Rangeland Research Center (CDRRC) lands. Sites were selected to represent the 5 major ecosystem types in the Chihuahuan Desert (upland grasslands, playa grasslands, mesquite-dominated shrublands, creosotebush-dominated shrublands, tarbush-dominated shrublands). For each ecosystem type, three sites were selected to represent the range in variability in production and plant diversity; thus the locations are not replicates. All sites are excluded from domestic grazing. Eleven sites are in non-grazed pastures, and at the other four sites 1 hectare areas around the observational plots were fenced in 1988. At all sites a grid of 49 (48 at one playa location) 1m x 1m replicate quadrats was laid out when sampling began in 1989. For each quadrat, aboveground biomass has been calculated from two data sources: 1) non-destructive horizontal cover and vertical height measurements of individual plants, or plant parts, within each quadrat, and 2) linear regression coefficients for each plant species derived from off-quadrat cover, height, and harvested biomass measurements. Non-destructive measurements (1) are taken during winter, spring, and fall measurement campaigns, then aggregated by species for each quadrat, and resulting dimensions are used to calculate species biomass (grams) by quadrat and season using using the species-specific regression coefficients derived from dataset 2. This is the most detailed biomass dataset available and can be used to derive values of net primary production between seasons or annually. Each dataset record contains calculated biomass (and related variables) by species, quadrat, season, and site. Data collection is ongoing with new observations in spring, fall, and winter of each year, but this data package may be updated less frequently. Attention: 1) For most species, these data are not appropriate for estimates of percentage cover or volume because of the way the data are collected. 2) Calculated values in this data package have changed over time as the methodology for estimating biomass has changed. If you are updating or adding to an earlier analysis of these data we recommend consulting with the dataset authors or a Jornada data manager. 3) Relating long-term NPP derived from this package with long-term precipitation is problematic given the importance of wet and dry periods and their effect on production in these ecosystems. 4) Data from 2013 and later are currently in provisional status and subject to change as we review the allometric equations used for estimating biomass. See Notes in the methods element for further details.

本数据包包含霍顿实验牧场（Jornada Experimental Range, JER）与奇瓦瓦沙漠牧场研究中心（Chihuahuan Desert Rangeland Research Center, CDRRC）所辖15个净初级生产力（Net Primary Production, NPP）研究样地的地上植被盖度、体积及计算得到的生物量数值。
所选样地旨在表征奇瓦瓦沙漠的5类主要生态系统类型：高地草原（upland grasslands）、浅滩草原（playa grasslands）、牧豆树占优灌丛（mesquite-dominated shrublands）、石炭酸灌木占优灌丛（creosotebush-dominated shrublands）以及塔布什灌丛（tarbush-dominated shrublands）。针对每类生态系统，选取3个样地以体现生产力与植物多样性的变化梯度，故这些样地并非重复样地。
所有样地均禁止家畜放牧：其中11个样地位于禁牧牧场内，剩余4个样地的观测样方周边1公顷区域于1988年完成围封。1989年启动采样工作时，所有样地均布设了由49个1m×1m重复样方组成的样网格（其中一处浅滩草原样地的样方数为48个）。
针对每个样方，地上生物量通过两类数据源计算得到：1）对每个样方内单株植物或植物器官开展非破坏性水平盖度与垂直高度测量；2）基于样方外的盖度、高度及收获式生物量测量结果，推导得到各植物物种的线性回归系数。
非破坏性测量工作于冬季、春季与秋季的野外调查中开展，随后按物种对每个样方的数据进行汇总，结合数据集2推导得到的物种特异性回归系数，即可计算得到每个样方、每个季节的物种生物量（单位：克）。
本数据集为当前可获取的最详尽的生物量数据集，可用于推导季节间或年度的净初级生产力数值。每条数据集记录均包含按物种、样方、季节及样地分类的估算生物量（及相关变量）。数据采集工作仍在持续，每年春季、秋季与冬季都会新增观测数据，但本数据包的更新频率相对较低。
注意：1）多数物种的此类数据不适用于盖度或体积百分比估算，因数据采集方式存在固有局限。2）随着生物量估算方法的迭代更新，本数据包内的估算数值曾多次调整。若您需基于此前的分析对本数据集进行更新或补充分析，建议咨询数据集作者或霍顿实验牧场的数据管理人员。3）鉴于干湿周期及其对这类生态系统生产力的关键影响，将本数据包得到的长期净初级生产力与长期降水量进行关联存在较大局限性。4）2013年及之后的数据目前处于临时状态，在我们审核用于生物量估算的异速生长方程期间，相关数据可能发生变更。更多细节请参见方法部分的注释。