Annual mean estimates of aboveground net primary production (NPP) at 15 sites at Jornada Basin LTER, 1989-ongoing

This package contains values of mean annual aboveground net primary
production (NPP, in grams per square meter per year) at 15 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. At each site, a 1 hectare area was
fenced in 1988 and a grid of 49 (48 at one playa location) 1m x 1m
replicate quadrats was laid out when sampling began in 1989. In
fall, winter, and spring periods aboveground biomass was calculated
for each species and quadrat at each NPP site. These calculations
rely on 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. NPP is then calculated as the positive biomass
increment between seasons. The annual totals in this dataset are
derived by summing mean site NPP values for winter (October -
February), spring (February - May), and fall (May - October)
increments for a single calendar year. Data collection is ongoing
with new annual NPP values calculated after the conclusion of each
growing season. Attention: 1) Calculated values in this data
package have changed over time as the methodology for estimating
biomass has changed. 2) Relating long-term NPP in this package with
long-term precipitation is problematic given the importance of wet
and dry periods and their effect on production in these ecosystems.
3) 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.

本数据集涵盖霍罗尼玛实验牧场（Jornada Experimental Range, JER）与奇瓦瓦荒漠牧场研究中心（Chihuahuan Desert Rangeland Research Center, CDRRC）辖内15个净初级生产力（net primary production, NPP）研究样地的年平均地上净初级生产力数值，单位为克·平方米⁻¹·年⁻¹。
本次选取的样地旨在覆盖奇瓦瓦荒漠的5类核心生态系统类型：高地草原、干盐湖草原（playa grasslands）、牧豆树占优灌丛（mesquite-dominated shrublands）、杂酚灌丛占优灌丛（creosotebush-dominated shrublands）与塔布什灌丛占优灌丛（tarbush-dominated shrublands）。每类生态系统各选取3个样地，以覆盖生产力与植物多样性的变异梯度，因此上述样地并非重复样地。
1988年，各研究样地内均划定1公顷围栏区域；1989年启动采样时，在该区域内布设了49个1m×1m的重复样方（其中一处干盐湖样地仅布设48个）。
在秋、冬、春三季，研究人员针对每个样地的各物种与各1m×1m样方计算其地上生物量。该计算依托两类数据来源：1）对每个样方内单株植物或植物器官的无损水平盖度与垂直高度测量数据；2）基于样方外获取的各植物物种盖度、高度与收获生物量数据推导得到的线性回归系数。
净初级生产力被定义为季际间生物量的正增长值。本数据集的年度总NPP由单个日历年的冬季（10月—次年2月）、春季（2月—5月）与秋季（5月—10月）的样地平均NPP增量求和得到。数据采集工作仍在持续，每个生长季结束后都会更新年度NPP数值。
注意事项：
1. 本数据包中的计算值随生物量估算方法的迭代优化而历经调整。
2. 鉴于干湿周期及其对这类荒漠生态系统生产力的关键影响，将本数据包中的长期NPP数据与长期降水数据进行关联分析存在局限性。
3. 2013年及之后的数据目前处于临时状态，在审核用于生物量估算的异速生长方程（allometric equations）后，相关数据仍可能发生变更。
更多细节请参阅方法模块中的注释内容。