General study design
There are 15 net primary production (NPP) monitoring sites located
in 5 vegetation communities that represent the major vegetation
types of the Jornada Basin. These vegetation zones and their
associated NPP sites (3 per zone) are: 1) creosotebush scrub
(CALI, GRAV, SAND), 2) tarbush flats (EAST, TAYL, WEST), 3)
grassland (BASN, IBPE, SUMM), 4) mesquite dunes (NORT, RABB,
WELL), and 5) playa (COLL, SMAL, TOBO). Grids of permanent 1
square meter replicate quadrats have been established at each site
for long-term monitoring of net primary production. Grids consist
of 49 quadrats arranged in a square 7 x 7 pattern, with quadrats
10 m apart. (Exception: P-COLL, the College Ranch playa, has 48
quadrats arranged in a 3 x 16 pattern). NPP observations at the
quadrats, including the non-destructive and off-grid harvests
described below, began in April 1989.
Biomass estimation methods
The objectives of the NPP study are to monitor spatial and
temporal patterns of aboveground biomass across a range of
ecosystem types; to allow the estimation of net primary production
and its variability in those ecosystems; and to provide a
quantitative description of plant community structure over time in
those ecosystems. This data package contains calculated plant
volume and aboveground biomass values for each species and quadrat
at all NPP sites during fall, winter, and spring sampling periods.
It is the most detailed biomass dataset available and it can be
(and has been) used to derive seasonal or annual values of NPP.
Biomass estimation methods, and changes to those methods and the
data, are described below.
Data used to calculate species volume and biomass at the NPP
quadrats come from two source datasets: 1) off-grid reference
measurements of aboveground plant volume (derived from cover and
height) and harvested aboveground plant biomass, and 2)
non-destructive plant volume measurements (again, from cover and
height) at permanent NPP quadrats (see "Source data
packages" for further details). The reference harvest dataset
is used to build linear regression models of plant biomass as a
function of plant volume. Depending on the outcome of a model
selection process involving F-testing of models with and without
interaction terms, these models may be specific to species, groups
of similar species, site, season, or some combination of these
factors. The non-destructive measurements of plant cover and
height are used to calculate species volume at each permanent
quadrat. For each of these species volumes, the selected
biomass-volume linear models are then used to estimate aboveground
biomass at the permanent quadrat.
Huenneke et al. (2001) describes both the non-destructive and
reference harvest sampling methods and the subsequent biomass
regression model selection. Descriptions of the NPP sites,
sampling design, and results from the first ~10 years of data
collection were initially laid out by Huenneke et al. (2002). Both
the data and the methods used in the study have evolved since, and
there are two additional publications that describe those changes.
In around 2008 it was determined that forcing the y-intercept of
the biomass-volume linear models through zero gave more sensible
estimates of quadrat biomass. This change is described by Muldavin
et al. (2008) and the regression models and resulting biomass
datasets were changed to be consistent with this study. Later,
Peters et al. (2012) described additional updates to the
biomass-volume regression coefficients that resulted from
additional harvests of selected Chihuahuan desert species. The
protocol for that publication also removed problematic species
(Yucca elata) from subsequent NPP
calculations, which affects NPP datasets derived from this one
(see "Derived data packages").
IMPORTANT NOTES
NOTE 1: These data are not appropriate for
estimates of percentage cover. NPP-associated percent cover
measurements were developed for and are used solely to provide the
best estimate of biomass production. Because the methodology
results in measurements of overlapping subcanopy systems and
canopies of adjacent individuals, NPP percent cover measurements
are not an appropriate measure of actual aerial plant cover. Doing
so will result in inflated numbers for the "actual"
vegetative cover.
NOTE 2: Calculated values in this data
package have changed over time as the methodology for deriving
biomass has changed. Data through 2003 was replaced online per
below on 9/22/2011. Analyses and results for ANPP differ from
previous uses of the data from 1989-1998 (Huenneke et al., 2002)
in three ways: (1) Yucca elata was removed prior to analysis
because its growth form results in large errors in biomass
estimates from year-to-year, (2) regressions between biomass and
plant volume used an intercept equal to 0 to be consistent with a
recent study in a similar system (Muldavin et al., 2008), and (3)
reference harvests obtained in extreme years resulted in adjusted
regression coefficients through time that reflect year-to-year
variation in ANPP. These changes result in ANPP values that are
smaller compared (Peters et al. 2012) with previous studies
(Huenneke et al., 2002).
NOTE 3: See Peters et al. GCB 2012 for the
relationships between ANPP and PPT in wet periods, and the
inappropriate use of long-term ANPP vs long-term PPT.
NOTE 4: Plant species codes in the
"spp" column of the data file are specific to the
Jornada Basin LTER. However, USDA Plants Database codes
(plants.usda.gov) have been assigned to most of these local codes
in a separate column. Further details for linking the local codes
to currently recognized taxa may be found in the "JRN
Vascular Plants" EDI package (knb-lter-jrn.210520001).
NOTE 5: In 2022, we have initiated a new
effort to evaluate the allometric equations used to estimate
biomass from field volumetric measurements, which will potentially
affect data in this dataset and other ANPP packages (such as
knb-lter-jrn.210011003). Therefore, values since 2012 are
provisional and subject to some change as this evaluation
proceeds. Please contact the JRN Information manager for further
information.
Source data packages
Biomass values in this dataset are calculated using two other data
sources, each assigned to their own JRN package IDs and regularly
maintained on EDI.
1. Non-destructive quadrat measurements of
plant cover and height collected in the field at NPP sites (EDI
package knb-lter-jrn.210011002).
2. Reference harvest measurements consisting
of plant cover, height, and harvested biomass taken outside the
NPP plots and used to generate regression models of biomass as a
function of volume (EDI package knb-lter-jrn.210011004).
Derived data packages
knb-lter-jrn.210011003 - Annual NPP summaries by site
References
Huenneke, Laura F., Dennis Clason, and Esteban Muldavin.
"Spatial heterogeneity in Chihuahuan Desert vegetation:
implications for sampling methods in semi-arid ecosystems."
Journal of Arid Environments 47, no. 3 (2001): 257-270.
Huenneke, Laura F., John P. Anderson, Marta Remmenga, and William
H. Schlesinger. "Desertification alters patterns of
aboveground net primary production in Chihuahuan ecosystems."
Global Change Biology 8, no. 3 (2002): 247-264.
Muldavin, Esteban H., Douglas I. Moore, Scott L. Collins, Karen R.
Wetherill, and David C. Lightfoot. "Aboveground net primary
production dynamics in a northern Chihuahuan Desert
ecosystem." Oecologia 155, no. 1 (2008): 123-132.
Peters, Debra PC, Jin Yao, Osvaldo E. Sala, and John P. Anderson.
"Directional climate change and potential reversal of
desertification in arid and semiarid ecosystems." Global
Change Biology 18, no. 1 (2012): 151-163.
