Jan 03, 2025
Aboveground net herbaceous plant production in grazinglands
- 1USDA-ARS;
- 2Archbold Biological Station
External link: https://ltar.ars.usda.gov
Protocol Citation: David Augustine, Elizabeth Boughton 2025. Aboveground net herbaceous plant production in grazinglands. protocols.io https://dx.doi.org/10.17504/protocols.io.q26g71qj1gwz/v1
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: Working
We use this protocol and it's working
Created: October 21, 2024
Last Modified: January 03, 2025
Protocol Integer ID: 110455
Keywords: Aboveground net primary production, Long-Term Agroecosystem Research, LTAR, USDA LTAR, Common Experiment, grazing lands, rangelands, grassland, yield
Funders Acknowledgements:
United States Department of Agriculture
Grant ID: None
Disclaimer
This research is a contribution from the Long-Term Agroecosystem Research (LTAR) network. LTAR is supported by the United States Department of Agriculture. The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable. USDA is an equal opportunity provider and employer.
Abstract
Aboveground Net Primary Production by herbaceous plants (ANHP) is a fundamental property of grazinglands, with consequences for secondary and tertiary productivity and ecosystem function. Estimation of ANHP in the presence of grazers is complicated by the fact that both plant growth and plant consumption occur simultaneously when grazers are present on the site. Because the aboveground portions of herbaceous plants die back to the root or crown during the dormant season in temperate zones, ANHP can be estimated by measuring the cumulative change in aboveground herbaceous biomass within plots where herbivory is temporarily excluded. Where plants species are present that retain live aboveground biomass through the dormant season (e.g. woody plants, cactus), alternative methods than what is described here may be needed if an estimate of aboveground production by these species is desired. Where aboveground biomass is harvested by species or by functional group, and a forage value (e.g. grazed vs. not grazed) can be assigned to each species or functional group, then aboveground forage production can also be calculated. In grasslands composed entirely of herbaceous species (gramminoids and forbs), ANHP is equivalent to aboveground net primary production. ANHP is a primary metric for the LTAR Grazinglands Common Experiments assessing the indicator of Yield, and the preferred unit of measurement is in kg/ha/year.
Objective:
Objective:
This protocol outlines methods for measuring Aboveground Net Herbaceous Plant Productivity (ANHP) over the course of a year to accommodate the wide range of plant species, timing of growth, and livestock use across rangeland LTAR sites. Methods increase in complexity, time, and labor demands where temporal variation in plant growth and/or livestock use is more complex, specifically if (a) there are multiple growing seasons per year, (b) species or functional groups grow asynchronously, or (c) aboveground live herbaceous biomass does not decline to zero each year (Sala and Austin 2000). It does not address production by woody or succulent plant species that retain live stems or organs aboveground during the dormant season. Also, complexity is added when accounting for the plant biomass consumed by domestic livestock, and the feedback effects of such consumption on plant growth rates during the remainder of the growing season or during subsequent growing seasons (McNaughton et al. 1996).
Timing of Sampling:
We assume for the use of this protocol in grazingland LTAR sites in the continental U.S. that there is a dormant season where live aboveground biomass of herbaceous species approaches zero during part of the year.
ANHP is estimated by the summed net annual increase in dry mass of live aboveground biomass of each plant species in the plant community, after accounting for biomass removed by herbivores during the growing season (Sala and Austin 2000, Knapp et al. 2007). ANPP consists of two components 1) green living biomass, 2) senesced biomass produced during the current year, which may be either
standing dead, or litter on the soil surface. The key point is that the biomass grew in the current year.
In the simplest case where (a) livestock grazing or other large vertebrate herbivory is absent, and invertebrate/small vertebrate herbivory is judged negligible, and (b) all species grow synchronously, ANHP can be estimated from the peak live aboveground herbaceous biomass measured at a single point in time each year, using the approaches described below. Table 3.2 in Knapp et al. 2007 provides a detailed summary of approaches.
Each site will need to assess the magnitude of asynchrony in plant growth pulses, which can arise from differences among species in phenology, life strategy, or because soil moisture fluctuations result in multiple growth pulses within a year. These sources of growth asynchrony will determine how many times during the year (and when) measurements of standing biomass are needed to attain an estimate of aboveground biomass production by each major species or functional group (e.g. cool season vs. warm season grasses). Where logistical constraints (time, labor) limit the number of times that peak biomass is measured annually, and this can affect the precision of the ANHP estimate, more intensive sampling or alternative methods should be used in one year to determine how the selected method affects precision of the ANHP estimate.
Accounting for Herbivory:
For LTAR sites grazed by livestock, measurements of ANHP must account for the amount of biomass consumed each year, and potential effects of such consumption on subsequent plant growth (McNaughton et al. 1996). Peak biomass is best estimated in plots where herbivores are temporarily excluded, but such exclusion eventually causes changes in plant architecture and physiology that can affect growth rates. Thus, the simplest approach is to establish a replicated set of moveable exclosures at a site, and sample peak biomass (or net changes in biomass) within the exclosures, but then move the exclosures whenever the rate of plant growth inside them begins to be affected by grazing exclusion (e.g. development of a much denser canopy inside cages, or a substantial change in leaf to stem ratios).
We recommend that:
(1) For all sites, exclosures should be moved to new locations immediately prior to the start of the growing season, or immediately prior to the start of the grazing season if livestock are not present on the site until after start of the growing season. A biomass measurement is not needed at this time.
(2) For productive grazinglands with high grazing intensity (e.g. >50% of ANPP) where post-defoliation regrowth is expected to be substantial, exclosures likely need to be moved (and biomass measured) two or more times per growing season. During the growing season, each time that exclosures are moved, aboveground biomass should be sampled in paired quadrats located both inside and outside the exclosure.
(2) For grazinglands with a single large growth pulse and/or moderate grazing intensity (eg. 20-50% of ANPP) exclosures should at a minimum be moved once per year (as in step 1 above), and then the aboveground biomass measured when the vegetation reach peak standing crop. If an estimate of forage consumption is not needed, then quadrats only need to be clipped inside the exclosures at this time.
(3) Size of a moveable exclosure is typcially 1 m3 or larger, to enable sampling of plant biomass from a quadrat placed at the center of the cage that is large enough to be representative of the plant community being sampled. Fencing should be tall enough and with mesh size small enough to prevent consumption of any of the enclosed vegetation, but mesh size should be as large as possible to minimize shading.
Measuring Standing Plant Biomass in Moveable Exclosures:
Standing biomass within a moveable exclosure (herafter referred to as a plot) can be measured at a given point in time by destructive harvest, or via indirect, non-destructive methods. Plot size and number needed to represent the fetch area of an EC tower or within a replicate of the common experiment will depend on the amount and scale of spatial heterogeneity within the target sampling area.
If using destructive harvests, place a quadrat of known size at the center of the exclosure and clip all aboveground plant biomass to crown level of perennial grasses or to ground level of plants without crowns. We recommend that harvested biomass should be separated by plant species or, at a minimum, by functional groups that differentiate between photosynthetic pathways (C3, C4, CAM) and life strategies (annual vs. perennial) and growth forms (graminoids vs. forbs). In many grasslands, standing dead biomass can be carried over from the previous year’s ANHP. We recommend each LTAR site develop a standardized protocol for technicians to differentiate between tissues produced in the previous year versus tissues that were produced in the current year but have begun to senesce at the time of harvest (typically identified based on color and degree of weathering). The former should be removed or separated from current-year growth prior to drying and weighing samples. Standing biomass produced in the current year for each species or functional group should be dried in an oven until a constant dry weight is achieved, and weighed.
Indirect, non-destructive estimates of aboveground biomass rely on calibrating an inexpensive index of plant biomass such as canopy light penetration with the more expensive harvest method (Knapp et al. 2007). A second method involves passing a set of metal pins or lasers through the plant canopy, and recording interceptions with plant tissues (Frank and McNaughton 1990). Species or groups of species that differ notably in leaf thickness and canopy architecture typically require separate calibrations, but such approaches can achieve r2 for the relationship between canopy interception and functional group biomass on the order of 0.83 – 0.96 (Frank and McNaughton 1990; Augustine 2003). For plant communities where plots often include substantial heterogeneity in plant cover, such as the Jornada
Experimental Range, a larger-scale version of canopy interception has been developed (Huenneke et al. 2001). In this method, 1 m2 plots are divided into a grid of 100 10 x 10 cm sections, and cover or projected surface area for each plant or plant part is estimated by counting the grid squares or portions of squares occupied by that plant or part (i.e. canopy interception is recorded with respect to 10 x 10 cm squares rather than interception with a pin or laser). When such indirect methods are used in long-term studies, calibrations should be checked with additional reference harvests in years with extreme or unusual weather conditions (Peters et al. 2012).
Calculating ANHP:
If biomass inside moveable exclosures is clipped just once per year at peak standing biomass, then ANHP is estimated as the mean aboveground biomass inside the exclosures.
If exclosures are moved more than once per year (e.g. on dates 1, 2, and 3, where date 1 is the start of the growing season), then ANHP is calculated as the mean aboveground biomass inside exclosures on date 2, plus the difference between mean biomass inside exclosures on date 3 and mean biomass outside exclosures on date 2.
If there are multiple growing seasons per year, then ANHP is calculated as the sum of growth inside cages in season 1 and growth inside cages in season 2. If biomass does not decline to zero prior to the start of season 2, then standing biomass must be clipped in grazed plots at the beginning of season 2, cages moved prior to the start of season 2, and biomass accumulation measured inside cages at peak biomass in season 2.
The preferred unit of measurements is in kg/ha/year.
Spatial Scale and Replication:
ANHP data will be used in at least two ways:
1) to quantify inter-annual variation in productivity across LTAR sites, and
2) to quantify within-site differences between treatments within the Common Experiment, or other experimental treatments.
The choice of how many plots to sample, the size of sampling plots, and their distribution on the landscape will vary with a number of factors, including vegetation structure and spatial heterogeneity, and the identity and magnitude of spatial gradients that may influence ANPP on the study site. For ANHP measurements associated with the eddy covariance towers, the spatial extent of sampling needs to represent the major sources of spatial variation within the flux tower fetch. Stratification of sampling should be employed where it will increase precision and sampling efficiency.
We suggest at a minimum:
1. Moveable grazing exclosures be distributed systematically across the fetch area, with a minimum of 15 exclosures at relatively homogenous sites.
2. Where the destructive harvest method is used, biomass should be clipped within one quadrat per exclosure. Quadrats could vary from 0.1 to 1 m2 depending upon plant size and density; elongated (e.g. 20 x 50 cm or 20 x 100 cm) quadrats are recommended to best accommodate small-scale heterogeneity in
plant and gap distributions.
3. Where larger plants are present, larger exclosures and quadrats may be necessary, and ideally will
encompass multiple individuals of the dominant species plus their interspaces.
4. Where destructive harvest conflicts with other measurements or research intents (e.g. if destructive harvest is not desired within the fetch of an EC tower), then sampling may be conducted at immediately adjacent, representative areas. Alternatively, non-destructive methods may be used to estimate biomass in moveable exclosures.
Coordination with other biological measurements:
Forage production: Where biomass is estimated by species, and each species can be assigned a forage value (e.g. grazed vs. ungrazed), then ANHP methods will also provide an estimate of forage production. If species vary temporally (e.g. among years) or among plant parts in their forage value, then sorting of ANHP could be conducted according to forage value in order to provide annual estimates of
both forage production and ANHP.
Plant nutrient content: Biomass harvests for ANHP can be used to provide tissues for plant nutrient content sampling; additional sorting by plant part may be required.
Plant-based measurement of herbivore consumption: Additional biomass harvests conducted in
paired quadrats outside the grazing exclosure can be used to measure livestock consumption rates in addition to ANHP (McNaughton et al. 1996). Depending on the grazing management regime
and plant growth/senescence patterns, this may require additional paired measurements of plant biomass in/out of grazing exclosures at other times of the year besides peak biomass, e.g. at the end of the grazing season.
Calibration of Remote Sensing of ANHP: Record the geospatial coordinates of the area sampled by the moveable exclosures, either as point coordinates of all exclosures or as a polygon of the area represented by the exclosures. This can be used to relate the measurement of ANHP to remotely sensed metrics of vegetation growth. Note that the satellites are primarily seeing grazed vegetation (assuming the exclosures are small and distributed systematically or randomly across the area they are sampling), which needs to be addressed in calibrations (Gaffney et al. 2018).
Protocol references
Augustine, D. J. 2003. Spatial heterogeneity in the herbaceous layer of a semi-arid savanna ecosystem. Plant Ecology 167:319-332.
Frank, D. A. and S. J. McNaughton. 1990. Aboveground biomass estimation with the canopy intercept method: a plant growth form caveat. Oikos 57:57-60.
Gaffney R, Porensky LM, Gao F, Irisarri JG, Durante M, Derner JD, et al. Using APAR to Predict Aboveground Plant Productivity in Semi-Aid Rangelands: Spatial and Temporal Relationships Differ. Remote Sensing (2018) 10(9). doi: 10.3390/rs10091474.
Knapp, A.K., J.M. Briggs, D.L. Childers, O.E. Sala, 2007. Estimating aboveground net primary production in grassland- and herbaceous-dominated ecosystems. Pp 27-48 In: Fahey, T.J. and A.K. Knapp, eds. Principles and standards for measuring primary production. Long-term Ecological Research Network series, Oxford University Press, New York.
Huenneke, L., D. Clason, and E. Muldavin. 2001. Spatial heterogeneity in Chihuahuan Desert
vegetation: implications for sampling methods in semi-arid ecosystems. Journal of Arid Environments 47:257-270.
McNaughton, S., D. Milchunas, and D. Frank. 1996. How can net primary productivity be measured in grazing ecosystems? Ecology 77:974-977.
Peters DPC, Yao J, Sala OE, Anderson JP. Directional Climate Change and Potential Reversal of Desertification in Arid and Semiarid Ecosystems. Global Change Biology (2012) 18(1):151-63. doi: 10.1111/j.1365-2486.2011.02498.x.
Sala OE, Austin AT. Methods of Estimating Aboveground Net Primary Production. In: Sala OE, Jackson R, Mooney H, Howarth R, editors. Methods in Ecosystem Science. New York, USA: Springer (2000).