Shrub encroachment is a global phenomenon. Both the causes and consequences of shrub encroachment vary regionally and globally. In the southwestern US a common native C3 shrub species, creosotebush, has invaded millions of hectares of arid and semi-arid C4-dominated grassland. At the Sevilleta LTER site, it appears that the grassland-shrubland ecotone is relatively stable, but infill by creosotebush continues to occur. The consequences of shrub encroachment have been and continue to be carefully documented, but the ecological drivers of shrub encroachment in the southwestern US are not well known.
One key factor that may promote shrub encroachment is grazing by domestic livestock. However, multiple environmental drivers have changed over the 150 years during which shrub expansion has occurred through the southwestern US. Temperatures are warmer, atmospheric CO2 has increased, drought and rainy cycles have occurred, and grazing pressure has decreased. From our prior research we know that prolonged drought greatly reduces the abundance of native grasses while having limited impact on the abundance of creosotebush in the grass-shrub ecotone. So once established, creosotebush populations are persistent and resistant to climate cycles. We also know that creosotebush seedlings tend to appear primarily when rainfall during the summer monsoon is well above average. However, high rainfall years also stimulate the growth of the dominant grasses creating a competitive environment that may not favor seedling establishment and survival. The purpose of the Mega-Monsoon Experiment (MegaME) is twofold. First, this experiment will determine if high rainfall years coupled with (simulated) grazing promote the establishment and growth of creosotebush seedlings in the grassland-shrubland ecotone at Sevilleta, thus promoting infill and expansion of creosotebush into native grassland. Second, MegaME will determine if a sequence of wet summer monsoons will promote the establishment and growth of native C4 grasses in areas where creosotebush is now dominant, thus demonstrating that high rainfall and dispersal limitation prevent grassland expansion into creosotebush shrubland.
Vegetation and soil measurements are taken in the spring and fall each year. Spring measurements are taken in May when spring annuals have reached peak biomass for the growing season. Fall measurements are taken in either September or October when summer annuals and all perennial species have reached peak biomass for the growing season, but prior to killing frosts. Vegetation cover is measured to assess growth and survival of grasses and shrubs. Bare soil and litter covers are also measured to monitor substrate changes that occur within the plots.
One meter2 vegetation quadrats are used to measure the cover of all plants present in each m2. There are 10 quads in each plot, checkered along on side of the plot. There is a tag on one rebar of each quad with the representative quad number.
General vegetation measurements
The cover is recorded for each species of live plant material inside the quadrat. Vegetation measurements are taken in two layers: a ground level layer that includes all grasses, forbs, sub-shrubs, and a litter and bare soil, and a “shrub” layer that includes the canopy of Larrea tridentata. The purpose of this approach is to include Larrea canopies, while allowing the cover values of the ground level layer to sum to approximately 100%. The dead plant covers are not included in the measurement, thus the total amount may not equal 100%. It is assumed that the remaining cover missing from the 100% is a combination of dead plant material.
The quadrat boundaries are delineated by the 1 m2 PVC-frame placed above the quadrat. Each PVC-frame is divided into 100 squares with nylon string. The dimensions of each square are 10cm x 10cm and represent 1 % of the total quadrat area or cover. The cover and height of all individual plants of a species that fall within the 1m2 quadrat are measured. Cover is quantified by counting the number of 10cm x 10cm squares intercepted by all individual plants of a particular species, and/or partial cover for individual plants < 1%.
Vegetation cover measurements
Cover measurements are made by summing the live cover values for all individual plants of a given species that fall within an infinite vertical column that is defined by the inside edge of the PVC-frame. This includes vegetation that is rooted outside of the frame but has foliage that extends into the vertical column defined by the PVC-frame. Again, cover is quantified by counting the number of 10cm x 10cm squares intercepted by each species. Do not duplicate overlapping canopies, just record the total canopy cover on a horizontal plane when looking down on the quadrat through the grid.
Larger cover values will vary but the smallest cover value recorded should never be below 0.1%. When dealing with individual plants that are < 1.00%, round the measurements to an increment of 0.1. Cover values between 1.00% and 10.00% should be rounded to increments of 1.0, and values > 10.00% are rounded to increments of 5.
Larrea tridentata canopy is estimated using the portion of the canopy that falls within the quadrat. The canopy edge is defined by a straight gravity line from the canopy to the ground (i.e. imagine a piece of string with a weight on the end being moved around the canopy edge). ForLarrea seedlings the code LSEED is used and is a separate measurement from the Larrea canopy measurements. The cover measurement for LSEED is simply a count of individuals, not actual cover, as it is assumed that they would have a cover of < 1.00%.
To determine the cover of a grass clump, envision a perimeter around the central mass or densest portion of the plant excluding individual long leaves, wispy ends or more open upper regions of the plant. Live tissue is frequently mixed with dead tissue in grass clumps.
The cover of forbs is the perimeter around the densest portion of the plant. Measure all foliage that was produced during the current season.
Cacti and Yucca
The cover of cacti and yucca is made by estimating a perimeter around the densest portion of the plant and recorded as a single cover. For cacti that consist of a cluster of pads or jointed stems (i.e., Opuntia phaecantha, Opuntia imbricata), estimate an average perimeter around the series of plant parts and record a single coverage measurement.
Vine cover (and some forbs) is often convoluted. Rather than attempt to estimate cover directly, take a frequency count of 10X10X10cm cubes that the vine is present in.
As with other vegetation measurements, the smallest cover value for seedlings should never be <0.1%. If the value of a seedling’s cover is less, round up to 0.1%.
Non-Vegetation cover measurements
Materials other than vegetation that are measured in the drought plots include soil and litter.
Measure the cover of the area occupied by abiotic substrates. Cover is quantified by summing the number of 10cm x 10cm squares intercepted by abiotic substrates. Cover values < 10.00% should be rounded to increments of and cover values > 10.00% should be recorded in increments of 5. If there is no soil in the quadrat, record “SOIL” in the species column for that quadrat and record a “0” for cover.
Measure the cover of the area occupied by litter, which is unattached dead plant material. Cover is quantified by summing the number of 10cm x 10cm squares intercepted by abiotic substrates. Cover values < 10.00% should be rounded to increments of 1 and cover values > 10.00% should be recorded in increments of 5. If there is no litter in the quadrat, record “LITT” in the species column for that quadrat and record a “0” for cover.
Clipping grass at Ecotone Site
After measurements are taken at the Ecotone Site, grass is clipped down to the soil and removed from half of the quads in each plot. The goal is to assess the impact of competition on successful creosote seedling germination. The following quads, # 2, 4, 6, 7, and 10, get clipped in every plot at the ecotone site.
The watering schedule varies based on seasonal rainfall. Our goal is to increase average monsoon precipitation (150mm) by 50%, so we shoot for a total of 225mm on the plots during the summer monsoon.
Additional Information on the personnel associated with the Data Collection:
Stephanie Baker 2014-present
Megan McClung 2014-present
Chandra Tucker 2014-present
Plant phenology or life-history pattern changes seasonally as plants grow, mature, flower, and produce fruit and seeds. Plant phenology follows seasonal patterns, yet annual variation may occur due to annual differences in the timing of rainfall and ambient temperature shifts. Foliage growth and fruit and seed production are important aspects of plant population dynamics and food resource availability for animals.
The germination rate of creosote (Larrea tridentata) on the Sevilleta appears to be very low. During the early years of the LTER project it was attempted to quantify such germination through the use of seedling plots which were monitored on a bi-annual basis (spring and fall). During the period from 1989 through 1992 there were no creosote seedling that germinated on the monitoring plots. In 1999 a rather sizeable population of small seedlings was observed in one quite localized area in the vicinity of Five Points. This rather large number of individuals in one very limited area raised several questions:1. When did this small community of individuals germinate- the supposition is that they had all germinated at the same time.2. What conditions existed in this particular location which allowed such prolific germination when there was no indication of virtually any other germination in this vicinty.3. The most immediate question was whether these seedlings would survive through the impending La Nina winter, spring and summer as many of the individuals did not appear to be very "healthy." To help answer these questions it was decided to begin a "small" monitoring project. All of the individuals in this small area thatappeared to be of the same age were marked with posts, located with GPS, measured (height) and photographed. This initial survey included 78 individuals. The population was resurveyed in August, 2000 and it was discovered that: 1. Many individuals were missed in the initial survey; 2. Most of the individuals (72 of 78) had survived the intervening 9 months; and 3. The average individual growth during this period was 1.4 cm. It is planned that this population will continue to be monitored (probably on an annual basis) to track survival and growth rate of these individuals.
Experimental Design: There has been little recent germination of creosote in the Five Points area. However, the discovery of a patch of relatively new seedlings in one localized area in Nov. 1999 prompted the initiation of this sampling. Continued measurement through time provides growth rates that can then be correlated with meteorological conditions.
Data Collection: Seedlings were marked by pounding galvanized pipes (about 1 m long) into the soil about 6" south of the seedlings. In some cases a single pipe was used to mark more than one seedling. Pipe numbers were written with black magic marker near the top of the pipe. Seedling numbers were written near ground level so they could be seen when photographed. Each pipe was GPS'd.
The height of each seedling was measured to the nearest mm. Measuring was done from the soil surface to a plane even with the top of the tallest leaves. Each seedling was photographed and a meter stick was laid horizontally on the ground (parallel to the plane of the photograph) against the corresponding pipe for scale. In some cases, multiple seedlings were included in one photograph. Arrows on the pipe indicated designated seedlings. These photos were taken with slide film and are currently in the possession of Doug Moore.
The initial cohort of seedlings from 1999 were remeasured on Aug 23, 2000 and seedlings 79-101 added to the data set. At that time, more seedlings were discovered. Seedlings 102-175 were measured on Sep 18, 2000.
Beginning in 2001, two diameters of the crown were measured; the first at the widest point and the second perpendicular to the first.
Between 2001 and 2004, the permenant marker that had been used to mark each pipe faded to the point that they could no longer be read. In 2004, numbered aluminum tags were attached to the pipes with wire but the numbers of the pipes and the corresponding samples no longer designate the same individual as prior to 2004.
Also, note the effect of the unprecedented freeze event of early 2011.
11/01/09 (JMM) Metadata updated and compiled from 1999 to 2009 data.
01/29/09 (YX) Metadata updated and compiled from 1999 to 2008 data.
01/28/2009 (YX) 99-2008 dataset was checked for errors and missing data.
Additional Information on the personnel associated with the Data Collection / Data Processing
Joy Francis, a post-doc with Jim Gosz, was instrumental in setting up this study.
1. Initiated - November 11, 1999, and Resurvey - August 23, 2000. 2. Marked missed seedlings - September 18, 2000. 3. Measured 1-79 - October 9, 2001-measured both height and 2 diameters. 4. Measured 129 - Dec.13, 2004 - Measured both height and 2 diameters. 5. Measured 1-129 - Sept. 22, 2005 - Measured both height and 2 diameters. 2006-2008 measured 1-129 for both height and 2 diameters.