Bouteloua Eriopoda) Is a Long-Lived Keystone Primary Producer Across Much of Semi-Arid Southwestern North America

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Abstract:

Black Grama (Bouteloua eriopoda) is a long-lived keystone primary producer across much of semi-arid southwestern North America. Though the ecology of this grass is relatively well studied, the potential effects of climate change on the species and ecosystem are more poorly understood. We examined size distribution and seed production of B. eriopoda following nine years of ongoing monsoon rainfall manipulation treatments. This Long Term Ecological Research (LTER) experiment is designed to imitate monsoon pulse precipitation patterns predicted by climate change models at Sevilleta National Wildlife Refuge, NM. Demographic measurements were collected in mid-June before the monsoon growing season began. Spikelets were collected once B. eriopoda florets went to seed in mid to late September. We found significantly higher mean tussock volume and height in plots with added precipitation compared to ambient control plots. Greater height was correlated with higher spikelet mass and average number of spikelets per inflorescence in long pulse treatment plots.

This indicates an increase in reproductive fitness accompanying increased survival with less frequent, more intense rainfall. These findings shed light on how semiarid ecosystems may respond to shifting precipitation pulses in the Southwest with regards to a changing climate.

Introduction:

Anthropogenic influences on climate are expected to continue changing weather patterns globally (Mach & Mastrandrea, 2014). One anticipated climate impact is the disruption of precipitation regimes. This shift will manifest in higher intensity precipitation events with longer intervals in between (Kharin et al. 2007). A ‘soil water bucket model’ (Knapp et al. 2008, Thomey et al. 2011) can be used to describe the manner in which this change in precipitation patterns will affect vegetation. The model describes upper layers of soil to maximum root depth as a bucket that fills after precipitation and slowly empties during dry periods. In mesic ecosystems, the bucket remains partially full most of the time with occasional periods of drought stress and anoxia. Hydric and xeric ecosystems experience frequent stress with a too full or empty bucket, respectively. This results in lower primary production in these ecosystems in comparison to mesic habitats. Frequent, small rainfall events are ideal in mesic systems, as they help to maintain optimum levels of hydration. More extreme rainfall will create more frequent periods of stress in mesic ecosystems, while in hydric and xeric ecosystems, this precipitation change is likely to benefit plants by creating longer intervals without stress.

In xeric ecosystems, changes in precipitation regimes are expected to extend both periods of ample water availability and periods of drought stress. These ecosystems are relatively well adapted to periods of drought, so it is expected that occasional heavy rainfall will improve Annual Net Primary Production (Thomey et al. 2011). With these changes, semiarid flora may need to alter water use strategies to utilize more drought avoidance strategies as opposed to a drought tolerance approach (Heschel & Riginos, 2005). Increased water use efficiency (WUE) would improve fitness during periods of rainfall, while decreased water use efficiency would help them endure through periods of drought. An increase in WUE plasticity would likely confer the greatest fitness and therefore should be selected for in a more variable precipitation regime.

Shrub encroachment is a major ecological concern and has been a topic of much research in semiarid grasslands (Elderidge et al. 2011; Knapp et al. 2008; Laliberte et al.

2004). Shrub encroachment drivers are largely athropogenic, including grazing as well as climatic changes, although it is unclear how a more extreme precipitation regime will impact woody plant advancement. Bouteloua dominated systems are experiencing ongoing pressure from Larrea tridentata (creosote bush) in New Mexico and across the southwest (Grover & Musick 1990). Creosote shrub encroachment has been found to negatively impact recruitment and reproductive fitness of black grama (Peters 2002), with decreases in seed counts and viability along creosote transition zones. High soil moisture is another important factor that impacts Bouteloua recruitment (Lauenroth et al.1994).

Understanding how changing precipitation will impact reproductive fitness and demography of Bouteloua populations will help to better understand the effects of compounded pressures on grasses along shrub encroachment fronts.

Demography of black grama and related species has been relatively well studied with regards to a variety of environmental variables. After fire, almost all black grama vegetation is typically destroyed. It regrows slowy but steadily, recovering half of its previous plant size after in five years. Bouteloua gracilis (blue grama) plant size, however, is not significantly affected by fire (Parmenter 2008). Black grama is also more sensitive to herbivory and grazing than blue grama, though in certain regions it will become the dominant species over long periods without disturbance (Gosz & Gosz 1995).

The effects of more intense, less frequent rainfall on ANPP have also been well studied, with black grama showing increases in ANPP under heavy rainfall treatments (Heisler-

White et al 2008; Thomey et al. 2011). However, the effects of anticipated precipitation changes on grassland demography are more poorly understood. Demographic studies on plant populations inform patterns of population size, survival rates, and mortality

(Hartnett & Bazazz 1985). Long term studies offer the clearest road to understanding these variables (Fair et al. 1999), especially in herbaceous clonal plants, in which growth ring analysis does not apply (de Witte & Stöcklin 2010). Size, although not always correlated smoothly with tree growth, is a useful and easily measured metric for age.

We aimed two answer four main experimental questions. (1) Does the effect of different rainfall treatments on soil moisture alter mean black grama individual size? (2)

How do demographic distributions differ between rainfall treatments? (3) How is black grama reproductive fitness affected by rainfall variability? (4) How are reproductive fitness and demography related? Materials/Methods:

Study Site

Sevilleta National Wildlife Refuge (SNWR), located at the junction of

Chihuahuan Desert, Conifer Woodland, Colorado Plateau Steppe and Great Plains

Grassland in central New Mexico, is home to a vast diversity of plants and animals

(Moreno-de las Heras et al. 2015). Converted from rangelands and set aside in 1973, the

Refuge contains many ecological experiments and meteorological stations as part of a

Long Term Ecological Research (LTER) program. This study was conducted at

34°20'38"N 106°43'37"W and an elevation of 1602m. The site lies in a distinct ecotone where Bouteloua eriopoda dominated grassland to the north transitions to a Larrea tridenta dominated shrubland to the south (Báez et. al., 2012). No L. tridenta individuals exist within study plots. Other significant plant species at the study site include

Muhlenbergia arenicola, Aristida purpurea, Sporobolus spp., Guttierezia sarothrae, and

Sphaeralcea wrightii. Average temperatures range from 1.6° C in January to 24.5° C in

July (Báez et. al., 2012). Precipitation at SNWR is highly variable spatially and temporally (Pennington and Collins, 2007), resulting in low aboveground net primary production (Xia et al. 2010). The site receives 240mm of annual precipitation on average and is subject to the North American Monsoon, which typically occurs from July through

September. During this period, 60% of yearly precipitation falls (Gosz et al. 1995). This rainfall regime divides the growing season into two periods. Winter accumulation of precipitation allows cool season grasses and ephemerals to flourish, while slow growing annuals, biennials and warm season C4 grasses including B. eriopoda thrive in the late season monsoons (Henderson et al. 1988).

Rainfall Manipulation

The MRME contains thirteen 9m×14m plots consisting of three ambient control precipitation plots, which receive only environmental precipitation, and five plots each of two different watering treatments (Fig. 1). One treatment receives an additional 5mm of water weekly throughout the monsoon season (July-Sept) and the other receives an additional 20mm of water only thrice during the monsoon season. Each receives the same amount of additional hydration annually. Water is administered in the morning, when there is less wind, via a hanging apparatus of pvc above the plots. Treatments have been ongoing since 2006 (Thomey et al. 2011).

Figure 1. MRME treatment plots with ongoing NPP (net primary production) Quadrats highlighted. (http://sev.lternet.edu/node/3222) Several ongoing LTER experiments are present in the MRME plots, including root ingrowth donuts measuring Belowground Primary Production, Net Primary Production and Nitrogen Fertilized Quadrats (Figure 1).

Climate/ Meteorological data

Monthly averages were calculated from meteorological data collected at a weather station approximately one kilometer south of the MRME site. Soil moisture probes are inserted at a depth of 30cm. We compared data from 2015 to long-term averages since the site was set up in 1999.

Study Species

Bouteloua eriopoda (Poaceae, Torr.) is a long-lived, perennial grass endemic to much of southwestern North America, including U.S. states Arizona, California,

Colorado, Kansas, Nevada, New Mexico, Oklahoma, Texas, Utah and Wyoming (USDA,

2016). It has an average lifespan of 2.2 years but individuals have been recorded living up to 28 years of age (Wright & Van Dyne, 1976). B. eriopoda has low seed viability and predominantly reproduces by stolon, a process which takes two years. In the first year, the mother plant will grow a stolon, which will take root in its second year. It is typically a low elevation grass that grows between 1000 and 1600m above sea level, but can occasionally be found at higher elevations. The grass is generally found in pure stands on dry sandy or gravelly soils and is very sensitive to grazing (Leithead et al.

1976). B. eriopoda is by far the most common species in the MRME study plot, and due to its high density was the only species we collected data for.

Size Distribution Measurements

Data collection was conducted between June 10th and June 24th, before monsoon season growth began.

The same 1m×3m region of each plot was sampled (Fig. 2). All Bouteloua eriopoda individuals that had at least 50% basal area contained within the 1m×3m subplot were measured in three dimensions and tagged with a numbered flag. Clumps of grass separated by 4cm or more at the base were regarded as individuals. When clumps were 4cm apart but connected by a stolon, they were measured as individuals. Root ingrowth donuts present in sampling subplots were ignored during data collection.

We measured the base diameter of each tussock in the longest possible direction as well across the base perpendicular to this line. All green or yellow (recently dead) grasses were included in tussock base measurements, but dead grey grasses were not included. Heights of grass clumps were also measured from the base of green foliage to the tallest point of the plant, dead or alive. Basal area was calculated under the assumption that the plants were roughly elliptical (dia1×dia2×π), and volume was estimated as an elliptical cylinder (dia1×dia2×π×h).

Seed Counts

Seed collection was conducted on the 8th, 18th, and 22nd of September 2015. Up to

16 individuals were randomly selected from each plot and all inflorescences from these individuals were collected. Inflorescences were clipped from culms within 5mm of the lowest floret. Loose seeds were collected and massed (to the nearest milligram) with intact inflorescences. Additional plant matter was discarded. The total number of spikelets per each inflorescence and the total mass of all inflorescences from an individual was recorded. These measures were used to calculate average number of spikelets per inflorescence and average spikelet mass for each individual.

Data Analyses

All statistical analyses were performed with JMP version 7.0.2 (SAS Institute,

Cary, NC). To determine treatment effects on plant sizes and fitness measures across plots, we used a linear mixed model ANOVA with plot nested within treatment and both as fixed effects. Plant size distributions were natural-log transformed to fit normality assumptions of the analysis. Planned contrasts for paired analyses within ANOVA were performed to determine differences between individual plots. Shapiro-Wilk tests were used to compare distributions of plant size and fitness measures with normal. We performed bivariate regression analyses to determine relationships between growth and reproductive fitness measures within treatments.

Results:

Climate

Precipitation was higher than average in 2015, with 279.3 mm of total rainfall.

Monthly air temperature averages in 2015 did not differ substantially from long-term means, but paired with high precipitation variability, some relevant disparities become apparent (Figure 2). Precipitation patterns in 2015 fell far from a typical monsoon season.

May and June were wetter and cooler than usual, which corresponded with higher than usual soil moisture. High precipitation in July reflected a typical monsoon, but was followed by a dry August and a very dry September, which corresponds with a drop in soil moisture and an increase in temperature. Additional precipitation in October helped to compensate for this dry period and caused a spike in soil moisture that persisted through the remainder of the calendar year.

It is important to note that some delay can be seen between precipitation and soil moisture patterns both in 2015 and on average, and that soil moisture also responds to temperature. In a typical year, soil moisture remains throughout the winter despite low precipitation and falls in May and June due to evaporation.

Demography

Neither area nor volume varied significantly with treatment. However, plant heights differed between treatments, with both watered plots containing taller plants in comparison to ambient control plots (Table 1). Small pulse plots showed slightly higher mean plant height on average, but this difference was not statistically significant (Figure

3, Table 1). 30" 25" 20" 15" 1999>2015"means" 10" 2015" 5" Average'Air'Temp'C' 0" Jan" Feb" Mar" Apr" May" Jun" Jul" Aug" Sep" Oct" Nov" Dec"

70" 60" 50" 40" 1999>2015"means" 30"

ppt'(mm)' 20" 2015" 10" 0" Jan" Feb" Mar" Apr" May" Jun" Jul" Aug" Sep" Oct" Nov" Dec"

25"

20"

15" 1999>2015"means" 10" 2015"

Soil'moisture'%' 5"

0" Jan" Feb" Mar" Apr" May" Jun" Jul" Aug" Sep" Oct" Nov" Dec"

Figure 2. Monthly averages of climate data collected approximately 1km south of the MRME site. Data is included for 2015 as well as long term means.

Treatment Plot C-L comparison C-S comparison L-S comparison log(total inf. mass) 0.2530 2.0045* 0.0697569418 0.2073872799 0.386081968 log(spikelet count) 0.1707 1.6471 0.0262391069 0.1073662895 0.2875259835 Avg. Spikelet mass 0.3532 4.3271**** 0.3473763856 0.556208544 0.2695292432 Avg. Spikelets/ inf. 2.7688 4.7994**** 0.4383583762 0.0291065483 0.0133160022 log(volume) 2.6485 7.6862**** 3.5160757612* 4.903900587* 0.3624290195 log(area) 0.8036 5.4440**** 0.9413164549 1.5466477803 0.1841949562 log(height) 5.8135** 15.6690**** 8.1948870334** 10.47602421*** 0.5561819628 *P<0.1, **P<.05, ***P<.01, ****P<.0001

Table 1. F values from Mixed model ANOVA of demographic and reproductive fitness measures between Treatment (fixed effect) and Plot (nested effect) with planned comparisons (t-tests) between individual treatments (C= Ambient Control, S=Small Pulse treatment 5mm/week, L=Large pulse treatment 20mm/month).

30" ""(b)" "(b)" 25"

20" (a)" 15"

Height'(cm)' 10"

0" Control" Small"Pulse" Large"Pulse"

Figure 3. Mean (±1 SE)heights of B. eriopoda individuals by treatment.

All demographic distributions varied significantly from normality, though it was possible to logarithmically transform volume to a normal distribution in the large pulse treatment (Table 2). Height was the measure with the greatest statistical distance from normality. Anecdotally, control plots visually contained much less vegetation cover, with many seedlings and few large, established individuals in comparison to watered plots.

Height distributions were bimodal in all three treatment categories, although the demographic distributions between treatments varied greatly (Figure 4). Few, if any individuals were taller than 60cm in any treatment. Very few individuals taller than 5 cm were present in ambient control plots compared to watered plots, although the presence of several larger individuals between 20cm and 50cm tall was unmistakable. The mode individual height in watered plots was between 10 and 15 cm, while the most common height in ambient control was between 0cm and 5cm. Differences in evenness of

Control 5mm/week 20mm/month log(volume) 0.955639**** 0.975068** 0.981400 log(area) 0.968302** 0.975278** 0.974092*** height 0.707078**** 0.914369**** 0.874548**** log(height) 0.892943**** 0.887335**** 0.938599**** Average spikelet mass 0.981457 0.989229 0.948991** Avg. spikelets/ inflorescence 0.984410 0.960276 0.965506 *P<0.1, **P<.05, ***P<.01, ****P<.0001

Table 2. W values of Shapiro-Wilk tests of normality. Control plot sample sizes for Average spikelet mass and Average spikelets/inflorescence were very small and these data should be ignored.

Ambient Control

150

100

50 Axis Count

0 10 20 30 40 50 Height (cm)

20mm/ month Large Pulse 100 75

25 Axis Count

0 10 20 30 40 50 60 70 80 Height (cm)

5mm/week Small Pulse 75

25 Count Axis Count

0 10 20 30 40 50 60 Height (cm)

Figure 4. Height distributions by treatment fitted with a smoothed nonparametric curve. Note that count axes vary between treatments. Note that axis scales are not identical. distribution were present as well. Ambient Control saw the most dramatically skewed size distributions, small pulse treated plots were the most evenly distributed, and large pulse treated plots fell in between. Between watered plots, counts of taller individuals were similar, but large pulse treatment plots contained about twice as many individuals in the 0cm to 15cm range.

Reproductive Fitness

No fitness measure varied significantly by treatment (Table 1). Distributions of fitness measures were not significantly different from normal, excluding average spikelet mass in large pulse treatment plots, which was right skewed (Table 2, Figure 5).

September, the month in which seeds were collected, was unseasonably dry (Figure 2).

At the time of seed collection, few plants in ambient control plots had produced seeds, so sample sizes in this category are too small to consider for our comparisons. In watered plots, which experienced a dry September to a lesser extent, many plants had produced seed by the time collection occurred (Table 3). Average spikelet mass and average number of spikelets per inflorescence did not vary with plant size measures, except for height in large pulse treatment plots, which showed significantly higher spikelet mass and number of spikelets per inflorescence in plants with greater height. This trend connects spikelet mass skewness with height distribution in large pulse plots (Figure 4, Figure 5,

Table 3). In small pulse treatment plots, average spikelet mass decreased slightly in individuals with greater basal areas and volumes. This trend was minute, but statistically present.

Ambient Control 2.0 1.5

1.0

0.5 Axis Count

.0022 .0023 .0024 .0025 .0026

5mm/week 20 15

5 Axis Count

.001.0015.002 .003.0035.004 .005

20mm/month

25 20 15 10 5 Axis Count

.001 .002 .003 .004 .005

** Figure 5. Distributions of average spikelet mass by treatment. Curve is fitted for a normal distribution. Control plot sample size is very small and should be ignored. Spikelet counts and total inflorescence mass increased with all measures of size

5mm/ week 20mm/ month Volume × total inf. mass 0.0000019** 0.0000095**** Area × total inf. mass 0.0000952** 0.0004395**** Height × total inf. mass 0.0091871*** 0.0153855****

Volume × spikelet ct. 0.0009275*** 0.0031102**** Area × spikelet ct. 0.0464653*** 0.1433387**** Height × spikelet ct. 3.1291259*** 4.4956138****

Volume × avg. sp. mass -3.591e-9** 3.8576e-9 Area × avg. sp. mass -1.845e-7** 0.0000002 Height × avg. sp. mass 0.0000079 0.0000262****

Volume × spik./ inf. -0.0000014 0.0000038 Area × spik./ inf. -0.000052 0.0001696 Height × spik./ inf. 0.0087550 0.0116135**

Table 3. Bivariate regression slopes of relationships between three demographic measures (Volume, Area, and Height) against four fitness measures (total inflorescence mass, spikelet count, average spikelet mass, and average number of spikelets per inflorescence). Control plots omitted due to small sample size.

Discussion:

More consistent water input increased B. eriopoda survivorship, though different rainfall treatments only increased mean height significantly (Table 1, Figure 3, Figure 4).

Water was a limiting factor for black grama height but not basal area, meaning that differences in ANPP are due to variation in height. Adding water also enables populations to maintain a higher proportion of larger individuals. Mortality appeared to increase with recruitment (Figure 4), but seedling survivorship may have been confounded by tradeoffs between reproduction by seed or stolon. Non-proportional reproductive fitness measures increased for taller plants under a more extreme precipitation regime, but decreased overall (Figure 5), implying selective pressure to take better advantage of large soil moisture pulses (Table 3).

Does the effect of different rainfall treatments on soil moisture alter mean black grama individual size?

Differences in mean plant height between watered and ambient plots align with the hypothesis that water availability is a limiting factor in this species and study site

(Table 1, Figure 3) (Julander 1945; Gosz &Gosz 1995). However, basal area did not differ between control and watered treatments, indicating that this particular trait may not be limited by water availability. Increased basal area may not correspond with a change in fitness, while more vertical growth could increase the range of seed dispersion for an individual, conferring higher fitness.

The lack of difference in mean size between watered treatments indicates that B. eriopoda populations are well adapted to a varied precipitation regime (Table 1, Figure 3). Previous study has shown that B. eriopoda aboveground net primary production

(ANPP) increases with fewer, more intense rainfall events (Thomey et al. 2011). The discrepancy between ANPP and mean plant size implies more complex demographic shifts in these populations.

How do demographic distributions differ between treatments?

The central dip in each height distribution may be explained by a difference in height between plants producing seed and plants not yet reproducing (Figure 4). The tallest part of a B. eriopoda individual is often a seed culm, so plants will grow significantly taller in the first year of seed production. Few or no plants were taller than

55-60 cm in any treatment, implying that individuals in watered treatments did not grow outside of their typical boundaries. Increased proportions of taller plants in watered plots indicate higher overall survival rates and demonstrate the necessity of additional water for maintaining a higher proportion of large individuals.

Differences in the modes of distributions indicate variability in plant mortality.

Ambient control plots had much greater numbers of seedlings 0mm-5mm, implying high recruitment, likely due to increased bare soil area and decreased intraspecific competition with larger individuals (Fair et al. 1999). Control plots also had high mortality rates beginning at germination. The mode of both watered plots fell between 10-15cm, indicating low seedling mortality before this size and increasing mortality above it. This pattern likely arises from more reproduction by stolon in watered plots. Large black grama individuals tended to reproduce by stolon much more than smaller individuals, therefore a larger proportion of stolon-sprouted offspring should appear in watered plots, where more large established individuals are present. The stolon will remain as a lifeline for resources until the daughter plant reaches a more stable size, decreasing overall offspring mortality. Above this height, mortality increases and surviving plants begin to produce seed, making them grow much taller in this season and creating the central drop seen in height distributions. The relatively even demographic distribution in small pulse plots is indicative of high survivorship across all plant ages, indicating that more consistent water input decreases mortality.

Increased plasticity in WUE will likely be selected for with less frequent, more intense rainfall. WUE plasticity is important for fitness because it allows for physiological flexibility in a plants growth potential. A fitness advantage will be conferred to individuals capable of decreasing WUE to take advantage of soil moisture after a heavy rain and increasing WUE during long dry periods. Individuals with less plastic WUE are unable to change their instantaneous water use strategy and consequent rates of growth. In a system where drought tolerance has long been the ideal mechanism for survival, drought avoidance strategies will be selected for more than they have in the past (Heschel & Riginos, 2005). Sensitivity of black grama growth to long-term drought has been demonstrated (Herbel et al. 1972), so WUE plasticity will become increasingly important in taking advantage of precipitation pulses.

How is black grama reproductive fitness affected by rainfall variability?

The lack of seed production in ambient control plots can likely be explained by the dry August and September of 2015. It is possible that more seed was produced after an especially wet October, but we conducted seed collection before this time. Reducing seed production for a season is potentially a resource-saving technique that makes sense for a long-lived grass like black grama, as an individual can put off seed production until a wetter year.

One would expect average spikelet mass and average number of spikelets per inflorescence to fit normal distributions, as these traits should vary randomly around an optimal mean within a population. In large pulse plots, positive skewness of average spikelet mass indicates that more individuals are producing smaller spikelets while a few individuals produce larger spikelets. This distribution indicates an ongoing shift toward smaller spikelet sizes in these plots. Spikelet size varies based on seed mass and number of seeds per spikelet. Both of these variables correspond with reproductive effort, so it appears that there is a trend toward decreasing seed production effort within the large pulse treatment. Allocating fewer resources toward seed production makes sense for individuals under a large, infrequent pulse precipitation regime, as survival rates for seedlings are lower than in a system with more consistent rainfall (Figure 4). These individuals are better off focusing more resources on stoloniferous cloning, which allows them to keep offspring attached to the parent plant until it reaches a size where it can be self sufficient. (Kleijn & Steinger 2002)

How are reproductive fitness and demography related?

As one would expect, larger plants are able to access more water, nutrients, and sunlight using these to producing a larger number and mass of seeds. Larger plants can also carry additional seeds as a simple function of having more room to do so (Table 3).

A look at the non-proportional reproductive measures of average spikelet mass and average spikelets per inflorescence reveals unexpected patterns. Positive correlation in the large pulse treatment between these two measures and and plant height can be explained by two main possibilities. One is that taller B. eriopoda individuals are more capable of harnessing the high amounts of water available during heavy precipitation events than smaller indviduals, and can use this additional resource to produce larger seeds. These indivduals perhaps have deeper root systems to compliment their greater height. Another explanation is that some individuals are predisposed to taking advantage of these large watering events and allot resources to both increased growth and seed production. It is possible that the more fit individuals in this explaination have lower

WUE or more plastic WUE, enabling them to take better advantage of this surplus soil moisture (Snyman & Fouche 1991).

Conservation Implications

The impacts of precipitation pulse variability on B. eriopoda demography and fitness would likely be amplified with the removal of additional water input, a significant confounding factor in this experiment. Though ANPP may increase in ungrazed ecosystems (Thomey et al. 2011), the effects of a more extreme precipitation regime with larger, less frequent events are not all beneficial. Average mean plant size did not increase, meaning that populations may not necessarily be stable over the long term.

Selective pressure on plants to take better advantage of heavy rainfall will eventually result in a better-adapted population but means that a portion of the population with maladaptive traits such as shallow root structure or low WUE plasticity will be harmed by precipitation shifts. Higher mortality and decreased reproductive fitness may expose black grama populations to the possibility of shrub encroachment and desertification

(Van Auken, 2000), reducing food availability for native grazers and granivores. These effects could result in a system with lower plant and animal biodiversity. Lower biodiversity can potential destabilize a system, making the system vulnerable to environmental change, resulting in lower overall biomass and changes in the suite of species that inhabit the ecosystem.

Sevilleta is a unique area of semiarid grassland in the US in that it is not used as rangeland. The vast majority of similar ecosystems do see livestock use, where overgrazing and trampling of seedlings are major threats to grass populations (Wright &

Van Dyne, 1976). In conjunction with the harmful effect of livestock, water availability stress is likely to exacerbate the harmful effects of livestock. These rangelands are a valuable resource as a place to raise livestock. Long-term management strategies must anticipate the increasing sensitivity of these ecosystems.

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