Fertility Islands

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Predictive modeling of spatial patterns of soil nutrients associated with fertility islands in the Mojave and Sonoran deserts. . Erika L. Mudrak, Jennifer L. Schafer, Andres Fuentes Ramirez, Claus Holzapfel, and Kirk A. Moloney. Fertility Islands. Shrub canopies provide windbreak - PowerPoint PPT Presentation

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Predictive modeling of spatial patterns of soil nutrients associated with fertility islands in the Mojave and Sonoran deserts. Erika L. Mudrak, Jennifer L. Schafer, Andres Fuentes Ramirez, Claus Holzapfel, and Kirk A. Moloney

The project Im going to be speaking about today is part of a larger collaborative effort between our group at Iowa State University and our colleagues at Rutgers University. Its a project funded by SERDP, the environmental research program of the department of defense in partnership with the Environmental Protection Agency and the Department of Energy. Were working on military bases in the southwestern US, where the desert shrubland vegetation is characterized by landscapes of regularly spaced shrubs. 1

Fertility IslandsShrub canopies provide windbreakprovide shade funnel and retain moisturenative annuals growincreased accumulation of organic matterIncreased soil nutrients under the shrubCreates resource heterogeneityStructurally defines the landscape

Larrea tridentatacreosote bushThese shrub create islands of fertility for annual desert plants by providing shade and a wind block, and therefore harboring a place of accumulating organic material and increased nutrients.

Heres a great example of the nutrient island concept. This is a picture of our study species, creosote bush, or Larrea tridentata. You see here that there is a cluster of annuals underneath the shrub. You might also notice there does also seem to be a fair amount of vegetation in the surrounding matrix, which motivates our research program. These plants are invasive species, (here Schismus and Errodium), that are able to colonize the matrix in between the shrubs, threatening to break down the natural fertility island pattern.As the invasive plants spread, it increases both the biomass and the connectivity of the nutrient island matrix, which threatens to change natural fire regimes, potentially impacting the composition and structure of native flora. 2

Project Goals:

Ultimate: Develop landscape-scale, spatially-explicit agent-based models - patterns of invasion by non-native annualseffect of fire cycle and climate change on these dynamicstest possible management plans Current: Characterization of landscape: perennial plant communitysoil nutrient availabilitywater availability

annual plant community

soil nutrient availabilityOur ultimate goals for the broader project are.

So we must provide a landscape on which to run our agent based model, Were in the first year of this project so we are currently working on a detailed characterization of the landscape [read]

Measuring fertility islands

Jackson and Caldwell 1993 Journal of Ecology

Thompson et al. 2005 Journal of Arid Environments

Li et al 2011 Ecological ResearchSchlesinger et al. 1996 Ecology

Lag distance (cm)Semi-variance ()There have been many different approaches to characterizing these fertility islands. The majority of studies I found do a simple comparison of soil properties under the shrub to that in the interspace.

In a landmark paper, Schlesinger et al used geostatistics in the form of semi-variograms to show that the distribution of soil nitrogen was autocorrelated at a scale close to the average size of shrubs for the overall study area.

However, both of these types of approaches are only descriptive in nature. Other studies have used intense sampling and interpolation techniques to make predictions about the distribution of soil nutrients in fertility islands, but the cost of sampling at this intense scale is often prohibitive. 4

Goal: Develop a model of soil nutrient concentration as a function of

distance from nearby shrubsdirection (N or S)the size of those nearby shrubslandscape heterogeneityunderlying autocorrelation structure. Soil nutrient distributionOur goal for this project was to efficiently sample the nutrient island characteristics in an area dominated by shrubs in order to

In our research site, we have complete information on the location and size of all Larrea from on-the-ground hand measurement. So were interested in moving from a shrub map like this (right picture) to a raster-like soil nutrient map like this (see insert), where the nutrients form hotspots under shrubs. 5

SonoranBarry M Goldwater AFAF

MojaveFt. Irwin NTC

Were working at two military bases in AZ and CA. In CA we have Ft Irwin, near Barstow, CA, and AZ, we have the Barry Goldwater Airforce Auxiliary Range, SW of Phoenix. Each desert is characterized by creosote bush, though in California other Ambrosia are almost co-dominant.

Here is shown the information that we have collected on the location and size of all Larrea shrubs in these study areas. Each of these research plots is 83m by 130m, but oriented differently due to roads in the area which provided us access to these sites. PRS (Plant Root Simulator)-probes

NH4+ + NO3-NH2PO4-P K+KCa2+CaMg2+ MgPlant available forms of macronutrients

Buried during growing season: Late January Late March2011To sample soil nutrient availability, we used Plant Root Simulator TM probes, which are ion-exchange resin membranes that mimicking the uptake of ions by plant.

Each sample contains two probes, one for anions and one for cations.

We chose to focus on the plant available forms of macronutrients.

7

distance from nearby shrubsthe size of those nearby shrubslandscape heterogeneitydirection (N or S)underlying autocorrelation structure PRS (Plant Root Simulator)-probes

NH4+ + NO3-NH2PO4-P K+KCa2+CaMg2+ MgPlant available forms of macronutrients?Im going to walk you through our sample design that places these PRS probes throughout our study area in a way that addresses each of our model interests as discussed before, shown here.

8Nutrient Leveldistance from nearby shrubsthe size of those nearby shrubslandscape position (trend)direction (N or S)underlying autocorrelation structure0 20 40 60 80 100 120 140 160 180 200 220 240x x x x x x x x x x x x x

Distance from shrub (cm)x= sample location

???Our first concern is the assumed decline in nutrients as you get farther away from the shrub. We decided to place samples in transects starting at the base of the shrub stem extending into the interspace.

We placed a pair of PRS probe ion exchange membranes spaced 20 cm apart along these transect.

Now the question of which shrubs to place these transects. 9

distance from nearby shrubsthe size of those nearby shrubssmall, medium, largelandscape heterogeneitydirection (N or S)underlying autocorrelation structure

We were also interested in the effect of shrub size on the distribution of these nutrient islands, so we stratified our focal shrubs among three size classes representing the middle 75% of the data. 10

distance from nearby shrubsthe size of those nearby shrubssmall, medium, largelandscape heterogeneity3 regionsdirection (N or S)underlying autocorrelation structure.

25 x 25mTo get at the overall landscape position or trend, we our restrited our choice of focal shrubs to these three regions, which were randomly chosen so that no region overlapped vertically or horizontally. This allowed us both to spread sampling throughout the entire study area, but also to look at autocorrerlation at medium distance lags. 11

distance from nearby shrubsthe size of those nearby shrubssmall, medium, largelandscape heterogeneity3 regionsdirection (N or S)underlying autocorrelation structure.

25 x 25mTo get at the overall landscape position or trend, we our restrited our choice of focal shrubs to these three regions, which were randomly chosen so that no region overlapped vertically or horizontally. This allowed us both to spread sampling throughout the entire study area, but also to look at autocorrerlation at medium distance lags. 12

distance from nearby shrubsthe size of those nearby shrubssmall, medium, largelandscape heterogeneity3 regionsdirection (N or S)underlying autocorrelation structure.

25 x 25mSeveral studies have shown that the effect of a shrub on a nutrient island depends on direction, which makes sense since in temperate zones in the northern hemisphere, shade is often cast more to the north. We added a direction treatment to our shrub sampling design, and placed transects to either the north or south of a shrub. 1318 shrubs 3 sizes 3 regions 2 directions

distance from nearby shrubsthe size of those nearby shrubssmall, medium, largelandscape heterogeneity3 regionsdirection (N or S)north, southunderlying autocorrelation structure

So given these three factors: size in small, medium, and large, the three regions, and two directions, we placed transects at the 18 shrubs shown here, with a balanced design between these factors. 14NPKCaMg

Mojave

mg/m2/63 daysmg/m2/63 daysmg/m2/63 daysmg/m2/63 daysmg/m2/63 days

Sonoran

mg/m2/46 daysmg/m2/46 daysmg/m2/46 daysmg/m2/46 daysmg/m2/46 daysHere I show the raw results, the nutrient concentrations plotted against distance from the shrub. Each line connects samples that were on the same transect. So you can kind of image the base of the shrub at the 0 mark, and the nutrient levels generally decline as you get farther away from the shrub.

Given the plots of these data, we felt that the decline of nutrients could be modeled in two ways- either as a negative exponential function, or a linear function.

We also noted that many of the nutrients didnt have a strong relationship to distance from shrub, so we included a model that didnt involve shrubs.

15Regional TrendNo shrub influence