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How surface rock cover affects water and nutrient availability of Sonoran Desert annual plants -- Honors Thesis

Publication date: 2015-08-10

Author(s):

  • Julea Shaw

Abstract:

Water and nutrient availability are the primary and secondary drivers of net primary productivity (NPP) in arid ecosystems. Although precipitation regulates water inputs, soil properties influence water availability for plant growth. Aridland soils are often covered with surface rocks, which can increase or decrease water availability by modifying evaporation, infiltration, light levels, and temperature. Due to the complexity of these direct and indirect mechanisms, the relationship between rock cover and NPP is not well understood. In this research we explore the relationship between rock cover, soil nutrient availability, and aboveground growth of desert annual plants over four years across a long-term nutrient enrichment experiment in the Sonoran Desert. We surveyed surface rock cover at fifteen sites in central Arizona that have been fertilized with nitrogen (N) and phosphorus (P), alone and in combination, for seven years. Using ANCOVA, we then explored the relative importance of rock cover, precipitation, and nutrient treatment on peak aboveground biomass of spring herbaceous annual plants that were collected in 2008, 2009, 2010, and 2013. We expected surface rocks to strengthen the positive relationship between precipitation, nutrient additions, and annual plant growth. Precipitation, nutrient additions, and surface rock cover together significantly influence growth of Sonoran Desert annual plants. As expected, nutrients and precipitation were the strongest drivers of annual plant biomass. Plant growth was positively related to N additions across all four years (ANCOVA, p <0.01); P in 2008 and 2010 years (p = 0.01 and 0.005, respectively);and precipitation in three of the four years (p < 0.05). Precipitation was the primary driver of plant biomass in the two driest years, 2009 and 2013 (partial eta2 = 0.51 and 0.52, respectively). Gravel (2-64 mm diameter) was only rock size class that was significantly related to annual plant biomass. Contrary to our expectations, gravel cover was negatively related to biomass in two of four years (2008 and 2009) with a relatively low effect size compared to rainfall and N (partial eta2=0.136 and 0.1, respectively), but comparable in effect size to P additions. The mechanism underlying the negative relationship between gravel cover and annual plant productivity is unclear. Although rocks can reduce evaporation, which would increase water availability, they may also reduce light availability and water infiltration, or increase soil temperature.


Keywords:


Temporal Coverage:

2007-10-01 to 2013-03-31

Geographic Coverage:

Geographic Description: Greater Phoenix metro area and surrounding desert
Bounding Coordinates:
Longitude:-112.534948992885 to -111.485173156806
Latitude:33.7257647695281 to 33.0132747503297

Contact:

Data Manager, Julie Ann Wrigley Global Institute of Sustainability, Arizona State University, 
PO Box 875402,TEMPE
 caplter.data@asu.edu

Methods used in producing this dataset: Show


Data Files (1) :

Tabular: data table

Description: Average rock cover, biomass, and precipitation for CNdep sites

Column Description Type Units
origOrder Sampling Order 1-236
year year of collection
nominalYear
siteName Sampling site ID
Enumeration:
  • LDP: Lost Dutchman State Park
  • UMP: Usery Mountain Park
  • SRR: Salt River
  • MCN: McDowell Mountain North
  • MCS: McDowell Mountain South
  • SME: South Mountain East
  • SMW: South Mountain West
  • PWP: Piestewa Peak
  • MVP: Mountain View Park
  • DBG: Desert Botanical Gardens
  • SNE: Sonoran Desert Nat. Mon. East
  • SNW: Sonoran Desert Nat. Mon. West
  • EMW: Estrella Mountain West
  • EME: Estrella Mountain East
  • WTM: White Tank Mountains
tmt Experimental fertilization treatment
Enumeration:
  • C: control
  • N: nitrogen added
  • P: phosphorous added
  • N+P: nitrogen + phosphorous
tmtCode Treatment code ID
Enumeration:
  • 1: Control
  • 2: N
  • 3: P
  • 4: N+P
region Site location
regionCode Region code ID
Enumeration:
  • 1: West
  • 2: Core
  • 3: East
ipBiomass average aboveground spring annual plant biomass from interplant subplots
gramsPerSquareMeter
pBiomass average aboveground spring annual plant biomass from under plant subplots
gramsPerSquareMeter
aveBiomass average aboveground spring annual plant biomass of IP and P subpots combined
gramsPerSquareMeter
rainfall Total rainfall from October 1 to March 31 from closest Maricopa County meteorological stations
millimeter
gravel Average % cover of embedded + surface gravel (rocks < 7.5 cm in diameter) on soil surface
dimensionless
embeddedGravel Average % cover of embedded gravel on soil surface
dimensionless
surfaceGravel Average % cover of surface gravel on soil surface
dimensionless
cobble Average % cover of embedded + surface cobble (rocks 7.5-25 cm in diameter) on soil surface
dimensionless
embeddedCobble. Average % cover of embedded cobble on soil surface
dimensionless
surfaceCobble Average % cover of surface cobble on soil surface
dimensionless
stone Average % cover of embedded + surface stone (rocks 25-60 cm in diameter) on soil surface
dimensionless
embeddedStone Average % cover of embedded stone on soil surface
dimensionless
surfaceStone Average % cover of surface stone on soil surface
dimensionless
boulder Average % cover of embedded + surface stone (rocks >60 cm in diameter) on soil surface
dimensionless
embeddedBoulder Average % cover of embedded boulder on soil surface
dimensionless
surfaceBoulder Average % cover of surface boulder on soil surface
dimensionless
plantInPlot Average estimated plant cover from visual percent estimates
dimensionless
bare Average bare soil from visual percent estimates
dimensionless
totalRockCover Gravel + Cobble + Stone + Boulder
dimensionless

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