Lab Exercise #2Gretchen Carlson

GEOG 711Chapel Hill

Integrated Carbon and Water Flux Simulation

9/10

Precipitation

Statement of the problem

This simulation explored how stand growth changed during dry and wet years in Chapel Hill. The dry year selected was 1988 and came after a series of other dry years, and the wet year was 1996 which followed another wet year.

1996 followed a wet year 1988 followed a dry year

Initialization File: 1980 - 2004 Output Parameters

Spin Up 6000 years Gross Primary Productivity

Normal Simulation 50 Years Net Primary Productivity

Soil Composition SOIL_SAND 30 Net Ecosystem Exchange

  SOIL_SILT 50 Evapotranspiration

  SOIL_CLAY 20 Soil water outflow

Soil Depth 1 meter   Precipitation

Leaf Area Index

Leaf Carbon

Vegetation Carbon

Soil Carbon

Soil Water

Soil Tension

Volumetric Soil Water

Snow Water

Fine Root Carbon

Live Stem Carbon

Sead Stem Carbon

Soil Water Transpiration

Soil Water Evaporation

Canopy Water Evaporation

Experimental Set up

Simulation1.Spin-up run for Chapel Hill2.Normal Simulation run for Chapel Hill for 50 years using 17 output parameters3.Dry year 1988 and wet year 1996 selected from data4.Wet and dry years analyzed for carbon and water flux

Results Soil carbon during 1988 (dry year) showed an overall increase, while soil carbon during 1996 (wet year) showed an overall decrease. Leaf carbon showed an opposite trend. 1996 showed an increase in leaf carbon, and 1988 showed a decrease in leaf carbon.

Changes in leaf carbon corresponded to changes in LAI for the year with 1996 showing more leaf carbon and more growth of LAI, and 1988 showing decreases in both LAI and leaf carbon.

Fine Root carbon varied throughout the year depending on soil water. During 1988 (dry year), soil water decreased from a high at the beginning of the year. Fine root carbon increased as soil water decreased until mid-summer when fine root carbon decreased. During 1996 (wet year), fine root carbon increased as soil water decreased during spring months. In mid summer around the same time of a decrease in 1988, fine root carbon began to decreased in 1996, but not nearly as much, and ultimately increased . During the dry year there was a net decrease in fine root carbon and during the wet year there was a net increase. These trends were similar for leaf carbon (not shown).

During the winter and spring of both years, soil carbon and soil water both followed similar tracks with a decrease in both soil water and soil carbon during the spring months.

In 1988 soil carbon increased during the summer months while soil water remained that same.

In 1996 soil carbon decreased after a heavy precipitation event saturated soils during the fall months.

1996 Soil Carbon vs. Soil Water

17.17

17.172

17.174

17.176

17.178

17.18

17.182

17.184

17.186

17.188

17.19

17.192

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SOILC (kg C m-2)

SOILW (kg W m-2)

1988 Soil Carbon vs. Soil Water

17.17

17.175

17.18

17.185

17.19

17.195

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100

150

200

250

300

SOILC (kg C m-2)

SOILW (kg W m-2)

GPP increased during both years in the spring. During mid-summer, negative soil psi correlated with decreased GPP. Less water led to less gross primary productivity. During the late fall and winter, GPP decreased during both years.

Conclusion and Discussion

More growth was observed during the wet year (1996) as reflected by increased LAI, increased carbon both in the leaves and fine roots, and higher GPP throughout the year, while conversely less growth was seen during the dry year (1988) using the same indicators. In both situations water appeared to be a limiting factor during summer months as shown by the relationship between soil psi and GPP.

Responses to changes in precipitation had a much greater impact during spring and summer then during fall and winter months when less growth occurred. Although 1996 was a wet year and followed a similarly wet year, much of the net precipitation between the years was from a single precipitation event in early fall. This event saturated soils during the fall months, when GPP was already decreasing for the year. How the timing of extreme events impacts growth for the year is worth further study. An extreme event in the spring which saturates the soil for much of the summer will likely have a far greater impact then one in the fall.

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0.001

0.002

0.003

0.004

0.005

0.006

0.007

1 21 41 61 81 101 121 141 161 181 201 221 241 261 281 301 321 341 361

HR

1996

1988

Heterotrophic respiration and Soil Carbon

Soil carbon during 1996 began decreasing in fall months around day 241 when Hurricane Fran hit NC. This coincides with a sharp increase in heterotrophic respiration.

In 1988 Soil carbon levels out and begins to decrease around day 200 along with heterotrophic respiration.

A little more exploration