# Presentation: 937242

## Carbon Fluxes in North-Central Florida:  

- <date/time>

**Carbon Fluxes in North-Central Florida:  **
**Using a Replicate Stand Approach to Assess Variation Across the Landscape**

** **Gregory Starr1, Timothy A. Martin1, Henry L. Gholz2, and Robert G. Gorman1

***1******University of Florida, School of Forest Resources and Conservation, Gainesville, Florida***
***2******National Science Foundation, Division of Environmental Biology, Arlington, Virginia***

**Methods:**
***Study sites:***
- The UF AmeriFlux project maintains three permanent flux towers in pine forests near Gainesville, Alachua County, FL:
- A regenerating slash pine (*Pinus elliottii *var. *elliottii*) plantation (planted winter 1998-1999)
- A mid-rotation *P. elliottii* plantation (planted winter 1989-1990)
- A naturally-regenerated *P. elliottii* / *P. palustris* stand managed to maintain pre-settlement structure and function, including periodic prescribed fires
- “Replicate” stands planted within one year of the permanent plantation sites were identified and targeted for campaign measurements with the portable eddy covariance system (Figure 1).**	****                **
***Eddy covariance***
-  Carbon dioxide, latent energy, and sensible heat fluxes were measured using a closed path eddy covariance systems in all stands (Clark et al., 1999, 2004,Gholz and Clark 2002, Powell *et.* *al* 2005).
- Data meeting the following criteria were filtered from the dataset:
- U* _<_0.2
- incomplete half hours of data
- Out of range data
- Data which fall out of normality range for sonic wind vectors
- Rain events within half hour data sets
***Meteorological data***
- sensors:
- Incident solar radiation (LI-200 pyranometer, Li-Cor, Inc., Lincoln, Nebraska)
- photosynthetically active radiation (quantum sensor LI-190, Li-Cor, Inc.)
- net radiation (#Q7, Radiation and Energy Balance Systems, Inc., Seattle, Washington)
- wind speed and direction (R.M. Young 3001-5, Campbell Scientific, Logan Utah.)
- temperature and relative humidity** **(ES-120, Omnidata International, Logan, Utah)
- precipitation (tipping bucket, Sierra Misco, Inc., Berkely, California)
- Soil energy storage was measured with three soil heat flux plates** **(HFT-3.1, Radiation and Energy Balance Systems, Inc.)** **buried 10 cm deep near the tower.
- Water table depth was measured continuously with a Steven’s gauge.
***Ecological measurements***
- Standard suite of ecological and inventory measurements at the permanent sites (see Clark et al. 1999 and 2004 for details).
- At roving sites, dormant season inventories will be used to derive biometric sequestration estimates; in addition, leaf litterfall and light interception will be measured in each replicate stand

**Introduction:**
**	**The appropriate scale for measurement of an “ecosystem carbon balance” is a fundamental issue, not only because boundaries of ecosystems are set arbitrarily but also because conclusions regarding carbon balance are dependent on the temporal and spatial scales of both controlling factors and of observation. The most common method now used for measurement of carbon fluxes, eddy covariance from a tower above the study ecosystem, has a variable and highly uncertain plot size, or footprint. In addition, high expense limits the numbers of flux towers and encourages their placement in the middle of reserves and other attractive study areas that may not be representative of the surrounding landscape. As a result, the robustness of large-scale carbon balance estimates derived from smaller-scale eddy covariance measurements is sometimes questionable.
- 	Since June, 1997, the University of Florida’s Ameriflux program has been using eddy covariance and supporting ecological measurements to quantify the carbon dynamics of intensively managed pine plantations in North Central Florida.  We utilize a chronosequence approach to understand changes in carbon sequestration across the slash pine management cycle, or rotation. This has involved deploying single, permanent towers in a rotation-aged plantation (~ 24 yrs old), a regenerating clearcut (ages 0-5 yrs), and a mid-rotation plantation (ages 10-15 yrs).  We have utilized information from these chronosequence measurements to estimate carbon balance over large spatial (900 km2) and temporal (25 yrs) scales in the region (Binford et al. *in press*).  Our estimates revealed interesting and interpretable temporal variations in regional C sequestration, but we were unable to estimate the uncertainty in our estimates, due to the lack of replication at the stand level.
- 	This poster describes a new initiative intended to improve our understanding of stand-to-stand variation in NEE in north Florida.  This approach involves deploying a  portable eddy covariance system for short (6-8 weeks) campaigns in “replicate” regenerating and mid-rotation pine plantations in north-central Florida. Replicate stands have been identified in consultation with land managers from the Rayonier and Plum Creek forest products companies.  The replicate stands are chosen to match the planting year of our permanent sites (+/- 1 year) and represent a range of variation in gross soil characteristics (e.g. presence/absence of spodic and argillic horizons, drainage class) and management regimes (e.g. degree of fertilization, herbicidal competition control, planting densities or tillage). This approach will enable us to compare fluxes between each "replicate" ecosystem and the same-aged ecosystem associated with our permanent sites over a range of environmental conditions.

**Progress and future work**
* *Initial 6-8 week campaigns have been completed for three replicate sites – two mid-rotation stands and one regenerating stand.* *
- An additional three stands have been selected for measurement.
- A total of 8 additional sites will be selected by projects end.
- Each replicate site will be the subject of two campaigns in contrasting seasons.
- Stand structure, management history, and soil information will be used to characterize and explain variation in NEE across the landscape.

**Results and Conclusions:**
- Selected replicate stands varied in structural characteristics, including height, diameter at breast height, stand density, and stem volume (Figure 2).
- The first replicate regenerating stand (Regen-Rov1, located approximately 30 miles from the permanent site) had lower stocking and stem volume than the permanent regenerating plantation site (Figure 2), and had dramatically different rates of NEE (Figure 3A and 3B).  The light saturated NEE parameter for the permanent regenerating stand was more than twice that of the replicate stand (Table 1), which resulted in substantial differences in C sequestration over a relatively short time period (Figure 3B).
- In contrast, the first replicate mid-rotation plantation (Mid rotation Rov1), which was located within a few km of the permanent mid-rotation plantation, was almost indistinguishable from the permanent stand in light response (Figure 3C, Table 1) and cumulative C sequestration (Figure 3D)

**Figure 2**. Stand structural characteristics for two regenerating and three mid-rotation slash pine plantations.

**Figure 1**. Map of north-central Florida showing the locations of the three permanent Gainesville, Florida AmeriFlux towers, as well as the locations of replicate stands in “regenerating” and “mid-rotation” age classes.

**References**
- Binford, M., H. Gholz, G. Starr, and T. Martin. *In press*. Regional carbon dynamics of the Southeastern Coastal Plain: balancing land-cover type, timber harvesting, environmental variation, and fire. Journal of Geophysical Research – Atmospheres.
- Clark, K.L., H.L. Gholz, J.B. Moncrieff, F. Cropley, and H.W. Loescher. 1999. Environmental controls over net exchanges of carbon dioxide from contrasting Florida ecosystems.* *Ecological Applications 9:936-948.
- Clark, K.L., H.L. Gholz, and M.S. Castro. 2004. Carbon dynamics along a chronosequence of slash pine plantations in north Florida.* *Ecological Applications 14:1154-1171.
- Gholz, H.L. and K.L. Clark. 2002. Energy exchange across a chronosequence of slash pine forests in Florida.* *Agricultural and Forest Meteorology 112:87-102.
- Powell, T.L., G. Starr, K.L. Clark, T.A. Martin, and H.L. Gholz. 2005. Ecosystem and understory water and energy exchange for a mature, naturally regenerated pine flatwoods forest in north Florida.* *Canadian Journal of Forest Research 35:1568-1580.