1. Management Effects on Soil Respiration in
North Carolina Coastal Plain Loblolly Pine Plantations
B13C
Michael Gavazzi1, Steve G. McNulty1, Emrys Treasure1 , Asko Noormets2,
1USDA Forest Service, Eastern Forest Environmental Assessment Center, Raleigh NC
2North Carolina State University, Raleigh, NC
Acknowledgements
This research was funded in part by NASA Grant NRA-04-OES-01 and the USDA Forest Service Northern Global Change Program
For more information contact Michael Gavazzi
(919) 515 – 2916,
Introduction
North Carolina’s coastal plain in characterized by inland areas with deep organic soils that store up to 80 kg carbon per square meter. Much of this area has been converted from bottomland hardwoods to loblolly pine (pinus taeda) plantations through intensive management, including ditching and bedding. While these practices encourage rapid pine growth and shorter rotations, their impact on soil respiration rates has not been fully quantified. Soil respiration has a strong influences on net ecosystem productivity (NEP), contributing approximately 70% of total ecosystem respiration in temperate forests. Management activities associated with clearcut harvest and stand thinning alter the vegetative and soil structure which can affect forest floor inputs, soil temperature, moisture, microbial activity, and diffusion rates, and can impede root growth. Studies have found no change, an increase and a decrease in soil efflux rates following disturbance. Evidence suggests that response is strongly related to management intensity. More research is needed to examine how forest management activities affect soil respiration rates and NEP.
Figures 1 a, b, c. Modeled soil efflux rates (Rs) from linear regression stepwise analysis (coefficients in Table 1). Graphs shown side by side for perspective only, not for direct comparisons between sites.
Tables 2 a, b, c. Total soil efflux (g C/m2/yr) by site, collar location and treatment (fertilization and thinning) if applicable for select years.
Methods
In 2005 we began measuring soil respiration on two sites near Plymouth, NC as part of a study to quantify ecosystem flux rates in managed coastal plain pine plantations. A 20-year old site (older pine site) had been in active pine management for over 50 years, with the current rotation thinned in the fall of 2009 and half the site fertilized in January Thinning involved the removal of every forth row and selective tree removal from the remaining rows. The other site (young pine site) had recently been converted from a bottomland hardwood forest to a pine plantation. A third site (recent clearcut) was added in the summer of 2009 soon after a clearcut harvest of mature loblolly pine.
Four plots were established on each site and six 20-cm pvc collars were installed in each plot. Collars were arranged approximately 60 degrees apart around a 7-meter radius plot. After thinning on the older pine site, the plot size increased to 15-meter radius and two addition plots were added to better represent stand conditions. Collar position was noted as either falling on the bed or inner bed area within the clearcut sites, and either in the thinned area or gaps created when whole rows were removed from the older pine site. An effort was made when possible to evenly distribute the number of collars between beds inner beds.
Soil CO2 efflux rates were measured using the LI-8100 Soil CO2 Flux System with the single chamber survey system (LI-COR Biosciences, Lincoln, NB). Measurements were taken bi-monthly during the winter and bi-weekly to monthly during the growing season as time and weather conditions permitted. Soil temperature (5 cm and 10 cm) and moisture (VWC to 20 cm) were measured next to each collar following the respiration measurement cycle. Soil temperature was measured every half-hour at four points in each plot with Hobo temperature probes (Onset, Cape Cod, MA) .
The influence of soil temperature and moisture on soil efflux rates was tested using multiple linear stepwise regression analysis (JMP software, SAS Institute). Effects were considered significant at p < 0.05 (Table 1). Significant regression estimates were used to model daily soil respiration rates from half-hour temperature measured in each plot and volumetric water content measured continuously at on-site meteorological stations (Figure 1 a, b, c). Treatment effect differences between years within a site were tested by comparing means with the Tukeys-Kramer method (Table 2 a, b, c)
Year
Total
Bed
Inner Bed
2005*
1561 A
2006
1111 B
831 a
1391 b
2007
965 C D
873 a
1057 b
2009
940 D
936 a
944 a
2011
1030 C
1137 a
924 b
Year
Total
Gap*
Thinned*
Fertilized
Unfertilized
Pre-thin Mean
738 A
2010
1104 B
731 a
1102 b
2011
1129 B
709 a
1018 b
981 a
1276 b
2012**
900
633 a
964 b
844 a
958 b
Year
Total
Bed
Inner Bed
2009*
916
689 a
1145 b
2010
1165 B
968 a
1364 b
2011
1298 C
1125 a
1463 b
* Measurements from June through December only.
Upper case letters not connected by the same letter are significantly different.
Lower case letters not connected by the same letter within years are significantly different.
* Gap and thinned means don’t average to total soil efflux due to the removal of measurements not clearly within either area.
** Measurements from January through October only.
Pre-thin means ranged from 613 to 824 g C/m2/yr.
Upper case letters not connected by the same letter are significantly different.
Lower case letters not connected by the same letter within years and treatments are significantly different.
* Too few collars on beds to test for significance.
Upper case letters not connected by the same letter are significantly different.
Lower case letters not connected by the same letter within years are significantly different.
Table Stepwise linear regression analysis between soil efflux rate and the following independent variables: soil temperature (Ts) at 5 and 10 cm and volumetric water content (VWC).
Site
Year T5
T10
VWC
Interaction R2
Recent Clearcut
2009
(0.041)
 0.35 (0.011) NS
NS 
0.97
2010
-0.30 (0.049)
(0.001)
-0.50 (0.002)
0.90
2011
(0.018)
0.24 (<0.001)
(0.006)
0.94
Young Pine Site
2005
0.40 (0.003)
(0.004)
 NS
0.79
2006
0.24 (0.003)
(<0.001)
-0.13 (0.020)
(0.001) 
0.93
2007
0.86
2008
0.17 (<0.001)
0.84
0.14 (0.003)
(0.0017)
-0.18 (0.008)
 (0.007)
0.87
0.15 (0.001)
(0.003)
(0.005)
0.89
(0.002)
0.22 (<0.001)
0.04 (0.019)
0.95
2012
0.19 (<0.001)
<0.001
0.002
Older Pine Site
0.16 (<0.001)
(<0.001) 
0.81
0.13 (<0.001)
(0.016)
0.78
(0.029)
0.92
0.18 (<0.001)
(0.015)
(0.013) 
0.14 (0.009)
-0.09(0.011)
 0.15 (0.002)
(0.002) 
-0.06 (0.006)
 (0.005)
0.31 (<0.001)
(0.010)
Figures 2 a, b. Modeled soil efflux rates (Rs) for bed and inner bed areas in the recently clearcut site.
Figures 4 a, b. Modeled soil efflux rates (Rs) for gaps and fertilized areas in the older pine site for selected years.
Figures 3 a, b. Modeled soil efflux rates (Rs) for bed and inner bed areas in younger pine site for selected years.
Results
There were strong linear relationships between soil temperature (Ts), volumetric water content (VWC) and soil efflux rates (Rs) (Table 1). Interactions between Ts and VWC were more common in the younger pine sites, and only occurred in the older pine site the second year after thinning and 2008 were two of the driest years on record at these sites, which may account for the lack of detected interaction between Ts and VWC during those years in the young pine site.
The highest efflux rates were recorded following each of the clearcut harvests, and Rs was similar between both sites two years after harvest. While Rs decreased in the third year after harvest on the young pine site, they continued to increase the third year after harvest on the recent clearcut.. The lower rates reported on the young pine site may be due to the extreme drought that occurred in Inner bed Rs was significantly higher than bed rates on the recent clearcut in each of the years measured. These inner beds were composed primarily of soil and decomposing logging slash mixed together during site prep, while the beds contained only soil and young pine seedlings with poorly developed root systems. The large difference in 2009 between bed and inner bed on the recent clearcut may be at attributed to measured flux rates taken prior to bedding, when the soil was compacted and VWC was high. Rs on the young pine site inner beds was significantly higher than the beds for the first four years after harvest. Rs was not significantly different the fifth and sixth years after harvest, but in the seventh year Rs was higher in the beds than innerbeds. This may be due to the pine root systems in the beds starting to contribute more to site Rs than the weeds and understory trees growing in the innerbeds.
The lowest Rs was recorded on the older pine site prior to thinning. Post-thinning Rs was significantly higher after thinning and this can likely be attributed to the higher soil temperatures recorded after thinning (data not shown). Significantly higher Rs was recorded in the thinned areas than the gaps. While decomposing root systems in the gaps should result in higher Rs, the compacted soil and higher VWC recorded in the gaps likely suppressed Rs compared to the thinned areas. Rs was higher on the unfertilized side of the older pine stand than the fertilized sided. These results have also been reported in other studies, and may be due to the fertilized pines allocating more carbon to aboveground than belowground biomass.
Conclusions
Soil efflux rates (Rs) were highly correlated with soil temperature and volumetric water content, but interaction between the two effects was more common in the younger sites than the older site.
Modeled Rs was highest following site disturbance on all sites. On the recent clearcut site, Rs was significantly greater each year after harvest in both the beds and inner beds, with higher rates recorded in the inner beds(mounds of soil and decomposing logging slash).
Rs in the young pines stand was highest the first two years after harvest, but not significantly different after that. After seven years, Rs on the beds in the young pine stand was higher than the inner beds.
Rs in the older pine stand was significantly greater after thinning compared to pre-thinned rates. Rs was has higher in the thinned areas compared to the gaps, and fertilization appears to suppress Rs.
Coefficients are shown only for effects used in the model.
Individual p values for all significant effects are shown in parentheses.
NS = not significant at p < 0.05.