Progress 04/19/01 to 03/31/07
Outputs
The objective of this project is to examine the impacts on soil respiration and nutrient dynamics of two disturbance agents in interior Alaskan forests: fire, currently the dominant stand-initiating disturbance, and logging, currently of minor import by any measure (but potentially growing). The project began in the summer of 2001. In previous reports, we summarized several lines of evidence suggesting that: 1.post-fire heterotrophic respiration declines relative to pre-fire conditions; 2.changes in forest floor chemistry are not responsible for the declines in heterotrophic respiration; 3.loss of root turnover (production and senescence of fine roots) appears to be the major cause of the decline in heterotrophic respiration. A persistent decline in heterotrophic soil respiration lessens total net post-fire carbon losses and hastens the return of positive carbon balance (i.e., the time at which photosynthesis has increased sufficiently to offset respiration losses). In
2006, the final year of the project, our focus shifted to include other constraints on soil carbon sequestration and release in boreal forests. Two of my graduate students, indirectly supported by this project, contributed to this effort. Evan Kane's PhD dissertation (completed August 2006) examined soil C stabilization, storage, and mineralization along a productivity gradient of black spruce forests across interior Alaska. His key findings:1.Productive forests have less total soil carbon than less productive forests, but a greater fraction of it is stabilized (not bio-available). With larger relative and absolute contents of labile soil organic matter, less productive forests are more likely to lose soil carbon under a changing climate; 2.The vast majority of soil C loss from black spruce forests is as gaseous CO2; very little C is lost from black spruce forests in leaching soil solution.; 3.Topography exerts a strong influence over not only the amount of CO2 generated by wildfire,
but also the amount of black carbon (charcoal) produced. Although soils on south-facing slopes lost more CO2 than those on north facing slopes, they also generated more black carbon, a biologically stable C form unlikely to be respired later. This is consistent with finding (1) above, and may be a mechanism contributing to productivity/stabilization relationships. Sarah Runck's MS research (anticipated completion in 2007) is showing that surface soil moisture strongly limits decomposition rates as well as forest productivity in upland mid-successional forests, with a net result of accumulating forest floor carbon over time.
Impacts
Attention increasingly is being focused on finding ways to sequester carbon to offset anthropogenic carbon emissions. Boreal forest soils are one potential locus of carbon sequestration, but also, as currently very large reservoirs of carbon, represent a very large risk of potential carbon source to the atmosphere under changing conditions. This program of research has constrained the likely post-fire response of carbon emissions by showing that fire does not cause accelerated decomposition and C emissions in boreal forests, and indeed slows rates of C release from soils relative to unburned forests.
Publications
- Kane, E.S., D.W. Valentine, G.J. Michaelson, J.D. Fox, and C-L. Ping. 2006. Controls over pathways of carbon efflux from soils along climate and black spruce productivity gradients in interior Alaska. Soil Biology & Biochemistry 38:1438-1450.
- Chapin, F.S. III, et al. 2006. Summary and Synthesis: Past and Future Changes in Alaska's Boreal Forest. Chapter 21 in Alaska's Changing Boreal Forest, Chapin, F.S. III, M. Oswood, K. Van Cleve and D. Verbyla, Eds. Oxford University Press, Oxford, UK.
- Valentine, D.W., K. Kielland, F.S. Chapin, III, A.D. McQuire, and K. Van Cleve. 2006. Patterns of Biogeochemistry in Alaskan Boreal Forests. Chapter 15 in Alaska's Changing Boreal Forest, Chapin, F.S. III, M. Oswood, K. Van Cleve and D. Verbyla, Eds. Oxford University Press, Oxford, UK.
- Kane, E.S. 2006. Mechanisms of soil carbon stabilization in black spruce forests of interior Alaska: soil temperature, soil water, and wildfire. PhD dissertation, University of Alaska Fairbanks. 141 pp.
- Kane, E.S., E.S. Kasischke, D.W. Valentine, M.R. Turetsky, A.D. McGuire. 2006. Topographic influences on wildfire consumption of soil organic carbon in black spruce forests of interior Alaska: implications for black carbon accumulation. Global Biogeochemical Cycles, submitted.
- Runck, S.A., D.W. Valentine, and J.A. Yarie. 2006. Sensitivity of soil organic carbon dynamics to long-term throughfall exclusion in interior Alaska. Poster presentation, Soil Science Society of America.
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Progress 01/01/05 to 12/31/05
Outputs
The objective of this project is to examine the impacts on soil respiration and nutrient dynamics of two disturbance agents in interior Alaskan forests: fire, currently the dominant stand-initiating disturbance, and logging, currently of minor import by any measure (but potentially growing). The project began in the summer of 2001. From 2001-2004, we measured soil respiration in burned and unburned black spruce forest plots in the Caribou-Poker Creeks Research Watershed (CPCRW), the site of the 1999 Frostfire experimental wildfire. We had clearly showed that soil respiration declined and remained low following wildfire compared unburned stands, but dense regrowing vegetation had begun to cause increasing artifacts associated with plant photosynthesis and respiration. We began measuring respiration in a contrasting series of lowland black spruce sites burned during the Survey Line fire of 2001, and in 2005 collected our 3rd year of measurements there. Because the
rock-free soils allowed installation of root exclusion collars, we have been able to distinguish heterotrophic and autotrophic respiration in this system. Although the data have been noisy, the relatively high degree of replication has nonetheless enabled us to show a statistically significant (p<0.05) decrease in heterotrophic respiration in burned soils compared to unburned soils. This suggests that fire results in an overall decrease in organic matter decomposition despite generally more favorable physical conditions (moisture and temperature) for microbial activity. Related work by Sarah Masco (Valentine's finishing MS student) showed that respiration rates in laboratory incubations of burned and unburned forest floor samples (Oe and Oa) were comparable for most of the duration of the experiment. The exception was very early in the incubation when unburned soils respired nearly twice as rapidly as burned soils, suggesting a pool of rapidly mineralized carbon was present in
unburned soils but absent in burned soils. As her samples were from the subsurface of the forest floor (i.e., no Oi), the only difference between burned and unburned soils was the presence in the former of recently senesced root litter. These results suggest that the rapid root turnover rates comprise a large fraction of heterotrophic soil respiration, and that wildfire reduces heterotrophic soil respiration by arresting this litter input.
Impacts
The real impacts of this project will accrue primarily to the public via management agencies, such as Alaskas Department of Natural Resources (especially Division of Forestry and Division of Lands). Projects designed to sequester carbon and sell carbon credits are increasing in number. To work in Alaska, any such scheme must be based on a clear understanding of and accounting for the future role of Alaskas boreal forest soils in sequestering or releasing carbon under changing disturbance regimes. This project, in concert with other related projects at UAF, is helping to provide that understanding.
Publications
- CASTELLS, E., J. PENUELAS, and D.W. VALENTINE. 2005. Effects of plant leachates from four boreal understory species on soil N mineralization, and white spruce (Picea glauca) germination and seedling growth. Annals of Botany 95(7):1247-1252.
- KANE, E.S., D.W. VALENTINE, E.A.G. SCHUUR, and K. DUTTA. 2006. Soil carbon stabilization along climate and stand productivity gradients in black spruce forests of interior Alaska. Canadian Journal of Forest Research, In Press.
- KANE, E.S., D.W. VALENTINE, G.J. MICHAELSON, J.D. FOX, and C.-L. PING. 2006. Controls over pathways of carbon efflux from soils along climate and black spruce productivity gradients in interior Alaska. Soil Biology & Biochemistry, In Press.
- VOGEL, J.G., and D.W. VALENTINE. 2006. Small root exclusion collars provide reasonable estimates of root respiration when measured during the growing season of installation. Canadian Journal of Forest Research, In Press.
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Progress 01/01/04 to 12/31/04
Outputs
The objective of this project is to examine the impacts on soil respiration and nutrient dynamics of two disturbance agents in interior Alaskan forests: fire, currently the dominant stand-initiating disturbance, and logging, currently of minor import by any measure (but potentially growing). The project began in the summer of 2001.During summer 2004, we obtained our first full season of respiration measurements using root exclosures in three replicate pairs of transects (one burn, one control) in the 2001 Survey Line fire area along the Tanana River. By excluding root respiration, we could separate the two major components of soil respiration (autotrophic associated with roots, heterotrophic associated with microbial decomposition) in order to assess how fire changes the rates of microbial activity. This provided the clearest evidence to date that post-fire soils experience an overall decrease of microbial respiration. In this case, that decrease was substantial:
about 22% of unburned heterotrophic respiration rates, which averaged 124 mg C m-2 h-1. This suggests that although total C loss following fire is higher than before owing to the lack of photosynthesis, that C loss is mitigated somewhat by slowed decomposition.Wildfires consumed nearly 2.7 million ha of Alaskas boreal forest during summer 2004, charring about one eighth of the forested area of Alaskas eastern Interior. This at once underscored the importance of fire-related research and provided widespread opportunities to establish research sites. We collaborated with other researchers associated with the NSF-funded Bonanza Creek Long Term Ecological Research (LTER) project to sample a network of recently burned (in some cases, still burning) sites. During summer 2005, we plan to obtain respiration measurements at these sites similar to the Frostfire and Survey Line fire sites that have been the focus of this project.
Impacts
The real impacts of this project will accrue primarily to the public via management agencies, such as Alaskas Department of Natural Resources (especially Division of Forestry and Division of Lands). Projects designed to sequester carbon and sell credit for it are increasing in number. To work in Alaska, any such scheme must be based on a clear understanding of and accounting for the future role of Alaskas boreal forest soils in sequestering or releasing carbon under changing disturbance regimes. This project, in concert with other related projects at UAF, will help provide that understanding.
Publications
- CASTELLS, E., J. PENUELAS, and D.W. VALENTINE. 2004. Are phenolic compounds released from Cistus albidus responsible for changes in N cycling at siliceous and calcareous soils? New Phytologist 162:187-195.
- VOGEL, J.G. 2004. Carbon cycling in three mature black spruce (Picea mariana (Mill.) B.S.P.) forests in interior Alaska. Ph.D. Dissertation, University of Alaska Fairbanks. 178 p.
- VOGEL, J.G., D.W. VALENTINE, and R.W. RUESS. 2004. Soil and root respiration in mature Alaskan black spruce forests that vary in soil organic matter decomposition rates. Can. J. For. Res. In press.
- VALENTINE, D.W. and T. QUINTAL. 2004. Fire decreases soil heterotrophic respiration in two contrasting black spruce stands. Poster presentation, International Boreal Forest Research Association
- VALENTINE, D.W., E. KANE, and T. QUINTAL. 2004. Heterotrophic respiration declines following wildfire in black spruce soils of interior Alaska. Oral presentation, Soil Science Society of America.
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Progress 01/01/03 to 12/31/03
Outputs
The objective of this project is to examine the impacts on soil respiration and nutrient dynamics of two disturbance agents in interior Alaskan forests: fire, currently the dominant stand-initiating disturbance, and logging, currently of minor import by any measure (but potentially growing). The project began in the summer of 2001. In summer 2003, we scaled back the frequency of measurements of soil respiration in burned and unburned black spruce forest plots in the Caribou-Poker Creeks Research Watershed (CPCRW), the site of the 1999 Frostfire experimental wildfire, as the pattern of reduced soil respiration had become clear over the previous several years. We also began measuring respiration in a contrasting series of lowland black spruce sites burned during the Survey Line fire of 2001. Based on our previous observation that the absence of root respiration was unlikely to account for the entirety of the decline in soil respiration, we included a series of root
exclusion experiments to quantify the root contribution to soil respiration. This turned out to be the case: excluding root respiration caused unburned soil respiration to decline by half, but unburned root-free respiration was still larger than soil respiration in burned areas. This suggests that fire results in an overall decrease in organic matter decomposition despite generally more favorable physical conditions (moisture and temperature) for microbial activity. Related work by Jason Vogel has shown that roots not only produce a large fraction of overall soil CO2 efflux, but also comprise a large and rapidly turning over fraction of soil organic matter in black spruce forests. Vogel found that total forest floor organic matter declined by 15-20 per cent over a three-year period during which roots were excluded. This was a surprising result because prevailing thought holds that the thick forest floor in black spruce forests is evidence of slow decomposition, but it now appears that
it is more likely the result of high rates of root inputs. Vogels results suggest that the rapid root turnover rates comprise a large fraction of heterotrophic soil respiration. In other words, removal of root inputs not only eliminates root respiration, but also restricts the supply of relatively labile C to soil microbial populations and thereby slows heterotrophic soil respiration as well. This is consistent with the observed declines in overall soil respiration in burned forest sites over and above simple elimination of root respiration.
Impacts
As this project began during summer 2001, its real impacts lie primarily in the future. They will accrue primarily to the public via management agencies, such as Alaskas Department of Natural Resources (especially Division of Forestry and Division of Lands). Discussions about carbon sequestration as a sellable product for land managers have become more commonplace and serious. To work in Alaska, any such scheme must be based on a clear understanding of and accounting for the future role of Alaskas boreal forest soils in sequestering or releasing carbon under changing disturbance regimes. This project, in concert with other related projects at UAF, will help provide that understanding.
Publications
- CASTELLS, E., J. PENUELAS, and D.W. VALENTINE. 2003. Influence of the phenolic compound bearing species Ledum palustre on soil N cycling in a boreal hardwood forest. Plant and Soil 251:155-166
- CASTELLS, E., J. PENUELAS, and D.W. VALENTINE. 2004. Are phenolic compounds released from Cistus albidus responsible for changes in N cycling at siliceous and calcareous soils? In press.
- VOGEL, J.G. and D. VALENTINE. 2003. The carbon cycling of mature Alaskan black spruce forests along a decomposition gradient. Poster presentation, Annual Alaska state meeting of the American Society for Microbiology.
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Progress 01/01/02 to 12/31/02
Outputs
In summer 2002, we continued bi-weekly measurements of soil respiration in burned and unburned black spruce forest plots in the Caribou-Poker Creeks Research Watershed (CPCRW), the site of the 1999 Frostfire experimental wildfire. Observed post-fire rates of soil respiration have shrunk to less than half of unburned respiration rates. Our summer 2002 estimates of root respiration cannot account for all of that difference, suggesting fire suppresses rates of heterotrophic soil respiration. Neither the conditions for decomposition nor organic carbon quality were negatively impacted by the fire, suggesting by difference that changes in overall organic carbon amounts and supply rates likely account for the observed effects.
Impacts
Future marketing of carbon credits produced by the Alaskan boreal forest requires a basic understanding of how soil carbon is sequestered and released. This project is helping provide a better understanding of how changing conditions, especially wildfire, which is significant in Alaska, affects soil composition. Information will benefit the public primarily through management agencies, such as Alaska's Department of Natural Resources and especially the Division of Forestry and the Division of Lands.
Publications
- CASTELLS, E., J. PENUELAS, and D.W. VALENTINE. 2002. Influence of the phenolic compound bearing species Ledum palustre on soil N cycling in a boreal hardwood forest. Plant and Soil in press.
- VALENTINE, D.W. 2002. Persistent decline in soil respiration following forest fire in interior Alaska. Oral presentation, Soil Science Society of America.
- VOGEL, J.G. and D.W. VALENTINE. 2002. Boreal black spruce and soil carbon exchange along an elevation gradient. Poster. American Geophysical Union.
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Progress 01/01/01 to 12/31/01
Outputs
The objective of this project is to examine the impacts on soil respiration and nutrient dynamics of two disturbance agents in interior Alaskan forests: fire, currently the dominant stand-initiating disturbance, and logging, currently of minor import by any measure (but potentially growing). The project began in the summer of 2001. We continued bi-weekly measurements of soil respiration in burned and unburned black spruce forest plots in the Caribou-Poker Creeks Research Watershed (CPCRW), the site of the 1999 Frostfire experimental wildfire. In the months following the fire, we observed slowing respiration rates in from the burned sites compared both to unburned controls and to pre-fire rates. Initial decreases in respiration could have resulted simply from the loss of subsurface root respiration. We expected that warmer soils under the black surfaces and open canopies left by the fire would soon generate conditions favorable to decomposition and hence respiratory
carbon dioxide losses. The post-fire depression in respiration rates has not only persisted, however, the summer 2001 measurements indicate that the difference between carbon dioxide losses in burned and unburned sites has grown. We also established new sites to capitalize on the large fires that burned up to the south bank of the Tanana River in June 2001, near existing research sites within the Bonanza Creek Experimental Forest. These sites are in both lowland black spruce and white spruce stands, and will provide an excellent opportunity to examine fire impacts on C balance in a substantially wetter area than the CPCRW.
Impacts
As this project began during summer 2001, its real impacts lie primarily in the future. They will accrue primarily to the public via management agencies, such as Alaska's Department of Natural Resources, especially Division of Forestry and Division of Lands. Discussions about carbon sequestration as a sellable product for land managers have become more commonplace and serious. To work in Alaska, any such scheme must be based on a clear understanding of and accounting for the future role of Alaska's boreal forest soils in sequestering or releasing carbon under changing disturbance regimes. This project, in concert with other related projects at UAF, will help provide that understanding.
Publications
- VALENTINE, D.W., R.D. BOONE, E.B. Sparrow. 2001. Fire has mixed effects on decomposition in boreal forests of interior Alaska. Poster presentation, Ecological Society of America.
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