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Title: Long-term carbon storage in a semi-natural British woodland
Author: Hale, Karen
ISNI:       0000 0004 5363 5528
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2015
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Atmospheric levels of CO2 are currently 395 ppm (dry air mole fraction measured at Mauna Loa, Hawaii), their highest concentration in 420,000 years. Forests play a major role in the global carbon (C) cycle by taking up inorganic C as CO2 through photosynthesis, converting it to organic compounds (biomass), and either storing it in living and dead organic matter (above and below ground: including trees, dead wood, litter, and soil) or returning it to the atmosphere by respiration, decay or fire. Globally, forests cover around 4.1 billion ha of the Earth’s surface and are estimated to contain up to 80% of all aboveground C and around 40% of all belowground (soils, litter, roots) terrestrial C. Forest C stocks have been reported to be increasing over the past 50 years in Europe and over the past 17 years in the United States. However, national forest inventories used to provide these data are often biased towards managed plantations, thereby leaving a knowledge gap regarding the dynamics of unmanaged, semi-natural forests. There are significant uncertainties about changes in C flux through time and the relative contributions of drivers such as land use, climate and atmospheric CO2. Decomposition of tree root C represents a potentially large C flux and contribution to the soil C sink when the input of dead and decaying root tissue, and root exudates, are greater than the output from respiration of roots, their symbionts, and the soil decomposer organisms. Therefore, quantifying decomposition rates and identifying primary controls of root decomposition are important for evaluating ecosystem function and possible responses to environmental change. This thesis explores long-term C dynamics in Lady Park Wood (LPW), an ancient semi-natural woodland situated in the counties of Monmouthshire and Gloucestershire, UK. We calculated changing tree biomass C stocks in LPW from 1945 to 2010. Separate estimates of tree biomass C, soil C and dead wood C were obtained to verify how C is apportioned among these types of forests. We used the dynamic vegetation model LPJ-GUESS to explore the likely contributions of temperature, CO2 and management to forest C stocks in this region during the last 65 years. A 30 month field experiment was conducted in LPW using oak roots of different diameter classes (<2 mm, 2-5 mm and 5-10 mm) in decomposition bags. These were buried in two locations: one with bare ground and one with the soil covered by ground layer vegetation, in order to quantify root decomposition rates. Lastly, we utilised long-term monitoring data from 2 other semi-natural woodlands in the UK to investigate whether LPW is a typical representation of live biomass C storage in these types of woodland. We then compared C storage in semi-natural forests with C storage in plantations and managed forests to see which type of forest stores the most C. Between 1945 and 2010, tree biomass (including roots) carbon stocks in LPW approximately doubled in the old-growth stands, (increasing from 8.92 kg C m-2 (0.025-quantile 7.21 k C m-2, 0.975-quantile 10.19 kg C m-2) to 17.50 kg C m-2 (0.025-quantile 14.09 kg C m-2, 0.975-quantile 20.24 kg C m-2)), and between 1977 and 2002 increased by almost 50% in the young-growth stands (from 6.30 kg C m-2 (0.025-quantile 5.39 kg C m-2, 0.975-quantile 7.23 kg C m-2) to 9.21 kg C m-2 (0.025-quantile 7.72 kg C m-2, 0.975-quantile 10.65 kg C m-2)). In the old-growth stands 60% (0.025-quantile 54%, 0.975-quantile 64%) of carbon was stored in tree biomass, 38% (0.025-quantile 34%, 0.975-quantile 43%) was stored in soil and 2% (0.025-quantile 1%, 0.975-quantile 4%) stored in coarse woody debris. In contrast, storage of carbon in the young-growth stands was allocated almost equally between tree biomass (53%, 0.025-quantile 48%, 0.975-quantile 57%) and soil (43%, 0.025-quantile 39%, 0.975-quantile 47%), with 4% (0.025-quantile 2%, 0.975-quantile 7%) stored in coarse woody debris. Results from LPJ-GUESS suggest that release from management was the major driver of carbon storage but CO2 also had a pronounced effect. Relatively little of the observed increase in carbon stocks was attributable to increased temperature. Similarly, little evidence of a temperature effect was found on root decomposition rates. Mean loss rates of roots buried in the location with ground vegetation were significantly higher than those of roots buried in the bare ground site. Large roots (5-10 mm) decomposed faster than medium (2-5 mm) or fine roots (<2 mm) over the first 18 months. A typical unthinned Sitka spruce plantation in the UK sequesters carbon faster than semi-natural forests, having accumulated 16 kg C m-2 by the end of its 60 year rotation, compared to Lady Park Wood which accumulated just 9.31 kg C m-2 over a 65 year period. However, semi-natural forests comprise much greater carbon stores over the long term. A time average equilibrium storage value (mean taken across the harvesting cycle) for unthinned Sitka spruce stands is 7.4 kg C m-2, whereas the mean storage value for semi-natural woodlands in this study is 17.5 kg C m-2. Although an increase in tree biomass carbon is consistent with European syntheses, this study suggests that semi-natural old-growth stands are storing more carbon than typical plantations, with tree biomass the most important compartment for carbon stores. There is clear evidence to suggest that semi-natural woodland may be an important and underestimated carbon stock in the UK.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available