Long-term carbon storage in a semi-natural British woodland
Long-term carbon storage in a semi-natural British woodland
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英国半天然林地的长期碳储存
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发表时间：
2015
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通讯作者：
K. Hale
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作者：
K. Hale
大气中的二氧化碳含量目前为395 ppm（在夏威夷的莫纳罗亚山测量的干空气摩尔分数），是42万年来的最高浓度。森林在全球碳（C）循环中发挥着重要作用，通过光合作用将无机C转化为CO2，将其转化为有机化合物（生物质），并将其储存在活的和死的有机物质中（地上和地下：包括树木，枯木，垃圾和土壤）或通过呼吸，腐烂或火灾将其返回大气。在全球范围内，森林覆盖了大约41亿公顷的地球表面，估计含有高达80%的地上碳和大约40%的地下（土壤，凋落物，根）陆地碳。据报道，过去50年来欧洲森林C储量不断增加，过去17年来美国森林C储量不断增加。然而，用于提供这些数据的国家森林清单往往偏向于有管理的种植园，从而在未管理的半自然森林的动态方面留下了知识空白。碳通量随时间的变化以及土地利用、气候和大气CO2等驱动因素的相对贡献存在很大的不确定性。当死亡和腐烂的根组织和根分泌物的输入大于根、其共生体和土壤分解生物的呼吸输出时，树根C的分解代表了潜在的大C通量和对土壤C汇的贡献。因此，定量分解速率和确定主要控制根分解是重要的生态系统功能和可能的响应环境变化的评估。本论文探讨了位于英国蒙茅斯郡和格洛斯特郡的一个古老的半天然林地-Lady Park Wood（LPW）的碳长期动态。我们计算了1945年至2010年LPW中树木生物量C储量的变化。树木生物量C，土壤C和枯木C的单独估计，以验证C是如何分配这些类型的森林。我们使用动态植被模型LPJ-GUESS来探讨在过去的65年里，温度，CO2和管理在这一地区的森林碳储量的可能贡献。采用不同直径级（<2 mm、2-5 mm和5-10 mm）的柞树根，在腐解袋中进行了为期30个月的田间试验。这些被埋在两个位置：一个是裸露的地面，一个是被地面植被覆盖的土壤，以量化根的分解率。最后，我们利用英国其他2个半天然林地的长期监测数据来调查LPW是否是这些类型林地中活生物量C储存的典型代表。然后，我们比较了半自然森林中的碳储量与人工林和人工林中的碳储量，以了解哪种类型的森林储存最多的碳。
从1945年到2010年，（包括根）LPW中的碳储量在古老的林分中大约翻了一番，（从8.92 kg C m-2（0.025分位数7.21 k C m-2，0.975分位数10.19 kg C m-2）增加到17.50 kg C m-2（0.025分位数14.09 kg C m-2，0.975分位数20.24 kg C m-2）），在1977 - 2002年期间增加了近50（从6.30 kg C m-2（0.025分位数5.39 kg C m-2，0.975分位数7.23 kg C m-2）到9.21 kg C m-2（0.025分位数7.72 kg C m-2，0.975分位数10.65 kg C m-2））。在老龄林分中，60%（0.025分位数54%，0.975分位数64%）的碳储存在树木生物量中，38%（0.025分位数34%，0.975分位数43%）储存在土壤中，2%（0.025分位数1%，0.975分位数4%）储存在粗木质残体中。与此相反，碳储量在乔木生物量（53%，0.025分位数48%，0.975分位数57%）和土壤（43%，0.025分位数39%，0.975分位数47%）之间分配几乎相等，与4%（0.025分位数2%，0.975分位数7%）存储在粗木质残体。LPJ-GUESS的结果表明，管理释放是碳储存的主要驱动力，但CO2也有明显的影响。所观察到的碳储存量增加中，温度升高所引起的增加相对较少。同样，很少有证据表明温度对根的分解率有影响。有地表植被的根系平均损失率显著高于倮地根系。在最初的18个月里，大根（5-10 mm）比中根（2-5 mm）或细根（<2 mm）分解得更快。
在英国，一个典型的未间伐的锡特卡云杉种植园比半自然森林更快地固碳，在其60年的轮伐期结束时积累了16 kg C m-2，而Lady Park Wood在65年的时间里积累了9.31 kg C m-2。然而，从长期来看，半天然森林的碳储存量要大得多。未间伐的锡特卡云杉林分的时间平均平衡储藏值（整个采伐周期的平均值）为7.4 kg C m-2，而本研究中半天然林地的平均储藏值为17.5 kg C m-2。虽然树木生物量碳的增加与欧洲的合成是一致的，这项研究表明，半自然的古老的生长林比典型的种植园储存更多的碳，与树木生物量的碳储存最重要的隔间。有明确的证据表明，在英国，半天然林地可能是一个重要的和被低估的碳储量。
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.