Old‐growth forests,carbon and climate change: Functions and management for tall open‐forests in two hotspots of temperate Australia

Abstract

The prognosis and utility under climate change are presented for two old‐growth, temperate forests in Australia, from ecological and carbon accounting perspectives. The tall open‐forests (TOFs) of south‐western Australia (SWA) are within Australia’s global biodiversity hotspot. The forest management and timber usage from the carbon‐dense old‐growth TOFs of Tasmania (TAS) have a high carbon efflux, rendering it a carbon hotspot. Under climate change the warmer, dryer climate in both areas will decrease carbon stocks directly; and indirectly through changes towards dryer forest types and through positive feedback. Near 2100, climate change will decrease soil organic carbon (SOC) significantly, e.g. by ～30% for SWA and at least 2% for TAS. The emissions from the next 20 years of logging old‐growth TOF in TAS, and conversion to harvesting cycles, will conservatively reach 66(±33) Mt‐CO₂‐equivalents in the long‐term – bolstering greenhouse gas emissions. Similar emissions will arise from rainforest SOC in TAS due to climate change. Careful management of old‐growth TOFs in these two hotspots, to help reduce carbon emissions and change in biodiversity, entails adopting approaches to forest, wood product and fire management which conserve old‐growth characteristics in forest stands. Plantation forestry on long‐cleared land and well‐targeted prescribed burning supplement effective carbon management.

Abbreviations: C, carbon; CBS, clearfell, burn and sow; CO₂‐e, CO₂ equivalents; CWD, coarse woody debris; DOC, dissolved organic carbon; GHG, greenhouse gas; Mt, megatonnes; SOC, soil organic carbon; SWA, south‐western Australia; SWAFR, Southwest Australian Floristic Region; TAS, Tasmania; TOF, tall open‐forest; t‐C ha^?1 yr^?1, tonnes of carbon per hectare per year     

The effects of foraging by the superb lyrebird (Menura novae-hollandiae) in Eucalyptus regnans forests at Beenak,Victoria

Abstract In an early spar-stage stand of Eucalyptus regnans at Beenak, Victoria, foraging by lyrebirds in bare floor areas on steep slopes results in a complex microtopography of excavations, accumulations and terracettes. About 200 t ha^?1 of litter and top soil may be displaced an average of 70 cm downhill per year. Magnetic ferruginous pisolite was used as a marker to monitor progressive soil movement over 3 years. Very little disturbance occurred in areas of dense ground fern, but in bare areas the whole forest floor may be turned over every 20 months. In the site studied, foraging activity by lyrebirds varied seasonally and topographically. Disturbance by other biotic agents was minimal. The mean depth of soil cultivation was about 10 cm and litter was frequently buried or mixed intimately with soil. Since buried leaf litter decays more quickly than that on the surface, lyrebird foraging is likely to increase the rate of nutrient cycling. The small, steep clifflets left at the uphill limits of each scratch microsite provide litter-free niches for the establishment of tree fern prothalli and shade-tolerant herbs. All stages in the growth of the rough tree fern, Cyathea australis, were present in bare floor areas, but in dense ground fern patches, young stages were confined to rotten logs and upturned root balls. Since dense tree fern development tends to diminish the cover of dense ground fern, lyrebird foraging activity may maintain an accessible food resource which would otherwise diminish with increased ground fern cover in these forests in the course of secondary succession after fire.     

The effect of fire and permafrost interactions on soil carbon accumulation in an upland black spruce ecosystem of interior Alaska: implications for post‐thaw carbon loss

High‐latitude regions store large amounts of organic carbon (OC) in active‐layer soils and permafrost, accounting for nearly half of the global belowground OC pool. In the boreal region, recent warming has promoted changes in the fire regime, which may exacerbate rates of permafrost thaw and alter soil OC dynamics in both organic and mineral soil. We examined how interactions between fire and permafrost govern rates of soil OC accumulation in organic horizons, mineral soil of the active layer, and near‐surface permafrost in a black spruce ecosystem of interior Alaska. To estimate OC accumulation rates, we used chronosequence, radiocarbon, and modeling approaches. We also developed a simple model to track long‐term changes in soil OC stocks over past fire cycles and to evaluate the response of OC stocks to future changes in the fire regime. Our chronosequence and radiocarbon data indicate that OC turnover varies with soil depth, with fastest turnover occurring in shallow organic horizons (～60 years) and slowest turnover in near‐surface permafrost (>3000 years). Modeling analysis indicates that OC accumulation in organic horizons was strongly governed by carbon losses via combustion and burial of charred remains in deep organic horizons. OC accumulation in mineral soil was influenced by active layer depth, which determined the proportion of mineral OC in a thawed or frozen state and thus, determined loss rates via decomposition. Our model results suggest that future changes in fire regime will result in substantial reductions in OC stocks, largely from the deep organic horizon. Additional OC losses will result from fire‐induced thawing of near‐surface permafrost. From these findings, we conclude that the vulnerability of deep OC stocks to future warming is closely linked to the sensitivity of permafrost to wildfire disturbance.     

The carbon costs of mitigating high‐severity wildfire in southwestern ponderosa pine

Forests provide climate change mitigation benefit by sequestering carbon during growth. This benefit can be reversed by both human and natural disturbances. While some disturbances such as hurricanes are beyond the control of humans, extensive research in dry, temperate forests indicates that wildfire severity can be altered as a function of forest fuels and stand structural manipulations. The purpose of this study was to determine if current aboveground forest carbon stocks in fire‐excluded southwestern ponderosa pine forest are higher than prefire exclusion carbon stocks reconstructed from 1876, quantify the carbon costs of thinning treatments to reduce high‐severity wildfire risk, and compare posttreatment (thinning and burning) carbon stocks with reconstructed 1876 carbon stocks. Our findings indicate that prefire exclusion forest carbon stocks ranged from 27.9 to 36.6 Mg C ha^?1 and that the current fire‐excluded forest structure contained on average 2.3 times as much live tree carbon. Posttreatment carbon stocks ranged from 37.9 to 50.6 Mg C ha^?1 as a function of thinning intensity. Previous work found that these thinning and burning treatments substantially increased the 6.1 m wind speed necessary for fire to move from the forest floor to the canopy (torching index) and the wind speed necessary for sustained crown fire (crowning index), thereby reducing potential fire severity. Given the projected drying and increase in fire prevalence in this region as a function of changing climatic conditions, the higher carbon stock in the fire‐excluded forest is unlikely to be sustainable. Treatments to reduce high‐severity wildfire risk require trade‐offs between carbon stock size and carbon stock stability.     

不饱和土壤CH4的吸收与氧化

不饱和土壤是已知唯一的 CH4 生物壑。综述了不饱和土壤 CH4 的吸收、氧化过程及其影响因素。不饱和土壤中 CH4 氧化的临界浓度低 ,因而甲烷氧化菌可氧化大气 CH4 并将其当作唯一的碳源和能源。土壤 CH4 吸收率与土壤湿度通常呈负相关关系。土壤湿度过高 ,大气 CH4 和 O2 向土壤中扩散受阻 ;或土壤湿度过低引起水分胁迫均导致甲烷氧化菌活性下降。NH 4对土壤中 CH4 氧化的抑制作用可归结为 NH3和 CH4 在甲烷单氧酶水平上的竞争、由氧化作用向硝化作用的转移以及 NH 4氧化生成的 NO- 2 的毒性。NH 4对 CH4 氧化的抑制作用与土壤有效氮含量成正比。各类氮肥对 CH4 氧化抑制作用 :化肥 >有机肥 ;铵态氮肥 >尿素。 NO- 3对 CH4 氧化没有抑制效应。阳离子代换量 (CEC)高的土壤 NH 4对 CH4 氧化的抑制作用轻。 CH4 氧化菌对大气 CH4 的高亲和力及 CH4 氧化所需较低的活化能导致其温度系数 Q1 0 较小。地温较低时 ,土壤氧化 CH4 的能力随温度升高而升高。当地温高于 CH4 氧化的最佳温度时 ,CH4 氧化菌难以与硝化细菌及其它微生物竞争利用土壤空气中的 O2 ,导致其活性降低。甲烷氧化菌对 p H值变化不敏感。团粒结构较好的壤土可保护 CH4 氧化菌免受干扰。未受干扰的森林土壤 CH4 氧化率的峰值一般出现在亚表     

土壤湿度对中国南部热带森林土壤甲烷吸收的影响

森林土壤是一个重要的大气甲烷的汇。然而,相较于寒带和温带,在热带尤其是东南亚地区,森林土壤甲烷通量的观测较少,这限制了目前对热带森林土壤甲烷通量与环境因子之间关系的认识,也给热带森林土壤甲烷汇的估算带来了一定的不确定性。在中国海南省吊罗山国家森林公园的热带森林土壤,采用激光光谱法测量了2016年9月至2018年9月逐月的土壤甲烷通量,并分析了其与周围环境因子的关系。结果表明：研究区土壤是甲烷的汇,山顶样地的年平均吸收量为0.95 kg CH₄-C hm^-2 a^-1,山脚样地的年平均吸收量为1.93 kg CH₄-C hm^-2 a^-1。干季(11月—次年4月)的甲烷吸收通量明显高于湿季(5—10月),占到全年甲烷吸收的68%。山顶样地年平均土壤湿度为19.2%,年内的波动较小(2.8%)。而山脚样地的年平均湿度相对较低,为12.7%,且年内波动大(5.4%)。土壤湿度是控制甲烷吸收最主要的环境因子,可以解释月际甲烷吸收变化的76%,甲烷吸收通量与土壤温度的相...     

Seasonal Dynamics of Atmospheric Methane Oxidation in Gray Forest Soils

Seasonal fluctuations in the methane fluxes in the soil–atmosphere system were determined for gray forest soils of Central Russia. Consumption of atmospheric methane was found to exceed methane emission in gray forest soils under forest and in the agrocenosis. The average annual rates of atmospheric methane consumption by the soil under forest and in the agrocenosis were 0.026 and 0.008 mg C-CH₄/(m² h), respectively. The annual rate of atmospheric methane oxidation in the gray forest soils of Moscow oblast was estimated to be 0.68 kton. Seasonal fluctuations in the methane oxidation activity were due to changes in the hydrothermal conditions and in the reserves of readily decomposable organic matter and mineral nitrogen, as well as to changes in the activity of methane oxidizers.