Effects of post-fire conditions on soil respiration in boreal forests with special reference to Northeast China forests

Abstract

Forest fires frequently occur in boreal forests, and their effects on forest ecosystems are often significant in terms of carbon flux related to climate changes. Soil respiration is the second largest carbon flux in boreal forests and the change in soil respiration is not negligible. Environmental factors controlling the soil respiration, for example, soil temperature, are altered by such fires. The abnormal increase in soil temperature has an important negative effect on soil microbes by reducing their activities or even by killing them directly with strong heat. On the other hand, although vegetation is directly disturbed by fires, the indirect changes in soil respiration are followed by changes in root activities and soil microbes. However, there is very limited information on soil respiration in the forests of Northeast China. This review, by combining what is known about fire influence on soil respiration in boreal forests from previous studies of post-fire effects on soil conditions, soil microbes, and forest regeneration, presents possible scenarios of the impact of anticipated post-fire changes in forest soil respiration in Northeast China.

Keywords

boreal forests / Northeast China / forest fires / forest regeneration / soil microbes / soil respiration

Introduction

Boreal forests cover about 9% (13.7×10⁶ km²) of the land between 45º and 75º north (Czimczik et al., 2005). They contain large amounts of stored soil carbon and are susceptible to periodic disturbances by forest fires (Sawamoto et al., 2000). Fire oxidizes organic matter and produces carbon dioxide (CO₂) and water, releasing tremendous amounts of energy as heat. Repeated low-intensity fires (ground fire) may shape forests by killing small trees, rejuvenating fire-tolerant grasses, and accelerating the cycling of nutrients. High-intensity fires (severe fire) kill the majority of trees, oxidize large quantities of nutrients such as nitrogen, and disturb plant-soil interactions for decades (Fisher and Binkley, 2000). Global climate changes are projected to be greatest in the northern regions, where forest fires are also becoming increasingly frequent. A better understanding of the effects of forest fire disturbances on carbon cycling is critically important, particularly in light of alterations to disturbance regimes that may occur with global climate changes (Litton et al., 2004).

The carbon cycle is characterized by carbon uptake and release. Carbon is uptaken by CO₂ assimilation in photosynthesis, and released by respiration of plants and heterotrophic organisms, as well as from burning vegetation (Schulze, 2002). Thus, understanding the effects of forest fire on the carbon cycle of boreal forests is essential to quantifying the role of boreal forests in the global carbon cycle. The carbon cycle in soil, the second largest carbon flux in boreal forests, is directly and indirectly affected by forest fires and is hypothesized to change during forest succession following forest fires (Wang et al., 2002). The flux of carbon from soils to atmosphere occurs primarily in the form of CO₂, and is the result of soil respiration. Soil respiration (or soil CO₂ efflux) is thought to represent 60%-80% of ecosystem respiration in boreal forests (Chapin et al., 2002; Raich and Potter, 1995). Carbon dioxide in the soil is produced by root respiration and by decomposition of litter and soil organic matter. Evidence from soil CO₂ profiles suggests that these environmental changes may result in increased decomposition of carbon previously immobilized by permafrost, potentially transforming a landscape that was once a net sink for carbon into a carbon source (O’Neill et al., 2003). Thus, the effects of forest fires on soil respiration should be considered in determining feedback to atmospheric climate dynamics (Bergner et al. 2004).

The soil respiration is controlled by environmental factors (soil temperature, soil moisture, etc.) and is mainly produced by the metabolic processes of root and soil microbes. It reflects the biological activities of the soil ecosystem. For example, surface radiometric temperature increased by up to 6 degrees and remained elevated even 15 years after a fire in the Canadian boreal forest. The CO₂ flux was reduced for a 15-year period after the fire, with the greatest reduction to ca. 25% of control areas during the year following the fire (Amiro et al., 2003). However, Burke et al. reported that in Canadian boreal forest soil, CO₂ effluxes from upland dominated by black spruce (Picea mariana) stands might not be immediately impacted by fire, possibly remaining at pre-burn levels by microbial decomposition of labile compounds released as a result of the fire (Burke et al., 1997). The relationship between vegetation and soil microbes is that of moving to a dynamic phase during the rehabilitation stage after disturbances by forest fires.

Chinese boreal forests, geographically distributed in the Daxing’anling Mountain of Northeast China, are the most southern part of the global boreal forest biome. The dominant species is larch (Larix gmelinii) with other major species including birth (Betula platyphylla), pine (Pinus sylvestris var. mongolica) and oak (Quercus mongolica) (Jiang et al., 2002). The forests of Northeast China cover about 31.4% of the forest areas and account for 40.7% of wood production in China (Liu et al., 1997). About 15106 hm² of forest areas in Northeast China suffer from forest fires every year (Yang and Wang, 2006). However, there are very few studies related to soil respiration after forest fires in Northeast China. In this review, we will first synthesize results from reported studies with respect to three aspects (soil condition, soil microbial communities, and forest regeneration) in an attempt to find the characteristics of soil respiration in global boreal forests disturbed by forest fires. Then we analyze studies on soil respiration in forests of Northeast China and discuss the prospect of studies on soil respiration, especially on the variations of post-fire soil respiration in this region.

Effects of post-fire on soil respiration

In the Siberian taiga ecosystem, soil respiration in severely burned forests is significantly lower than in unburned forests, and values are intermediate in mildly burned forests (Sawamoto et al., 2000). Soil respiration rates range from a low level of 156 g C•m^-2 in a 7-year-old stand to a high level of 254 g C•m^-2 in a 22-year-old stand during the growing season in the jack pine (Pinus banksiana) forests in Michigan, USA (Yermakov and Rothstein, 2006). CO₂ flux measurements (from towers, aircraft and remote sensing/modeling) show that the regenerating boreal forests in western Canada have a low initial flux which increases with time after a fire (Amiro et al., 2003). These studies suggest that severe fires decrease soil respiration at first and then soil respiration increases with time.

What is the trend of soil respiration changes following forest fires in boreal forests? This is a quite complicated question to be answered since many environmental factors combine together to affect soil respiration. We will approach soil respiration from the characteristics and dynamics of soil physical conditions, soil microbes and forest regeneration combined with soil changes influenced by disturbance from forest fires.

Soil physical conditions

The effects of forest fire on soils have intrigued scientists since the beginning of forest soil science. The oxidation of organic matter releases large amounts of energy as heat. This tremendous release of energy raises the temperature of the soil. Neal et al. (1965) reported that soil temperatures (at a depth of 5 cm) in a burned portion of a Douglas-fir (Pseudotsuga menziesii) clearcut averaged 6°C higher than the unburned portion. Forest fires may also increase soil moisture by decreasing water use by the vegetation. Water permeability is often diminished following fires due to the plugging of surface pores and increased fire-induced water repellency (Krammes and DeBano, 1965).

Losses of nutrients after forest fires are modified by conditions of differing fire intensities, soil characteristics, topographies, and climatic patterns. The supply of nutrient cations (calcium, magnesium, and potassium) generally increases following forest fires. This increase results from their direct release from burning organic matter and the subsequent increase in organic matter decomposition. The concentrations of soil ammonium and nitrate generally increase greatly after forest fires. However, it often lasts for only a few growing seasons or less (Adams et al., 1986;Christensen, 1987). Subsequent changes in nitrogen availability following fires depend on the quantity of nitrogen loss in the fire, changes in rates of microbial mineralization after the fire, and the competition for mineralized nitrogen between microbes and plants (Fisher and Binkley, 2000).

Nutrient leaching rates also increase after forest fires. The leaching of nutrients from soil after forest fires is influenced by the increased ions available to the uptake and retention by plants, absorptive properties of the forest floor and soil (both microbial and mineral), and patterns of precipitation and evapotranspiration. Accelerated erosion following fires can cause significant nutrient loss (Wright and Bailey, 1982) because of changes in vegetation, soil properties, hydrology, and geomorphic processes.

Soil pH typically increases immediately after fires and then declines to pre-fire levels over a period of months, years, or decades at most (Fisher and Binkley, 2000). Subsequently, changes in soil pH influence the availability of some nutrients, both through direct geochemical effects and indirect effects on microbial activity.

Soil microbial properties

Forest fires significantly alter the composition and activity of soil microbes, but there is little information available on these changes. Here we mainly mention mycorrhizal fungi and nitrogen fixation bacteria, which are very important for plant regeneration in boreal forests.

Environmental conditions of the residual forest floor may be very different after forest fires. Charred and darkened organic materials may absorb radiation more than unburned materials would, resulting in warmer conditions. Environmental factors that most directly affect carbon exchange vary depending upon burn status. In unburned stands, soil respiration is highly correlated with seasonal patterns of soil temperature. In burned stands, soil becomes significantly warmer, about 4-8 degrees warmer than in unburned stands. The increased soil temperature after forest fires may lead to enhanced diffusion of CO₂ by microbial activity between the atmosphere and forest soils. Decomposition is generally expected to increase after fires because of increased temperatures at the soil surface and increased soil moisture (Bissett and Parkinson, 1980; Van Cleve and Dyrness, 1985).

A pulse of increased nutrient availability typically follows forest fires as a result of reduced competition among plants, increased release of elements from organic matter, and altered activity of soil biota. Increased microbial activity in burned soils may increase the release of phosphorus from organic matter, and competition for the increased supply of phosphorus between microbes and plants could determine the extent of enhancement of plant nutrition. In addition to the direct losses of phosphorus during forest fires, subsequent effects on phosphorus cycling include changes in pH and microbial phosphorus transformations. Increases in pH should generally increase phosphorus availability when phosphorus is bound to ions and aluminum and should decrease the availability of phosphorus bound with calcium (Lindsay, 1979). Changes in microbial properties of soils after fires have been documented, but few generalizations seem supportable. Zhuang et al. (2002) reported that along with differences in fire and climate history, a number of other factors influence the carbon dynamics. These factors include nitrogen fixation, the growth of moss, and changes in the organic layer, soil drainage, and fire severity.

Mycorrhizal species that are primarily associated with the organic horizons under forest fire disturbance regimes are more susceptible to the deleterious effects of forest fires than those non-mycorrhizal species found in the mineral layer. Fire intensity appears to be a major factor in determining the response of the microbial community. Severe fires have been shown to reduce mycorrhizal fungal diversity (Danielson, 1984; Torres and Honrubia, 1997). Several studies have highlighted the importance of resistant propagules of ectomycorrhizal fungi (Baar et al., 1999) and arbuscular mycorrhizal fungi (Horton et al., 1998) in the establishment of tree seedlings in areas affected by severe fires. In contrast, low intensity fires, where many larger host plants can survive, seem to have a limited long-term effect upon mycorrhizal fungal diversity and community composition (Jonsson et al., 1999). However, Stendell et al. (1999) demonstrated that a low intensity fire could in the short term have a very marked effect upon community structure where the dominant species within an ectomycorrhizal community are primarily associated with the organic layers. Many of the ectomycorrhizal fungi which appeared dominantly on the post-fire seedlings were those which formed only a small proportion of the mycorrhizae in the forest before the fire, or those present only as resistant propagules within the soil (Taylor and Bruns, 1999). The propagules were thought to be spores, mycelial fragments and perhaps sclerotia that survived the fires due to their position within the soil profile. These fungi were able to proliferate and become the dominant components of the ectomycorrhizal community. However, Treseder et al. (2004) reported that fires did not noticeably reduce the abundance of arbuscular mycorrhizal fungi and ectomycorrhizal colonization required up to 15 years to return to pre-fire levels. As a result, dominant mycorrhizal groups shifted from arbuscular to ectomycorrhizal fungi as plant succession progressed.

Most studies on the effects of fires on non-symbiotic nitrogen fixation have found that the rates are so low as to be important only on a time scale of decades or centuries, and none has shown ecologically meaningful increases in rate following fires. Wei and Kimmins (1998) estimated that free-living nitrogen fixation would be twice greater in burned stands due to a wildfire compared to that without fires in a lodgepole pine (Pinus contorta) forest. Some studies (Woodmansee and Wallach, 1981; Boerner, 1982;Christensen, 1987) have speculated that non-symbiotic nitrogen fixation increases after forest fires.

Bacterial functional diversity was greatest in the oldest sites. Altogether, microbes that can mineralize organic compounds (i.e., ectomycorrhizae and bacteria) recover more slowly than those that cannot (i.e. arbuscular mycorrhizae) (Fisher and Binkley, 2000). Microbial succession may influence soil carbon and nitrogen dynamics in the first several years following forest fires, by augmenting carbon storage in glomalin while inhibiting mineralization of organic compounds. Bissett and Parkinson (1980) found no difference in microbial biomass between burned and unburned plots after 6 years, but the ratio of bacteria to fungi was higher in the burned plots in a spruce-fir forest in Alberta, British Columbia, Canada.

Although many studies indicate that soil respiration decreases following fires, several models assume that heterotrophic respiration increases. The post-fire conditions may stimulate microbial respiration because of higher nutrients and substrates in remnant soils and enhanced soil temperature. Whole soil microbial respiration within 1-m depth was reported to be higher in severely burned forests than in unburned forests. In fact, forest fires increased the soil microbial respiration in the Siberian larch forests (Sawamoto et al., 2000). The microbial respiration estimated after the fire was almost three times as high as that calculated before the fires (Kim and Tanaka, 2003). Furthermore, evidence from soil CO₂ profiles suggests that environmental changes may have resulted in enhanced decomposition of carbon previously immobilized by permafrost, potentially transforming a landscape that was once a net carbon sink into a carbon source in Siberia taiga, Far East Russia (Sawamoto et al., 2000).

Forest regeneration

The intensity of a fire is critical for rehabilitation in forest ecosystems. Severe fires kill the majority of trees, oxidize large quantities of nutrients and disturb plant-soil interactions for decades; whereas low-intensity fires may kill small trees and accelerate the cycling of nutrients.

Despite the loss of nutrients in fires, nutrient availability to plants typically increases after fire as a result of heat-induced release of nutrients, reduced competition among plants, and perhaps sustained changes in soil conditions (such as temperature and water content). Forest fires leave behind large amounts of ash, typically ranging from 2 to 15 mg•ha^-1 (Raison et al., 1985). The concentrations of nutrients are unusually high in ash. Some of the nutrients in ash are water soluble or readily released by microbial activity, and this should provide a large supply of available nutrients for the recovery of the vegetation. Other nutrients in ash, such as phosphorus, may not be readily soluble and may become only slowly available (Giardina et al., 1995). The availability of phosphorus limits the production in many ecosystems around the world, yet effects of forest fires on phosphorus availability have been poorly documented. Plant nutrition after fires depends on changes in both nutrient availability and in the distribution and activity of plant roots and mycorrhizal fungi. Few studies have examined root responses to the nutritional effects of fires.

The effect of fires on soil respiration changes with plant regeneration and soil microbial community. The post-fire increases in temperature and substrate quality was reported to increase decomposition of humic materials in the first 7 years after fires (O’Neill et al., 2006). In that study, Bryophyte species exhibited a distinct successional pattern in the first five decades after a fire, which corresponded to decreased soil temperature and increased carbon accumulation in organic soils. Potential rates of carbon exchange in mosses were greater in early successional species and declined as the stand matured. Residual sources of CO₂ increased as a function of stand age, reflecting increased contributions from roots as the stand recovered from disturbance (O’Neill et al., 2006). The observed recovery of soil respiration rates to pre-burn levels by 7 years post-burn is probably due to the respiration of plant regeneration and the combined effects of elevated soil temperatures and improved litter quality with microbial activities in the soil (Burke et al., 1997).

Advances in the study of forest soil respiration in Northeast China

The Northeast China forests have large distribution areas and a huge amount of carbon storage. They play an important role in both local and national carbon budgets in China. The changes in soil respiration are highly related to carbon budgets; however, few data are available on soil respiration for this region.

Relevant studies have dramatically increased since 1991 due to the recent social and scientific need to mitigate climate change. By 2006, the number of papers published on soil respiration was 337, worldwide (Fig. 1); while 13 papers related to the estimation of soil respiration in Northeast China were published by that time (Table. 1). Among these 13 publications, 9 studies were conducted in the Changbai Mountain (42º24’N, 128º28’E), and only four were performed in Mao’ershan (45º24’N, 127º40’E), which belongs to Chinese boreal forests. For example, Yang and Wang (2006) reported that the seasonality of soil respiration was mainly driven by soil temperature and soil moisture for 6 secondary temperate forest types in Mao’ershan. They found that broad-leaved forests had a higher soil respiration rate than coniferous forests. Jiang et al. compared the soil respiration in two larch plantations of different ages: 17 years and 31 years, and separated rhizosphere respiration from total soil respiration by the trenching method. Their results demonstrated that significantly different soil respiration rates between the two plantations were due to different forest ages (Jiang et al., 2005). Rhizosphere and soil respiration were highly related to soil temperature but not to soil water content. However, relevant data were too limited to reveal the conditions of soil respiration in Chinese boreal forests. Furthermore, there is presently a nearly blank research area on soil respiration following forest fires in northeast China. The study of global carbon cycling and soil respiration is probably still a young field. It urgently needs more contributions from Chinese soil and ecological scientists to reduce the gap between China and the world.

Challenges facing the study of soil respiration after forest fires in Northeast China

Forest fires typically destroy forest communities, reduce their growth, remove nutrients, and increase decomposition of soil organic matter from forests in the short term. Effects of forest fire on soil respiration (by changing root respiration and soil microbial respiration) are attracting increasing interest since soil respiration is highly related to the global carbon dynamics. However, what are the overall long-term effects of post-fire conditions on soil respiration in the Northeast China forests? Little information has been collected on this question, and the available data suggests that no single generalization will be appropriate across forests and soils.

Many studies support the idea that soil respiration and root respiration are reduced by forest fires and later they increase with time. However, the changes in soil microbial respiration are still unclear and further work is needed.

There are many methods for measuring soil respiration, with large differences in accuracy, spatial and temporal resolution, and applicability. Hence, the choice of a specific technique is often a trade-off between requirements and feasibility (Janssens, 2000). In general, it can be divided into direct measurement and indirect measurement. The chamber method is the most direct approach to determine carbon flux from the soil surface or forest floor (Davidson et al., 2002). The method is categorized into two types as static and dynamic, depending on the presence or absence of air circulation. This method is flexible in selecting sample locations and isolating specific ecosystem components. It has however been criticized on account of the chamber effects (Mosier, 1990), which mainly include soil disturbance, pressure variations, and temperature and moisture changes when the chamber is placed. Soil respiration has also been estimated by several kinds of indirect methods, including the micrometeorological method (the Eddy covariance technique, the flux-gradient method, the Bown ratio/energy balance method, the aerodynamic method), the mass balance technique, and the soil CO₂ profile method. To choose a proper method, we should take into account the targets and conditions of our research. The Chinese Terrestrial Ecosystem Flux Research Network (ChinaFLUX) is a long-term national network of micrometeorological flux measurement sites that measures the net exchange of CO₂, water vapor, and energy between the biosphere and atmosphere. ChinaFLUX has promoted the study of carbon cycling in China. Some studies were conducted in the Changbai Mountain because there is a Flux tower in that station. However, the range of ChinaFLUX is limited and few data on soil respiration are available in the forests of Northeast China. Moreover, the quality and quantity of the studies need to be improved.

The temporal variations in soil respiration result from variations in environmental conditions after fires. Moreover, following forest fires, the soil respiration on forest grounds becomes more sensitive to fluctuations in surface moisture conditions. Carbon loss may be accentuated if microbial activity increases after fires, owing to elevated soil temperatures and soil moisture. Therefore, to estimate the post-fire soil respiration, changes in respiration rate of plant roots and soil microbes and environmental factors, such as soil temperature and moisture, as related to fire intensity, should be considered together.

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