Post-fire tree stress and growth following smoldering duff fires

Understanding the proximate causes of post-fire conifer mortality due to smoldering duff fires is essential to the restoration and management of coniferous forests throughout North America. To better understand duff fire-caused mortality, we investigated tree stress and radial growth following experimental fires in a long-unburned forest on deep sands in northern Florida, USA. We burned basal fuels surrounding 80 mature Pinus palustris Mill. in a randomized experiment comparing the effects of basal burning treatments on stem vascular meristems; surficial roots; root and stem combinations; and a non-smoldering control. We examined the effects of duration of lethal temperatures (>60 °C) on subsequent pine radial growth and root non-structural carbohydrates (starch and sugar). Duff and mineral soil temperatures in the experimental fires consistently exceeded 60 °C for over an hour following ignition, with lethal temperatures of shorter duration recorded 20 cm below the mineral soil surface. Duff heating was best explained by day-of-burn Oe horizon moisture (P = 0.01), although little variation was explained (R² = 0.24). Post-fire changes in latewood radial increment in the year following fires was related to duration of temperatures >60 °C 10 cm deep in the mineral soil (P = 0.07), but explained little variability in post-fire growth (R² = 0.17). In contrast, changes in non-structural carbohydrate content in coarse roots (2–5 mm diameter) 120 days following burning were more strongly correlated with the duration of lethal heating 5 cm below the mineral soil surface (P = 0.02; R² = 0.53). Results from this study implicate the role of mineral soil heating in the post-fire decline of mature longleaf pine following restoration fires in sandy soils.     

Post-harvest windthrow and recruitment of large woody debris in riparian buffers on Vancouver Island

Large woody debris (LWD) provides structural complexity to small streams. Riparian buffers are intended to provide long-term vegetation cover and supplies of LWD, but post-harvest windthrow often occurs. To evaluate the impacts of windthrow in riparian buffers and identify the components for a small stream LWD recruitment model, we sampled 26 streams in immature and older stands in wind-exposed areas of southwestern and northern Vancouver Island. These treed buffer strips had been exposed following clearcut harvest of adjacent timber on both sides 1–20 years previously. For stream sections 100 m long in each buffer, all logs greater than 7.5 cm diameter that spanned at least part of stream channel were measured. A total of 658 logs were recorded. Windthrown trees were comparable in characteristics to the trees that made up the buffer. The majority of logs derived from windthrown trees were oriented perpendicular to the stream channel and were suspended above the stream channel. Even 20 years after harvesting, two-thirds of the logs were still suspended above the stream. Logs in older buffers were more decayed, and the decay rate depended on tree species and initial diameter. Log height above stream was negatively correlated with log decay class and time since logging. Log length declined with time since harvest exposure and decay class. Sediment was exposed on upturned roots and within mineral soil pits. The volume of soil retained on upturned rootwads declined over time, but some soil remained even after 20 years. Very little of this exposed sediment was close enough to the creek to result in sediment delivery.     

Decay dynamic of coarse and fine woody debris of a beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) forest in Central Germany

Decay rates of woody debris were estimated and used to model the decay of various diameters of branches and stems in a beech stand in Central Germany. In addition, use of wood density, volume and mass loss to quantitatively describe the degree of decay was tested. The mass loss during decay could be described by a simple exponential function. Under the presented climatic conditions, beech coarse woody debris (CWD) with a diameter >10 cm decays completely in about 35 years. In the first 8 years of decay the mass loss is determined by the decrease in wood density, and subsequently by the loss in volume. Estimation of wood density allows the first three of the four classes of decay to be distinguished, while trees in the last two decay classes could be distinguished using wood volume. Beech fine woody debris with a diameter between 1 and 10 cm decays within about 18 years. The litter fraction of <1 cm is part of the humus layer after 4 years. If there are goals for the amounts, types and dimensions of woody debris to be provided for conservation of biological biodiversity and other ecological functions in managed beech forests, this study offer indications for how long existing woody debris can meet its functions and how frequent new input of CWD is required.     

Effects of preparation of clear‐cut forest sites on the soil mycobiota with consequences for Scots pine growth and health

Fungal abundance and diversity were assessed in two differently managed forest soils. In soil 1, chipped woody debris had been spread on the top soil and mixed with the soil by standard deep ploughing to 50 cm. In soil 2, woody debris was removed and there was no ground preparation. Ten years later, the different methods of management resulted in unexpectedly distinctive habitat heterogeneity with a high variation in structure of the mycobiota communities. Incorporation of woody debris into soil resulted in lower abundance of fungi but greater fungal diversity (which may enhance ecosystem stability) and was beneficial for the growth of Scots pines. Removal of post‐harvest woody debris from the surface of the clear‐cut site with no subsequent ground preparation, compared with incorporating chipped debris and deep ploughing, resulted in the soil of the subsequent 10‐year‐old Scots pine plantation having: (a) 159‐fold greater abundance of fungi, (b) lower fungal diversity, (c) different fungal community composition, (d) higher proportion of mycorrhizal fungi, (e) no root rot pathogens, (f) less tree biomass. The study showed that removal of post‐harvest woody debris and no ground preparation is potentially beneficial for the health of young coniferous trees.     

Succession of mosses,liverworts and ferns on coarse woody debris,in relation to forest age and log decay in Tasmanian wet eucalypt forest

In managed forest landscapes, understanding successional processes is critical to management for sustainable biodiversity. Coarse woody debris is a key substrate for forest biodiversity, particularly because it undergoes complex succession reflecting the effects of changes in both forest structure and substrate characteristics. The present study used a chronosequence approach to investigate succession of mosses, liverworts and ferns on coarse woody debris following clearfell, burn, sow native forest silviculture in wet eucalypt forest in Tasmania, focussing on discriminating between the effects of forest age and log decay. It also compared successional processes following wildfire with those following clearfell, burn, sow silviculture. Forest regenerating after the latter form of regeneration showed clear ecological succession up to 43 years (the limit of available sites), characterised by increasing diversity and cover, and clearly delineated specialisation among species with regard to successional stage. Analyses of subsets of the full data-set indicated that the effects of forest age dominated this succession, with minimal effects of substrate change independent of forest age. Analysis of within forest microenvironments were consistent with the inference that microenvironmental changes related to forest age drive major successional changes in these forests. Comparative analysis indicated similarity between successional states in post-wildfire and post- clearfell, burn, sow regeneration after 43 years for logs of the same decay stages, and continuing succession on post-wildfire sites to at least 110 years. Overall, these data suggest that management to sustain fern and bryophyte diversity should ensure that areas of forest beyond 110 years are represented in the landscape at appropriate spatial scales.     

Biomass and production of fine and coarse roots of trees under agrisilvicultural practices in north-east India

An understanding of the rooting pattern of tree species used in agroforestry systems is essential for the development and management of systems involving them. Seasonal variation, depth wise and lateral distribution of biomass in roots of different diameter classes and their annual production were studied using sequential core sampling. The investigations were carried out in four tree species under tree only and tree+crop situations at ICAR Research Farm, Barapani (Meghalya), India. The tree species were mandarin (Citrus reticulata), alder (Alnus nepalensis), cherry (Prunus cerasoides) and albizia (Paraserianthes falcataria). The contribution of fine roots to the total root biomass ranged from 87% in albizia to 77% in mandarin. The bulk of the fine roots (38% to 47%) in the four tree species was concentrated in the upper 10 cm soil layer, but the coarse roots were concentrated in 10–20 cm soil depth in alder (46%) and albizia (51%) and at 0–10 cm in cherry (41%) and mandarin (48%). In all the four tree species, biomass of both fine- and coarse-roots followed a unimodal growth curve by showing a gradual increase from spring (pre-rainy) season to autumn (post rainy) season. Biomass to necromass ratio varied between 2 to 3 in the four tree species. The maximum (3.2) ratio was observed during spring and the minimum (2) in the rainy season. In alder and albizia, the fine roots were distributed only up to 1 m distance from the tree trunk but in the other two species they were found at a distance up to 1.5 m from the tree trunk. The annual fine root production varied from 3.6 Mg ha^–1 to 6.2 Mg ha^–1 and total production from 4.2 to 8.4 Mg ha^–1 in albizia to mandarin, respectively. Cherry and mandarin had a large number of woody roots in the surface layers which pose physical hindrance during soil working and intercultural operations under agroforestry. But the high biomass of roots of these two species may be advantageous for sequential or spatially separated agroforestry systems. However, alder and albizia have the most desirable rooting characteristics for agroforestry systems.This revised version was published online in November 2005 with corrections to the Cover Date.     

Forest structure and woody plant species composition along a fire chronosequence in mixed pine–oak forest in the Sierra Madre Oriental, Northeast Mexico

Although wildfires are occurring frequently in the pine–oak forests in the Sierra Madre Oriental (northeastern Mexico), data on post-fire succession and forest structure are still rare. Our objectives were to (1) assess the changes in woody plant species composition after fire and to (2) to relate successional patterns to environmental variability. Based on their fire history 23 plots were selected in the Parque Ecológico Chipinque (PECH). Changes in forest structure across the chronosequence of burned stands were deduced from density, height and diameter measurements of trees and shrubs (>5 cm in diameter) in all plots of 1000 m². Differences in woody plant species composition among the plots were evaluated using Shannon evenness measure and the Whittaker's measure and by Hierarchical cluster analysis and Detrended Correspondence Analysis. Hierarchical cluster analysis showed a high similarity among all recently burned plots, independed of the aspect. Multivariate analysis showed that local environmental factors, including time since fire, continue to structure species composition. Oak species (mainly Quercus rysophylla) resprouted successfully after fire and dominated young post-fire stands. Pine species (Pinus pseudotrobus and Pinus teocote) only appeared 18 years after fire and were the dominating species in mature stands (62 years after fire). In contrast, woody plant species composition in older stands tended also to be influenced by factors such as aspect and by the potential solar radiation (PSR) during the growing season. The results demonstrate that in the PECH, natural regeneration is sufficient and woody plant species composition will be similar to pre-fire conditions after 60 years of succession. We conclude that the park managers should consider incorporating natural disturbance regimes into their management practices.     

Response of vegetation and birds to severe wind disturbance and salvage logging in a southern boreal forest

Vegetation and birds were inventoried on the same plot before and after a severe windstorm in 1999 disturbed a mature black spruce (Picea mariana)–jack pine (Pinus banksiana) forest in northern Minnesota. Following the storm, another plot was established in an adjacent portion of the forest that was salvage-logged. Birds were inventoried on both plots through 2002. The original unsalvaged plot was prescribed-burned in 2004, but vegetation was surveyed through 2003, and through 2005 on the salvaged plot. We examined the effects of wind disturbance by comparing the pre-storm bird and vegetation communities with those developing afterwards through 2002 and 2003, respectively, and the effects of salvage logging by comparing vegetation and the bird community on the unsalvaged plot with those in the salvaged area. Wind reduced the canopy of the forest by over 90% with a temporary increase in the shrub layer, mostly resulting from tip-ups. Several plant species, including jack pine and beaked hazel (Corylus americana), appeared temporarily in the ground layer (<1 m height), but did not persist through 2003. Quaking aspen (Populus tremuloides) root sprouts were abundant in 2001, but decreased dramatically by 2003. Delayed mortality of tipped trees resulted in reduction of the shrub layer to pre-storm levels, and release of advanced regeneration black spruce and balsam fir (Abies balsamea). Bird species using the forest changed from dominance by canopy-foraging species to ground-brush foraging species, with an overall increase in bird diversity. Salvage logging resulted in significant reduction in coarse woody debris, and successful recruitment of jack pine seedlings. Quaking aspen sprouts were nearly 30 times more abundant in the salvage-logged area compared to the unsalvaged control. Ruderal species, especially red raspberry (Rubus ideaus), fringed bindweed (Polygonum cilinode), and several sedges (Carex spp.), were significantly more abundant after salvage logging. The bird community, on the other hand, was greatly diminished by salvage logging, with a reduction in diversity, density, and overall richness of species.     

Short-term effects of fire on soil properties inPinus densiflora stands

This study was performed to investigate a short time change (one week after fire) on soil properties due to the fire inPinus densiflora Sieb. et Zucc stands of the Kosung area in Kangwon Province in Korea. Twenty seven sampling plots [16 burned (8 low intensity fire, 8 high intensity fire) and 11 unburned plots] were chosen. Mineral soil samples from three depths (0–5, 5–15, and 15–25 cm) under the forest floor were collected. Forest fire in the area affected soil chemical properties. Soil pH, total nitrogen, available phosphorus, potassium, calcium, and magnesium in the surface soil (0–5 cm) of the burned area compared with the unburned area increased, but there was no marked change in the subsurface soil (5–25 cm). Organic matter, total nitrogen, available phosphorus, and exchangeable cations in the surface soil were generally lower in the high than in the low intensity fire areas. This indicates that these nutrients on the high intensity fire may be volatilized. The results suggest that change in soil chemical properties in the area was restricted mainly to the surface soil and was different between the high and the low intensity fire types.     

The colonization of decaying logs by vascular plants and the consequences of fallen logs for herb layer diversity in a lowland alluvial forest

The presence of coarse woody debris in forests is an important naturalness criterion as well as a biodiversity indicator. Decaying logs provide habitat for many organisms including vascular plants. The aims of this study are to describe the colonization of decaying logs by vascular plants in a lowland alluvial forest and to assess the impact of fallen logs on the biodiversity of the herb layer. We investigated 493 logs in the Ranšpurk National Nature Reserve in southern Moravia. For each log, we recorded species of vascular plants rooting on its surface. Decaying logs were characterized by their dimensions, woody species and decay class. Vascular plants rooted on 279 surveyed logs. Decaying logs were colonized by 79 species of vascular plants. Terrain relief was found to be a significant factor for colonizing species. Our study did not find any significant spatial pattern of colonized logs with regard to the distribution of vascular plants, intensity of colonization or the ecological demands of colonizers. The colonization of lying logs by vascular plants was significantly influenced by the species of logs, their surface area and decay class. Species composition of vascular plants on decaying logs and that on mineral soil were significantly different. Demonstrably higher frequencies of Dryopteris carthusiana and Impatiens parviflora were found on decaying logs. The presence of decaying logs and the microsites they create is important for total forest biodiversity including the diversity of vascular plants. We recommend increasing the proportion of decaying logs even in commercial forests.

     

Loss of carbon sequestration potential after several decades of shifting cultivation in the Southern Yucatán

Cumulative losses from shifting cultivation in the tropics can affect the local to regional to global balance of carbon and nutrient cycles. We determined whether shifting cultivation in the Southern Yucatán causes feedbacks that limit future forest productivity and carbon sequestration potential. Specifically, we tested how the recovery of carbon stocks changes with each additional cultivation-fallow cycle. Live aboveground biomass, coarse woody debris, fine woody debris, forest floor litter and soil were sampled in 53 sites (39 secondary forests 2–25 years old, with one to four cultivation-fallow cycles, and 14 mature forests) along a precipitation gradient in Campeche and Quintana Roo, Mexico. From the first to the third or fourth cultivation-fallow cycle, mean carbon stocks in live aboveground biomass debris declined 64%. From the first to the third cycle, coarse woody debris declined by 85%. Despite declining inputs to soil with each cultivation-fallow cycle, soil carbon stocks did not further decline after the initial conversion from mature to secondary forest. The combined aboveground and soil carbon stock declined almost 36% after conversion from mature forest, however two additional cultivation cycles did not promote further significant decline, largely because of the stability of the soil carbon pool. Although age was the dominant factor in predicting total carbon stocks of secondary forests under shifting cultivation, the number of cultivation-fallow cycles should not be neglected. Understanding change beyond the first cycle of deforestation will enhance forest management at a local scale by improving predictions of secondary forest productivity and related agricultural productivity. A multi-cycle approach to deforestation is critical for regional and national evaluation of forest-based carbon sequestration. Finally, models of the global carbon cycle can be better constrained with more accurate quantification of carbon fluxes from land-use change.     

Effect of fire disturbance on active organic carbon of <Emphasis Type="Italic">Larix gmelinii</Emphasis> forest soil in Northeastern China

Active organic carbon in soil has high biological activity and plays an important role in forest soil ecosystem structure and function. Fire is an important disturbance factor in many forest ecosystems and occurs frequently over forested soils. However, little is known about its impact on soil active organic carbon (SAOC), which is important to the global carbon cycle. To investigate this issue, we studied the active organic carbon in soils in the Larix gmelinii forests of the Da Xing’an Mountains (Greater Xing’an Mountains) in Northeastern China, which had been burned by high-intensity wildfire in two different years (2002 and 2008). Soil samples were collected monthly during the 2011 growing season from over 12 sample plots in burned and unburned soils and then analyzed to examine the dynamics of SAOC. Our results showed that active organic carbon content changed greatly after fire disturbance in relation to the amount of time elapsed since the fire. There were significant differences in microbial biomass carbon, dissolved organic carbon, light fraction organic carbon, particulate organic carbon between burned and unburned sample plots in 2002 and 2008 (p < 0.05). The correlations between active organic carbon and environmental factors such as water content, pH value and temperature of soils, and correlations between each carbon component changed after fire disturbance, also in relation to time since the fire. The seasonal dynamics of SAOC in all of the sample plots changed after fire disturbance; peak values appeared during the growing season. In plots burned in 2002 and 2008, the magnitude and occurrence time of peak values differed. Our findings provide basic data regarding the impact of fire disturbance on boreal forest soil-carbon cycling, carbon-balance mechanisms, and carbon contributions of forest ecosystem after wildfire disturbance.     

Testing woody fuel consumption models for application in Australian southern eucalypt forest fires

Five models for the consumption of coarse woody debris or woody fuels with a diameter larger than 0.6 cm were assessed for application in Australian southern eucalypt forest fires including: CONSUME models for (1) activity fuels, (2) natural western woody and (3) natural southern woody fuels, (4) the BURNUP model and (5) the recommendation by the Australian National Carbon Accounting System which assumes 50% woody fuel consumption. These models were assessed using field data collected as part of the woody fuel consumption project (WFCP) in south-west Western Australia and northern-central Victoria. Three additional datasets were also sourced to increase variability in forest type, fuel complex and fire characteristics. These datasets comprised data from south-west Western Australia collected as part of Project Aquarius, the Warra Long Term Ecological Research site in Tasmania and Tumbarumba in south-eastern New South Wales. Combined the dataset represents a range of fire behaviour characteristic of prescribed burning conditions with a maximum fireline intensity of almost 4000 kW m⁻¹.     

Densification, stand-replacement wildfire, and extirpation of mixed conifer forest in Cuyamaca Rancho State Park, southern California

A century of fire suppression culminated in wildfire on 28 October 2003 that stand-replaced nearly an entire 4000 ha “sky island” of mixed conifer forest (MCF) on Cuyamaca Mountain in the Peninsular Range of southern California. We studied the fire affected Cuyamaca Rancho State Park (CRSP), which represents a microcosm of the MCF covering approximately 5.5 × 10⁶ ha (14%) of California, to evaluate how fire suppression unintentionally destabilizes this ecosystem. We document significant changes in forest composition, tree density, and stem diameter class distribution over a 75-year period at CRSP by replicating ground-based measurements sampled in 1932 for the Weislander Vegetation Type Map (VTM) survey. Average conifer density more than doubled, from 271 ± 82 trees ha⁻¹ (standard error) to 716 ± 79 ha⁻¹. Repeat aerial photographs for 1928 and 1995 also show significant increase in canopy cover from 47 ± 2% to 89 ± 1%. Changes comprise mostly ingrowth of shade-tolerant Calocedrus decurrens [Torr.] Floren. in the smallest stem diameter class (10–29.9 cm dbh). The 1932 density of overstory conifer trees (>60 cm dbh) and 1928 canopy cover at CRSP were similar to modern MCF in the Sierra San Pedro Mártir (SSPM), 200 km S in Baja California, Mexico, where fire suppression had not been practiced, verifying that the historical data from the early twentieth century represent a valid “baseline” for evaluating changes in forest structure. Forest successions after modern crown fires in southern California demonstrate that MCF is replaced by oak woodlands and shrubs. Post-fire regeneration in severely burned stands at CRSP includes abundant basal sprouting of Quercus chrysolepis Liebm. and Quercus kelloggii Newb., but only few seedlings of Abies concolor [Gord. and Glend.] Lindl (average 16 ± 14 ha⁻¹), while whole stands of C. decurrens, Pinus lambertiana Dougl., and Pinus ponderosa Laws. were extirpated. Prescribed burning failed to mitigate the crown fire hazard in MCF at CRSP because the low-intensity surface fires were small relative to the overall forest area, and did not thin the dense understory of sapling and pole-size trees. We propose that larger, more intense prescribed understory burns are needed to conserve California's MCF.     

喜马拉雅山西部Askot野生动物保护区内粗木质残体沿山高度分布格局与现状

在印度Askot野生动物保护区Goriganga流域的3个分水岭(Charigad,Dogarhigad和upper Gosigad)内,沿着海拔高度900～2600m设置5条曲线调查样带(样带A、B、C、D和E),调查了粗木质残体的分布格局和状况.海拔高度每升高100 m设置一块1 hm2的样地.结果表明,不同演替阶段的粗木质残体百分比贡献率按降低顺序排列依次是:枯立木--相位Ⅰ>相位Ⅱ>相位Ⅳ>相位Ⅲ;而原木-相位Ⅲ>相位Ⅱ>相位Ⅳ.调查样带A内喜马拉雅长叶松(Pinus roxburghii)林内枯立木密度在1500m处较高,调查样带B通麦栎(Quercus lanata)林的枯立木密度在2300米处较高(10个/hm2).喜马拉雅长叶松林的枯立木和原木总获得量为13.9 t,其中枯立木和原木分别占41%和59%;而通麦栎林枯立木和原木总量仅为5.6 t,枯立木和原木分别占60%和40%.此外,粗木质残体的存在,有利于为当地生长的兰花营造良好的生长环境.在喜马拉雅长叶松林中等高度区域内,高密度的枯立木和原木导致该区内物种丰富度较低,地被物密度也较低.这主要是由于该区光线充足、土壤水分含量低,只有优势种才能占领这样生境.     

Coarse woody debris dynamics in a post-fire jack pine chronosequence and its relation with site productivity

The long-term relationships between coarse woody debris (CWD) dynamics, soil characteristics and site productivity have, so far, received little attention. The objectives of the study were to describe CWD dynamics along a post-fire chronosequence (43–86 years after fire) in jack pine (Pinus banksiana Lamb.) stands, assess the importance of buried CWD in terms of soil available water holding capacity (AWHC), and investigate relationships between CWD, AWHC, nutrient retention and site productivity.

Twelve jack pine stands on sandy, mesic sites of glaciolacustrine origin were surveyed. Buried wood volume within the forest floor varied between 1 and 57 m³ ha⁻¹ (4–92% of total site CWD volume) and showed no relationship with time. Downed log mass accumulation followed a “U shaped” successional pattern with time since fire. Buried wood AWHC was negligible compared with that of the 0–20 cm mineral soil layer. The most productive sites were characterised by higher forest floor dry weight, effective CEC and water holding capacity in the mineral soil. Path analyses of relationships between organic matter content, CWD and forest floor CEC showed that CEC was conditioned by forest floor organic matter and buried wood content.     

Biofuel harvests, coarse woody debris, and biodiversity - A meta-analysis

Forest harvest operations often produce large amounts of harvest residue which typically becomes fine (foliage, small limbs and trees) and coarse woody debris (snags and downed logs). If removed at harvest, residual biomass has potential to be a local energy source and to produce marketable biofuel feedstock. But, CWD in particular serves critical life-history functions (e.g., breeding, foraging, basking) for a variety of organisms. Unfortunately, little is known about how forest biodiversity would respond to large scale removal of harvest residues. We calculated 745 biodiversity effect sizes from 26 studies involving manipulations of CWD (i.e., removed or added downed woody debris and/or snags). Diversity and abundance of both cavity- and open-nesting birds were substantially and consistently lower in treatments with lower amounts of downed CWD and/or standing snags, as was biomass of invertebrates. However, cumulative effect sizes for other taxa were not as large, were based on fewer studies, and varied among manipulation types. Little is currently known about biodiversity response to harvest of fine woody debris. Predicting the effects of biomass harvests on forest biodiversity is uncertain at best until more is known about how operational harvests actually change fine and coarse woody debris levels over long time periods. Pilot biomass harvests report post-harvest changes in CWD levels much smaller than the experimental changes involved in the studies we analyzed. Thus, operational biomass harvests may not change CWD levels enough to appreciably influence forest biodiversity, especially when following biomass harvest guidelines that require leaving a portion of harvest residues. Multi-scale studies can help reduce this uncertainty by investigating how biodiversity responses scale from the small scale of manipulative experiments (i.e., 10-ha plots) to operational forest management and how biodiversity response to CWD levels might vary at different spatial and temporal scales and in different landscape contexts.     

A quantitative framework for evaluating the mass balance of in-stream organic debris

A quantitative framework is developed for analyzing the mass budget of in-stream woody debris. Wood budgets are necessary for defining the relative importance of different recruitment processes over short and long periods, for designing spatially explicit simulation models, and for estimating the range of variability. The framework is used to analyze century-long patterns of large woody debris in streams that are governed by episodic forest death (fire and wind), forest growth and chronic mortality, bank erosion, mass wasting, decay, and stream transport. Simplified mathematical expressions are used to represent climatic, hydrologic, geomorphic, and biotic processes. Results are expressed in terms of time series and probability distributions. Predictions include that in areas of longer fire rotation (500 years) toppling of fire-killed trees comprises only 15% of the long-term wood budget yet chronic stand mortality that affect the large standing forest biomass ensures continuous large volumes of wood in streams. In contrast, toppling of fire-killed trees in forest environments with shorter fire rotations (150 years) comprise about 50% of the wood budget and indicates that field observers have a significantly higher chance of encountering low wood volumes in streams. Wood recruitment by bank erosion should increase irregularly downstream and bank erosion recruitment should exceed mortality recruitment at a bank erosion rate of approximately 5 cm per year. Recruitment from debris flows represents the single largest point source of woody debris to streams. The rarity of debris flows, in conjunction with a 3% per year annual decay rate, limits the contribution of wood from debris flows to about 12% of the long-term wood budget. Fluvial transport of wood promotes an increase in both inter-jam spacing and jam volume downstream. The proportion of woody debris transported into a reach in comparison to lateral recruitment approaches an asymptotic maximum of 50% when tree height approaches channel width. The relationships among process rates, their spatial variance across landscapes, and the resulting probability distributions of long-term patterns of wood abundance are proposed as a set of general theoretical principles. New data on wood supply and storage at the network scale are needed to fully test the predictions made in this analysis.     

Soil and microbial respiration in a loblolly pine plantation in response to seven years of irrigation and fertilization

Because soil CO₂ efflux or soil respiration (R_S) is the major component of forest carbon fluxes, the effects of forest management on R_S and microbial biomass carbon (C), microbial respiration (R_H), microbial activity and fine root biomass were studied over two years in a loblolly pine (Pinus taeda L.) plantation located near Aiken, SC. Stands were six-years-old at the beginning of the study and were subjected to irrigation (no irrigation versus irrigation) and fertilization (no fertilization versus fertilization) treatments since planting. Soil respiration ranged from 2 to 6 μmol m⁻² s⁻¹ and was strongly and linearly related to soil temperature. Soil moisture and C inputs to the soil (coarse woody debris and litter mass) which may influence R_H were significantly but only weakly related to R_S. No interaction effects between irrigation and fertilization were observed for R_S and microbial variables. Irrigation increased R_S, fine root mass and microbial biomass C. In contrast, fertilization increased R_H, microbial biomass C and microbial activity but reduced fine root biomass and had no influence on R_S. Predicted annual soil C efflux ranged from 8.8 to 10.7 Mg C ha⁻¹ year⁻¹ and was lower than net primary productivity (NPP) in all stands except the non-fertilized treatment. The influence of forest management on R_S was small or insignificant relative to biomass accumulation suggesting that NPP controls the transition between a carbon source and sink in rapidly growing pine systems.