Forest Floor Moisture Content Research Articles

Scientific support to prescribed underburning in southern Europe: What do we know?

Prescribed burning is a technically demanding and usually highly scrutinized and debated practice. Barriers of various natures have constrained the development of prescribed burning in forests (PUB) in southern Europe, with insufficient research and outreach among the contributing factors. This paper synthesizes PUB knowledge in the region and identifies research needs. PUB research in the western Mediterranean basin was fostered by international cooperative projects that studied the ecological and management ramifications of low-intensity burning for fire hazard mitigation. Effects of PUB on soil and vegetation are minor and short-lived and regulated through forest floor moisture content, fire intensity, tree resistance to fire, and ignition patterns. Generic burn prescriptions are available and specific burn windows targeting site-specific burn objectives can be developed with the existing software tools. However, the need to increase the depth and breadth of PUB research is apparent. Current knowledge is based upon pine forests, particularly Pinus pinaster, as past research has overlooked hardwoods; was obtained across a limited number of research teams and study sites; and essentially reflects short-term treatments. Fuel consumption by PUB effectively decreases fire potential, but post-treatment fuel dynamics and effects on wildfire spread and severity warrant further study. Future work should devote more attention to the socioeconomic, biodiversity and carbon storage implications of PUB and should expand to encompass cumulative effects and the whole PUB regime and its variation; long-term experiments and monitored management programs are crucial to this end.

Temporal dynamics of carbon and nitrogen in the surface soil and forest floor under different prescribed burning regimes

Prescribed burning has been widely used in the management of forests for reducing wildfire risk, and can have significant effects on soil carbon (C) and nitrogen (N) pools and their temporal changes. This study aimed to investigate the impacts of different burning frequency regimes on the temporal dynamics of C and N in the topsoil and forest floors. The experimental site was a 39year old wet scherophyll forest prescribed burning trial at Peachester, southeast Queensland, Australia, with treatments of no burning (NB) since 1969, 2 yearly burning (2yrB) and 4 yearly burning (4yrB) since 1972. Each of three burning treatments had four replications and these plots were randomly distributed over an area of the forest with similar vegetation and soil characteristics. Soil (0–10cm) and forest floors were sampled monthly for 6months prior to the next scheduled burning for both burning treatments to minimize the effects of fire recency. Prescribed burning significantly (P<0.01) affected most C and N variables in both soils and forest floors. The 2yrB treatment had significantly lower (P<0.01) soil total C, total C:N ratio, microbial biomass C (MBC) and N (MBN), MBC:MBN ratio, dissolved organic C (DOC) and N (DON), NO3−-N, inorganic N and L layer total N, DON and NO3−-N, compared with the NB and 4yrB treatments. However, there were no overall significant differences in these variables between the NB and 4yrB treatments. Sampling month significantly (P<0.01) affected C and N variable in both soils and forest floors except for soil total C and N and F layer MBC. Temporal dynamics of most of these labile C and N variables were highly related to soil and forest floor moisture content, seven day mean air temperature (MAT) and cumulative rainfall prior to sampling date. However, fire effects were independent of sampling month, as there were no significant interactions between them for most response variables measured. This study has clearly demonstrated that more frequent burning (2yr burning) had negative impact on soil and forest floor C and nutrient pools. Insignificant differences observed in soil C and N and forest floor N pools between less frequent burning (4yrB) and NB treatments highlighted that the prescribed burning at four year interval gave sufficient time for recovery of these soil and forest floor nutrients to pre-burn levels.

Changes in soil chemical and biological properties after thinning and prescribed fire for ecosystem restoration in a Rocky Mountain Douglas-fir forest

Practices such as thinning followed by prescribed burning, often termed ‘ecosystem restoration practices’, are being used in Rocky Mountain forests to prevent uncontrolled wildfire and restore forests to pre-settlement conditions. Prior to burning, surface fuels may be left or collected into piles, which may affect fire temperatures and attendant effects on the underlying soil. The objective of this study is to determine which pre-fire fuel management treatments best reduce fuel loadings without causing fire temperatures high enough to impair soil chemical and biological properties. Five fuel-management treatments were compared: large piles, small piles, cut and leave, slash-free areas around mature leave-trees, and unburned control. We measured key properties of forest floors and mineral soil (forest floor depth, soil pH, carbon and nutrient levels, and microbial abundances) prior to and during the first year after fire, and explored relationships among fuel loadings, fire temperatures and changes in these soil properties. Fire temperatures were above 300°C for more than 3h in the large-pile treatment but were lower and of shorter duration in the small-pile and cut-and-leave treatments. The most severe fire effects occurred around the leave-trees where temperatures were above 200°C for more than 2h, the forest floor was completely consumed, and the mature trees were killed. In the forest floors, abundances of all microbial groups were reduced and pH and availabilities of Ca2+, Mg2+ and PO43- were increased in all burned treatments. Forest floor C and N contents were reduced in burned plots by an average of 39% and 44% respectively, and availabilities of nitrate and sulphate were increased in the leave-tree areas only. There were few significant changes in mineral soil properties – pH and availabilities of NO3-, Mg2+ and SO42- increased in leave-tree areas whilst PO43- and K+ increased under large piles. Microbial abundances had not recovered to pre-fire levels in any burned treatments after one year, which may be attributed to the persistence of significant increases in pH. Prior to the fire, microbial abundances were most closely related to N concentration in the forest floor, and C and N concentrations in the mineral soil; after fire, microbial abundances were most closely related to pH of the forest floor. Forest floor consumption and attendant changes in chemical and biological properties were most closely related to pre-fire moisture content, indicating that forest-floor moisture content may be as critical as fuel loading in determining impacts of prescribed fire on soil.

Effects of tree canopy on litter decomposition rates of Abies nordmanniana, Picea orientalis and Pinus sylvestris

Effects of tree canopy (distance from the stem) on litter decay rates were investigated in the field for 4 years. Needle litter of Nordmann fir [Abies nordmanniana (Stev.) Spach.], Oriental spruce [Picea orientalis (L.) Link] and Scots pine (Pinus sylvestris L.) species were placed at three distances from their stems (<1 m, 1–2 m and >2 m). The litter showed highest decomposition rates at the distance of >2 m and lowest rates at <1 m for all species. Initial lignin concentrations were most strongly correlated with the decay rates, but among the microclimatic and soil factors, forest floor moisture content was the best predictor. The results showed that litter quality variables define the potential rates of microbial decomposition rates within and between trees, but adverse environmental conditions created by a tree canopy can also significantly affect decomposition rates. The closer distance to the stem significantly reduces litter decay rates by changing environmental conditions (especially forest floor moisture content) which can reduce microbial activity, and thus, the effects of tree canopy on litter decomposition rates should be taken into an account when calculating or comparing litter decomposition dynamics of forest ecosystems.

The microbial communities of aspen and spruce forest floors are resistant to changes in litter inputs and microclimate

Previous studies have shown that forest floors from stands dominated by trembling aspen (ASPEN; Populus tremuloides Michx.) tend to support a greater microbial biomass with a different microbial community structure than forest floors from stands dominated by white spruce (SPRUCE; Picea glauca (Moench) Voss). A reciprocal transfer experiment, in concert with coarse and fine mesh bags that allowed or excluded fine root in-growth, was used to examine how the composition of these forest floor microbial communities respond to changes in belowground inputs from fine roots, aboveground inputs (e.g. from litter and through-fall) and soil microclimatic conditions over 1 year. Neither the microbial biomass nor the microbial community structure (assessed using phospholipid fatty acid analyses and substrate-induced respiration techniques) of forest floors of ASPEN or SPRUCE origin were altered by reciprocal transfer to SPRUCE or ASPEN stands, with or without fine root inputs. Despite the lack of changes in microbial community structure, the stand type during incubation had a strong effect on forest floor moisture content and concentrations of nitrate, while mesh size had a significant effect on forest floor pH and the abundance of mesofauna. Thus, changes in microbial community structure did not co-occur with changes in other characteristics of these forest floors. The resistance of the forest floor microbial communities to change may be a function of the high C contents of these soils. Further treatment effects may have been detected if the study had been extended beyond 1 year. Reciprocal transfer studies using coarse and fine mesh bags allow transferred soils to respond to fluctuations in microclimate, organic inputs and soil biota and, therefore, hold considerable promise for studies examining the influence of disturbances on soil properties.

Inferred effects of cloud deposition on forest floor nutrient cycling and microbial properties along a short elevation gradient

Cloud water deposition often increases with elevation, and it is widely accepted that this cloud water increases acid loading to upland forest ecosystems. A study was undertaken in south-eastern Québec to determine if a 250 m elevation gradient (i.e. 420–665 m), along a uniform sugar-maple stand on the slope of Mount Orford, corresponded to a pH gradient in the forest floor and to predictable changes in soil nutrient availability and microbial properties. Precipitation data from a nearby study, and a photographic survey, provided presumptive evidence that this elevation gradient corresponded to a strong gradient in cloud water deposition. Forest floor temperature did not differ significantly across elevations. Forest floor moisture content was significantly higher, whereas pH and exchangeable Ca and Mg were significantly lower, at the higher elevations. Average seasonal net nitrification rates, determined by long-term laboratory incubations, did not differ significantly across elevations, whereas average seasonal net ammonification rates were significantly higher at higher elevations. Basal respiration rates and microbial biomass did not differ significantly across elevations, but metabolic quotient was significantly higher at higher elevations indicating possible environmental stress on forest floor microbial communities due to cloud water deposition. Anaerobic N mineralisation rates were significantly higher at higher elevations suggesting that N-limited microbial communities frequently exposed to cloud cover can be important short-term sinks for atmospheric N, thereby contributing to increase the active-N fraction of forest floors. We conclude that, where no significant changes in vegetation or temperature occur, elevation gradients can still be used to understand the spatial variability of nutrient cycles and microbial properties.

Forest floor microbial biomass across a northern hardwood successional sequence

Microbial dynamics partially control forest productivity, yet are poorly understood in the northern hardwood forest. Following forest cutting, microbial biomass may reflect changes in the forest floor environment. Using direct counts, forest floor microbial biomass was studied in a sequence of northern hardwood forest stands ranging in age since clearcutting from 3 to more than 120 yr. The organic horizon was collected 3 times in each of 1994 and 1995, in June, August and October. Samples were analyzed for active fungi, active bacteria, pH, forest floor moisture content and organic fraction. Forest floor temperature was measured in the field with each collection; forest floor mass was measured once in each stand. In 1995, forest floor samples were also analyzed for total N, nitrate and ammonium. Amounts of active microbial biomass (fungi plus bacteria) ranged from 38 to 103 μg g −1 organic matter; active microbial biomass per unit area ranged from 40 to 800 mg m −2 forest floor organic matter. Microbial biomass varied significantly among sampling periods in 1994, but not in 1995, and was greater in early- and late-successional stands than in mid-successional stands. Microbial biomass was not very responsive to the environmental factors measured in this study. Moisture content was the environmental variable that most often contributed to variation in microbial biomass, and together with additional factors explained only 23% of the variation in 1994, and only 27% in 1995. Lower microbial biomass in mid-successional stands suggests that microbial dynamics in this forest are not controlled by factors directly related to forest harvesting.

Nutrient management in a Pinusradiata plantation after thinning: the effect of thinning and residues on nutrient distribution, mineral nitrogen fluxes, and extractable phosphorus

The effects of thinning and residues on nutrient distribution, N fluxes, and extractable P were studied for 3 years after the first thinning of a 10-year-old Pinusradiata D. Don plantation. Treatments were (i) unthinned, (ii) thinned to 52% of basal area with all residues removed (zero residue; ZR), (iii) thinned with all residues retained and spread uniformly (normal residue; NR), and (iv) thinned with all residues retained plus the addition of those removed from the ZR treatment, approximately doubling the amount of residue in treatment NR (high residue; HR). The total nutrient contents of trees (above ground), residues, forest floor, and the top 0.30 m of mineral soil were 1206 kg N•ha−1, 126 kg P•ha−1, 828 kg K•ha−1, and 1272 kg Ca•ha−1 after thinning. Residues contained 11, 10, 10, and 7% of site N, P, K, and Ca, respectively. Thinning without residue retention (unthinned cf. ZR) elevated mean monthly maximum soil temperatures at a depth of 50 mm by up to 5.3 °C in summer and 1 °C in winter. Thinning had no effect on forest floor or mineral soil (0–0.30 m) moisture content. In the unthinned treatment an average of 25 kg N•ha−1•year−1 was mineralized, all of which was taken up by the trees, and there was no leaching. Thinning caused a 10% increase in rates of N mineralization and uptake and a small transient increase in soil mineral N concentrations, but had no effect on leaching. Maintenance of prethinning levels of N uptake by the remaining trees (after a 48% reduction in basal area) suggests that uptake per tree was approximately doubled. Maintenance of N uptake after thinning was consistent with maintenance of basal area growth (m2•ha−1) and an increase in foliar N concentrations. Thinning had no effect on extractable P in the forest floor or mineral soil. The retention of residues (NR and HR) decreased mean monthly maximum and elevated mean monthly minimum soil temperatures relative to the ZR treatment, and the effect increased with the amount of residue. Forest floor moisture content increased with the amount of residue, but residue retention had no effect on mineral soil moisture. Rates of mineralization were higher in the presence of residue and annual averages were 28, 33, and 42 kg N•ha−1•year−1 for ZR, NR, and HR treatments, respectively. Despite increased N mineralization in the presence of residue there was no leaching and all the additional N was taken up by the trees. Basal area increment increased in the presence of residue (P = 0.235) and was 10.7 (ZR), 11.1 (NR), and 11.8 (HR) m2•ha−1 over 4 years of measurement.

Conversion of dense lodgepole pine stands in west-central British Columbia into young lodgepole pine plantations using prescribed fire. 1. Biomass consumption during burning treatments

The ecological effects of different treatments used to convert dense Pinuscontorta var. latifolia Engelm. stands into young P. contorta plantations are determined. The treatments used were felling the trees with a bulldozer and either broadcast burning the slash or bulldozing the slash into windrows, which were then burned. Burns were conducted under different fuel moisture conditions and state of fuel curing to achieve four classes of fire severity. The preburn surface fuel load was relatively high due largely to fallen dead woody materials (10−21 kg/m2). The biomass of the forest floor (5−10 kg/m2) was similar to that of the tree slash (5−13 kg/m2), while the understory vegetation was a minor component (0.3−0.5 kg/m2). The quantity of slash and understory vegetation consumed by burning increased with the preburn mass of the same components. Forest floor consumption depended primarily on the preburn forest floor mass for windrow burns and on forest floor moisture content as well as preburn forest floor mass for broadcast burns. Fire severity generally did not have a strong influence on biomass consumption, although it did significantly influence forest floor consumption. There was a general trend, however, of increasing biomass consumption in broadcast burns with increasing fire severity. Windrow burning consumed more biomass than did broadcast burning under similar weather and fuel moisture conditions. Windrow burning resulted in uniformly high biomass consumption that was relatively independent of fuel moisture over the wide range of fuel moistures studied.

Prediction of forest-floor moisture content on jack pine cutovers

Moisture contents of organic forest-floor materials were studied by strata on a clear-cut jack pine (Pinusbanksiana Lamb.) site in relation to slash distribution and changes in both duff moisture code and fine fuel moisture code, the two weather-based components of the Canadian Forest Fire Weather Index System. The best-fitting curvilinear regression (Y = a + b/X) of the duff moisture code showed distinctive patterns of variation, so that both the surface and subsurface forest-floor strata were consistently moister under slash cover than in slash openings, and for each of these two slash conditions, the subsurface forest-floor strata were consistently moister than the surface strata. Combinations of these materials down to mineral soil showed intermediate moisture contents both in slash openings and under slash cover. The straight line regression (Y = a + bX) of the fine fuel moisture code was the best-fitting moisture content relationship for the surface forest-floor materials, but only in slash openings; under slash cover, the curvilinear regression (Y = a + b/X) of the duff moisture code was more suitable. The coefficients of correlation, r, as derived from these varied regressions, were all very highly significant at P &lt; 0.001.

Selected Parameters of Fire Behavior and Pinus banksiana Lamb. Regeneration in Eastern Ontario

Fire behavior variables were quantified in eastern Ontario jack pine (Pinus banksiana Lamb.) ecosystems and used to interpret observed fire impacts and effects. A series of seven fires, ranging in frontal fire intensity from 70 to 17 000 W/m, were documented. Forest floor moisture content prior to burning was negatively correlated with weight of forest floor consumed per unit area (r2 = 0.97) and per cent mineral soil bared (r2 = 0.95). Frontal fire intensity was positively correlated with per cent tree mortality (r2 = 0.98) and mean height of char (r2 = 0.76). Frontal fire intensities of 17 000 kW/m resulted in seedling numbers of 30 000 to over 50 000 ha−1 considered to be more than adequate for establishing the next generation of crop trees. Jack pine mean seedling height, 13 to 16 years after fire, was also positively correlated with frontal fire intensity (r2 = 0.82), ranging from 0.5 to 3.8 m on lowest and highest intensity burns, respectively. Similar relationships were found when seedling height was regressed against per cent tree mortality (r2 = 0.62) and forest floor consumption (r2 = 0.79).Results are discussed in terms of ecological requirements of the species, particularly during the regeneration phase, and it is concluded that quantification of fire behavior observations is mandatory if burning conditions are to be understood and/or duplicated by the land manager for the attainment of a given forest management objective.

FOREST FLOOR VARIABILITY AND FACTOR ANALYSIS: A CASE STUDY

A case study is presented to illustrate how factor analysis can be used to quantify probable causes of physical and chemical forest-floor variability. Twenty forest-floor variables were examined for a 30 × 30-m sampling plot located within a mature, naturally regenerated conifer stand. "Slough" (the degree of accumulation of decayed stumps, logs, branches, etc.), litter (hardwood- vs. softwood-type in terms of base-status) and insolation (degree of canopy closure) factors appeared to underlie the observed variability pattern. Factor identification was facilitated by the following variable association pattern: (i) total C with total Fe, Al, N, P, K; (ii) total C with LFH-mass and thickness; (iii) water-extractable P with 2 N potassium-chloride-extractable NH4-H; (iv) total and 1 N ammonium-acetate-extractable Ca with pH; and (v) ground cover vegetation density with forest floor moisture content and thickness. Key words: Forest floor, lateral variability, factor analysis