Years Of Afforestation Research Articles

Shifts in soil organic carbon and nitrogen dynamics for afforestation in central China

The afforestation has been proposed as a primary means of sequestering carbon (C) from the atmosphere, thereby mitigating climate change. However, consequences of afforestation on soil organic C and nitrogen (N) dynamics due to spatial heterogeneity are not fully understood. The objectives of this study were to identify the consequences of 18 years of afforestation on C and N dynamics in the Danjiangkou Reservoir area of central China. Soil samples from the woodland, shrubland, cropland and adjacent open area soils (i.e. the control) were separated into four aggregate sizes (>2000μm, 250–2000μm, 53–250μm and <53μm), and three density fractions [free light fraction (LF), intra-aggregate particulate organic matter (iPOM) and mineral-associated organic matter (mSOM)]. All fractions were analyzed for their C and N content, and δ13C and δ15N values. Afforestation enhanced the soil C and N storage, primarily due to increases in soil C and N storage of macroaggregates (>2000μm) with the largest (65–87%) fraction in iPOM. The C: N ratios generally increased from the cropland to shrubland to woodland across all fractions. The 13C values indicated that the fastest decay rates (k=0.024yr−1) in LF of macroaggregates (>2000μm) was in woodland, but the fastest decay rates (0.114–0.137yr−1) in iPOM of all aggregates were observed in cropland. Meanwhile, the most enriched δ15N values (0.87–3.81) were found in cropland soil. Our results suggest that afforestation could alter weight distribution of soil fractions and increase the soil organic C and N storage, thereby affecting the turnover and stability of SOM.

Evaluación de la restauración de la diversidad fúngica en un área reforestada con Polylepis australis (Rosaceae): un estudio de caso

Polylepis australis forest had been significantly disturbed and afforestation is an alternative to restore these ecosystems. The aim of this study is to evaluate the restoration of the fungal community in P. australis afforestation. Our hypothesis is that the fungal community varies between forest situations: high degraded, mature forest, and restored forest. Fungal variables (richness, abundance and evenness of macromicetes) were compared between a native area, an afforestation and a degraded land. To this end, 15 plots of 5x5 m were established and five of them were sampled in each season. Macromicetes fruiting bodies were collected and classified into different morpho-taxonomical groups. Principal Components Analyses and correlation analyses between the fungal variables and the plot’s structure were performed as well as correlation analysis between the fungal variables and soil chemical variables. Richness, abundance and evenness were significantly higher in the native area, intermediate in the afforestation and lower in the degraded land. The native area’s plots showed high tree cover, the afforestation ́s plots high graminoid and young trees cover, and the degraded land’s plots high forbs cover. Values of fungal variables were higher at higher tree cover and were positively correlated with soil phosphorus content and pH. The results show that, after 12 years of afforestation with P. australis, the fungal diversity in that ecosystem is higher than the observed in reference degraded land but is far from the observed in a native area. It can be hypothesized that fungi are sensitive bioindicators of forest degradation and restoration.

Dynamics of soil aggregate-associated organic carbon along an afforestation chronosequence

The objectives of this study were to determine the dynamics of aggregate-associated organic carbon (OC) along an afforestation chronosequence on abandoned farmland of China, and to examine the contributions of changes in aggregate-associated OC to changes in total soil OC. We investigated the dynamics of OC associated with aggregates along an afforestation chronosequence. Water-stable aggregates were isolated, and the OC concentrations in total soil and the aggregates were measured. Averaged across the entire chronosequence, afforestation led to 116, 128 and 108 % average increases in OC concentrations in macroaggregates, microaggregates and the <0.053 mm size class, respectively, in the top 20 cm of soil. The OC stocks in macroaggregates increased by averages of 651 and 473 % at 0–10 and 10–20 cm depths, respectively, mostly within the first 24 years. The OC stocks in microaggregates decreased during the first 35 years and then increased during 48–200 years of afforestation. Averaged across the entire chronosequence, the increases in OC stocks in macroaggregates accounted for 83 and 100 % of the total increase in OC stocks in soils at 0–10 and 10–20 cm depths, respectively. Our results indicated that the accumulation of OC in soils after afforestation on abandoned farmland was mainly due to the accumulation of OC in macroaggregates.

Pinus ponderosaalters nitrogen dynamics and diminishes the climate footprint in natural ecosystems ofPatagonia

SummaryUnderstanding climate effects on plant–soil interactions in terrestrial ecosystems remains challenging due to the fact that floristic composition covaries with climate, particularly along rainfall gradients. It is difficult to separate effects of precipitationper sefrom those mediated indirectly through changes in species composition. As such, afforestation (the intentional planting of woody species) in terrestrial ecosystems provides an ecological opportunity to assess the relative importance of climate and vegetation controls on ecosystem processes.We investigated the impacts of 35 years of afforestation on ecosystemNdynamics, in ecosystems ranging from arid shrub‐steppe to closed‐canopy forest inPatagonia,Argentina. Sites of natural vegetation and adjacent sites planted with a single exotic species,Pinus ponderosa,were identified in five precipitation regimes along a continuous gradient of 250–2200 mm mean annual precipitation (MAP). We evaluatedCandNparameters of vegetation and soil, as well as natural abundance of13Cand15Nin leaves, roots, ectomycorrhizae (EcM) and soils.In natural vegetation, most leaf traits (%N,C:Nratios, leaf mass per area, δ15Nvalues) demonstrated strong significant relationships withMAP, while these relationships were nearly absent in afforested sites. In addition, theEcMof native southern beech and pine trees were significantly enriched in15Nrelative to leaves at all sites where they were present. While soilCandNpools in both vegetation types increased withMAP, overall pool sizes were significantly reduced in afforested sites.Synthesis. Observed relationships between leaf traits and precipitation in natural vegetation may be driven largely by shifts in species composition and plant–soil interactions, rather than direct effects of precipitation. Our results suggest that a change in the species composition of the dominant vegetation is sufficient to alterCandNcycling independently of climate constraints: pine afforestation homogenizedNdynamics across sites spanning an order of magnitude ofMAP. These results highlight the important control of ectomycorrhizal associations in affectingCandNdynamics. Additionally, they serve to demonstrate that altering natural species composition alone is sufficient to cause large, detectable impacts onNturnover independently of direct climate effects.

Conservation of natural and cultural heritage in Dunhuang, China

A country's natural and cultural heritage is created in a unique environment, and for this heritage to survive, it must be preserved in a similar environment. In China, 60years of afforestation to combat desertification near the Mogao Grottoes and the Crescent Moon Spring have shown that attempts to protect the regional environment have not protected these heritage sites, and may actually have endangered them. Conserving the environment's original state may be the most effective solution for heritage conservation where sites have survived under a specific set of physical and environmental conditions for hundreds or thousands of years and may not be able to survive a new environment. Man-made changes should only be attempted with great care to avoid damaging the conditions that have preserved the natural or cultural heritage in the long term.

Revegetation as an efficient means of increasing soil aggregate stability on the Loess Plateau (China)

Soil aggregate stability influences several aspects of soil physical behavior, such as water infiltration and soil erosion (Amézketa, 1999). We investigated the soil aggregate stability characteristics in the framework of the ‘Grain for Green’ vegetation rehabilitation project at the Loess Plateau (China) by using the Le Bissonnais (1996) and the modified Yoder (1936) methods. Both non-grazed grassland and afforestation revegetations were considered.The size distribution mode was always >2mm for the fast wetting test (FW) in the non-grazed grassland communities. This fraction accounted for approximately 40% to 80% of the total soil weight. The wet stirring (WS) test showed a distribution similar to that determined by the FW test. For the slow wetting (SW) test, 80% or more of the non-grazed grassland soil fragments was >2mm.The mean weighted diameter (MWD) that was determined by the Le Bissonnais' method was different among the tests and land uses. For the FW test, all the plant communities were significantly different from that of the recently abandoned grazing on grassland at the 0–20 and 20–40cm depths. In the Artemisia sacrorum community, the maximum MWD was approximately 3mm for the 0–20cm depth. There were no significant differences among the plant communities after 3years of afforestation and 7years of non-grazing of the grassland. The MWDs were lower in the afforestation area than in the non-grazed grassland area. The maximum MWD value from the FW test was approximately 1.8mm and was significantly lower (<0.5mm) for cropland. The MWD of the modified Yoder method was positively related to the slow wetting and wet stirring (WS) tests of the Le Bissonnais' method (n=20 and r=0.83 and 0.87, respectively).In the Loess Plateau, revegetations by non-grazed grassland and afforestation are efficient means of increasing aggregate stability and decreasing soil erodibility. The aggregate stability under non-grazed grasslands is higher than that under afforestation. The effect of revegetation is persistent, which makes it a suitable long-term management practice. Compared with the modified Yoder's method, the FW test of the Le Bissonnais' method is better at determining aggregate stability differences among land uses and is recommended for future studies.

Dynamics and stabilization of soil organic carbon after nineteen years of afforestation in valley‐type savannah in southwest China

AbstractSerious concerns about carbon (C) sequestration capacity and the stabilization of sequestered C in forested soils have emerged in the context of global climate change. The organic C in soil and in soil fractions at four sampling times in Acacia auriculiformis plantations afforested in 1991 were investigated with a combination of density fractionation and acid hydrolysis techniques. The results showed that the accumulation of C in the forested soils had accelerated because the afforestation of wasteland with A. auriculiformis. The C accumulation rates of the surface and subsurface soils averaged 0.38 and 0.17 t C/ha/yr, respectively, during the 19 yr following the afforestation. The percentage of organic C in heavy fraction relative to total soil organic C at the surface soil was 71% in 2003. This value was significantly (P &lt; 0.05) higher than that in 2010 (68%). The chemical recalcitrant C index of light fraction was significantly (P &lt; 0.05) higher than that of heavy fraction in 2003 regardless of soil depth, but both decreased with time. ca. 58–68% of the newly sequestered C was protected by physical mechanism, and 41–50% was transferred into the acid nonhydrolysable fraction during the 12–19 yr after the trees were planted. The chemical stability of the physically protected C remained lower than that of the unprotected C following the afforestation in the valley‐type savannah. However, both the stability values showed a decline with time.

Tree species controls on soil carbon sequestration and carbon stability following 20 years of afforestation in a valley-type savanna

In the farmland/forest ecotone of southwestern China, many areas are experiencing afforestation and subsequent shift in the ecosystem carbon (C) stocks. The effects of five tree species on soil organic C (SOC) accumulation and C stability following 20years of afforestation in a valley-type savanna since 1991 were investigated in fractions below 0.25mm. The bulk soils were fractionated with a combination of density fractionation and acid hydrolysis techniques.The results showed that SOC densities in the afforested stands have accumulated since the changes in the land uses. The surface soil C sequestration rates varied substantially among the five stands and ranged from 0.13tCha−1year−1 to 0.47tCha−1year−1 during the two decades of afforestation. The percentage of organic C in the heavy fraction (via density fractionation) relative to the total SOC was 61.9–68.0% under the five stands after 20years of afforestation. The overall biochemical recalcitrant C density accounted for 37.6–49.9% of the total SOC in these stands in 2011.The tree species controls on the soil C sequestration and on the C in the separated fractions following 20years of afforestation in a valley-type savanna. However, the biochemical stability of the physically protected C remained lower than that of the unprotected C, irrespective of the tree species. Among the five studied tree species, Leucaena leucacephala was the most suitable tree species for afforestation in the valley-type savanna for the higher C sequestration rates in the soils and for the moderate stability of SOC, which potentially, at least theoretically, lead to the high biological utilisation (e.g. nutrient release) of soil organic matter when SOC decays.

Effects of afforestation with four unmixed plant species on the soil–water interactions in a semiarid Mediterranean region (Sicily, Italy)

Purpose An assessment of the effect of plant cover on the properties of four afforested soils in central Sicily was performed with the aim of discriminating among them after 60 years of afforestation.

Reducing topsoil salinity and raising carbon stocks through afforestation in Khorezm, Uzbekistan

Agricultural mismanagement of irrigated drylands results in severe soil degradation. Afforestation is an option for ameliorating such degraded land. We evaluated the impact afforestation has on the topsoil (0–20 cm) of salinized degraded cropland in regards to salinity, aggregate stability, and soil organic carbon (SOC) stocks in Uzbekistan, Central Asia. The effects of tree plantations established under either furrow or drip irrigation were studied four years following afforestation and two years after irrigation ceased. For comparative study we also sampled fallow land, land with 80 years of tree growth, natural forest, desert ecosystems, and paddy rice fields. Initial furrow irrigation showed to be most effective in improving soil fertility after four years of afforestation; the respective plantations of Populus euphratica and Ulmus pumila showed significant levels of reduced soil salinity and increased aggregate stability and improved SOC stocks. The comparison of the long-term afforested land with the short-term equivalent suggested a C sequestration rate of 0.09–0.15 t C ha−1 year−1. The SOC stocks of the long-term afforestation site exceeded those of the native forest. Hence, a rehabilitation of salt-affected cropland is feasible following the conversion into occasionally irrigated tree plantations, although it takes decades to reach steady-state conditions.

Changes in Soil Particulate Organic Matter, Microbial Biomass, and Activity Following Afforestation of Marginal Agricultural Lands in a Semi-Arid Area of Northeast China

Afforestation of agricultural lands has been one of the major land use changes in China in recent decades. To better understand the effect of such land use change on soil quality, we investigated selected soil physical, chemical and microbial properties (0-15 cm depth) in marginal agricultural land and a chronosequence of poplar (Populus euramericana cv. 'N3016') plantations (5-, 10-, 15- and 20-years old) in a semi-arid area of Northeast China. Soil bulk density significantly declined after conversion of agricultural lands to poplar plantations. Soil total organic carbon (TOC) and nitrogen (TN) concentrations, microbial biomass C (MBC) and potential N mineralization rate (PNM) decreased initially following afforestation of agricultural lands, and then increased with stand development. However, soil metabolic quotient (qCO(2)) exhibited a reverse trend. In addition, soil particulate organic matter C (POM-C) and N (POM-N) concentrations showed no significant changes in the first 10 years following afforestation, and then increased with stand age. These findings demonstrated that soil quality declined initially following afforestation of agricultural lands in semi-arid regions, and then recovered with stand development. Following 15 years of afforestation, many soil quality parameters recovered to the values found in agricultural land. We propose that change in soil quality with stand age should be considered in determining optimum rotation length of plantations and best management practices for afforestation programs.

Water balance in afforestation chronosequences of common oak and Norway spruce on former arable land in Denmark and southern Sweden

Precipitation, throughfall and soil moisture were measured, and interception, transpiration and water recharge were estimated in four afforestation chronosequences on former arable land at two Danish locations (Vestskoven and Gejlvang) and at one southern Swedish location (Tönnersjöheden). Afforestation was performed using Norway spruce ( Picea abies (Karst.) L.) and common oak ( Quercus robur L.) at Vestskoven and only Norway spruce at Gejlvang and Tönnersjöheden. Four to five stands of different ages (5–92 years) were studied in each of these chronosequences. Hydrological fluxes were calculated using the soil hydrological model SWAP. Throughfall flux and soil water content were used for calibration of the model. The simulated water recharge decreased with increased stand age within 30–40 years of afforestation. This was mainly due to increased interception evaporation with age. The annual water recharge was higher below oak stands (149–192 mm yr −1) than below spruce stands (107–191 mm yr −1) of similar age. The relative water recharge was also considerably higher from the sandy glaciofluvial soils at Gejlvang and Tönnersjöheden than from the sandy loamy till soils at Vestskoven.

Labile soil organic matter pools in a calciorthent after 18 years of afforestation by different plantations

Labile soil organic matter pools (LSOMs) are the fine indicators of soil quality which are influenced by changes in management practices. The suitability of forest tree species is essential for soil quality improvement of a nutrient deficient calciorthent with very high percentage of free CaCO 3 (34%). Six multipurpose tree species were selected to investigate the effect of afforestation after 18 years of plantations on size and dynamics of LSOMs, e.g. dissolved organic carbon (DOC), microbial biomass carbon (MBC) and light fraction carbon (LFC) and also on soil respiration. LSOMs were estimated during four seasons: summer (June), autumn (September), winter (December) and spring (March) in 0–15 cm and 15–30 cm soil depths and in situ soil respiration was also measured during the corresponding periods. Afforestation by Eucalyptus tereticornis (Smith) increased soil organic carbon (SOC) of the surface 30 cm soil layer by 142% which was at par with Terminalia arjuna Bedd. and Albizia procera (Roxb.) Benth while 63% increase was found by Pongamia pinnata (Linn.) over control plot (3.10 kg ha −1 SOC). Soil under T. arjuna had the highest increase in the mean annual concentration of DOC (by 201%), MBC (by 413%) and LFC (by 263%) over control in the 30 cm soil layer followed by E. tereticornis. Among the LSOMs, LFC had the maximum contribution to SOC (9.61–11.71%) in the afforested plots up to 30 cm soil layer. The relationships among the three labile pools were highly significant. Mean annual soil respiration was highest in E. tereticornis plot which was at par with Acacia lenticularis (L.) Willd and A. procera, while it was lowest in T. arjuna plot. Thus, MBC was the most sensitive to afforestation and LFC was the largest reservoir of carbon among the three LSOMs. The results suggest that afforestation by the six multipurpose tree species had increased the LSOMs and soil respiration to different magnitudes. T. arjuna was found to be the most suitable plantation with highest increase in LSOMs and lowest respiration among the different plantations.

Evolution of soil microbial biomass in restoration process of Robinia pseudoacacia plantations in an eroded environment

Vegetation recovery is a key measure to improve ecosystems in the Loess Plateau in China. To understand the evolution of soil microorganisms in forest plantations in the hilly areas of the Loess Plateau, the soil microbial biomass, microbial respiration and physical and chemical properties of the soil of Robinia pseudoacacia plantations were studied. In this study, eight forest soils of different age classes were used to study the evolution of soil microbial biomass, while a farmland and a native forest community of Platycladus orientalis L. were chosen as controls. By measuring soil microbial biomass, metabolic quotient, and physical and chemical properties, it can be concluded that soil quality was improved steadily after planting. Soil microbial biomass of C, N and P (SMBC, SMBN and SMBP) increased significantly after 10 to 15 years of afforestation and vegetation recovery. A relatively stable state of soil microbial biomass was maintained in near-mature or mature plantations. There was an increase of soil microbial biomass appearing at the end of the mature stage. After 50 years of afforestation and vegetation recovery, compared with those in farmland, the soil microbial biomass of C, N and P increased by 213%, 201% and 83% respectively, but only accounting for 51%, 55% and 61% of the increase in P. orientalis forest. Microbial soil respiration was enhanced in the early stages, and then weakened in the later stage after restoration, which was different from the change of soil organic carbon. The metabolic quotient (qCO2) was significantly higher in the soils of the P. orientalis forest than that in farmland at the early restoration stage and then decreased rapidly. After 25 years of afforestation and vegetation recovery, qCO2 in soils of the R. pseudoacacia forest was lower than that in the farmland soil, and reached a minimum after 50 years, which was close to that of the P. orientalis forest. A significant relationship was found among soil microbial biomass, qCO2 and physical and chemical properties and restoration duration. Therefore, we conclude that it is possible to artificially improve the ecological environment and soil quality in the hilly area of the Loess Plateau; a long time, even more than 100 years, is needed to reach the climax of the present natural forest.

Soil δ15N and nutrients under exotic tree plantations in the southwestern Ethiopian highlands

This study was undertaken to evaluate the effect of tree species on soil properties. Soil δ 15N, total N, C:N ratio, base cations and available P were analyzed under farmland, native forest and tree plantations established on abandoned farmland at Belete forest. Changes associated with land use were evaluated using a comparative approach. Forest clearing followed by continuous cultivation of annual crops resulted in a significant decline in total N, base cations and available P within 0–10 or 0–20 cm depth. Assuming a linear rate of loss, total N declined by 90.6 kg ha −1 year −1 or by 38% (6.8 Mg ha −1) of the level in native forest during the 75 years of cultivation. The 15N enrichment of soil and litter N in the farmland may indicate losses of N through leaching and harvest. The degree of change in soil properties under exotic trees after 20 years of afforestation of former farmland depended on tree species. Total N within 0–50 cm depth increased by 5.7 Mg ha −1 under Cupressus lusitanica and 2.0 Mg ha −1 under Pinus patula. The decrease in δ 15N value with afforestation was greater for C. lusitanica than for P. patula, indicating that N cycling under C. lusitanica progressed more towards ‘native forest like’ conditions. Under C. lusitanica, exchangeable Ca 2+ increased significantly at 0–5 cm. Exchangeable K + increased significantly within 0–30 cm depth under both C. lusitanica and P. patula. The increased C:N ratio under these tree species was attributed to recent organic matter (OM) addition. The soil under Eucalyptus grandis, established on former mixed land use (pasture plus farmland), had nearly similar δ 15N, TN, exchangeable Ca 2+ and K + to native forest. The residual effect of 15N-depleted C 4 pasture grasses might explain the lack of difference in soil δ 15N profile below 10 cm depth between E. grandis and native forest. Greater OM inputs by E. grandis probably account for the greater C:N ratio relative to native forest soil.

Rates of forest floor decomposition and soil forming processes as indicators of forest ecosystem functioning on a polluted dredged sediment landfill

Results of intensive monitoring of forest floor decomposition rates and soil forming processes after afforestation of a calcareous upland dredged sediment landfill with an oxidised surface soil are presented. Nutrient status of the sediment substrate favours tree growth and allows for afforestation and thus an integration in the landscape. Soil processes on the landfill resulted in small differences between the surface and the subsurface soil layer, although higher soil organic carbon and Cd concentrations in the surface soil were observed. So far, tree species had a minor impact on soil processes. Relative to the uncontaminated covered part of the site and to general references, forest floor decomposition was found to be relatively fast for sycamore maple and pedunculate oak. Despite the pollution status of the dredged sediment landfill, the sediment substrate was favourable for forest floor decomposition. This might indicate that the soil nutrition status and the high carbonate status override the negative impact of soil pollution with metals and other pollutants. Application of a covering layer composed of uncontaminated soil material resulted in lower Cd concentrations in earthworms, but concentrations were still higher than for references. We conclude from the observations that polluted but fertile soils allow for afforestation and for regular forest floor decomposition with normal or slightly elevated metal concentrations. Only long-term observations of such new forests will lead to a correct site-specific assessment of the actual ecological risks, but after 16 years of landfilling and 12 years of afforestation no adverse effects were observed.

Impact of land use and seasonal freezing on morphological and physical properties of silty Norwegian soils

Land use and seasonal freezing is assumed to influence the transport of water and particles through the soil. The aim of this study was to identify, measure and interpret changes in soil pedality that could influence the transport of water and particles through the soil profile. The morphological, physical and chemical properties of three soils on a plateau in the Vansemb area, south eastern Norway, were compared. The soils have 60 to 90 cm thick silty sediments deposited above a marine clay substratum. One site had always been under natural forest vegetation. The second site had been afforested with spruce for 40 years after centuries of cattle grazing without ploughing. The third site had been under cultivation for about 70 years. Because of winter freezing, the upper horizons display a platy pedality created by the growth of ice segregation blades. This is followed by a desiccation structuration with vertical shrinking fissures lining a prismatic pedality in deeper horizons. As frost penetrates deeper in soils under crop and grassland, the platy structures are deeper in these sites and overlap with the vertical fissure system. As a result of tillage operations, the horizons below the plough layer became denser. This decrease in porosity is to some degree counterbalanced by the active ice segregation and the earthworm activity. Tillage operations are also responsible for the filling of the vertical fissure system with particles from the horizons above. The shift to crop or grassland increased bioturbation, mainly by earthworm populations. This activity remained at least partly preserved, even after 40 years of afforestation. Burrowing animals were absent from the mineral horizons of the forest soil. Comparative studies, as this one, ascertain the orientation of soil evolution; and thus provide essential information for soil management with minimal degradation and particle transport.

The Effects of Tree Species and Site on the Solubility of Cd, Cu, Ni, Pb and Zn in Soils

Afforestation of former agricultural land changes soil characteristics such as pH and organic matter content, which may affect heavy metal solubility in the soil. In this study the effects of different tree species on heavy metal solubility were investigated at four 34 years old adjacent stands of beech (Fagus sylvatica L.), grand fir (Abies grandis Lindl.), Norway spruce (Picea abies (L.) Karst.) and oak (Quercus robur L.) planted on former agricultural land at four different sites in Denmark. The sites differ in soil characteristics and represent two texture classes (loamy sand and sandy loam). Soil pH and soil organic matter content was measured in the 16 stands and soil solution was isolated by centrifugation from three depths at four different occasions. Dissolved organic carbon (DOC), pH in the soil solution and the soil solution concentrations (availability) of Cd, Cu, Ni, Pb and Zn were determined. Analysis of variance showed that the tree species affects soil pH and organic matter content in the topsoil, but not in the lower horizons. Norway spruce and grand fir acidify more than beech and oak, and the highest amount of accumulated soil organic matter is in the topsoil under Norway spruce. The effects of tree species on soil solution pH and DOC resemble the effect on soil pH and organic matter content. Grand fir enhances the solubility of Cd and Zn in the topsoil with the lower solubility found under beech and oak and Norway spruce enhances the solubility of Cu, Ni and Pb in the top horizons. The lowest solubility of Ni and Pb is found under beech and oak, whereas the lowest Cu concentrations in the soil solution are found under grand fir. After 34 years of afforestation no effects of tree species on the concentrations of heavy metals in the soil solution from the C-horizons were found. The tree species effect on the concentration of Cd, Cu and Ni in the soil solution depends on the soil characteristics with the higher concentrations found in sandy loam soils, whereas no effect of site on the solution concentration was found for Pb and Zn. It was not possible to find a clear correlation between the soil solution concentrations of heavy metals, pH and DOC concentration.

Planted green ash ( Fraxinus pennsylvanica Marsh.) and herbaceous vegetation responses to initial competition control during the first 3 years of afforestation

Formerly row-cropped bottomland sites that have been planted with hardwoods through afforestation programs are typically associated with low tree stocking, slow tree growth, and an enduring influence of agricultural weeds in the herbaceous stratum. Consequently, many of the intended benefits of a forestry land use are delayed or forfeited. This study evaluates the growth of planted green ash and the development of volunteer herbaceous communities on a bottomland site in southern Illinois, USA in response to two different operational herbicide (pre-emergence sulfometuron and post-emergence glyphosate) and tillage treatments, applied individually and in combination. Both herbicide treatments increased green ash height and diameter growth while tillage produced no response. At the end of three growing seasons, the post-emergence glyphosate treatment increased forb cover. Total grass cover was not impacted by herbicide treatments after the second growing season. During the second and third growing seasons, the pre-emergence sulfometuron treatment increased broomsedge ( Andropogon virginicus L.) cover and decreased goldenrod ( Solidago nemoralis Ait.) cover relative to post-emergence glyphosate and no herbicide treatments. Johnsongrass ( Sorghum halepense L.) cover decreased dramatically following the first growing season in all treatments. Horseweed ( Conyza canadensis L.) exhibited a complex response to silvicultural treatments through the course of the study. Commonly used silvicultural treatments that yielded similar and positive tree growth responses differed in their impact on herbaceous community composition through the first 3 years of stand development. These results suggest that treatment choice may have long-term implications for overstory canopy development on these and similar afforested bottomland sites characterized by presence of agricultural weeds and low tree stocking.

Change in soil carbon following afforestation

Quantifying changes in soil C may be an important consideration under large-scale afforestation or reforestation. We reviewed global data on changes in soil C following afforestation, available from 43 published or unpublished studies, encompassing 204 sites. Data were highly variable, with soil C either increasing or decreasing, particularly in young (<10-year) forest stands. Because studies varied in the number of years since forest establishment and the initial soil C content, we calculated change in soil C as a weighted-average (i.e. sum of C change divided by sum of years since forest establishment) relative to the soil C content under previous agricultural systems at <10, >10 and <30 cm sampling depths. On average, soil C in the <10 cm (or <30 cm) layers generally decreased by 3.46% per year (or 0.63% per year) relative to the initial soil C content during the first 5 years of afforestation, followed by a decrease in the rate of decline and eventually recovery to C contents found in agricultural soils at about age 30. In plantations older than 30 years, C content was similar to that under the previous agricultural systems within the surface 10 cm of soil, yet at other sampling depths, soil C had increased by between 0.50 and 0.86% per year. Amounts of C lost or gained by soil are generally small compared with accumulation of C in tree biomass. The most important factors affecting change in soil C were previous land use, climate and the type of forest established. Results suggest that most soil C was lost when softwoods, particularly Pinus radiata plantations, were established on ex-improved pastoral land in temperate regions. Accumulation of soil C was greatest when deciduous hardwoods, or N 2-fixing species (either as an understorey or as a plantation), were established on ex-cropped land in tropical or subtropical regions. Long-term management regimes (e.g. stocking, weed control, thinning, fertiliser application and fire management) may also influence accumulation of soil C. Accumulation is maximised by maintaining longer (20–50 years) forest rotations. Furthermore, inclusion of litter in calculations reversed the observed average decrease in soil C, so that amount of C in soil and litter layer was greater than under preceding pasture.