Spatial heterogeneity of understory vegetation and soil in an Alaskan upland boreal forest fire chronosequence

In this study we characterized spatial heterogeneity of soil carbon and nitrogen pools, soil moisture, and soil pH of the first 15?cm of the soil profile; depth of the organic horizon; forest floor covers; and understory vegetation abundances in three sites (1999, 1987 and 1920 wildfires) of a boreal forest chronosequence of interior Alaska. We also investigated the cross-dependence between understory vegetation distribution and soil characteristics. Our results showed higher microbial respiration rates and microbial biomass in the oldest site and greater net N mineralization rates in the mid-successional site. Although spatial heterogeneity was absent at the scale studied for the majority of soil variables (60%), understory vegetation abundances and forest floor cover, spatial heterogeneity decreased with time after fire for the depth of organic horizon, soil microbial biomass, N mineralization rates and feathermoss cover. Our results also showed that increasing time after fire decreased the number of correlations between understory vegetation and soil characteristics while it increased between forest floor covers and soil characteristics. Overall, our study suggest that fire initially creates a patchy mosaic of forest floor cover, from fire hot spots, where high intensity burning exposes mineral soil, to practically unburned areas with intact mosses and lichens. As time since fire passes, forest floor cover and soil characteristics tend to become more uniform as understory species fill in severely burned areas.     

Assessing the relative fire proneness of different forest types in Portugal

Abstract

We have assessed the fire proneness of the main forest types in Portugal classified according to the main species, using three different approaches: the use of resource selection ratios applied to burned patches, the proportion of randomly located plots that were burned and the proportion of burned National Forest Inventory plots. The results allowed ranking fire proneness according to the following decreasing order: maritime pine forests, eucalyptus forests, unspecified broadleaf forests, unspecified conifer forests, cork oak forests, chestnut forests, holm oak forests and stone pine forests. In order to understand the obtained results we have assessed the structure of the different forest types using the percent cover of seven vegetation layers (C1–C7), a Tree Dominance Index, a Height Index and a Cover Index (IC). Structural variables and stand composition were used to predict fire probability according to binary logistic modelling. Only four structural variables and stand composition provided significant results, the latter being the most important variable for explaining fire probability. These models were used to predict fire probability for different stand types as a function of IC.     

浙江天童常绿阔叶林不同演替阶段地表凋落物的C:N:P化学计量特征

地表凋落物在森林物质循环中起着重要作用, 但是目前缺乏对其不同分解层次中碳(C)、氮(N)、磷(P)演替动态的研究。该文以浙江天童常绿阔叶林为研究对象, 用空间代替时间序列的方法, 通过测定5个演替阶段地表凋落物不同分解层次的凋落物量、有机碳库和氮磷养分库的储量及C:N:P化学计量特征, 探讨地表凋落物特征的演替动态。结果表明: 1)随着演替的进行, 地表凋落物量和有机碳储量呈现下降的趋势。2)在各演替阶段, 有机碳含量在各分解层表现出未分解层(L) > 半分解层(F) > 已分解层(Y)的趋势; 有机碳储量均表现为Y < F。3)演替前期群落氮含量和储量显著低于演替中后期群落; 不同分解层的氮含量在各演替阶段皆表现为: Y > F > L, 且各层氮含量随着演替的进行均趋于升高。4)磷含量在演替中期群落最低, 各演替阶段不同分解层的磷含量皆表现为Y > F > L。磷储量的演替趋势不明显。L层磷储量随着演替进行趋于降低。5)随着演替进行, 凋落物C:N、C:P和N:P皆趋于下降(p < 0.05)。在各分解层之间, C:N和C:P皆表现为Y < F < L, N:P差异不显著。总之, 随着演替进行, 天童常绿阔叶林地表凋落物量降低, 有机碳库及氮磷养分库的含量趋于升高, 储量趋向降低, C:N:P趋于下降, 体现了生态系统碳和养分循环随着演替进行在不断优化。     

Decomposition and nitrogen release from the foliage litter of fir (Abies sachalinensis) and oak (Quercus crispula) under different forest canopies in Hokkaido, Japan

When two tree species co-occur, decomposition and nitrogen (N) release from the foliage litter depend on two factors: the forest floor conditions under each canopy type and the species composition of the litter. We conducted an experiment using fir and oak to answer several questions regarding decomposition beneath canopies of the two species and the effects of litter species composition on decomposition. We compared the rates of decomposition and N release from three different litters (fir needle, oak leaf, and a mixture of the two) in 1-mm-mesh litterbags on the forest floor under three different canopies (a 40-year-old fir plantation, large oak trees, and mixed fir and oak trees) in Hokkaido, Japan, over a 2-year period. Beneath each of these canopy types, the litter decomposition rate and percentage of N remaining in the litterbags containing a mixture of fir and oak litter were not significantly different from the expected values calculated for litterbags containing litter from a single tree species. Oak leaf litter decomposed significantly faster than fir needle litter beneath each canopy type. The litter decomposition rate was significantly higher beneath the fir canopy than under the oak canopy, and was intermediate under the mixed canopy of fir and oak. No net N release, that is, a decrease in the total N compared to the original amount, was detected from fir litter under each canopy type or from oak leaf litter beneath the oak canopy. N increased over the original amount in the fir litter beneath the oak canopy and the mixed canopy of fir and oak, but N was released from the oak litter under the fir canopy and the mixed canopy of fir and oak. These results suggest that oak leaf litter blown onto fir forest floor enhances nutrient cycling, and this might be a positive effect of a mixed stand of conifer and broad-leaved trees.     

C:N:P stoichiometry in forest floor litter of evergreen broad-leaved forests at different successional stages in Tiantong,Zhejiang, eastern China

地表凋落物在森林物质循环中起着重要作用, 但是目前缺乏对其不同分解层次中碳(C)、氮(N)、磷(P)演替动态的研究。该文以浙江天童常绿阔叶林为研究对象, 用空间代替时间序列的方法, 通过测定5个演替阶段地表凋落物不同分解层次的凋落物量、有机碳库和氮磷养分库的储量及C:N:P化学计量特征, 探讨地表凋落物特征的演替动态。结果表明: 1)随着演替的进行, 地表凋落物量和有机碳储量呈现下降的趋势。2)在各演替阶段, 有机碳含量在各分解层表现出未分解层(L) > 半分解层(F) > 已分解层(Y)的趋势; 有机碳储量均表现为Y < F。3)演替前期群落氮含量和储量显著低于演替中后期群落; 不同分解层的氮含量在各演替阶段皆表现为: Y > F > L, 且各层氮含量随着演替的进行均趋于升高。4)磷含量在演替中期群落最低, 各演替阶段不同分解层的磷含量皆表现为Y > F > L。磷储量的演替趋势不明显。L层磷储量随着演替进行趋于降低。5)随着演替进行, 凋落物C:N、C:P和N:P皆趋于下降(p < 0.05)。在各分解层之间, C:N和C:P皆表现为Y < F < L, N:P差异不显著。总之, 随着演替进行, 天童常绿阔叶林地表凋落物量降低, 有机碳库及氮磷养分库的含量趋于升高, 储量趋向降低, C:N:P趋于下降, 体现了生态系统碳和养分循环随着演替进行在不断优化。     

Litter dynamics recover faster than arthropod biodiversity during tropical forest succession

Litterfall and litter decomposition are key elements of nutrient cycling in tropical forests, a process in which decomposer communities such as macro-arthropods play a critical role. Understanding the rate and extent to which ecosystem function and biodiversity recover during succession is useful to managing the growing area of tropical successional forest globally. Using a replicated chronosequence of forest succession (5–15, 15–30, 30–45 years, and primary forest) on abandoned pastures in lowland tropical wet forest, we examined litterfall, litter chemistry, and effects of macro-arthropod exclusion on decomposition of two litter types (primary and 5- to 15-years-old secondary forest). Further, we assessed macro-arthropod diversity and community composition across the chronosequence. Overstory cover, litterfall, and litter nutrients reached levels similar to primary forest within 15–30 years. Young secondary forest litter (5–15 years) had lower initial N and P content, higher C:N, and decayed 60 percent faster than primary forest litter. The presence of macro-arthropods strongly mediated decomposition and nutrient release rates, increasing litter mass loss by 35–44 percent, N released by 53 percent, and P release by 84 percent. Forest age had no effect on soil nutrients, rates of litter decomposition, nutrient release, or macro-arthropod influence. In contrast, abundance and community composition of macro-arthropods remained significantly lower and distinct in all ages of secondary compared with primary forest. Order richness was lower in 5–15 years of secondary compared with primary forest. Our results suggest that in highly productive tropical wet forest, functional recovery of litter dynamics precedes recovery of decomposer community structure and biodiversity.     

Changes in soil properties after afforestation of former intensively managed soils with oak and Norway spruce

In many European countries, surplus agricultural production and ecological problems due to intensive soil cultivation have increased the interest in afforestation of arable soils. Many environmental consequences which might rise from this alternative land-use are only known from forest establishment on less intensively managed or marginal soils. The present study deals with changes in soil properties following afforestation of nutrient-rich arable soils. A chronosequence study was carried out comprising seven Norway spruce (Picea abies (Karst.) L.) and seven oak (Quercus robur L.) stands established from 1969 to 1997 on former horticultural and agricultural soils in the vicinity of Copenhagen, Denmark. For comparison, a permanent pasture and a ca. 200-year-old mixed deciduous forest were included. This paper reports on changes in pH values, base saturation (BS_eff), exchangeable calcium, soil N pools (N_min contents), and C/N ratios in the Ap-horizon (0–25 cm) and the accumulated forest floor. The results suggest that afforestation slowly modifies soil properties of former arable soils. Land-use history seems to influence soil properties more than the selected tree species. An effect of tree species was only found in the forest floor parameters. Soil acidification was the most apparent change along the chronosequence in terms of a pH decrease from 6 to 4 in the upper 5 cm soil. Forest floor pH varied only slightly around 5. Nitrogen storage in the Ap-horizon remained almost constant at 5.5 Mg N ha^–1. This was less than in the mineral soil of the ca. 200-year-old forest. In the permanent pasture, N storage was somewhat higher in 0–15 cm depth than in afforested stands of comparable age. Nitrogen storage in the forest floor of the 0–30-year-old stands increased in connection with the build-up of forest floor mass. The increase was approximately five times greater under spruce than oak. Mineral soil C/N ratios ranged from 10 to 15 in all stands and tended to increase in older stands only in 0–5 cm depth. Forest floor C/N ratios were higher in spruce stands (26.4) as compared to oak stands (22.7). All stands except the youngest within a single tree species had comparable C/N ratios.     

Biogeochemical cycles in natural forest and conifer plantations in the high mountains of Colombia

Plant litter production and decomposition are two important processes in forest ecosystems, since they provide the main organic matter input to soil and regulate nutrient cycling. With the aim to study these processes, litterfall, standing litter and nutrient return were studied for three years in an oak forest (Quercus humboldtii), pine (Pinus patula) and cypress (Cupressus lusitanica) plantations, located in highlands of the Central Cordillera of Colombia. Evaluation methods included: fine litter collection at fortnightly intervals using litter traps; the litter layer samples at the end of each sampling year and chemical analyses of both litterfall and standing litter. Fine litter fall observed was similar in oak forest (7.5 Mg ha/y) and in pine (7.8 Mg ha/y), but very low in cypress (3.5 Mg ha/y). Litter standing was 1.76, 1.73 and 1.3 Mg ha/y in oak, pine and cypress, respectively. The mean residence time of the standing litter was of 3.3 years for cypress, 2.1 years for pine and 1.8 years for oak forests. In contrast, the total amount of retained elements (N, P, S, Ca, Mg, K, Cu, Fe, Mn and Zn) in the standing litter was higher in pine (115 kg/ha), followed by oak (78 kg/ha) and cypress (24 kg/ha). Oak forests showed the lowest mean residence time of nutrients and the highest nutrients return to the soil as a consequence of a faster decomposition. Thus, a higher nutrient supply to soils from oaks than from tree plantations, seems to be an ecological advantage for recovering and maintaining the main ecosystem functioning features, which needs to be taken into account in restoration programs in this highly degraded Andean mountains.     

Mass loss and nitrogen dynamics in decomposing acid forest litter in the Netherlands at increased nitrogen deposition

Litterbag experiments were carried out in five forest ecosystems in the Netherlands to study weight loss and nitrogen dynamics during the first two years of decomposition of leaf and needle litter. All forests were characterized by a relatively high atmospheric nitrogen input by throughfall, ranging from 22–55 kg N ha^–1 yr^–1.Correlation analysis of all seven leaf and needle litters revealed no significant relation between the measured litter quality indices (nitrogen and lignin concentration, lignin-to-nitrogen ratio) and the decomposition rate. A significant linear relation was found between initial lignin-to-nitrogen ratio and critical nitrogen concentration, suggesting an effect of litter quality on nitrogen dynamics.Comparison of the decomposition of oak leaves in a nitrogen-limited and a nitrogen-saturated forest suggested an increased nitrogen availability. The differences in capacities to retain atmospheric nitrogen inputs between these two sites could be explained by differences in net nitrogen immobilization in first year decomposing oak leaves: in the nitrogen-limited oak forest a major part (55%) of the nitrogen input by throughfall was immobilized in the first year oak leaf litter.The three coniferous forests consisted of two monocultures of Douglas fir and a mixed stand of Douglas fir and Scots pine. Despite comparable litter quality in the Douglas fir needles in all sites, completely different nitrogen dynamics were found.     

Nature and nurture in the dynamics of C, N and P during litter decomposition in Canadian forests

We measured changes in carbon (C), nitrogen (N) and phosphorus (P) concentrations and mass of 10 foliar litters decomposing over 12 years at 21 sites across Canada, ranging from subarctic to temperate, to evaluate the influence of litter quality (nature) and forest floor (nurture) on N and P dynamics. Most litters lost P faster than N, relative to C, except in one litter which had a high initial C:P quotient (2,122). Net N loss occurred at mass C:N quotients of between 33 and 68, positively correlated with the C:N quotient in the original litter, and net P loss likely occurred at C:P quotients between 800 and 1,200. Forest floor properties also influenced N and P dynamics: the higher the C:N or C:P quotient in the surface soil organic matter, the smaller the proportion of initial N or P left in the decomposing litter, relative to C. There was a convergence of C:N and C:P quotients as the litters decomposed, with an overall mass ratio of 427:17:1 when the litters reached 20% original C remaining. These results, covering a wide range of sites and litters and thus decomposition rates, showed that the C:N:P quotients followed similar trajectories and converged as the litters decomposed. The relative loss of N and P was affected by both the initial litter nutrient concentration and the chemistry of the site forest floor, with the former being more important than the latter, resulting in spatial variations in nutrient content of the forest floor.     

Phosphorus limitation and competitive capacities of Pinus halepensis and Quercus ilex subsp. rotundifolia on different soils

Aleppo pine (Pinus halepensis) and the evergreen holm oak (Quercus ilex) dominate forest areas of the Mediterranean Basin. Both species regenerate abundantly after fires: pine through seedlings and holm oak through resprouts. Cumulative nutrient losses caused by frequent fires may have decreased soil nutrient availability in such areas. To assess the role of nitrogen and phosphorus as limiting factors for growth of these species during post-fire recovery, a field fertilisation and competition experiment was conducted in a 5-year post-fire shrubland on calcareous soil, where naturally-regenerated saplings of Aleppo pine and resprouts of interior holm oak (Quercus ilex subsp. rotundifolia) coexist. Three years after fertilisation, relative basal area increment was 56% greater in pines fertilised with 250 kg P ha^–1 than in non fertilised ones. N fertilisation had small or no effects. Interactions between N and P fertilisation were not observed. Growth of Aleppo pine only increased with P fertilisation when neighbours were removed. Hence, the negative effect of neighbours on growth was greater when P availability was enhanced by fertilisation. In contrast, holm oak was able to grow more (110%) in response to increased P supply even without neighbour removal. A common garden experiment was then conducted with potted seedlings to investigate whether the suggested higher competitive capacity of holm oak for P held under a range of P amendments on different soils and competitive situations. P fertilisation increased seedling biomass yield of both species. When P availability increased, a negative effect of neighbours on growth was observed for holm oak and in 70 a lesser extent for Aleppo pine. In conclusion, in the field, holm oak resprouts showed higher competitive ability for P uptake compared to Aleppo pine saplings, but in potted seedlings in common garden conditions this trend was not observed. Therefore holm oak is not always competitively superior to Aleppo pine for P. Potted seedlings of both species had a notable plasticity in shoot/root biomass allocation, but only holm oak increased its proportional allocation to roots when neighbours were present. P availability can be a key factor in growth and competitive relations of these two species, but effects differ depending on soil type, individual age, regeneration type (i.e., seedling versus resprouts), and competitive situation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Do Boreal Forests Need Fire Disturbance to Maintain Productivity?

Fire is considered as a major driver of ecosystem processes of the boreal forest with important effects on soil and forest productivity. When the interval between successive forest fires is long, a thick organic layer can develop and eventually interfere with processes involved in tree nutrient uptake. We thus hypothesized that the organic layer of well-drained boreal stands increases with time since last fire and that thick organic layers are associated with low values of soil temperature, nutrient availability, and site productivity. This was tested on a chronosequence composed of 90 boreal stands ranging from 1 to more than 2000 years after fire within which we measured organic layer thickness (OLT), mineral soil and foliage nutrient concentrations, soil temperature, ground cover of Sphagnum sp. and Ericaceae sp., leaf area index, aboveground biomass production, and growth efficiency index (GEI). The OLT increased during the first 64 years after fire but stayed statistically constant thereafter. This initial increase in OLT was accompanied by an increase in the C/N ratio and decreases in soil temperature, foliar N, and GEI. The absence of a significant decrease in productivity from 80 to 2000 years post-fire suggests that these characteristics reach a steady state early in the chronosequence that persists in the absence of major disturbances or changes in site conditions. These results imply that management practices may not be necessary to maintain boreal forest productivity in the absence of fire on well-drained sites.     

Seed germination and seedling performance of two Mediterranean tree species,holm oak (shape Quercus ilex L.) and Aleppo pine (shape Pinus halepensis Mill.): a multifactor experimental approach

A two-level multifactor experimental approach was used to compare seed germination and seedling performance of two Mediterranean tree species: the early successional Aleppo pine (Pinus halepensis Mill.) and the late successional holm oak (Quercus ilex L.). In a first experiment germination rate was evaluated under the combined effects of shade, nitrogen availability, and pine or holm oak leaf litter. In a second experiment we tested for the effects of shade, nutrient availability, and litter type on seedling survival, growth and biomass allocation. Holm oak showed higher germination rates under shaded than under unshaded conditions, while Aleppo pine showed no differences between shaded and unshaded conditions. Nitrogen availability and litter type had no significant effect on germination of either species. Both species showed increased RGR, but also higher mortality rates, when grown in an enriched nutrient environment. While Aleppo pine showed no differences in RGR and mortality rate under different shading levels, RGR decreased and mortality increased for holm oak in full light. Increased radiation decreased LAR, SLA and height:diameter ratio, and increased RWR and R/S in both species, although Aleppo pine showed more pronounced changes. Unlike Aleppo pine, holm oak responded to increased nutrient availability by decreasing R/S and increasing LAR. From these results, no seed-seedling conflicts were found in either species, but a trade-off does seem to exist for holm oak between biomass allocation traits deployed in response to increased nutrient availability and radiation. Aleppo pine outperformed holm oak under most environmental conditions tested and showed a wider regeneration niche.     

Fire Severity and Long-term Ecosystem Biomass Dynamics in Coniferous Boreal Forests of Eastern Canada

The objective of this study was to characterize the effects of soil burn severity and initial tree composition on long-term forest floor dynamics and ecosystem biomass partitioning within the Picea mariana [Mill.] BSP-feathermoss bioclimatic domain of northwestern Quebec. Changes in forest floor organic matter and ecosystem biomass partitioning were evaluated along a 2,355-year chronosequence of extant stands. Dendroecological and paleoecological methods were used to determine the time since the last fire, the soil burn severity of the last fire (high vs. low severity), and the post-fire tree composition of each stand (P. mariana vs. Pinus banksiana Lamb). In this paper, soil burn severity refers to the thickness of the organic matter layer accumulated above the mineral soil that was not burned by the last fire. In stands originating from high severity fires, the post-fire dominance by Pinus banksiana or P. mariana had little effect on the change in forest floor thickness and tree biomass. In contrast, stands established after low severity fires accumulated during the first century after fire 73% thicker forest floors and produced 50% less tree biomass than stands established after high severity fires. Standing tree biomass increased until approximately 100 years after high severity fires, and then decreased at a logarithmic rate in the millennial absence of fire. Forest floor thickness also showed a rapid initial accumulation rate, and continued to increase in the millennial absence of fire at a much slower rate. However, because forest floor density increased through time, the overall rate of increase in forest floor biomass (58 g m⁻² y⁻¹) remained constant for numerous centuries after fire (700 years). Although young stands (< 200 years) have more than 60% of ecosystem biomass locked-up in living biomass, older stands (> 200 years) sequester the majority (> 80%) of it in their forest floor. The results from this study illustrate that, under similar edaphic conditions, a single gradient related to time since disturbance is insufficient to account for the full spectrum of ecosystem biomass dynamics occurring in eastern boreal forests and highlights the importance of considering soil burn severity. Although fire severity induces diverging ecosystem biomass dynamics in the short term, the extended absence of fire brings about a convergence in terms of ecosystem biomass accumulation and partitioning.     

Conifer seedling recruitment in a southeastern Canadian boreal forest: the importance of substrate

Abstract. In order to explain conifer species recruitment in Canada's southeastern boreal forest, we characterized conifer regeneration microsites and determined how these microsites vary in abundance during succession. Microsite abundance was evaluated in deciduous, mixed and coniferous stands along a 234-yr postfire chronosequence. Conifers were most often found in relatively well-illuminated microsites, devoid of litter, especially broad-leaf litter, and with a reduced cover of lower vegetation (< 50 cm tall). Although associated with moss-rich forest floor substrates, Abies balsamea was the most ubiquitously distributed species. Picea glauca and especially Thuja occidentalis seedlings were frequently found on rotten logs. Light measurements did not show differences among seedling species nor between stand types. The percentage cover of broad-leaf litter decreased significantly during succession. Also, rotten logs covered with moss occupied a significantly larger area in the mid-successionnal stands than in early successional deciduous or late successional coniferous stands. The results suggest that the presence of specific forest floor substrate types is a factor explaining low conifer recruitment under deciduous stands, conifer codominance in the mid-successional stage, and delayed Thuja recolonization after fire. Results also suggest that some facilitation mechanism is responsible for the observed directional succession.     

Nutrient Dynamics in the Litter-Fall and Forest Floor of Evergreen Broad-Leaved Forest and Pinus yunnanensis Forest in Central Yunnan

The quantity of litter-fall and the standing crop of H sublayer in the forest floor of evergreen broad-leaved forest were higher than that of Pinus yunnanensis forest. The quantity of the forest floor changed with the quantity of litter-fall, and thera was a dynamic equilibrium between the litter-fall and forest floor. The content of nutrient elements of the forest floor was higher than that of litter-fall in both types. The order of nutrient element content in the forest floor differed from that in litter-fall. In the evergreen broad-leaved forest, the order was N > Ca > Si>K>Mg>Al>P>Fe>Mn>Zn>Cu,and in Pinus yunnanensis forest it was Ca>N>Si>K>Al>Mg >Fe>P>Mn>Zn>Cu. But the order of nutrient element content in the forest floor had changed: in evergreen broad-leaved forest it was Si > N > Al > Ca > Fe > K>Mg>P>Mn>Zn>Cu, the Pinus yunnanensis forest it was Si > AI>N>Ca>Fe>K>M>P>Mn>Zn>Cu. With increase in the intensity of litter decomposition Ash, Si and Al had obviously increased, N, Fe, P and K had also increased in varying degrees, but there was some reduction in Ca, Zn and Mg. The ratio of carbon and nitrogen in litter decreased' with increase in litter decomposition.     

Afforestation effects on SOC in former cropland: oak and spruce chronosequences resampled after 13 years

Chronosequences are commonly used to assess soil organic carbon (SOC) sequestration after land‐use change, but SOC dynamics predicted by this space‐for‐time substitution approach have rarely been validated by resampling. We conducted a combined chronosequence/resampling study in a former cropland area (Vestskoven) afforested with oak (Quercus robur) and Norway spruce (Picea abies) over the past 40 years. The aims of this study were (i) to compare present and previous chronosequence trends in forest floor and top mineral soil (0–25 cm) C stocks; (ii) to compare chronosequence estimates with current rates of C stock change based on resampling at the stand level; (iii) to estimate SOC changes in the subsoil (25–50 cm); and (iv) to assess the influence of two tree species on SOC dynamics. The two chronosequence trajectories for forest floor C stocks revealed consistently higher rates of C sequestration in spruce than oak. The chronosequence trajectory was validated by resampling and current rates of forest floor C sequestration decreased with stand age. Chronosequence trends in topsoil SOC in 2011 did not differ significantly from those reported in 1998, however, there was a shift from a negative rate (1998: ?0.3 Mg C ha^?1 yr^?1) to no change in 2011. In contrast SOC stocks in the subsoil increased with stand age, however, not significantly (P = 0.1), suggesting different C dynamics in and below the former plough layer. Current rates of C change estimated by repeated sampling decreased with stand age in forest floors but increased in the topsoil. The contrasting temporal change in forest floor and mineral soil C sequestration rates indicate a shift in C source‐sink strength after approximately 40 years. We conclude that afforestation of former cropland within the temperate region may induce soil C loss during the first decades followed by a recovery phase of yet unknown duration.     

The effects of the moss layer on the decomposition of intercepted vascular plant litter across a post-fire boreal forest chronosequence

Aims

Feather mosses form a thick ground layer in boreal forests that can intercept incoming litter fall. This interception may influence the decomposition of incoming litter but this has been little explored. We investigated how the moss layer influences decomposition of intercepted litter along a 362-year fire driven forest chronosequence in northern Sweden across which soil fertility declines.

Methods

We placed leaf litter from three plant species into plots in which mosses and dwarf shrubs were either experimentally removed or left intact, at each of ten stands across the chronosequence. After one year we measured litter mass loss, and litter nitrogen and phosphorous.

Results

Litter decomposed consistently faster, and had higher nitrogen and phosphorus, in the presence of mosses and at greater depth in the moss layer. Despite an increase in moss depth across the chronosequence we did not find consistent increases in effects of moss removal on litter decomposition or on litter N or P.

Conclusions

Our findings identify a clear role of the moss layer in boreal forests in promoting the decomposition of intercepted leaf litter, and highlight that this role is relatively consistent across chronosequence stages that vary greatly in productivity and moss depth.     

Landscape dynamics in Mediterranean oak forests under global change: understanding the role of anthropogenic and environmental drivers across forest types

The Mediterranean region is projected to be extremely vulnerable to global change, which will affect the distribution of typical forest types such as native oak forests. However, our understanding of Mediterranean oak forest responses to future conditions is still very limited by the lack of knowledge on oak forest dynamics and species‐specific responses to multiple drivers. We compared the long‐term (1966–2006) forest persistence and land cover change among evergreen (cork oak and holm oak) and deciduous oak forests and evaluated the importance of anthropogenic and environmental drivers on observed changes for Portugal. We used National Forest Inventories to quantify the changes in oak forests and explored the drivers of change using multinomial logistic regression analysis and an information theoretical approach. We found distinct trends among oak forest types, reflecting the differences in oak economic value, protection status and management schemes: cork oak forests were the most persistent (62%), changing mostly to pines and eucalypt; holm oak forests were less persistent (53.2%), changing mostly to agriculture; and deciduous oak forests were the least persistent (45.7%), changing mostly to shrublands. Drivers of change had distinct importance across oak forest types, but drivers from anthropogenic origin (wildfires, population density, and land accessibility) were always among the most important. Climatic extremes were also important predictors of oak forest changes, namely extreme temperatures for evergreen oak forests and deficit of precipitation for deciduous oak forests. Our results indicate that under increasing human pressure and forecasted climate change, evergreen oak forests will continue declining and deciduous oak forests will be replaced by forests dominated by more xeric species. In the long run, multiple disturbances may change competitive dominance from oak forests to pyrophytic shrublands. A better understanding of forest dynamics and the inclusion of anthropogenic drivers on models of vegetation change will improve predicting the future of Mediterranean oak forests.     

Increasing Red Maple Leaf Litter Alters Decomposition Rates and Nitrogen Cycling in Historically Oak-Dominated Forests of the Eastern U.S.

Without canopy-opening fire disturbances, shade-tolerant, fire-sensitive species like red maple (Acer rubrum L.) proliferate in many historically oak-dominated forests of the eastern U.S. Here, we evaluate potential implications of increased red maple dominance in upland oak forests of Kentucky on rates of leaf litter decomposition and nitrogen (N) cycling. Over 5 years, we evaluated mass loss of leaf litter and changes in total N and carbon (C) within six leaf litter treatments comprised of scarlet oak, chestnut oak, and red maple, and three mixed treatments of increasing red maple contribution to the leaf litter pool (25, 50, and 75% red maple). Over a 1.5-year period, we conducted a plot-level leaf litter manipulation study using the same treatments plus a control and assessed changes in net nitrification, ammonification, and N mineralization within leaf litter and upper (0–5 cm depth) mineral soil horizons. Red maple leaf litter contained more “fast” decomposing material and initially lost mass faster than either oak species. All litter treatments immobilized N during initial stages of decomposition, but the degree of immobilization decreased with decreasing red maple contribution. The leaf litter plot-level experiment confirmed slower N mineralization rates for red maple only plots compared to chestnut oak plots. As red maple increases, initial leaf litter decomposition rates will increase, leading to lower fuel loads and more N immobilization from the surrounding environment. These changes may reduce forest flammability and resource availability and promote red maple expansion and thereby the “mesophication” of eastern forests of the U.S.