Litter decomposition rate is dependent on litter Mn concentrations

A statistically significant linear relationship was found between annual mass loss of foliar litter in the late stages of decomposition and Mn concentration in the litter. We used existing decomposition data on needle and leaf decomposition of Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta var. contorta), Norway spruce (Picea abies (L.) Karst.), silver birch (Betula pendula L.), and grey alder (Alnus incana L.) from Sweden and Aleppo pine (Pinus halepensis Mill.) from Libya, to represent boreal, temperate, and Mediterranean climates. The later the decomposition stage as indicated by higher sulfuric-acid lignin concentrations, the better were the linear relationships between litter mass loss and Mn concentrations. We conclude that Mn concentrations in litter have an influence on litter mass-loss rates in very late decomposition stages (up to 5 years), provided that the litter has high enough Mn concentration. The relationship may be dependent on species as the relationship is stronger with species that take up high enough amounts of Mn.     

Carbon sequestration rates in organic layers of boreal and temperate forest soils — Sweden as a case study

Aim The aim of this work was to estimate C sequestration rates in the organic matter layer in Swedish forests. Location The region encompassed the forested area (23 × 10⁶ ha) of Sweden ranging from about 55° N to 69° N. Methods We used the concept of limit values to estimate recalcitrant litter remains, and combined it with amount of litter fall. Four groups of tree species were identified (pine, spruce, birch and ‘other deciduous species’). Annual actual evapotranspiration (AET) was estimated for 5 × 5 km grids covering Sweden. For each grid, data of forested area and main species composition were available. The annual input of foliar litter into each grid was calculated using empirical relationships between AET and foliar litter fall in the four groups. Litter input was combined with average limit values for decomposition for the four groups of litter, based on empirical data. Finally, C sequestration rate was calculated using a constant factor of the C concentration in the litter decomposed to the limit value, thus forming soil organic matter (SOM). Results We obtained a value of 4.8 × 10⁶ metric tons of C annually sequestered in SOM in soils of mature forests in Sweden, with an average of 180 kg ha^?1 and a range from 40 to 410 kg ha^?1. Norway spruce forests accumulated annually an average of 200 kg C ha^?1. The pine and birch groups had an average of 150 kg ha^?1 and for the group of other deciduous trees, which is limited to south Sweden, the C sequestration was around 400 kg ha^?1. Conclusions There is a clear C sequestration gradient over Sweden with the highest C sequestration in the south‐west, mainly corresponding to the gradient in litter fall. The limit‐value method appears useful for scaling up to a regional level to describe the C sequestration in SOM. A development of the limit value approach in combination with process‐orientated dynamic models may have a predictive value.     

Factors influencing limit values for pine needle litter decomposition: a synthesis for boreal and temperate pine forest systems

We synthesized available data for decomposition of pine (Pinus) needle litter in pine forests to determine the litter chemical characteristics and climate factors that explained variation in the limit value, i.e. the level of accumulated mass loss at which the decomposition process either continues at a very low rate or possibly stops. Our data base included 56 separate studies on decomposition of pine needle litter, spanning Scots pine, lodgepole pine, Aleppo pine, stone pine and white pine, mainly incubated at the site of collection. Studies had 5 to 19 samplings, on average 10, and the decomposition was followed to a mass loss ranging from 47 to 83%, on average 67%. The periods from 3.0 to 5.4 years, on average 3.9 years, were of sufficient duration to allow estimates of limit values of decomposition. We used a linear mixed model with regression effects to relate limit values to potential explanatory variables, namely the sites’ long-term mean annual temperature (MAT) and mean annual precipitation (MAP) and to substrate-chemistry factors. Regarding the latter, we explored two models; one that included initial concentrations of water solubles, lignin, N, P, K, Ca, Mg, and Mn and one that included only lignin, N, Ca, and Mn to focus on those nutrients known to influence lignin degradation. Using backward elimination significant explanatory variables were determined. For litter decomposed in its site of origin we found the limit value to depend mainly on the initial concentration of Mn, with higher Mn concentrations resulting in higher accumulated mass loss. Thus, litter with higher Mn reached a higher limit value and left a smaller stable fraction. This is likely due to the fact that Mn is an essential component of ligninolytic enzymes important for degrading litter in the later stages of decomposition. Manganese has received little attention in decomposition studies to date. Given its significance in this synthesis, the role of Mn in influencing variation in the late stages of decomposition among ecosystems and among litters of other genera besides Pinus deserves further attention.     

Advances in the positional cloning of nodulation genes in soybean

The effect of liming on the decomposition of Norway spruce needle litter was studied in 40–60-year-old Norway spruce stands. Finely-ground limestone had been spread about 30 years ago at a dose of 2 t ha^–1 and reliming was carried out about 20 yr later at a dose of 4 t ha^–1. Needle litter was collected from both control and limed plots, and it was placed in litter bags in the middle of the humus layer of the plot from which they originated, and similarly to the other plot in May. Litter bags were sampled after 4, 12 and 16 months. The site of origin of the needle litter, whether from control plot or from limed plot, affected mainly the early stages of decomposition. Initially the effect of liming was seen as decreased concentration of water soluble material and then, during decomposition, as decreased mass loss and decreased degradation of lignin, and increased C/N ratio. The incubation site, whether the control or the limed plot, did not affect decomposition significantly.Decomposition of Scots pine needles in a young Scots pine plantation was also studied. The treatments were: 2 t ha^–1 of finely-ground limestone and 2.5 t ha^–1 of bark ash spread 8 months before this study. The treatments did not affect decomposition much, but some stimulation of the treatments on decomposition was observed. Compared to spruce needles, the C/N ratio of pine seedles was lower, they contained less lignin and more water soluble material, and decomposed faster in the first summer.     

Nitrogen availability affects saprotrophic basidiomycetes decomposing pine needles in a long term laboratory study

Fungi, especially basidiomycetous litter decomposers, are pivotal to the turnover of soil organic matter in forest soils. Many litter decomposing fungi have a well-developed capacity to translocate resources in their mycelia, a feature that may significantly affect carbon (C) and nitrogen (N) dynamics in decomposing litter. In an eight-month long laboratory study we investigated how the external availability of N affected the decomposition of Scots pine needles, fungal biomass production, N retention and N-mineralization by two litter decomposing fungi – Marasmius androsaceus and Mycena epipterygia. Glycine additions had a general, positive effect on fungal biomass production and increased accumulated needle mass loss after 8 months, suggesting that low N availability may limit fungal growth and activity in decomposing pine litter. Changes in the needle N pool reflected the dynamics of the fungal mycelium. During late decomposition stages, redistribution of mycelium and N out from the decomposed needles was observed for M. epipterygia, suggesting autophagous self degradation.     

Sensitivity of French temperate coniferous forests to climate variability and extreme events (Abies alba,Picea abies and Pinus sylvestris)

Questions: (1) How do extreme climatic events and climate variability influence radial growth of conifers (silver fir, Norway spruce, Scots pine)? (2) How do elevation and soil water capacity (SWC) modulate sensitivity to climate? Location: The sampled conifer stands are in France, in western lowland and mountain forests, at elevations from 400 to 1700 m, and an SWC from 50 to 190 mm. Methods: We established stand chronologies for total ring width, earlywood and latewood width for the 33 studied stands (985 trees in total). Responses to climate were analysed using pointer years and bootstrapped response functions. Principal component analysis was applied to pointer years and response function coefficients in order to elucidate the ecological structure of the studied stands. Results: Extreme winter frosts are responsible for greater growth reductions in silver fir than in Norway spruce, especially at the upper elevation, while Scots pine was the least sensitive species. Exceptional spring droughts caused a notable growth decrease, especially when local conditions were dry (altitude<1000 m and SWC<100 mm for silver fir, western lowlands for Scots pine). Earlywood of silver fir depended on previous September and November and current‐year February temperature, after which current June and July water supply influenced latewood. Earlywood of Norway spruce was influenced by previous September temperature, after which current spring and summer droughts influenced both ring components. In Scots pine, earlywood and latewood depended on the current summer water balance. Local conditions mainly modulated latewood formation. Conclusions: If the climate becomes drier, low‐elevation dry stands or trees growing in western lowlands may face problems, as their growth is highly dependent on soil moisture availability.     

Direct and indirect effects of climate on decomposition in native ecosystems from central Argentina

Abstract Climate affects litter decomposition directly through temperature and moisture, determining the ecosystem potential decomposition, and indirectly through its effect on plant community composition and litter quality, determining litter potential decomposition. It would be expected that both the direct and indirect effects of climate on decomposition act in the same direction along gradients of actual evapotranspiration (AET). However, studies from semiarid ecosystems challenge this idea, suggesting that the climatic conditions that favour decomposition activity, and the consequent ecosystem potential decomposition, do not necessarily lead to litter being easier to decompose. We explored the decomposition patterns of four arid to subhumid native ecosystems with different AET in central‐western Argentina and we analysed if ecosystem potential decomposition (climatic direct effect), nutrient availability and leaf litter potential decomposition (climatic indirect effect) all increased with AET. In general, the direct effect of climate (AET) on decomposition (i.e. ecosystem potential decomposition), showed a similar pattern to nutrient availability in soils (higher for xerophytic and mountain woodlands and lower for the other ecosystems), but different from the pattern of leaf litter potential decomposition. However, the range of variation in the ecosystem potential decomposition was much higher than the range of variation in litter potential decomposition, indicating that the direct effect of climate on decomposition was far stronger than the indirect effect through litter quality. Our results provide additional experimental evidence supporting the direct control of climate over decomposition, and therefore nutrient cycling. For the ecosystems considered, those with the highest AET are the ecosystems with the highest potential decomposition. But what is more interesting is that our results suggest that the indirect control of climate over decomposition through vegetation characteristics and decomposability does not follow the same trend as the direct effect of climate. This finding has important implications in the prediction of the effects of climate change on semiarid ecosystems.     

Do vascular plants and bryophytes respond differently to coniferous invasion of coastal heathlands?

Invasion by non-native conifers may pose a threat to local biodiversity, but knowledge about introduced conifer effects on Northern Hemisphere ecosystems is scarce. The coastal heathlands of north-west Europe are threatened by invasion of native and introduced tree species. We assess how spread of the introduced conifer Sitka spruce (Picea sitchensis (Bong.) Carr.) into European coastal heathlands affect two major functional groups; vascular plants and bryophytes, and how these effects relate to the environmental changes imposed by the developing tree canopies. We compared the impact of introduced Sitka spruce and native Scots pine (Pinus sylvestris L.) by analysing effects on species richness and turnover of vascular plants and bryophytes along fine-scale transects from individual tree stems into open heathland vegetation. Environmental impacts were assessed by measured environmental variables, and the responses of the two species groups were assessed by calculating changes in their respective mean Ellenberg indicator values. Species richness decreased beneath both conifers, related to decreased light and increased nitrogen and pH. Whereas vascular plants responded negatively to poor light conditions beneath dense and low Sitka spruce canopies, bryophytes were more negatively affected by the warmer and drier microclimates beneath Scots pine. Introduced Sitka spruce impacts the sub-canopy environment differently from the native Scots pine, and the two functional plant groups responded differently to these impacts. This suggests that future forests are likely to differ in species richness and composition, depending on whether succession is based on native or introduced coniferous trees.     

The effect of single tree species on soil microbial activities related to C and N cycling in the Siberian artificial afforestation experiment

The effects of grassland conversion to forest vegetation and of individual tree species on microbial activity in Siberia are largely unstudied. Here, we examined the effects of the six most commonly dominant tree species in Siberian forests (Scots pine, spruce, Arolla pine, larch, aspen and birch) on soil C and N mineralization, N₂O-reduction and N₂O production during denitrification 30 years after planting. We also documented the effect of grassland conversion to different tree species on microbial activities at different soil depths and their relationships to soil chemical properties. The effects of tree species and grassland conversion were more pronounced on N than on C transformations. Tree species and grassland conversion did significantly alter substrate-induced respiration (SIR) and basal respiration, but the differences were not as large as those observed for N transformations. Variances in SIR and basal respiration within species were markedly lower than those in N transformations. Net N mineralization, net nitrification, and denitrification potential were highest under Arolla pine and larch, intermediate under deciduous aspen and birch, and lowest beneath spruce and Scots pine. Tree species caused similar effects on denitrification potential, net N mineralization, and net nitrification, but effects on N₂O reduction rate were idiosyncratic, indicating a decoupling of N₂O production and reduction. We predict that deciduous species should produce more N₂O in the field than conifers, and that Siberian forests will produce more N₂O if global climate change alters tree species composition. Basal respiration and SIR showed inverse responses to tree species: when basal respiration increased in response to a given tree species, SIR declined. SIR may have been controlled by NH₄ ⁺ availability and related therefore to N mineralization, which was negatively affected by grassland conversion. Basal respiration appeared to be less limited by NH₄ ⁺ and controlled mostly by readily available organic C (DOC), which was higher in concentration under forests than in grassland and therefore basal respiration was higher in forested soils. We conclude that in the Siberian artificial afforestation experiment, soil C mineralization was not limited by N.     

Differential selection of North American and Scandinavian conifer browse by northwestern moose (Alces alces andersoni) in winter

Scandinavian moose (Alces alces) eat Scots pine (Pinus sylvestris) in winter. Although North American moose are known to eat conifers such as true firs (Abies spp.) in winter, substantial consumption of pine by moose in North America has not been documented. Here, we document short-term winter preferences of human-habituated northwestern moose (Alces alces andersoni) for branches of mature North American and European conifer species as determined by a cafeteria-style feeding trial. Moose selected for species such as Douglas fir (Pseudotsuga menziesii; from which they took the smallest bite diameters) while avoiding species such as lodgepole pine (Pinus contorta; from which they took the largest bites) and hybrid white spruce (Picea glauca × engelmanii). The amount of species-specific biomass consumed by moose was negatively correlated with bite diameters taken from branches of those species and did not appear to be significantly influenced by differences in twig morphology between species. Our trial suggests that northwestern moose readily consume conifers in winter and, from the species we tested, prefer Douglas fir. While no clear preference existed between Scots pine and lodgepole pine, moose avoided lodgepole pine, but not Scots pine, relative to Douglas fir. Our trial suggests that northwestern moose are more likely to feed on the branches of Douglas fir than pine, which may be of interest to foresters managing conifers within the North American range of moose, particularly where Scots pine are being considered for planting.     

Cross‐biome transplants of plant litter show decomposition models extend to a broader climatic range but lose predictability at the decadal time scale

We analyzed results from 10‐year long field incubations of foliar and fine root litter from the Long‐term Intersite Decomposition Experiment Team (LIDET) study. We tested whether a variety of climate and litter quality variables could be used to develop regression models of decomposition parameters across wide ranges in litter quality and climate and whether these models changed over short to long time periods. Six genera of foliar and three genera of root litters were studied with a 10‐fold range in the ratio of acid unhydrolyzable fraction (AUF, or ‘lignin’) to N. Litter was incubated at 27 field sites across numerous terrestrial biomes including arctic and alpine tundra, temperate and tropical forests, grasslands and warm deserts. We used three separate mathematical models of first‐order (exponential) decomposition, emphasizing either the first year or the entire decade. One model included the proportion of relatively stable material as an asymptote. For short‐term (first‐year) decomposition, nonlinear regressions of exponential or power function form were obtained with r² values of 0.82 and 0.64 for foliar and fine‐root litter, respectively, across all biomes included. AUF and AUF : N ratio were the most explanative litter quality variables, while the combined temperature‐moisture terms AET (actual evapotranspiration) and CDI (climatic decomposition index) were best for climatic effects. Regressions contained some systematic bias for grasslands and arctic and boreal sites, but not for humid tropical forests or temperate deciduous and coniferous forests. The ability of the regression approach to fit climate‐driven decomposition models of the 10‐year field results was dramatically reduced from the ability to capture drivers of short‐term decomposition. Future work will require conceptual and methodological improvements to investigate processes controlling decadal‐scale litter decomposition, including the formation of a relatively stable fraction and its subsequent decomposition.     

Comparative dendroclimatic study of Scots pine, Norway spruce, and silver fir in the Vrancea Range, Eastern Carpathian Mountains

Using dendroclimatical methods, we compared the growth response to climate fluctuations of three of the main Romanian Carpathian Mountains coniferous species, Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies Karst.) and silver fir (Abies alba Mill.), growing intermixed in a unique stand. Climate and soil conditions were therefore the same for all the trees studied. The experimental site was chosen to be representative of the south-exposed sites in the Romanian Carpathian Mountains, where pine occurs naturally. In order to investigate the consequences of fluctuations in climate at different temporal scales, we examined both inter-annual and decadal time-steps. An index of soil water deficit was computed to investigate the consequences of drought. Our study reveals that species exhibited different responses to precipitation, temperature or drought. Overall, pine was the most sensitive to precipitation, while spruce showed a higher response to temperature at high frequency for both the current and the previous growing seasons, and to soil water deficit. Fir was the least sensitive species of the comparison. However, for all species, decadal modulations of growth show precipitation as a common and strong driver on the dry south-facing slopes. The results show that Scots pine would be affected more than fir by increased drought frequency and could in the future be replaced on the xeric sites.     

The effects of SO2 and O3 on the foliar nutrition of Scots pine, Norway spruce and Sitka spruce in the Liphook open-air fumigation experiment

Foliar elements were analysed in Scots pine, Sitka spruce and Norway spruce over a 6 year period before and during continuous exposure to SO₂ and O₃ in an open-air fumigation experiment. Sulphur dioxide treatment elevated foliar sulphur concentration in all species, and there were increases in foliar nitrogen in the two spruce species but not in pine. The concentrations of cations were frequently increased by SO₂ treatment, but there was no correlation between the sulphur concentration of needles and their total cation charge. SO₂-related elevations of foliar magnesium were correlated with the concentration of this element in soil solution, but the mechanism by which other cations were enhanced remains unclear. The only consistent effects on nutrient ratios were for SO₂ treatments to increase sulphur/cation ratios.     

Limit values for plant litter decomposing in two contrasting soils—influence of litter elemental composition

The decomposition dynamics of four types of needle litter and three types of leaf litter were followed for 3 years at two very contrasting coniferous forest systems, a nutrient-rich silver fir (Abies alba Mill.) forest in south Italy (Monte Taburno) and a nutrient-poor Scots pine (Pinus sylvestris L.) forest in central Sweden (Jädraås). Decomposition of the same litter type at the two sites did not differ in the early stages but proceeded further at the nutrient-rich forest site than at the nutrient-poor one. Limit values for decomposition were calculated and the differences for the same litter type between the two contrasting coniferous systems were investigated. At both sites six of the seven litter types gave significant (asymptotic) limit values for decomposition, which varied with litter type. For four litter types out of six the limit values differed significantly between the two sites and were always higher at the nutrient-rich site (Monte Taburno). Using all available data for litters incubated at the two sites revealed that at the nutrient-poor site (Jädraås) there was a significant negative relationship between litter N levels and limit values and there was also a significant negative relationship between initial concentrations of heavy metals (e.g. Zn, Cd, Cu) and limit values. In contrast, at the site Monte Taburno, rich in nutrients and in heavy metals, there was no such relationship.     

Decomposition of secondary compounds from needle litter of Scots pine grown under elevated CO2 and O3

Elevated atmospheric carbon dioxide (CO₂) and ozone (O₃) concentrations have both been shown to affect plant tissue quality, which in turn could affect litter decomposition and carbon (C) and nutrient cycling. In order to evaluate effects of climate change on litter chemistry, needle litter was collected from Scots pine (Pinus sylvestris L.) saplings exposed to elevated CO₂ or O₃ concentration and their combination over three growing seasons in open‐top chambers. The decomposition of needle litter was followed for 19 months in a pine forest. During decomposition, needle samples for secondary compound analysis were collected and the mass loss of needles was followed. Main nutrients and total phenolics were analysed from litter in the beginning and at the end of the experiment. After 19‐month decomposition, the accumulated mass loss was about 34%; however, no significant differences were found in decomposition rates of needle litter between various treatments. Concentrations of total monoterpenes were about 4%, total resin acids 21% and total phenolics 14% of the initial concentrations in litter after 19‐month decomposition. In the beginning of litter decomposition, concentrations of individual monoterpenes –α‐pinene and β‐pinene – were significantly higher in needle litter grown under elevated CO₂. However, concentrations of total monoterpenes during the whole decomposition period were not significantly affected by CO₂ or O₃ treatments. Concentrations of some individual and total resin acids were higher in needle litter grown under elevated CO₂ or O₃ than under ambient air. There were no significant differences in concentrations of total phenolics as well as nitrogen (N) and the main nutrient concentrations between treatments during decomposition. High concentrations of monoterpenes and resin acids in needles might slightly delay C recycling in forest soils. It is concluded that elevated CO₂ and O₃ concentrations do not have remarkable impacts on litter decomposition processes in Scots pine forests.     

Tree Species Traits Influence Soil Physical,Chemical, and Biological Properties in High Elevation Forests

Background

Previous studies have shown that plants often have species-specific effects on soil properties. In high elevation forests in the Southern Rocky Mountains, North America, areas that are dominated by a single tree species are often adjacent to areas dominated by another tree species. Here, we assessed soil properties beneath adjacent stands of trembling aspen, lodgepole pine, and Engelmann spruce, which are dominant tree species in this region and are distributed widely in North America. We hypothesized that soil properties would differ among stands dominated by different tree species and expected that aspen stands would have higher soil temperatures due to their open structure, which, combined with higher quality litter, would result in increased soil respiration rates, nitrogen availability, and microbial biomass, and differences in soil faunal community composition.

Methodology/Principal Findings

We assessed soil physical, chemical, and biological properties at four sites where stands of aspen, pine, and spruce occurred in close proximity to one-another in the San Juan Mountains, Colorado. Leaf litter quality differed among the tree species, with the highest nitrogen (N) concentration and lowest lignin∶N in aspen litter. Nitrogen concentration was similar in pine and spruce litter, but lignin∶N was highest in pine litter. Soil temperature and moisture were highest in aspen stands, which, in combination with higher litter quality, probably contributed to faster soil respiration rates from stands of aspen. Soil carbon and N content, ammonium concentration, and microbial biomass did not differ among tree species, but nitrate concentration was highest in aspen soil and lowest in spruce soil. In addition, soil fungal, bacterial, and nematode community composition and rotifer, collembolan, and mesostigmatid mite abundance differed among the tree species, while the total abundance of nematodes, tardigrades, oribatid mites, and prostigmatid mites did not.

Conclusions/Significance

Although some soil characteristics were unaffected by tree species identity, our results clearly demonstrate that these dominant tree species are associated with soils that differ in several physical, chemical, and biotic properties. Ongoing environmental changes in this region, e.g. changes in fire regime, frequency of insect outbreaks, changes in precipitation patterns and snowpack, and land-use change, may alter the relative abundance of these tree species over coming decades, which in turn will likely alter the soils.     

High rate of adaptive evolution in two widespread European pines

Comparing related organisms with differing ecological requirements and evolutionary histories can shed light on the mechanisms and drivers underlying genetic adaptation. Here, by examining a common set of hundreds of loci, we compare patterns of nucleotide diversity and molecular adaptation of two European conifers (Scots pine and maritime pine) living in contrasted environments and characterized by distinct population genetic structure (low and clinal in Scots pine, high and ecotypic in maritime pine) and demographic histories. We found higher nucleotide diversity in Scots pine than in maritime pine, whereas rates of new adaptive substitutions (ω_a), as estimated from the distribution of fitness effects, were similar across species and among the highest found in plants. Sample size and population genetic structure did not appear to have resulted in significant bias in estimates of ω_a. Moreover, population contraction–expansion dynamics for each species did not affect differentially the rate of adaptive substitution in these two pines. Several methodological and biological factors may underlie the unusually high rate of adaptive evolution of Scots pine and maritime pine. By providing two new case studies with contrasting evolutionary histories, we contribute to disentangling the multiple factors potentially affecting adaptive evolution in natural plant populations.     

Climate‐diameter growth relationships of black spruce and jack pine trees in boreal Ontario,Canada

To predict the long‐term effects of climate change – global warming and changes in precipitation – on the diameter (radial) growth of jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana [Mill.] B.S.P.) trees in boreal Ontario, we modified an existing diameter growth model to include climate variables. Diameter chronologies of 927 jack pine and 1173 black spruce trees, growing in the area from 47°N to 50°N and 80°W to 92°W, were used to develop diameter growth models in a nonlinear mixed‐effects approach. Our results showed that the variables long‐term average of mean growing season temperature, precipitation during wettest quarter, and total precipitation during growing season were significant (alpha = 0.05) in explaining variation in diameter growth of the sample trees. Model results indicated that higher temperatures during the growing season would increase the diameter growth of jack pine trees, but decrease that of black spruce trees. More precipitation during the wettest quarter would favor the diameter growth of both species. On the other hand, a wetter growing season, which may decrease radiation inputs, increase nutrient leaching, and reduce the decomposition rate, would reduce the diameter growth of both species. Moreover, our results indicated that future (2041–2070) diameter growth rate may differ from current (1971–2000) growth rates for both species, with conditions being more favorable for jack pine than black spruce trees. Expected future changes in the growth rate of boreal trees need to be considered in forest management decisions. We recommend that knowledge of climate–growth relationships, as represented by models, be combined with learning from adaptive management to reduce the risks and uncertainties associated with forest management decisions.     

雪被斑块对川西高山森林凋落叶N和P释放的影响

采用凋落物网袋法,研究冬季不同关键时期雪被斑块对川西高山森林6种代表性树种凋落物分解过程中N和P释放的影响.结果表明: 整个雪被覆盖季节凋落物N表现为富集,P表现为释放,且雪被融化期P释放速率最大.厚型和中型雪被斑块下凋落物P释放速率大于薄型和无雪被斑块,而薄型和无雪被斑块下凋落物的N释放速率明显较高.6种凋落物N释放率和释放速率与日均温呈显著负相关;除岷江冷杉外,其他树种凋落物P释放率和释放速率与日均温表现为正相关.气候变暖情景下冬季雪被覆盖的减小将促进高山森林冬季凋落物分解过程中N释放,抑制P释放.     

Composition of riparian litter input regulates organic matter decomposition: Implications for headwater stream functioning in a managed forest landscape

Although the importance of stream condition for leaf litter decomposition has been extensively studied, little is known about how processing rates change in response to altered riparian vegetation community composition. We investigated patterns of plant litter input and decomposition across 20 boreal headwater streams that varied in proportions of riparian deciduous and coniferous trees. We measured a suite of in‐stream physical and chemical characteristics, as well as the amount and type of litter inputs from riparian vegetation, and related these to decomposition rates of native (alder, birch, and spruce) and introduced (lodgepole pine) litter species incubated in coarse‐ and fine‐mesh bags. Total litter inputs ranged more than fivefold among sites and increased with the proportion of deciduous vegetation in the riparian zone. In line with differences in initial litter quality, mean decomposition rate was highest for alder, followed by birch, spruce, and lodgepole pine (12, 55, and 68% lower rates, respectively). Further, these rates were greater in coarse‐mesh bags that allow colonization by macroinvertebrates. Variance in decomposition rate among sites for different species was best explained by different sets of environmental conditions, but litter‐input composition (i.e., quality) was overall highly important. On average, native litter decomposed faster in sites with higher‐quality litter input and (with the exception of spruce) higher concentrations of dissolved nutrients and open canopies. By contrast, lodgepole pine decomposed more rapidly in sites receiving lower‐quality litter inputs. Birch litter decomposition rate in coarse‐mesh bags was best predicted by the same environmental variables as in fine‐mesh bags, with additional positive influences of macroinvertebrate species richness. Hence, to facilitate energy turnover in boreal headwaters, forest management with focus on conifer production should aim at increasing the presence of native deciduous trees along streams, as they promote conditions that favor higher decomposition rates of terrestrial plant litter.