Aging Mongolian pine plantations face high risks of drought-induced growth decline:evidence from both individual tree and forest stand measurements

Discerning vulnerability differences among different aged trees to drought-driven growth decline or to mortality is critical to implement age-specific countermeasures for forest management in water-limited areas.An important species for afforestation in dry environments of northern China,Mongolian pine(Pinus sylvestris var.mongolica Litv.)has recently exhibited growth decline and dieback on many sites,particularly pronounced in old-growth plantations.However,changes in response to drought stress by this species with age as well as the underlying mechanisms are poorly understood.In this study,tree-ring data and remotely sensed vegetation data were combined to investigate variations in growth at individual tree and stand scales for young(9-13 years)and aging(35-52 years)plantations of Mongolian pine in a water-limited area of northern China.A recent decline in tree-ring width in the older plantation also had lower values in satellited-derived normalized difference vegetation indices and normalized difference water indices relative to the younger plantations.In addition,all measured growth-related metrics were strongly correlated with the self-calibrating Palmer drought severity index during the growing season in the older plantation.Sensitivity of growth to drought of the older plantation might be attributed to more severe hydraulic limitations,as reflected by their lower sapwood-and leaf-specific hydraulic conductivities.Our study presents a comprehensive view on changes of growth with age by integrating multiple methods and provides an explanation from the perspective of plant hydraulics for growth decline with age.The results indicate that old-growth Mongolian pine plantations in water-limited environments may face increased growth declines under the context of climate warming and drying.

Responses of biomass to the addition of water, nitrogen and phosphorus in Keerqin sandy grassland, Inner Mongolia, China

沙的草地的生产率上的水，氮和磷的效果被调查与一充分发现在东南的 Keerqin 砂土限制草地生产率的自然恢复的主要因素的因子试验。总共，八个处理作为水增加(W)被设计，氮肥增加(N)，磷化肥增加(P)，水+氮肥增加( WN )，水+磷化肥增加( WP )，氮肥+磷化肥增加( NP )，浇+氮肥+磷化肥增加( WNP )和控制( CK )。每个处理被复制六次并且随机把阴谋分到 48 (4 m × 4 m ) 那被一个 2-m 缓冲区分开。生产率的恢复被氮因素仅仅为 Keerqin 砂土并且不的沙的草地限制的结果表演走水路限制了并且磷。相对 CK 阴谋，生物资源并且未葬所有阴谋的主要生产率(ANPP ) 与氮肥增加了的网显著地被提高(P 【 0.05 ) 在 2005 生长期。草根质量在地下的生物资源是主导的。现在的学习可能低估在北中国捕捞草地的主要生产率，由于地下的生物资源大小的限制。

Small-Scale Spatial Variability of Soil Nutrients and Vegetation Properties in Semi-Arid Northern China

A field experiment was conducted at Kezuohouqi County, Inner Mongolia Autonomous Region of China, which was located on the southeastern edge of the Horqin Sandy Land, to study the spatial variability of soil nutrients for a smallscale, nutrient-poor, sandy site in a semi-arid region of northern China; to investigate whether or not there were 'islands of fertility' at the experimental site; and to determine the key nutrient elements that sustained ecosystem stability. Results obtained from geostatistical analysis indicated that the spatial distribution pattern of soil total nitrogen (STN) was far different from those of soil organic matter (SOM), total phosphorus (STP), and total potassium (STK). Compared to SOM, STP, and STK, STN had a lower structural heterogeneity ratio and a longer range, while other elements were all similar. In addition, STN had an isotropic spatial structure, whereas the others had an anisotropic spatial structure. The spatial structure patterns of herbage species, cover,and height also differed, indicating that spatial variability was subjected to different ecological factors. Differences in the spatial variability patterns among soil nutrients and vegetation properties showed that soil nutrients for a small-scale were not the primary limiting factors that influenced herbage spatial distribution patterns. Incorporating spatial distribution patterns of tree species, namely, Pinus sylvestris var. mongolica Litv. and shrub Lespedeza bicolor Turcz. in a research plot and using fractal dimension,SOM, STP, and STK were shown to contribute to the 'islands of fertility' phenomenon, however STN was not, possibly meaning that nitrogen was a key limiting element. Therefore, during restoration of similar ecosystems more attention should be given to soil nitrogen.

Soil animals and nitrogen mineralization under sand-fixation plantations in Zhanggutai region,China

The effects of soil animals on soil nitrogen (N) mineralization and its availability were studied by investigating soil animal groups and their amounts of macro-faunas sorted by hand, and middle and microfaunas distinguished with Tullgren and Baermann methods under three Pinus sylvestris var. mongolica Litv. plantations in Zhanggutai sandy land, China. In addition, soil N mineralization rate was also measured with PVC closed-top tube in situ incubation method. The soil animals collected during growing season belonged to 13 orders, 5 groups, 4 phyla, whose average density was 86 249.17 individuals·m^(-2). There were significant differences in soil animal species, densities, diversities and evenness among three plantations. Permanent grazing resulted in decrease of soil animal species and diversity. The average ammonification, nitrification and mineralization rates were 0.48 g·m^(-2)·a^(-1), 3.68 g·m^(-2)·a^(-1) and 4.16 g·m^(-2)·a^(-1), respectively. The ammonification rate in near-mature forest was higher than that in middle-age forests, while the order of nitrification and net mineralization rates was: middle-age forest without grazing < middle-age forest with grazing < near-mature forest with grazing (P<0.05). Soil N mineralization rate increased with soil animal amounts, but no significant relationship with diversity. The contribution of soil animals to N mineralization was different for different ecosystems due to influences of complex factors including grazing, soil characteristics, the quality and amount of litter on N mineralization.

Effect of Land Cover Change on Soil Phosphorus Fractions in Southeastern Horqin Sandy Land, Northern China

In the past 50 years, large areas of the Horqin sandy land were afforested to prevent desertification. Although the afforestation policy appears successful, many people now doubt whether it is suitable to plant trees with high density on the poor soils in semiarid regions. Little is known about the impacts of afforestation on the sandy soil properties, although the evaluation of these impacts is fundamental to judge the rationality of afforestation policy. Soil phosphorus (P) fractions, acid phosphomonoesterase activities, and other soil chemical properties were compared among five adjoining typical ecosystems on poor sandy soils in southeastern Horqin sandy land. The ecosystems studied are natural elm savanna, degraded grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, Chinese pine (Pinus tabulaeformis) plantation, and mixed plantation of Mongolian pine and poplar (Populus simonii). The results showed that organic P dominated soil P (47%–65%) was the principal source of available P. The degradation of elm savanna to grassland significantly reduced soil pH and resulted in an overall reduction in soil fertility, although slightly increased labile inorganic P. Grassland afforestation had no significant influence on soil pH, organic carbon, and total N but significantly reduced total P. Impacts of grassland afforestation on soil P fractions depended on tree species. Natural elm savanna had higher soil P conserving ability than artificial plantations. Therefore, with the aim of developing a sustainable ecosystem, we suggested that vegetations with low nutrient demand (particularly P) and effcient nutrient cycling would be more suitable for ecosystem restoration in the semiarid region.

Soil Inorganic Nitrogen and Microbial Biomass Carbon and Nitrogen Under Pine Plantations in Zhanggutai Sandy Soil

The dynamics of soil inorganic nitrogen (NH4+ -N and NO-3 -N) and microbial biomass carbon (Cmic) and nitrogen (Nmic) under 30-year-old fenced Pinus sylvestris L. var. mongolica Litvin (SF), unfenced P. sylvestris L. var. mongolica Litvin (SUF), and unfenced Pinus densiflora Siebold et Zucc. (DUF) plantations in the Zhanggutai sandy soil of China were studied during Apr. to Oct. 2004 by the in situ closed-top core incubation method. All mentioned C and N indices in each stand type fluctuated over time. The ranges of inorganic N, Cmic, and Nmic contents in the three stand types were 0.7–2.6, 40.0–128.9, and 5.4–15.2 μg g-1, respectively. The average contents of soil NH4+ -N and Cmic under the three 30-year-old pine plantations were not different. However, soil NO3 -N and total inorganic N contents decreased in the order of SUF ≥ SF ≥ DUF, the Nmic content was in the order of SF = SUF > DUF, and the Cmic:Nmic ratio was in the order of SUF = DUF > SF. Seasonal variations were observed in soil inorganic N, microbial biomass, and plant growth. These seasonal variations had certain correlations with microbe and plant N use in the soil, and their competition for NH+4 -N was mostly regulated by soil N availability. The influence of tree species on inorganic N and Nmic were mainly because of differences in litter quality. Lack of grazing decreased the Cmic:Nmic ratio owing to decreased carbon output and increased the ability of soil to supply N. The soil N supply under the P. sylvestris var. mongolica plantation was lower than under the P. densiflora plantation.

Effects of ultraviolet(UV) radiation and litter layer thickness on litter decomposition of two tree species in a semi-arid site of Northeast China

Forests and grasslands in arid and semi-arid regions receive high-intensity ultraviolet（UV） radiation year-round. However, how the UV radiation affects the litter decomposition on the forest floor remains unclear. Here, we conducted a field-based experiment in 2011 in the southeastern Horqin Sandy Land, Northeast China, to investigate the effects of UV radiation, litter layer thickness, and their interaction on the mass loss and chemical properties of decomposing litter from Xiaozhuan poplar（Populus × xiaozhuanica） and Mongolian pine（Pinus sylvestris var. mongolica） plantation trees. We found that UV radiation accelerated the decomposition rates of both the Xiaozhuan poplar litter and Mongolian pine litter. For both species, the thick-layered litter had a lower mass loss than the thin-layered litter. The interaction between UV radiation and litter layer thickness significantly affected the litter mass loss of both tree species. However, the effects of UV radiation on the chemical properties of decomposing litter differed between the two species, which may be attributed to the contrasting initial leaf litter chemical properties and morphology. UV radiation mostly had positive effects on the lignin concentration and lignin/N ratio of Xiaozhuan poplar litter, while it had negative effects on the N concentration of Mongolian pine litter. Moreover, litter layer thickness and its interaction with UV radiation showed mostly positive effects on the N concentration and lignin/N ratio of Xiaozhuan poplar litter and the ratios of C/N and lignin/N of Mongolian pine litter, and mostly negative effects on the C/N ratio of Xiaozhuan poplar litter and the N concentration of Mongolian pine litter. Together, these results reveal the important roles played by UV radiation and litter layer thickness in the process of litter decomposition in this semi-arid region, and highlight how changes in the litter layer thickness can exert strong influences on the photodegradation of litter in tree plantations.

Phosphorus fractions and phosphomonoesterase activities in sandy soils under a temperate savanna and a neighboring Mongolian pine plantation

To assess the effects of savanna afforestation on soil phosphorus （P） transformations in eastern Horqin Sandy Land, China, P fractions and phosphomonoesterase activities were examined in two soil horizons （0-5 cm and 5-20 cm） under a savanna and an adjacent 30-year-old Mongolian pine （Pinus sylvestris L. vat. mongolica Litv.） plantation on a P-deficient semi arid sandy soil. The results showed that all soil P fractions and phosphomonoestcrase activities decreased with soil depth at both sites except that labile organic P under the plantation was constant with soil depth, In contrast to savanna, soils trader Mongolian pine plantation had lower phosphomonoesterase activities and concentrations of all P fractions （with an exception of Al-P）, lower proportions of organic P and Ca-P in total P, and higher proportions of labile P, Al-P and Fe-P in total P. These results suggested that P transformations mainly occurred in surface soils, and P recycled through litterfall was the most important source of plant available P. Mongolian pine afforestation enhanced the bioavailability of both organic P and Ca-P. simultaneously reduced soil P pools. indicating that protection of forest floor and P fertilization are necessary to maintain the sustainable functioning of Mongolian pine plantations.

Decomposition of Mongolian pine litter in the presence of understory species in semi-arid northeast China

The effects of understory plant litter on domi- nant tree litter decomposition are not well documented especially in semi-arid forests. In this study, we used a microcosm experiment to examine the effects of two understory species （Artemisia scoparia and Setaria viridis） litter on the mass loss and N release of Mongolian pine （Pinus sylvestris var. mongolica） litter in Keerqin Sandy Lands, northeast China, and identified the influencing mechanism from the chemical quality of decomposing litter. Four litter combinations were set up： one monocul- ture of Mongolian pine and three mixtures of Mongolian pine and one or two understory species in equal mass proportions of each species. Total C, total N, lignin, cel- lulose and polyphenol concentrations, and mass loss of pine litter were analyzed at days 84 and 182 of incubation.The chemistry of pine litter not only changed with the stages of decomposition, but was also strongly influenced by the presence of understory species during decomposition. Both understory species promoted mass loss of pine litter at 84 days, while only the simultaneous presence of two understory species promoted mass loss of pine litter at 182 days. Mass loss of pine litter was negatively correlated with initial ratios of C/N, lignin/N and polyphenol/N of litter combinations during the entire incubation period; at 182 days it was negatively correlated with polyphenol concentration and ratios of C/N and polyphenol/N of litter combinations at 84 days of incubation. Nitrogen release of pine litter was promoted in the presence of understory species. Nitrogen release at 84 days was negatively correlated with initial N concentration; at 182 days it was negatively correlated with initial polyphenol concentration of litter combinations and positively correlated with lignin concentration of litter com- binations at 84 days of incubation. Our results suggest that the presence ofunderstory species causes substantial changes in chemical components of pine litter that can exert strong influences on subsequent decomposition of pine litter.

Rhizosphere organic phosphorus fractions of Simon poplar and Mongolian pine plantations in a semiarid sandy land of northeastern China

The aim of this study was to investigate the role of rhizosphere organic phosphorus （P） in soil P supply in semiarid forests and the effects of tree species on rihizosphere organic P. We examined organic P fractions in rhizosphere and bulk soils of mono-specific Simon poplar （Populus simonii） and Mongolian pine （Pinus sylvestris var. mongolica） plantations in a semiarid sandy soil of Horqin Sandy Land in Northeast China. Total organic P （TPo） accounted for 76% of total P across the two stands. The concentration of organic P （Po） fractions decreased in the order of NaOH-Po〉Res-Po〉HCI-Po〉NaHCO3-Po in both plantations. The concentration of NaHCO3-Po was 38% and 43% lower in rhizosphere soil than in bulk soil in Simon poplar and Mongolian pine plantations, respectively. In contrast, total P, TPo and NaOH-Po significantly accumulated in rhizosphere soil in Simon poplar plantations, but no change in Mongolian pine plantations. Soil recalcitrant organic P fractions were positively correlated with soil organic carbon. The results suggest that rhizosphere labile organic P was an important source of plant-available P in this semiarid region, but the dynamic of rhizosphere recalcitrant organic P fractions varied with tree species and was correlated to organic carbon dynamics.

Responses of plant diversity and species composition to the cessation of fertilization in a sandy grassland

Nitrogen was the main limiting nutrient of net primary production in the southeastern Keerqin Sandy Lands, Northeast China. Species richness declined and biomass increased after five consecutive years of nitrogen fertilization of these sandy grasslands （2004-2008）. After fertilization had been stopped for three years （2009-2011）, we surveyed vegetation on previously fertilized plots to quantify changes in commu- nity composition. Respect species richness showed an increasing trend over time since the cessation of fertilization. Respect vegetation height and coverage showed decreasing trends over time since the cessation of fertilization. Species composition changed after fertilization ceased, the dominant species shifting from Cannabis sativa, Phragmites communis and Chenopodium acuminaturn in 2008 to Cannabis sativa, Phragmites communis and Artemisia scoparia in 2011. Dominance of dominant species declined from 66.2% in 2008 to 57.5% in 2011. The importance value of annual plants in the earlier nitrogen addition plots was higher than in control plots, but the differences were not significant in 2011. The importance value of perennial plants differed significantly between treatments from 2009 to 2011. The reversion rate not only differed be- tween community characteristics, but also between functional groups in the same community characteristic. Although the residual effect of nitrogen addition on vegetation was still observed three years after fertilization ceased, the vegetation showed signs of recovery.

Three-dimensional (3D) structure model and its parameters for poplar shelterbelts

The spatial functions of surface area density(vegetative surface area per unit canopy volume) and cubic density(vegetative volume per unit canopy volume) have been used as two three-dimensional(3D) structural descriptors for shelterbelt.The functions were defined by models as a general case.However,sub-models such as surface area,volume,and corresponding distributions were not explicitly defined for poplar trees,which are a dominant woody species in shelterbelts all over China,and this limits applications of the models in China and elsewhere.In order to define and develop these sub-models for shelterbelts,poplar trees were destructively sampled from multiple-row shelterbelts and then were measured for their surface area and volume.Using these measurements,we estimated parameters to define their equations explicitly.Based on the architecture and planting patterns of trees in shelterbelts,the distribution of the surface areas and volumes vertically and across the width for different tree heights were constructed for the three components of trunks,branches and leaves.Incorporating the defined equations into the models,we described the 3D structure of a multiple-row poplar shelterbelt.The results showed that,the spatial change in magnitude of surface area density(0.215-10.131 m2/m3) or cubic density(0.00007-0.04667 m3/m3) in shelterbelts is large and their distributions are not uniform.The assumption for boundary-layer flow modeling efforts that the 3D distribution of shelterbelt structure was uniform is not the case in field.The 3D structure model not only can be used to model the flow field as influenced by each tree component,but also can express the entire aerodynamic characteristics of a shelterbelt.The methodologies and equations that are developed in this study can be applied to estimate the 3D structure of a shelterbelt with a design similar to our studied poplar shelterbelts in terms of species composition and planting patterns.The fitted models can be used to describe the 3D aerodynamic structure of field shelterbelts.Furthermore,an improved description of shelterbelt structure has great potential to improve the simulation of boundary layer flows as influenced by that shelterbelt.Such insights can eventually be used to quantify the design of shelterbelt structure and/or adjustment for managing the function of shelterbelts and their effects on microclimate.

Establishment of mixed plantations of Pinus sylvestris var.mongolica and Populus×xiaozhuanica may not be appropriate:evidence from litter decomposition

Aims Mongolian pine(Pinus sylvestris var.mongolica)and Xiaozhuan poplar(Populus×xiaozhuanica)are two predominant afforestation tree species in the semi-arid sandy lands of northeast china,which are characterized by poor soil nutrients.Plant litter decomposition plays a critical role in regulating nutrient cycling in terrestrial ecosystems.Admixture of broadleaf litter to conifer litter is expected to improve litter decomposition and soil fertility,and thus productivity.However,the effects on the decomposition of litter mixture of the above two tree species are not well understood.therefore,it is essential to assess the decomposition performance of litter mixture with the aim of improving forest nutrient management and the establishment of mixed plantation.Appropriate forest manage-ment practice is critical for the sustainability of site productivity in plantation forests.Methods We conducted a field litterbag decomposition transplant experi-ment for single pine litter,single poplar litter and their mixture in a pine stand,a poplar stand and an adjacent grassland for 16 months in the Keerqin Sandy Lands,northeast china.Important Findings After 16 months of incubation,there remained significantly more litter mass of pine(73.8%)than of poplar(67.2%).the mass remaining was positively correlated with litter carbon(c):nitrogen(N),c:phosphorus(P)and lignin:N ratios,and negatively with litter N and P concentrations,which suggests that initial litter chemical properties were an important factor affecting litter decay.Generally,net N and P immobilizations were observed during decomposition.this indicates that litter decomposition in this area was N-limited as N was progressively immobilized,and then tended to induce P limitation.thus,we strongly recommend prohibiting litter harvesting by local residents to maintain soil fertility in this nutrient-poor area.Our results do not support the home-field advantage hypothesis,as illustrated by the fact that,in most cases,mass loss of litter from native habitat was comparable to that in transplanted habitats during decomposition.Furthermore,a dominant additive effect was detected,indicating that the establishment of mixed plantation may not be appropriate for these two species.

Leaf traits indicate survival strategies among 42 dominant plant species in a dry, sandy habitat, China

The objective of this paper was to assess the congruency of leaf traits and soil characteristics and to analyze the survival strategies of different plant functional types in response to drought and nutrient-poor environ-ments in the southeastern Ke’erqin Sandy Lands in China.Six leaf traits—leaf thickness(TH),density(DN),specific leaf area(SLA),leaf dry weight to fresh weight ratio(DW/FW),leaf N concentration(N mass),and N resorption efficiency(NRE_(mass))-of 42 plant species were investi-gated at four sites.The correlations between leaf traits and soil characteristics-organic C(OC),total N(TN),total P(TP),and soil moisture(SM)-were examined.We found that the six leaf traits across all the 42 species showed large variations and that DW/FW was negatively correlated with OC,TN,TP,and SM(P<0.05),while other leaf traits showed no significant correlations with soil characteristics.To find the dissimilarity to accommodate environment,a hierarchical agglomerative clustering analysis was made of all the species.All the species clustered into three groups except the Scutellaria baicalensis.Species of group III might be most tolerant of an arid environment,and species of group II might avoid nutrient stress in the nutrient-poor environment,while group I was somewhat intermediate.Therefore,species from the different groups may be selected for use in vegetation restoration of different sites based on soil moisture and nutrient conditions.

High adaptability of Pinus sylvestris var.mongolica to drought-induced soil nutrient deficiency

Background:Drought can exert a profound influence on soil nutrient availability,and understanding whether and how tree species adapt to this change is a critical priority for predicting the consequence of climate change on forest structure and function.The objective of this study was to examine the adaptability of Mongolian pine(Pinus sylvestris var.mongolica)to drought‑induced changes in soil nutrient availability from the perspective of root functions.Methods:We conducted a 7‑year precipitation manipulation experiment with three levels of throughfall reduction(0%,30%,and 50%)to simulate different drought intensities.We measured soil physicochemical properties and fineroot nutrient concentrations and biomass,and calculated the stoichiometric homeostatic regulation coefficient(1/H)of fine roots.Results:Drought reduced soil organic carbon(C),nitrogen(N),phosphorous(P)and inorganic N concentrations,as well as ratios of total N to total P,and available N to available P in the 0–20 cm soil layer.In contrast,drought had no significant effect on fine‑root N and P concentrations,and fine‑root biomass in the 0–40 cm soil layer.Fine roots displayed high homeostatic regulation coefficients of N(with 1/H values of 0.19 and 0)and P(with 1/H values of 0.33 and 0)concentrations in 0–20 and 20–40 cm soil layers,respectively.Conclusions:Our results indicate that drought leads to soil nutrient deficiency and the decoupling between N and P cycling,and provide evidence that Mongolian pine has high adaptability to drought‑induced decrease in soil nutrient availability by maintaining great fine‑root biomass to ensure sufficient nutrient uptake.