Vegetation C–N–P accumulation and allocation patterns at the community level in early restored plantations in the loess hilly-gully region

Huifeng Wu, Baoan Hu, Ying Ma, Wenkai Shi, Xiaoqin Cheng, Fengfeng Kang,Hairong Han,＊

a School of Ecology and Nature Conservation, Beijing Forestry University, Beijing, 100083, China

b Qilaotu Mountain National Observation and Research Station of Chinese Forest Ecosystem, Chifeng, 024400, China

c Shanxi Forestry and Grassland Engineering Station, Taiyuan, 030000, China

Keywords:

ABSTRACT Accumulation of vegetation biomass is a crucial process for carbon fixation in the early stage of afforestation and a primary driving force for subsequent ecological functions.Accurately assessing the storage and allocation of elements in plantations is essential for their management and estimating carbon sink capacity.However, current knowledge of the storage and allocation patterns of elements within plant organs at the community level is limited.To clarify the distribution patterns of elements in plant organs at the community level,we measured the biomass within plant organs of five typical plantations in the early stage of afforestation in the loess hilly-gully region.We assessed the main drivers of element accumulation and distribution by employing redundancy analysis and random forest.Results revealed significant differences in biomass storages among plantations and a significant effect of plantation type on the storages of elements within plant organs.Furthermore, the dominant factors influencing C–N–P storage and allocation at the community level were found to be inconsistent.While the storage of elements was mainly influenced by stand openness, total soil nitrogen, and plant diversity, the allocation of elements in organs was mainly influenced by stand openness and soil water content.Overall,the spatial structure of the community had an important influence on both element storage and allocation,but soil conditions played a more important role in element allocation than in storage.Random forest results showed that at the community level, factors influencing element storage and allocation within plant organs often differed.The regulation of elemental storage could be regulated by the major growth demand resources, while the allocation was regulated by other limiting class factors, which often differed from those that had a significant effect on element storage.The differences in plant organ elemental storage and allocation drivers at the community level reflect community adaptation strategies and the regulation of resources by ecosystems in combination with plants.Our study provides valuable insights for enhancing plantation C sink estimates and serves as a reference for regulating element storage and allocation at the local scale.

1.Introduction

Afforestation is an important ecological restoration project that is commonly used worldwide and has been expected to act as a carbon(C)sink to mitigate climate change in recent years(Naudts et al.,2016;Tong et al., 2018; Di Sacco et al., 2021).Afforestation has been proven to increase C sequestration in different dimensions of ecosystems(Wang and Huang, 2020; Wang et al., 2021), while also significantly altering the C allocation patterns of ecosystems(Litton et al.,2007;Lutter et al.,2016).However, restored ecosystems primarily store C in the above-ground portion of the whole ecosystem during the early stages of plantation(Uri et al.,2014;Lutter et al.,2016).This means that in early plantations,nutrient sequestration by the plant community is an important C fixation mechanism.Different C allocation at the organ level could lead to large differences in estimates of global woody carbon fluxes and stocks (Ise et al., 2010; Jevon and Lang, 2022; Zhou et al., 2022), which has significant implications for forest management and policies aimed at increasing carbon sequestration.

The storage and allocation of vegetation biomass in terrestrial ecosystems connects the active biochemical processes(photosynthesis)and long-term storage patterns(carbon sequestration)of ecosystem material cycling processes and is an important hub of the material cycling process.For example,the rate of decomposition and the factors that drive it vary across different plant organs(Guo et al., 2023), which suggests that the allocation of biomass plays a critical role in determining the efficiency of nutrient cycling in ecosystems.Moreover, biomass storage and allocations of biomass are related to a variety of ecological functions such as plant growth,reproduction,and defense(Zhao et al.,2019),and are also a reflection of species resource acquisition and environmental adaptation strategies(Zhang et al.,2020a).

Plants usually allocate biomass reasonably among leaves,stems,and roots according to their growth requirements, which are regulated by several key factors, including nutrient availability, light, and water(Poorter et al., 2012; Yue et al., 2021).At large scales climate and weather conditions can have a significant impact on the growth and productivity of plants.For example,drought conditions can limit biomass production, while favorable growing conditions can lead to increased biomass storage and allocation.In the global forest carbon stock,44%is attributed to the soil(0–1 m),while 42%is allocated to the living plants above and below ground(Pan et al.,2011).Given the prevailing context of global climate change, it is imperative to recognize the vital significance of the belowground component (comprising soil and roots) as it serves both as a C pool and a critical determinant of plant growth within the forest ecosystem.Drought and elevated CO2could encourage plants to increase root biomass allocation to obtain water and nutrients (Eziz et al., 2017).Nitrogen (N) deposition and rainfall changes also alter vegetation above-and belowground biomass allocation patterns(Li et al.,2019).There were also studies showing that temperature drives the global-scale allocation patterns of forest biomass among leaves, stems,and roots in plants(Reich et al.,2014).In addition,the age of the forest can also affect the C allocation pattern of the entire forest ecosystem and plant organs (Fonseca et al., 2011; Deng et al., 2017).Older trees typically store more biomass and allocate more resources for the production and storage of biomass compared to younger trees(Zhou et al.,2016).In plantations,the management practices of reforestation could also impact the storage and allocation of biomass (Du Toit, 2008).For instance,practices like thinning and pruning helped to promote growth and biomass production, while improper management led to reduced productivity and biomass storage (Ge et al., 2015).Additionally, fertilizer application affected stand growth and nutrient use efficiency,enhanced wood density (Sicard et al., 2006), and thus influenced the spatial allocation of biomass(Rance et al.,2009).

The ratio-based optimal partitioning hypothesis and the isometric allocation hypothesis are widely recognized as biomass allocation patterns at the individual plant level(Bloom et al.,1985;Niklas,2005).The biomass allocation patterns and the factors influencing them at the community level are still in need of further exploration.In addition, in recent years more attention has been paid to the allocation of C, while less attention has been paid to the storage and allocation of important nutrients for plant growth, such as N and phosphorus (P), especially upscaling from the species to community level (Zhao et al., 2019).The distribution of plant biomass is accompanied by the allocation of multiple elements, and stoichiometric coupling among C, N, and P is crucial for maintaining growth-related ecosystem functions (Wieder et al., 2015;Tian et al., 2019; Zhang et al., 2020a).This is important for accurately assessing C sink storage in plantation forests and for reasonably regulating the C sink function.

While numerous studies have explored the relationship between forest biomass and allocation concerning climate on a large scale, little variation in climate exists at small scales.Temperature and rainfall are not applicable for regulating biomass storage and allocation at the scale of plantations.Additionally, what is easy to regulate at this scale is the community's spatial structure and soil water and fertilizer conditions.The hypotheses were as follows: (1) different types of plantations significantly impact the elemental storage and allocation at the community level; (2) factors influencing element storage and allocation in various plant organs at the community level vary; (3) the storage and allocation of elements at the community level are governed by distinct factors, which may differ from those at the individual level.We conducted a systematic ecological survey on five types of plantations in the Loess Plateau which are widely distributed afforestation.This study aimed to clarify the biomass storage and allocation of the plantation ecosystem at the level of plant organs, and investigate the effects of environmental factors, including community spatial structure, soil physical and chemical properties such as moisture content (SMC), bulk density (BD), particle size, pH, N and P content, on the element distribution (C–N–P).It is hoped that our research can promote a more accurate estimation of C storage and provide guidance for regulating the storage and allocation of plant organ-level elements at the local scale.

2.Materials and methods

2.1.Plantation location and environmental variables

Our study was conducted at Wuqi Forestry Farm, Poplar High-yield Forest Bureau, Shanxi Province, China (Fig.1).The study area is located in the hilly and gully region of the Loess Plateau (Wu et al.,2022),and covers a total experimental area of 1.56 ha(40°07′–40°24′N,112°52′–113°31′E).The region is characterized by a significant presence of artificial forests consisting of large quantities of Pinus tabuliformis(PT),Pinus sylvestris(PS),and Caragana korshinskii (CaK).The annual average precipitation and temperature in the area are 350 mm and 7.3°C(http://data.cma.cn), and the climate belongs to the semi-arid continental climate.

In July 2021, we conducted a comprehensive ecological field investigation on the experimental site.This investigation encompassed various aspects, including the planting time, geographical location, vegetation growth status, soil physicochemical properties, and community spatial structure of the plantation (Table S1).We conducted a sampling and investigation on five plantations, comprising three monoculture plantations(PT,PS,CaK)and two mixed plantations(PT+CaK and PS+CaK).Each plot was established with a size of 20 m × 20 m, containing five nested subplots of shrubs with an area of 5 m×5 m and ten subplots of herbs with an area of 1 m × 1 m.We recorded information for all the plants within the sample plot, including species name, quantity, height,diameter at breast height, crown width, and other relevant details.We computed Shannon's diversity index (H) to assess the species diversity within the community.Furthermore, we utilized a canopy analyzer(WinSCANOPY 2010) to quantify and analyze the structural features of the forest interior space,obtaining important information such as forest canopy openness and the forest canopy photosynthetic photon flux density(PPFD)(Wu et al.,2023).We collected mixed soil samples from the 0–20 cm soil layer within the plantation for analysis of pH as well as nutrient content, specifically soil organic carbon (SOC), soil total nitrogen (STN), and soil total phosphorus (STP).We used Mastersizer 3000(Malvern Instruments, Malvern, England) laser particle size analyzer to determine the soil mechanical composition of mixed soil samples, and obtained the median particle size of the soil(median particle size,MPS).In addition,we used a 100 cubic centimeter soil corer to collect intact soil samples for analysis of soil physical properties such as moisture content(SMC), and bulk density (BD).The pretreatment and methods used to determine soil physical and chemical properties were based on the measurement methods outlined in Bao(2000).

2.2.Vegetation sampling and nutrient analysis

Fig.1.Location and distribution map of sampling sites in the Loess Plateau.The abbreviations for artificial forest types are as follows:PT,Pinus tabuliformis;PS,Pinus sylvestris; CaK, Caragana korshinskii; PT + CaK, Mixed plantations of Pinus tabuliformis and Caragana korshinskii; PS + CaK, Mixed plantations of Pinus sylvestris and Caragana korshinskii.

We conducted a biomass survey of all vegetation in the target plantation plot using the standard wood method,the full harvest method,and the biomass empirical equation model(Wu et al.,2022).Diverging from earlier research, our present investigation centers on scrutinizing the allocation patterns of afforestation species and their derived plants in plantation settings, alongside exploring how modifications to planting techniques may influence the growth and biomass proportion of co-existing plant species.Additionally,our study places greater emphasis on the allocation of nutrient reserves at the plant organ level in plantations.To calculate nutrient reserves more accurately, we distinguished between new and mature leaves and branches and collected and measured the nutrient content of each type separately.The nutrient content of the branches and trunks of the trees was measured,and they were aggregated into the stem component when calculating nutrient storage.When sampling plant rhizomes and measuring nutrient content,the distinction between coarse and fine roots was made, but they were combined when calculating organ nutrient reserves.When summarizing the biomass allocation of the whole plantation system, we included the above-ground parts of herbaceous plants in the leaf category.The nutrient content of each plant organ sample was determined after digestion.Carbon content was determined using the potassium dichromate oxidation method (Bao, 2000).The plant's total nitrogen and phosphorus were measured with SEAL AutoAnalyzer3 (Seal Analytic,Germany)following the operating specifications.

2.3.Statistical analyses

One-way ANOVA was performed to compare the differences in biomass and elemental storage among different artificial forests.Before the analysis, normality, and homogeneity of variance were tested, and the LSD multiple comparison method was used to indicate the significance of differences.As the dimensional difference between different environmental factors was large,data standardization was conducted in subsequent analyses.

Redundancy analysis (RDA) was performed using CANOCO 5 (htt p://www.canoco.com/) to explore the effects of environmental factors on elemental storage and allocation.Additionally,a random forest model was used to predict the effects of environmental factors on elemental storage and allocation,and“incmse”(average variance gain)was used to evaluate the relative importance of different environmental factors.Specifically,incmse refers to the increase in the average variance of the entire random forest after splitting a certain feature compared to the average variance when that feature is not split during the construction of the random forest.Therefore,a larger incmse indicates a more important feature for the prediction ability of the random forest.The random forest model was built using the‘rfPermute’package in R 4.2.1.

3.Results

3.1.Biomass storage and allocation characteristics within plantation communities

We compared the total biomass and spatial allocation of biomass in plantations with different vegetation types(Table 1).The results showed that there are significant differences in total biomass among different plantations.After mixed planting of P.tabuliformis, P.sylvestris, and C.korshinskii,the overall biomass decreased significantly.Although CaK had the smallest total biomass(19.54±2.98 t?ha-1),its root system and litter biomass were higher compared to other plantations.In contrast,the monoculture plantation of PT had the largest leaf biomass(6.42 ± 0.64 t?ha-1) which was significantly greater than other plantations.The monoculture plantation of PS had the largest stem biomass(38.40±1.87 t?ha-1) which was significantly greater than other plantations.The monoculture plantations of PT and CaK had the largest root biomass(7.77 ± 2.24 and 7.44 ± 0.80 t?ha-1, respectively), which were significantly greater than other plantations.Additionally, the monoculture plantation of PT had the largest litter biomass(5.61±1.22 t?ha-1)which was significantly greater than other plantations.In summary,changes in vegetation types had a significant impact on the biomass of different organs in plantations.

In addition to the total biomass storage of the plantation, we also examined the biomass allocation patterns of artificially planted species and other co-occurring natural plant species in the plantation community, as well as the proportion of biomass allocation among different organs within the whole community (Fig.2).Although monoculture plantations of PT and PS exhibited significantly higher total biomasscompared to mixed plantations, our study revealed that the biomass of natural plants in mixed plantations accounted for a greater proportion.Therefore, planting mixed plantations was a viable option to obtain a larger proportion of natural plant biomass.Additionally, the results indicated that after the mixture of P.tabuliformis, P.sylvestris, and C.korshinskii,the proportion of biomass in the stem decreased,while the proportion of biomass in the leaves, roots, and litter increased.The allocation law of biomass storage showed that vegetation types changed the allocation pattern of biomass in the entire plant community within the plantation.

Table 1 Biomass allocation and difference of different types of plantations(Mean±SD).

3.2.C–N–P storage and allocation characteristics within plantation communities

We showed the storage and allocation proportions of C–N–P in different plant organs across various plantation plant communities(Fig.3).Overall,the reserve sizes and allocation proportions of different elements varied across plantation communities.The CaK plantation had the smallest carbon storage, but had high levels of N and P reserves.Conversely, monoculture plantations of PT and PS, which had higher C storage, exhibited lower levels of nitrogen.In terms of C storage and allocation, the stems accounted for the largest proportion, especially in arbor monoculture plantations, where the proportion was significantly higher than in other types of plantations.Compared to C,the storage and allocation of N increased overall in leaf and root storage, while the proportion of stem storage decreased.However, the storage and allocation of P were similar to that of N,as seen in the case of P.tabulaeformis and P.sylvestris, transitioning from monoculture to mixed plantations resulted in a reduction of stem proportion to a certain extent and an increase in root proportion.In general, alterations in plantation types resulted in changes not only to the size of element storages,but also had a pronounced effect on the allocation patterns of elements within the plant community.

3.3.The relationship between environmental factors and whole-community C–N–P storage and allocation in plantations

The impact of environmental factors,including forest spatial structure and soil water and fertilizer status,on the storage and allocation of C,N,and P in plantations was analyzed using RDA (Fig.4).We conducted a conditional effect analysis to evaluate the R2values of environmental factors and determine their impact on target biological indicators(Fig.4a).Our results showed that the most significant environmental factors influencing the nutrient element (C–N–P) storage of the entire community were openness(Explains=58.8%,p=0.002),STN(Explains=12.8%,p=0.002),and H(Explains=5.8%,p=0.002).However,in the individual effect results, openness, PPFD, STN, and SMC had a significant impact on the storage of elements in plantation plant communities,with respective contributions of 67.1%,44.8%,42.6%,21.5%,and 13.0%.

We found that under the conditional effects (Fig.4b), the environmental factors that had a significant impact on the allocation of elements(C–N–P) in plant organs in the entire plant community were openness(Explains=4.2%,p=0.002),SMC(Explains)=19.4%,p=0.002,and stand density (SDNY) (Explains = 6.5%, p = 0.002).However, in the results of individual effects, openness, SMC, STN, PPFD, and stand density significantly impacted the nutrient allocation of plant organs in artificial forest communities,contributing 24.2%,20.0%,15.0%,14.9%,and 13.7%,respectively.

3.4.Random forest modeling reveals drivers of C–N–P storage and allocation across plant organs in plantations

In this study, we used the ‘rfPermute’ random forest algorithm to perform feature selection and determine the relative importance of each feature, which enabled us to identify the most useful features for predicting and evaluating element reserves and allocation.The figures(Figs.5 and 6)display the relative importance of various environmental factors in the storage and allocation of C–N–P in different plant organs in early plantations.

Fig.3.Organ-level allocation of carbon,nitrogen,and phosphorus in the whole plant community across different types of plantations.We compared the differences in element reserves at the same organ level and denoted statistically significant differences at the 0.05 level using different lowercase letters.

Fig.4.Relationships between environmental factors and whole-plant community C/N/P storages (a) and allocation at the organ-level (b): results of redundancy analysis (RDA).MPS, soil median particle size; SMC, moisture content; SOC, soil organic carbon; STN, soil total nitrogen; STP, soil total phosphorus; PPFD, forest photosynthetic photon flux density; BD, soil bulk density.

Fig.5.Contribution and importance of environmental factors in predicting plant community organ- and element-specific storages using random forest models, with significant relationships indicated by asterisks at p < 0.05.BD, soil bulk density; H, Shannon's diversity index; MPS, soil median particle size; OPEN, forest canopy openness; PPFD, forest photosynthetic photon flux density; SNDY, stand density; SMC, moisture content; SOC, soil organic carbon; STN, soil total nitrogen; STP, soil total phosphorus.

The overall results of the random forest showed the relative importance of different environmental factors for predicting the storage of C–N–P in different organs varied (Fig.4).Specifically, openness and PPFD were found to be significantly important for predicting C storage in leaves, while leaves N storage was mainly influenced by SMC, and P storage in leaves was more influenced by soil T/N/P content.Regarding stems, H, SDNY, and STP were found to have significant roles in predicting C storage, while stems N storage was mainly influenced by PPFD, SDNY, and MPS, and P storage in stems was more influenced by community spatial structure such as PPFD, SDNY, openness, and H.In terms of litter, soil nutrient content and plant diversity played an important role in predicting carbon storage,while N storage had a better response relationship with community structural characteristics such as PPFD and H.Additionally, PPFD and STP had an important role in P storage of the litter,and SMC and STN also had a greater influence.As for roots,N storage had a better correspondence with environmental factors and was more influenced by factors such as PPFD, stand density, SMC,and STN, while SMC was more important for root C storage.However,environmental factors overall were less effective in predicting root P storage (R2=24.05).

The allocation of C–N–P storage in different organs of the community was influenced by various environmental factors(Fig.6),and this result significantly differed from the relationship between storage and environmental factors observed in Fig.5.MPS and STN were significant predictors of C allocation pattern in leaves of the community,while leaf N allocation was primarily influenced by stand density and MPS.The allocation of P storage,on the other hand,was mainly affected by spatial structure factors such as openness,PPFD,and stand density.H and stand density played significant and essential roles in predicting the allocation of stem C storage within the community.Meanwhile, stand density and SOC had significant effects on the allocation of stem N storage,whereas P storage was more influenced by soil factors,such as MPS,STP,and STN.Soil nutrient content and pH were both essential predictors of C and N storage allocation in the litter with better response relationships.In contrast,the allocation of P storage in the litter was mainly influenced by community structure factors such as H and openness.The allocation of root N storage in the community corresponded well with the community structure,especially PPFD,stand density,and H,which were particularly important for the allocation of root C and N storage.While environmental factors as a whole had an important influence on the prediction of roots P storage,it was more related to soil properties such as MPS,pH,SOC,and STN.

Fig.6.Contribution and importance of environmental factors in predicting plant community organ-and element-specific allocation using random forest models,with significant relationships indicated by asterisks at p < 0.05.BD, soil bulk density; H, Shannon's diversity index; MPS, soil median particle size; OPEN, forest canopy openness; PPFD, forest photosynthetic photon flux density; SNDY, stand density; SMC, moisture content; SOC, soil organic carbon; STN, soil total nitrogen; STP, soil total phosphorus.

4.Discussion

4.1.Ecological interpretation of the allocation of biomass and storage of C–N–P elements in plantation ecosystems

Biomass storage and allocation in plantations are crucial for ecosystem function and stability(Aerts and Honnay,2011;Hulvey et al.,2013; Yu et al., 2020).By understanding how biomass is stored and allocated in plantations, we can better assess their productivity and carbon sequestration capacity.Our results indicated that while the total biomass storage and allocation at the organ level varied among different plantations,the plant organ biomass ratio exhibited more stability at the community level.This could be a reflection of the ecosystem's precise regulation of environmental resources.Water scarcity, in particular, remains a dominant factor in biomass distribution at the plant organ level on the Loess Plateau.Spatial structure plays a significant role in both vegetation type response and species distribution, affecting the distribution of essential resources such as light and water, which are critical for plant growth and development(Laliberte et al.,2008;Hitsuma et al.,2021).Furthermore, species diversity is the initial expression of the result of multiple factors within the community,which ultimately causes changes in the distribution pattern of elemental stocks at the community level in the form of species.

In addition, the results of different vegetation types in plantations show that the type of vegetation significantly influenced the biomass stock of the plantation community, particularly between monoculture and mixed forests (Table 1).We also observed that the choice of silvicultural species had a significant impact on the biomass.Different growth habits and biomass reserve strategies of each tree species contributed to the significant differences in plant organ biomass allocation at the community level (Figs.2 and 3).In our redundancy analysis results(Fig.4a), we found that openness and PPFD significantly affected elemental storage, which corresponds to the significant impact of vegetation type on biomass.The introduction of C.korshinskii into the plantation resulted in an enriched vertical structure and an increased understory PPFD, as the mixing of tree species significantly altered the spatial structure of the stand.Specifically, the biomass distribution of C.korshinskii was different from that of P.tabuliformis and P.sylvestris,and the increased planting of C.korshinskii changed the biomass distribution pattern at the community level.Moreover, the change in community spatial structure after the introduction of C.korshinskii provided more living space for plants and altered resource allocation,as reflected in the RDA results, where species diversity made an important contribution to biomass allocation (Fig.4).In addition, soil properties are critical factors in plant growth and can serve as limiting conditions for the distribution of various plants (Pintaldi et al., 2016; Zhang et al.,2021).In areas such as the Loess Plateau,where soils are poor,STN,pH,and SMC all play a crucial role in the distribution of plant species and the allocation strategy of plant resources.Consequently,these soil properties can significantly impact the elemental storage of the community.Previous research has shown that N and P concentrations were higher in actively growing sites than in structural sites(Yang and Luo,2021).This finding explains the clear variation in N and P allocation in plant organs in Fig.2,particularly in leaves and roots,where growth activity is higher.This confirms our second hypothesis that the factors influencing element storage and allocation in various plant organs are different.We also conducted an interesting analysis where we compared the biomass of plants in different plantations, including those naturally occurring(remaining plants) in addition to the artificially planted species.We observed that the proportion of remaining plant biomass increased in mixed plantations, which we attribute to their more open spatial structure that facilitates the growth of natural plants.The presence of natural herbaceous plants in the forest understory also has a significant water and soil conservation effect (Zhang et al., 2020b, 2021).Our findings suggest that forestry management should consider selecting more derived plants to maximize ecological benefits,especially when resources are limited.

4.2.Drivers of C–N–P element storages and allocation at the organ level in plantation ecosystems

The rational allocation of resources is a strategy used by plants to adapt to their environment (Bonser, 2013; Yan et al., 2016a; She et al.,2017).They allocate biomass and elements among their leaves, stems,roots, and other structures in response to environmental conditions(Poorter et al., 2012; Reich et al., 2014; Yan et al., 2016a).We used a random forest model to assess more precisely the impact of local-scale environmental characteristics on the storage and allocation of C–N–P in plant organs(see Figs.4 and 5).Our findings revealed that community spatial structures,such as openness and PPFD,were crucial for predicting leaf C/P storage,while SMC played a significant role in predicting leaf N storage.The reason for this is that an open spatial structure with high PPFD can provide more light energy,promoting photosynthesis and plant growth,which,in turn,increases plant carbon fixation.And SMC has an important influence on plant N uptake and N distribution in soils (He et al., 2021; Song et al., 2022).The planting density of tree species greatly determines the proportion of different species in the mixed forest,which in turn affects the C storage of stems.Meanwhile, N and P are nutrients absorbed by plants from the soil, which is inherently poor in this region,and the community structure is a redistribution of resources that influences the storage of N and P.Soil water and fertilizer conditions can influence the decomposition rate and biodiversity of the litter (Liu et al., 2016; Wang et al.,2017;Petraglia et al.,2018), which in turn affects the C,N and P storages of the litter.And PPFD has a greater effect on the C and P storages of the litter because photosynthesis is an important energy source for plant growth and nutrient cycling,and PPFD can affect plant photosynthesis (Naumburg and Ellsworth, 2002), influence plant growth, and change the input source of the litter and thus change its C and P storage.Additionally, water is essential for plant growth and metabolism,and it has an impact on the activity of microorganisms in the soil (Ahmed et al., 2019).Higher soil moisture helps to increase the activity of plant roots(Gao et al.,2018),which in turn increases their C and N storage.The spatial structure of the community is a response to the spatial distribution and positional relationships among species within the community, influencing light and root distribution, which in turn influences plant photosynthesis and growth rate,and thus the carbon and nitrogen storage of the root system.The poor overall fit between P storage and environmental factors may be due to the slow uptake and translocation of P.

The allometric scaling and stoichiometric homeostasis provides a framework for the formation of elemental allocation patterns at the community level but does not seem to be sufficient for these reasons.Zhao et al.(2019) proposed that community-level element allocation is influenced by the common architectural and biochemical constraints on nutrient acquisition and transport,and that ecosystems regulate element allocation with a “conservative allocation strategy”.As C is mainly derived from photosynthesis, while N and P are soil nutrients, C distribution is likely more related to factors affecting light distribution,while N and P distribution is related to soil properties, unless plant nutrient distribution is significantly related to plant structure and transport.In contrast, our results indicate that C allocation is more related to soil properties,while N and P allocation is related to spatial structural factors within the community.This result supports our hypothesis three,which suggests that element allocation at the community level differs from element allocation at the individual level.The underground component(soil and roots) of forest ecosystems serves as a vast C pool, with soil alone contributing to approximately 44%of the total C storage in forest ecosystems (Pan et al., 2011).Soil not only provides the direct environment for root growth but also serves as an important source of nutrients required for plant growth, thus influencing plant organ traits(Rusch et al.,2010).This explains the critical role of soil properties in the distribution and storage of C in both aboveground and underground parts(Fig.6).This supports the idea that plant nutrient distribution is shaped by common architectural and biochemical constraints on nutrient acquisition and transport.The function and activity of plant organs also influence element allocation, and some studies suggest that actively growing sites have higher N and P concentrations than structural sites(Yan et al., 2016b; Wang et al., 2018; Zhao et al., 2019).Furthermore,some research has shown that N and P concentrations were higher in actively growing sites than in structural sites(Yang and Luo,2021).This may explain why element allocation in leaves and roots tends to be influenced by growth factors such as light and water,while allocation in stems is influenced by the stable soil environment.Our findings reveal a shift in the determinants of elemental storage and allocation in plant organs at the community level, reflecting the adaptation strategies of plant communities to the environment and the regulation of environmental resources by ecosystems.

5.Conclusion

The storage and allocation of biomass are critical for carbon sequestration and other ecological benefits in plantation ecosystems.The type of plantation and silvicultural species significantly influence biomass storage and allocation.The dominant factors influencing C–N–P storage and allocation at the community level were inconsistent.In general,the community spatial structure had important effects on both the storage and allocation of elements,while soil conditions were more important in elements allocation.This insight is crucial for regulating element storage and cycling at the community level and provides a new pathway for managing carbon sequestration in plantations from the perspective of carbon distribution in forest ecosystems.The storage of elements at the local scale could be regulated by the main growth-demand elements,while the allocation could be regulated by other limiting classes of factors that are often not the ones having a significant impact on storage.The storage and distribution of elements in an ecosystem are generally regulated by a combination of factors that work in concert, ultimately shaping the stable state of the ecosystem.The differences in organ-level storage and allocation drivers reflect community adaptation strategies.Our study provides detailed patterns of plant organ element storage at the community level,which can facilitate accurate assessment of carbon sinks in plantations and serve as a reference for element allocation and storage regulation in plantations at the local scale.

Funding

This work was supported by the National Key Research and Development Program of China(No.2019YFA0607304).

Availability of data

Data are available on request from the authors.

Authors’ contributions

Wu Huifeng designed the research.Wu Huifeng,Hu Baoan,Ma Ying,and Shi Wenkai performed the filed experiments.Wu Huifeng analyzed data and wrote the manuscript.Kang Fengfeng,Cheng Xiaoqin,and Han Hairong reviewed the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful to the foresters at Wuqi Forest Farm for their assistance in data collection and for sharing their experiences working in the local forests.We thank the two anonymous reviewers for their constructive comments and suggestions.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.i.org/10.1016/j.fecs.2023.100132.