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Forestry Advance Access published online on March 15, 2007
Forestry, doi:10.1093/forestry/cpm002

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Deposition patterns in bulk precipitation and throughfall in a subtropical mixed forest in central-south China

Gong Zhang^1,2, Guang-Ming Zeng^1,*, Chun-Yan Du¹, Yi-Min Jiang¹, Xiao-Kang Su¹, Ren-Jun Xiang², Lu Huang¹, Min Xu¹ and Chang Zhang¹

¹ College of Environmental Science and Engineering, Hunan University, Hunan province, Changsha 410082, PR China
² Hunan Research Academy of Environmental Sciences, Hunan province, Changsha 410004, PR China

^* Corresponding author. E-mail: zgming{at}hnu.cn

	Summary

Top Summary Introduction Materials and methods Results Discussion Conclusion Acknowledgements References

 
The chemistry and deposition pattern in bulk precipitation and throughfall (TF) were examined and evaluated based on the 3-year observations in Shaoshan subtropical deciduous-conifer mixed forest in central-south China. The TF chemistry was notably changed when passing through canopies, which probably was attributed to the dry deposition (DD) on leaf surface and the canopy exchanges. Base cations' (Ca²⁺, Mg²⁺ and K⁺) fluxes were significantly enriched in TF, in particular for K⁺. The annual K⁺ canopy exchange was 12 times larger than DD, and canopy exchange of Ca²⁺, Mg²⁺ and K⁺ was four times as high as the DD. The canopy exchange of base cations in association with weak acid accounted for 28.4 per cent of total leached base cations, which was one of the important factors to modify the TF chemistry.

	Introduction

Top Summary Introduction Materials and methods Results Discussion Conclusion Acknowledgements References

 
Input of acidity by rain to aquatic and terrestrial ecosystems has gained considerable public and scientific interests (Galloway and Likens, 1976; Johannes et al., 1986; Lindberg et al., 1986; Cao et al., 1989; Draaijers et al., 1994; Zeng et al., 2005; Zhang et al., 2006a). Precipitation chemistry is often modified considerably as it passes through the canopy, especially in areas with high atmospheric dry deposition (DD) inputs (Bredemeier, 1988; Solange et al., 1999). Because of the difficulty in measuring DD inputs, throughfall (TF) measurements are often used as an estimate of atmospheric deposition to forests (Edmonds et al., 1991; Draaijers and Erisman, 1995). The modifications of chemistry in TF depend on the following processes: the chemicals in precipitation (Lovett et al., 1996), the evaporation or interception of water by canopies (Bruijnzeel et al., 1993; Zhang et al., 2006b), the excretion of weak acids (Draaijers et al., 1994), the dry period between rainfall events (Lindberg et al., 1990) and the uptake and leaching of ions (Zeng et al., 2005). Leaching from the canopy increases with rainfall acidity (Fan et al., 1999), particularly for pH < 3.0 (Hamburg and Lin, 1998; Zeng et al., 2005), which is especially true for divalent cations, such as Mg²⁺ and Ca²⁺ (Cheng et al., 1989). The deficiency of Mg²⁺ in foliage, as a result of rainfall acidity, has been postulated in Europe and US (Ulrich and Pankrath, 1983; Potter et al., 1991). However, the leaching of Ca²⁺ from leaf cell membranes has been proposed as a possible cause of forest decline, by increasing the sensitivity to frost (Draaijers et al., 1994; Dehaye et al., 1999). Rainfall acidity can be neutralized by ion exchange with H⁺. The exchange of rainwater H⁺ for canopy Ca²⁺ and Mg²⁺ can be up to 40 per cent of the pH neutralization (Sayre and Fahey, 1999). Organic acids in canopy have been suggested as acid buffering agents in TF (Draaijers et al., 1994; Dehaye et al., 1999; Lin et al., 2000a). However, little information is available on the precise chemical and quantitative mechanisms of total neutralization processes through a canopy (Butler and Likens, 1995).

Numerous methods have been used to measure or calculate DD on forest canopies, with multiple regression models (Lin et al., 2000a; Watmough and Dillon, 2003) and the Na-ratio approach (Bredemeier, 1988; Draaijers et al., 1994; Draaijers and Erisman, 1995), the most widely used. Both have been successfully applied to a variety of forests in the temperate regions (Johannes et al., 1986; Edmonds et al., 1991), but few of them have been used in (sub) tropical forests (Mayer and Ulrich 1974; Lin et al., 2000b; Zeng et al., 2005). The objectives of this study are (1) to evaluate the seasonal chemistry of bulk precipitation (BP) and TF via the statistical methods; (2) to determine the contribution of DD and canopy exchange to the precipitation and TF fluxes and (3) to examine the importance of canopy exchange of weak acids in TF pattern in Shaoshan forest in central-south China.

	Materials and methods

Top Summary Introduction Materials and methods Results Discussion Conclusion Acknowledgements References

 
Study area

The data were collected from ten 30 x 30 m plots in Shaoshan forest (27° 87' N, 112° 91' E, 290 m a.s.l.) with a two-layer canopy structure in Hunan province from January 2000 to December 2002. Shaoshan forest is 30 km away from the nearest town, Xiangtan city in central-south China (Figure 1). The subtropical monsoon climate of Hunan is symbolized by cold in winter and hot in summer, abundant but unevenly distributed rainfall (more in summer than winter), and high humidity. Relative humidity is up to 84 per cent in spring and 90 per cent in summer. There is an annual mean rainfall of 1550 mm and an annual average temperature of 17.0°C in the observed years in Shaoshan forest.

View larger version (24K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 1. Location of the study site and disposition of the 10 plots in Shaoshan forest (a) and layout of TF collectors in a plot (b).

 
Fir (Cunninghamia lanceolata), massoniana (Pinus massoniana), camphor wood (Cinmamomum camphora) and bamboo (Phyllostachys pubescens) are the tree canopy components. Fir (Cunninghamia lanceolata) approximately accounts for 44 per cent, massoniana (Pinus massoniana) 31 per cent, camphor (Cinmamomum camphora) 20 per cent and bamboo (Phyllostachys pubescens) 5 per cent of the total stand volume of 300 m³ ha^–1. The projected canopy coverage of the stand is 82 per cent; the forest trees are generally 20–70 years old. The subcanopy is dominated by camellia (Camellia japonica), oleander (Nerium indicum) and holly (Euonumus japonicus).

Sampling sites

A standard wet-only collector from Meteorologiska Institutionen Stockholms Universitet (Stockholm, Sweden) was placed on the top of a 10-m height tower within the TF plots. The wet deposition samples were collected daily, but the daily samples were pooled to weekly samples prior to chemical analysis. For the 10 (A–J plots) plots in the studied forest, three plots (A–C plots) are set in the lower parts of the forest (25–50 m a.s.l.), five plots (D–H plots) in the middle parts (75–100 m a.s.l.) and two plots (I and J plots) in the upper parts (125–170 m a.s.l.) (Figure 1). TF collectors (17.0 cm diameter) were placed 1.0 m above the ground. Collectors were placed under canopies and kept in the dark. The TF collector is made of a plastic bottle (2 l), a plastic funnel (d = 11.5 cm), a connector with a filter (nylon screen) and a mounting equipment. The filter is replaced by a new one and the funnels are rinsed twice using distilled water (100 ml) after each weekly collection. At weekly intervals, the collected TF volume in the 16 collectors per plot was pooled and weighed. Chemical analysis for TF was done at monthly intervals in pooled samples. Pooled samples were stored in the refrigerator at 4°C and filtered (0.45 µm membrane filter) prior to analysis.

Laboratory analyses

All the analytical methods were based on standard procedures described in European Monitoring and Evaluation Programme manual (1996). SO, NO, Cl^–, Na⁺ and NH were determined by using ion chromatography (Dionex-320, Dionex, CA, USA). Ca²⁺, Mg²⁺ and K⁺ were determined by flame atomic absorption spectrophotometer (SH-3800, Shimadzu Corporation, Japan) in laboratory, while the conductivity and pH value were measured at 25°C by pH meter (PHS-3C, Shanghai REX Instrument Factory, Shanghai, PR China) in unfiltered solutions (see the detailed description in Zhang et al., 2006a).

Calculation and statistics

Continuous 3-year observations of TF and BP fluxes in our forest stands allowed the application of the Na-ratio method (e.g. canopy budget model), developed by Ulrich and Pankrath (1983), with the aim of estimating the contribution of DD and the canopy leaching in TF. In the method, Na is assumed not to be leached from canopy and it is used as a tracer of DD of particles such as Ca²⁺, Mg²⁺ and K⁺. DD can be calculated through (TF_Na – BP_Na)/BP_Na (DD factor) multiplied by the ionic flux in BP (Draaijers and Erisman, 1995; Zhang et al., 2006a). A second assumption of the model is that the canopy uptake (CU) of NH and H⁺ equals the total canopy leaching of Ca²⁺, Mg²⁺ and K⁺ minus the leaching of Ca²⁺, Mg²⁺ and K⁺ associated with excretion of weak acids (i.e. Draaijers and Erisman, 1995; Zeng et al., 2005). Thus, knowing the CU of (NH + H⁺), the DD flux of (NH + H⁺) can be estimated from TF + CU – BP. The CU of NH can be obtained based on the assumption that the canopy exchange efficiency of H⁺ was six times as high as NH (Bredemeier, 1988; Draaijers et al., 1994). Canopy exchange of NO equals the canopy exchange of nitrogen minus the NH (Christ et al., 1995). The canopy exchange of nitrogen can be calculated via the method proposed by Erisman et al. (2002). Finally, it is assumed that canopy-leaching SO and Cl^– are negligible and, therefore, the DD contribution of SO and Cl^– equals the net TF flux. The net TF flux can be obtained from TF – BP. Although some assumptions of the model are arguable, some comparing studies have shown good agreement between the results of this model and those obtained by other approaches such as inferential techniques or artificial surfaces or the multiple regression models (Draaijers et al., 1992; Butler and Likens, 1995; Lin et al., 2000b).

To better understand the chemical transformation of rainfall after the passage through the canopies and to study the complex interrelationships among the ions, we examine the correlation coefficients (i.e. Pearson correlation coefficient) among the chemical compositions of BP and TF, respectively. Furthermore, a principal component analysis was employed as the factor extraction tool to reduce the dimensionality and to identify the factors regulating the variations in BP and TF based on the 3-year monitoring data in Shaoshan forest. The Statistical Program for Social Sciences program (SPSS 10.0 for Windows) was used to carry out the two analyses.

The measured conductivities were in excellent agreement with the calculated conductivities for both wet and TF samples as indicated by high linear correlation coefficients, r², that were 0.96 and 0.98, respectively, for BP and TF. The fluxes (meq m^–2 per period) of TF and open-field precipitation were calculated by multiplying the volume-weighted concentration with the amount of water on a seasonal basis. The data series of this study are the averaged values of the same season in the three observed years. The difference between the sum of cations and the sum of anions is assumed to be the estimation of weak organic acids (Zhang et al., 2006a).

Differences in ion concentrations and fluxes in precipitation and TF among the 10 forest plots were examined using one-way analysis of variance (SPSS 10.0 for Windows).

	Results

Top Summary Introduction Materials and methods Results Discussion Conclusion Acknowledgements References

 
Chemistry in BP and TF

No statistical difference among the 10 plots in rain quantity in precipitation (1550 mm year^–1) as well as in TF (1218 mm year^–1) was found in Shaoshan forest.

The relative low concentration of Na⁺, Cl^– and Mg²⁺ in BP indicated that the sea, which was 1000 km from the study site, had only minor effects on the results. The most abundant ion in BP was SO in the four seasons, followed by NH and Ca²⁺ in spring and winter and by Ca²⁺ and NH in summer and autumn (Table 1). Annual concentration of SO amounted to 62.9 and 77.8 per cent of anions in BP and TF. NH accounted for 53.9 and 28.9 per cent, and Ca²⁺ for 25.6 and 38.5 per cent of cations in precipitation and TF, respectively.

View this table: [in this window] [in a new window]   Table 1: Seasonal volume-weighted mean values of concentration (µeq l–1) of BP and TF samples collected in Shaoshan forest during the observed periods (P < 0.05)a

 
The ionic concentrations were significantly enriched in TF. SO was higher in TF than in BP, whereas, H⁺ in TF was notably lower than that in precipitation (Table 1). NH and weak acids were increased in TF in each season. The highest enrichment of base cations in TF compared with BP was Ca²⁺ in spring (2.7-fold), K⁺ in autumn (7.6-fold) and Mg²⁺ in winter (3.2-fold), respectively.

The seasonal SO/NO ratio in BP varied from 2.1 to 6.8, which indicated that the contribution of sulphur compounds to local acid rain pollution was larger than nitrogen compounds. The range of the ratios in TF was 4.4–10.3 and generally higher than BP (Table 1).

DD and canopy exchange

Table 2 presents seasonal ionic flux in BP, TF, net TF flux, DD and canopy exchange. The annual mean contribution of DD (187.1 meq m^–2 year^–1) based on the Na-ratio method to precipitation (810.1 meq m^–2 year^–1) and TF (1255.9 meq m^–2 year^–1) was 23.1 and 14.9 per cent, respectively. The annual DD and the canopy exchange of base cations (Ca²⁺, Mg²⁺ and K⁺) were 52.6 meq m^–2 year^–1 and 205.7 meq m^–2 year^–1, respectively, accounting for 11.0 and 42.8 per cent of TF flux of base cations (480.6 meq m^–2 year^–1). Annual K⁺ canopy exchange was 12 times larger than DD, and canopy exchange of Ca²⁺, Mg²⁺ and K⁺ was almost four times as high as DD. The DD of H⁺ and NH (44.9 meq m^–2 year^–1) accounted for 34.6 per cent of the TF flux (129.8 meq m^–2 year^–1). It was noted that the canopy exchange of weak acid accounted for 28.4 per cent of total base cations, indicating that 28.4 per cent of base cations in TF was leached with the excretion of weak acids. The negative canopy exchange of N-NO indicated the retention in the canopy, which was similar to NH.

View this table: [in this window] [in a new window]   Table 2: Annual mean fluxes of BP, TF and net throughfall (NTF) and DD and canopy exchange (CE) (meq m–2 year–1) and rainwater (P) (mm) in Shaoshan forest (P < 0.05)a

 
Correlation between ions in precipitationand TF

There were high correlation coefficients between SO and NO in BP and in TF, respectively (Tables 3 and 4). The correlation coefficients between NH and SO, NH and NO were higher than 0.8 (Tables 3 and 4) and the coefficients between Ca²⁺ and Mg²⁺, Mg²⁺ and K⁺, Na⁺ and Mg²⁺ in TF were 0.93, 0.88 and 0.90, respectively. The low correlative coefficient between Cl^– and base cations in TF implied negligible canopy leaching of Cl^–.

View this table: [in this window] [in a new window]   Table 3: Pearson correlation coefficients of the ions in BP in Shaoshan forest

 

View this table: [in this window] [in a new window]   Table 4: Pearson correlation coefficients of the ions in TF in Shaoshan forest

 
Principal components of precipitation and TF

The factor loadings for the compositions in BP and TF were listed in Tables 5 and 6. Three factors were extracted in BP and that explained 98.5 per cent cumulative variance of the total variance (Table 5). Factor 1 was composed of Ca²⁺, Mg²⁺, K⁺, Na⁺ and SO accounted for 69.5 per cent of total variance in BP. Factor 2 was made up by NO, NH. Factor 3 comprised of K⁺ and Ca²⁺, which was assumed to come from the incineration of vegetation and waste in the vicinity of the studied stand (Hamburg and Lin, 1998; Finér et al., 2004; Jiang et al., 2004).

View this table: [in this window] [in a new window]   Table 5: Results of principal component approach on BP in Shaoshan forest

 

View this table: [in this window] [in a new window]   Table 6: Results of principal component approach on TF in Shaoshan forest

 
Three factors were extracted from the chemicals in TF in Shaoshan forest, which accounted for 96.0 per cent of the total variance (Table 6). SO, NO, K⁺, Na⁺ and Mg²⁺ constituted Factor 1 and explained 65.9 per cent of the total variance. Factor 2 was composed of NH and Ca²⁺, and the strong positive association between ammonium and calcium appears in TF but is absent in BP indicates the possibility of some exchange of canopy calcium for dry-deposited ammonium. Factor 3 was composed by Ca²⁺ and K⁺ which are the main nutrients of vegetation.

	Discussion

Top Summary Introduction Materials and methods Results Discussion Conclusion Acknowledgements References

 
DD and canopy leaching

The ion enrichments in TF have been widely examined, which was generally attributed to the wash-off of atmospheric DD, the canopy leaching of base cations, the transportation of pollutants and the anthropogenic activities (Ulrich and Pankrath, 1983; Johannes et al., 1986; Edmonds et al., 1991; Dehaye et al., 1999; Sayre and Fahey, 1999). The high correlation between Ca²⁺ and SO, Ca²⁺ and NO in BP may suggest the potential pedogenic sources (Lin et al., 2000a; Finér et al., 2004). Principal components in precipitation revealed that the most constituents of the soil dust or soil particle from local environment as well as local anthropogenic activities to a certain extent influenced precipitation chemistry. The component of Factor 1 extracted from TF not only illustrated the soil particles in TF but also suggested the H⁺-induced canopy exchanges. Factor 2 was assumed to derive from the fertilizer of agricultural activities (Jiang et al., 2004).

The leaching of base cations in forests was attributed to the exchange in association with H⁺ and weak acids in rainwater. Proton in rainwater is capable of leaching Ca²⁺ and K⁺ in vegetation on the wetted foliage surfaces (Potter et al., 1991; Zeng et al., 2005). High H⁺ concentration in rainwater accelerates the canopy ionic leaching (Dehaye et al., 1999; Nadia and Étienne, 2000; Balasubramanian et al., 2001). The flux of Ca²⁺ was up to five times higher in TF than in BP in north China forests because of the canopy leaching process and DD (Cao et al., 1989). Lindberg et al. (1986) reported that the DD was affected by the length of dry period and the amount of canopy leaching was related to the amount of precipitation, especially for K⁺. Moreover, Campo et al., (2000) have reported that the leaching amount of K⁺ was positive related to the amount of precipitation. Edmonds et al. (1991) and Potter et al. (1991) have reported that the accelerated canopy leaching may bring about potential harmful effects on forest ecosystems.

The strong correlation between SO and base cations in TF was attributed mainly to the DD. However, the canopy leaching of SO associated with base cations should not be neglected. Although many authors assumed that the canopy leaching of SO was negligible (Butler and Likens, 1995; Lin et al., 2000b), the leaching evidences of SO have been reported in eastern Finland forests (Piirainen et al., 2002; Finér et al., 2004).

Patterns in precipitation and TF

The contribution of 23.1 per cent for DD to BP was lower than that of 50 per cent in temperate forests (Watmough and Dillon, 2003), but which was similar to that in Fushan forest (Lin et al., 2000a; 2003). The contribution of DD of base cations in TF was 14.9 per cent, which was less than the 20–60 per cent in temperate forests (Watmough and Dillon, 2003). The DD of (H⁺ + NH) accounted for 34.6 per cent of BP, which was higher than the 24 per cent in northern Italian forest (Balestrini and Tagliaferri, 2001).

The Na-ratio method indicated a negative canopy effect for H⁺ and NO, which indicated that the ions were retained by the canopy leaves. The proportion of deposited nitrogen which was taken up by the canopies was higher in young, fast growing stands which had a high N requirement, compared with that of old and poorly growing stands (Nadia and Étienne, 2000). Canopy nitrogen retention is widely observed (Brown and Lund, 1994; Lin et al., 1997), but the underlying mechanisms remain unclear and explanations are somewhat controversial. Some researchers believe that nitrogen retention is a biologically mediated mechanism rather than simple chemical exchange or passive diffusion (Schaeferm and Reiners, 1990; Campo et al., 2000). Lin et al. (2000b) suggested that the passive movement across a concentration gradient played an important role not only in canopy nitrogen retention but also for other ions in BP, such as Mg²⁺, HCO and Cl^–.

Precipitation and TF chemistry in similar forests

The highest concentrations in both BP and TF were SO at Zhenwushan forest in Chongqing with 469.1 and 1008.9 µeq l^–1 (Table 7), which implied that there was severe acid rain pollution in this region (Zhang et al., 1996). The precipitation chemistry at Zhenwushan and Baiyunshan forests was unbalanced, but the TF chemistry was balanced (Xu et al., 2001), in contrast, that in Mangshan forest kept balanced well both in BP and TF (Feng et al., 2001). Fushan forest and Nanping plantations were with balanced chemistry in precipitation but with unbalanced in TF (Fan et al., 1999; Lin et al., 2000a) (Table 7). Unfortunately, the authors failed to investigate the causes of the unbalanced TF chemistry. The unbalanced chemistry in TF was generally attributed to the increment of cations, in particular for Ca²⁺ and K⁺ in the present forest study. The exact origin of Ca²⁺ in BP is unclear at this stage but could be derived from road dust, cement factories and long-distance transported dust. Forest researches are generally limited by the complexity of forest itself and the outside environmental factors, such as tree species, climate, meteorological conditions and topography (Erisman et al., 2002).

View this table: [in this window] [in a new window]   Table 7: Annual mean concentration of ions (µeq l–1) in BP and TF in forests

 
Application of Na-ratio method

The Na-ratio approach based on temperate forest ecosystems may lead to spurious results while applying to subtropical forest, which is derived from the assumptions in the calculation of canopy exchange and base cations’ deposition. The particles containing Ca²⁺, Mg²⁺ and K⁺ are with the same deposition efficiency as particles containing Na⁺, which may cause the underestimates of Ca²⁺ and Mg²⁺ and the overestimates of K⁺ in temperate forests (Draaijers et al., 1994). The Na-ratio approach tends to overestimate DD because canopy leaching of Na⁺ is assumed to be zero, which is unlikely in the (sub) tropical forest canopies, especially in the regions with high humidity and frequent rainfall events. Na-ratio method should work better in China than in Europe, because in Europe the source of Na is mostly sea salt, whereas Ca and K come largely from terrestrial dust and are not necessarily related. In China, the aerosols are more likely to come from the same sources. Although there are some agreements between Shaoshan subtropical forest and temperate forests, the results in Fushan subtropical forest were in contrast with temperate forest (Lin et al., 2000a). To improve the application of Na-ratio method in forests, the ratios in atmospheric particles, instead of in BP, should be used whenever possible.

	Conclusion

Top Summary Introduction Materials and methods Results Discussion Conclusion Acknowledgements References

 
This paper highlights an under-investigated feature of forest ecosystems which has important implications on ecological and biogeochemical processes on the forest floor and beyond. Patterns in precipitation and TF also affect the processes in Shaoshan forest in central-south China. The conclusions of this study are as follows:

1 The results of Na-ratio approach suggested that the DD accounted for 23.1 per cent in BP and 14.9 per cent in TF. The contribution of DD of base cations in precipitation and TF was 11.0 and 42.8 per cent. The DD of (H⁺ + NH) accounted for 34.6 per cent of TF flux.

2 The annual K⁺ canopy exchange was approximately two times larger than DD, and canopy exchange of Ca²⁺, Mg²⁺ and K⁺ was almost four times as high as DD. The canopy exchange of base cations in association with weak acid accounted for 28.4 per cent of total leached base cations.

3 The correlation coefficients and the principal components in precipitation and TF revealed that the most constituents of the soil dust or soil particle from local environment as well as local anthropogenic activities influenced precipitation chemistry.

	Acknowledgements

Top Summary Introduction Materials and methods Results Discussion Conclusion Acknowledgements References

 
The study was financially supported by the National Natural Science Foundation of China (Grant No. 70171055, 50179011), the Natural Foundation for Distinguished Young Scholars (Grant No. 50225926), the Doctoral Foundation of Ministry of Education of China (20020532017), the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of Ministry of Education, PR China, in 2000. We also thank the two anonymous referees for their constructive suggestions, which have greatly improved the quality of this manuscript.

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