Difference between rainfall and throughfall chemistry for different forest stands in the Qinling Mountains, China


 This study compared the effects of four forest canopies on throughfall chemistry in the Qinling Mountains, China. Rainfall and throughfall samples were collected in stands of Quercus aliena (Qa) var. Acuteserrata, Pinus tabulaeformis (Pt), P. armandii (Pa), and mixed broad-leaved (Mb) trees from 2009 to 2011. The results indicated that the pH of the rainfall, which was mildly acidic, increased as it passed through the forest canopy. The pH increased more within the broad-leaved forest canopy than the coniferous forest. Concentrations of decreased as rainfall passed through the Qa canopy but increased after passing through the other species. The concentrations of and Zn, Cd and Pb decreased as rainfall passed through the four canopies. The coniferous forest canopy was more effective than the broad-leaved forest in reducing in rainwater. The decreases in Cd concentrations were similar among the four canopies. The Pb concentration decreased the most among the heavy metals, and the order of the decrease was Qa > Pt > Pa > Mb. The results may provide a basis for the selection of tree species for afforestation in water sources in the Qinling Mountains and similar areas.


INTRODUCTION
Many countries and regions in the world use forests as industrial and domestic water supply sources. Even if these forests are protected, pollutants can be transported over long distances by the wind (Zhang et al. ). Rainfall captures some pollutants and brings them into the forest ecosystem. The forest canopy can absorb some of the pollutants, causing NH þ 4 , NO x , and some heavy metal element concentrations in throughfall water to decrease (Ferm forest canopy is the first part of the forest that comes into contact with precipitation, it is highly dynamic in the growing season, and the tree leaves have a large surface area (leaf area index ¼ 3-10 m 2 m À2 ; Ulrich et al. ) that is conducive to absorption (Gandois et al. ). Therefore, the forest canopy is a key layer of forest ecosystems in improving water quality (Tan et   This study was undertaken to quantify changes in water chemistry as rainfall passed through four main types of forest stand canopies in the Huoditang natural forest region, which is located in the central part of the Qinling Mountains, China. The study was conducted in a concentrated area where the pollution sources were the same, the local level of atmospheric pollution was affected by both common air pollutants and heavy metals, and environmental conditions were similar throughout. Thus, the results of this study may provide an accurate basis for the selection of tree species for afforestation projects in watersheds in the Qinling Mountains and act as a reference for similar areas. The results also may provide an understanding about the effects of these four different tree stand canopies on water quality and may be used in the management of forests as water sources.

MATERIALS AND METHODS
The study area The Qinling Mountains are an important water source for the middle segment of China's South-to-North Water Transfer Project, which is a national strategic project. The mountains are primarily located in northwest China's Shaanxi Province. The mountains have rich deposits of Pb and Zn. The production rate of these metals in the Qinling Mountains ranks seventh nationally.
Heavy metal pollution from mining and smelting has become a serious problem in some parts of the Qinling Mountains (Zhang ), and acid rain has also become common in recent years in the surrounding areas (Zhang et al. ). Acid rain impacts Chongqing municipality the most (Hong ; Wu et al. ; Zhang ), but it also occurs in parts of Henan, Shaanxi, and Hubei provinces (Zhao et al. ; Wang et al. ). Some of these areas are far from the Qinling Mountains; however, wind can transport atmospheric pollutants to these mountains from great distances (Zhang et al. ). The Huoditang forest region is located in a warm temperate zone and has a humid mountain climate. Mean annual temperatures range from 8 to 10 C. The mean annual precipitation is 1,130 mm. The rainy season is between July and September and accounts for 53% of the annual precipitation. Snowfall occurs between late October and early April, and the precipitation in this period accounts for 13% of the annual precipitation. The climate is dry in spring and humid in autumn. The average annual relative humidity is 77%.
The forest was harvested during the 1960s and 1970s; much of the area is now covered by dense natural secondary growth. The dominant tree species are Qa var. acuteserrata, Pinus tabulaeformis (Pt), P. armandii (Pa), Betula albosinensis, B. luminifera, Picea wilsonii, Abies fargesii, and Populus davidiana. The forest cover is 92% (the rest is roads and Huoditang forest farm office areas), and the canopy closure is >0.7. There is no cultivated land in or around the Huoditang forest region.

Water sampling
We determined the effect of the forest canopies on rainfall water chemistry by comparing rainfall and throughfall con- Rainfall and throughfall water samples were collected in 8.5 L plastic buckets (funnel diameter is 20 cm) fitted with multiperforated PVC film covers to permit rainfall or throughfall water but prevent insects or vegetative debris The buckets were emptied and rinsed with clean stream water and then deionized water before each collection. Each collection represents one individual event.
All water samples were placed into 500 mL polyethylene bottles, taken to the laboratory within 24 h, and then stored below 0 C until analysis.

Rainfall and throughfall pH
Rainfall pH in the Huoditang forest averaged 5.84 (Table 2) during the 3-year study period and ranged from a low of 3.93 respectively. These results indicate that when rainfall is weakly acidic (pH < 6.0), the Mb canopy increased the pH of the throughfall the most, followed by the Qa canopy, the Pt canopy, and then the Pa canopy.    Rainfall NO À 3 concentrations averaged 0.21 mg L À1 during the study period (Table 2), ranging from 0.00 to 1.10 mg L À1 (Figure 3). As rainfall passed through the forest canopies, the average NO À 3 concentrations of rainfall declined by 20, 29, 53, and 27% in the Qa, Pt, Pa, and Mb stands, respectively. This indicates that all four forest canopies generally absorbed NO À 3 from rainfall. The decreases in average NO À 3 concentration were greater in the coniferous stands than in the broad-leaved stands. This implies that coniferous canopies were better than broad-leaved canopies at absorbing NO À 3 from rainfall. Furthermore, coniferous canopies were more effective than broad-leaved canopies in buffering rainfall NO À 3 pollution. This result is supported by the presence of fewer throughfall NO À 3 concentration extreme outliers for conifers than for broadleaved trees in Figure 3.
Rainfall NH þ 4 concentrations averaged 0.40 mg L À1 during the study period, ranging from 0 to 1.48 mg L À1 (Table 2; Figure 3). The NH þ 4 concentrations increased by an average of only 5% as rainfall passed though the Qa canopy, but decreased by 42, 57, and 17% as rainfall passed through the Pt, Pa, and Mb canopies, respectively.
All the canopies absorbed NH þ 4 from rainfall, with the exception of the Qa canopy. The possible explanation for this exception is that NH þ 4 in leaf tissues could be exchanged more easily for H þ in rainwater as weakly acidic rainfall passed through the Qa canopy than the other canopies.
This dynamic is the same as base cations (Ca 2þ þ Mg 2þ þ K þ ), as previously mentioned.
Rainfall PO 3À 4 concentrations averaged 0.09 mg L À1 during the study period, ranging from 0.03 to 0.14 mg L À1 (  Low N and P concentrations in throughfall can not only improve the quality of drinking water derived from it, but these low concentrations can also lower the risk of eutrophication within downstream water. From this viewpoint, coniferous canopies improve water quality more than broad-leaved canopies.

Base cation concentrations
The rainfall Ca 2þ concentration averaged 1.94 mg L À1 in the Huoditang forest during the study period ( Table 2). As rainfall passed through the forest canopies, the average rainfall Ca 2þ concentrations increased by 47, 16, 26, and 52% in the Qa, Pt, Pa, and Mb stands, respectively. These increases demonstrate that rainfall leached Ca 2þ from all four forest canopies. Furthermore, the largest Ca 2þ losses were from the Mb trees, followed by the Qa (Table 2; Figure 4); this is perhaps because leaf Ca concentrations are higher in broad-leaved trees than in coniferous trees (Table 3). Alternatively, conifers secrete greater amounts of viscous oil than broad-leaved tree species. Leaf Ca concentrations of Pt are higher than those of Pa; however, the average throughfall Ca 2þ concentration was lower in the Pt stand than in the Pa stand. One explanation is that Pt leaves may secrete a greater amount of viscous oil than Pa leaves (Chen et al. ).
Rainfall Mg 2þ concentrations averaged 0.63 mg L À1 in the Huoditang forest during the study period ( Table 2). As rainfall passed through the forest canopies, average rainfall Mg 2þ concentrations increased by 215, 79, 178, and 187% in the Qa, Pt, Pa, and Mb stands, respectively. The changes in Mg 2þ concentration as rainfall passed through the forest canopy were similar to the changes in the Ca 2þ concentration. The reason that more Mg 2þ was leached from the broad-leaved canopies than from the coniferous canopies is probably the same as previously discussed for Ca 2þ . The average throughfall Mg 2þ concentration was lower in the Pt stand than in the Pa stand. The reason for this result is also the same as Ca 2þ .
Rainfall K þ concentrations averaged 2.59 mg L À1 , the highest of the base cation concentrations, during this study ( Table 2). The relative changes in the K þ concentration as rainfall passed through the forest canopies were similar to those observed for Ca 2þ and Mg 2þ , although there were more outliers (Figure 4). Specifically, as rainfall passed through the canopy, average rainfall K þ concentrations increased by 76, 10, 55, and 54% in the Qa, Pt, Pa, and Mb stands, respectively. These results generally show that more K þ was leached from broad-leaved canopies than from coniferous canopies (Table 2; Figure 4).
The reason for this result was similar to our previous explanation for Ca 2þ . Throughfall in the Pt stand had the lowest K þ concentration in this study. One explanation for this result is the high concentrations of viscous oils in Pt There were more outliers for throughfall K þ concentrations than for the other base cation concentrations ( Figure 4), which imply that K þ was more easily leached as rainfall passed through the stand canopies. The reason for this finding probably is that K is not as tightly bound to structural tissues or enzyme complexes as Mg and Ca (Balestrini & Tagliaferri ).

Cadmium and lead concentrations
In the Huoditang forest, the average rainfall Cd concentration was 4.73 μg L À1 (Table 2). This amount was close to 5.0 μg L À1 , the maximum allowable concentration of Cd in drinking water stipulated in China's Standards for Drinking Water Quality (GB5749-2006). Rainfall Pb concentrations during the study period averaged 18.19 μg L À1 ( Table 2).
The concentration was higher than the safe drinking water standard of 10.0 μg L À1 . As rainfall passed through the forest canopies, average Cd concentrations decreased dramatically; thus, the average throughfall Cd concentrations were much lower than China's safe water drinking standard.
The average decreases were 71, 72, 74, and 83% in the Qa, Pt, Pa, and Mb stands, respectively. There were two extreme outliers for Cd concentrations in rainfall ( Figure 5). These extreme outliers imply that air in the Huoditang forest was polluted with Cd. The average rainfall Pb concentration was the highest among the heavy metals tested in this study (Table 2). As rainfall passed through the forest canopies, the average rainfall Pb concentrations decreased by 69, 64, 63, and 44% in the Qa, Pt, Pa, and Mb stands, respectively.
The Qa canopy tended to reduce rainfall Pb concentrations the most in this study; however, the differences among the four canopies types were relatively small (Table 2; Figure 5).
The reason for the decline in Cd concentration as rainfall passed through the forest canopies is unclear.
There are three possible explanations for the large decreases in average Pb concentrations as rainfall moved through the forest canopies (Shang et al. ). First, about 50% of Pb absorbed from rainfall by leaves is deposited in the plant cuticle and cannot be moved. The second explanation is that Pb absorbed from rainfall reacts easily with surplus nonprotein sulfhydryl in plant cells to form insoluble compounds. Finally, some Pb absorbed from rainfall may form crystals that accumulate slowly on cell walls and are then deposited in the form of dense particles. The average rainfall Pb concentration was the highest among the heavy metals, and two outliers shown for the rainfall Pb concentration in Figure 5 indicate that air in the Huoditang forest was polluted with Pb as well.

Zinc and copper concentrations
Rainfall Zn concentrations averaged 8.39 μg L À1 during the study period (Table 2), which is much lower than the maximum allowable Zn concentration (1,000.0 μg L À1 ) in drinking water stipulated in China's Standards for Drinking  Zhang et al. (1996).  (Table 2). Specifically, the average Zn concentrations decreased by 40, 5, 10, and 57% in the Qa, Pt, Pa, and Mb stands, respectively. The larger decreases occurred as water passed through the broad-leaved canopies (Table 2; Figure 6).
This implied that the broad-leaved canopies absorbed more Zn from rainfall than the coniferous canopies, perhaps because the broad-leaved trees required more Zn. This is supported by Avila & Rodrigo (), who reported that the canopy of a holm oak forest absorbed large amounts of Zn and Cd.
The Cu rainfall concentration during the study period averaged 3.41 μg L À1 ( concentrations. This finding is also supported by the throughfall Cu concentration interquartile range being the largest for the Pt in Figure 6. An explanation for this result might be that the stand density and canopy closure were greater in the Pt stand than in the other stands (Table 1)

CONCLUSIONS
(1) Rainfall pH, which was weakly acidic, increased as rainfall passed through the Qa, Pt, Pa, and Mb stands. The largest increase occurred as rainfall passed through the canopy of the Mb stand, followed by the Qa, Pt, and finally Pa stands.
(2) Concentrations of SO 2À 4 decreased as rainfall passed through the Qa canopy, but increased as rainfall passed through the others. The SO 2À 4 concentration had the highest increase as rainfall passed through the Pt canopy followed by the Pa. Concentrations of NO À 3 decreased as rainfall passed through all four forest canopies. The order of the decrease was Pa > Pt > Mb > Qa.
Concentrations of NH þ 4 increased slightly as rainfall passed through the Qa canopy, but decreased as rainfall passed through the others. The Pa canopy caused the largest decrease in NH þ 4 , followed by the Pt, and finally the Mb. The PO 3À 4 concentration increased as rainfall passed through all four canopy types. The order of the increase was Qa > Mb > Pa > Pt.
(3) Base cation concentrations increased as weakly acidic rainfall passed through the forest canopies. The order of the increase was Qa > Mb > Pt > Pa.
(4) Concentrations of Zn, Cd, and Pb decreased as rainfall passed through the four forest canopies. The decrease in Pb concentration was greater than the decreases in Cd and Zn concentrations. Throughfall Zn concentrations were the lowest in the Mb stand, followed by the Qa, Pt, and finally Pa stands. The four canopy types had similar effects on rainfall Cd concentrations.
As rainfall passed through the forest canopies, the rainfall Pb concentrations decreased by an average of 12.61, 11.55, 11.54, and 7.40 μg L À1 in the Qa, Pt, Pa, and Mb stands, respectively. Rainfall Cu concentrations decreased as rainfall passed through the Qa and Pa canopies, but increased as rainfall passed through the Pt and Mb canopies.