Hydrological Characteristics Of A Moso-Bamboo (Phyllostachys Pubescens) Forest In South China
YANHUI WANG AND YONGMIN LIU
The Institute of’ Forestry, The Chinese Academy of’ Forestry, Beljing, Chino
Hydrological Characteristics Of A Moso-Bamboo (Phyllostachys Pubescens) Forest In South China
Since 1986 the multiple benefits of moso-bamboo forest, a special forest type found mainly in south China, have been investigated in a small 11.7 ha watershed in Fenyi County, Jiangxi Province. The mean annual precipitation in the study area is 1593.3mm. For the 0-60cm soil layer the average soil bulk density is 1.00g/cm3, and the mean values for other soil properties are: total porosity 71.74%; non-capillary porosity 5.81 %; and water retention capacity 430 mm. The maximum effective water retention capacity of 313mm is 28% higher than that for Chinese fir (Cunningharnia lancedata) plantations and natural broadleaved forest in the neighbouring area. The parameters fb, ji and k, in Horton’s infiltration equation, measured using the double-ring method under drought conditions, are 29.10 mm/min, 8.28 mm/min and 0.239 1, respectively. These infiltration properties are more favourable than those under nearby Chinese fir plantations.
Compared with a Chinese fir plantation, the canopy interception ratio of moso-bamboo is lower, but the stemflow ratio is higher. The annual canopy interception ratio is 11.1%. Because of snowfall, the interception ratios in January, February and March are higher, with values of 12.1-17.2%, whereas during the period of leaf fall in April, May and June the interception ratios are lower with values of 9.2-9.5%. During the other months they are relatively constant. The annual stemflow ratio is 4.4%. Again, because of snowfall, the stemflow ratios in January, February and March are lower with values of 2.8-2.9%, whereas during the remaining months they are fairly constant.
Runoff analysis shows that the annual runoff ratio in this research watershed is 54.8%, but the ratio for quick runoff, composed of direct runoff and surface runoff, is only 0.8%. The upper interflow ratio is 15% and the ratio for the slow runoff composed of deeper interflow and underflow is 39%. The moso-bamboo forest is very effective in reducing peak runoff and increasing low flows. The annual nutrient element inputs (kg/ha) to the moso-bamboo forest ecosystem associated with throughfall and stemflow are N 17.7, P 0.38, K 56.5, Ca 31.4, Mg 4.8 and SiOz 26.2, respectively. All the measured element inputs, with the exception of P, are higher than those associated with precipitation in the open, where typical values are N 10.1, P 0.89, K 18.8, Ca 25.8, Mg 3.1 and SiOz 10.1.
The annual outputs in streamflow are N 3.0, P 0.28, K 16.6, Ca 38.9, Mg 8.3 and SiOz 125.7, indicating that for N, P and K the moso-bamboo forest ecosystem is an accumulating system, whereas for Ca, Mg and SiOz the reverse applies. All the pH values associated with precipitation in the open, throughfall. stemflow. surface runoff from runoff plots and streamflow in the research watershed vary between 6.45 and 7.60 and are close to neutral.
KEY WORDS: South China; bamboo forest; forest hydrology; water chemistry
Moso-bamboo forest is a special forest type occurring primarily in south China. Its occurrence accounts for 2.1% of the total forest area and 70% of the total bamboo forest area in China, and 17.5% of the total bamboo forest area of the world. It is an important component of the forest resources of China. Compared with other forest types, moso-bamboo forest has five advantages: (a) rapid growth; (b) short cutting inter- vals; (c) high biomass yield; (d) multiple uses of bamboo wood and the forest floor; and (e) a strong self- restoration capacity, which ensures its perpetual existence when treated in the right way. Moso-bamboo forests therefore play an increasingly important part in both maintaining the local ecological balance and in rural economic development.
There have been many studies and reports about the economic benefits and the optimum management of moso-bamboo forests, but little information exists on their ecological and hydrological benefits. To evaluate scientifically these ecological and hydrological benefits, a long-term research project involving several types of forest ecosystem in south China, including moso-bamboo, Chinese fir (Cunninghamia lanceofata) and Masson's pine (Pinus massoniana) was initiated by the Institute of Forestry of the Chinese Academy of Forestry. This paper reports the results of the hydrological investigations of moso-bamboo forest.
STUDY AREA AND RESEARCH METHODS
Natural conditions in the research area
The research area is located in Fenyi County, Jiangxi Province, and lies in the central area of the region where moso-bamboo forest is found. The mean annual air temperature is 17.9"C, with values of 5.3"C in January and 29.0"C in July. The mean annual precipitation is 1593.3 mm. The small research watershed has an area of 11.7 ha and an elevation range of 445-620 m. The main gully is 300 m long, with a gradient of 0.4098. The mountain yellow earth soils are underlain by weathered slates, and at depth by relatively unweathered and therefore relatively impermeable limestones. The vegetation is semi-intensively managed pure moso-bamboo forest, with a density of 4100 poles per hectare and an average circumference at eye level of 33.2cm. This small watershed is typical of the local area.
Soil physical and hydrological properties
Because the roots of the moso-bamboo forest extend to a maximum depth of 60-70 cm and because soils are of a similar depth, tests were undertaken on 60cm profiles comprising four layers: 0-10, 10-20, 20-40 and 40-60cm. The bulk density, total capillary porosity and non-capillary porosity were measured at 10 sites with a cylindrical soil sampler. The infiltration capacity was measured with a double ring infiltro- meter. Before the infiltration tests we measured the soil water content, which is termed the previous soil water content, and during infiltration we measured the water temperature, which is required to convert the measured infiltration rates to a standard temperature of 10°C (Nanjing Institute of Soil Science, 1978). This made it possible to compare the infiltration capacities measured under different conditions. Because of the dense root system in the upper soil layer, the depth of penetration of the infiltrometer ring was limited to 15 cm. The water depth in the ring was maintained at 9 cm. The measured data were fitted to Horton's infiltration equation.
Redistribution of precipitation in the vegetation canopy
The gross precipitation was measured in a clearing within the watershed using a rainfall recorder. At the centre of the watershed a 100 m2 plot was selected, and 15 rain gauges were installed to measure throughfall and five different sized bamboos were selected for stemflow measurements. Water balance calculations were used to estimate the canopy interception. The water retention capacities of the surface vegetation and litter were determined by collecting samples and measuring their water retention.
Runof characteristics of the research watershed
This watershed forms a naturally closed system. A 90" triangular weir was installed at its outlet and, to document the different runoff components, four 20 m x 10 m runoff plots were constructed on a nearby hillslope, where the vegetation, soil, topography and other conditions were similar. These enabled surface runoff to be measured.
Input-output of important nutrient elements
The main components of the hydrological system, including gross precipitation, throughfall, stemflow, surface runoff from the runoff plots and streamflow were sampled and analysed monthly. The chemical analysis included pH and the concentrations of N, P, K, Ca, Mg and SO2. These measurements allowed the input-output budget of the watershed to be calculated and the effect of the bamboo forest on this bud- get to be evaluated.
RESULTS AND ANALYSIS
Soil physica f and hydrological properties
Soil physical properties. The average bulk density and porosity values for all sampling points in the watershed and for all points on the central hillside, as well as values for a 13 year old Chinese fir plantation and a natural broadleaved forest located within 500 m of the research watershed, are listed and compared in Table I. Based on this comparison it is clear that the soil physical properties of the moso-bamboo forest are very favourable. In other studies (Cao Qungen, 1989) similar situations have been reported, wherein the upper soil layer affected by the roots of the moso-bamboo forest possesses soil properties which are better than those of the broadleaved forest and much better than those of grassland, whereas in the deeper soil layers the contrary situation is found because of the deeper influence of the root system of the broadleaved forest.
Water retention capacity. The water retention capacity represents the water content of saturated soil after 48 hours soaking (cf. Wang Yanhui, 1990). The effective water retention capacity is the difference between the initial and saturated soil water contents. The initial soil water content used in this paper is the lowest value measured during the research period. The values of effective water retention capacity reported in this paper can therefore be viewed as maximum values. The average value for all points in the watershed and all points located on the central hillside as well as the measured values for the Chinese fir plantation and the natural broadleaved forest located outside the research watershed are listed and compared in Table II. It can be clearly seen that both the total and the effective water retention capacities of the moso-bamboo forest are higher than those of the Chinese fir plantation and the broadleaved forest, especially in the case of the effective values, which are 28% higher.
Infltration parameters. In Horton's infiltration equation, the final infiltration ratef, is a measured value, whereas the initial infiltration rate fa and the decay parameter k are fitted values. The average infiltration parameters for all the points in the watershed and for all points on the central hillside as well as the values for the Chinese fir plantation and the natural broadleaved forest are presented and compared in Table III. Based on this comparison it is evident that although the infiltration capacity of the bamboo forest is lower than that of natural broadleaved forest, it is substantially better than that of the Chinese fir plantation. The infiltration capacity of moso-bamboo forest was also found to be much greater than that of a Chinese fir plantation and Masson’s pine plantation measured in experiments undertaken at another experimental station nearby (Ma Xuehua and Yang Guangying, 1990; Ma Xuehua et al., 1992). Compared with grassland (cf. Cao Qungen, 1989), both the fo andf, values associated with moso-bamboo forest are more favourable and the time needed to reach f, is much longer (30 minutes) than that for grassland (1 5 minutes).
Redistribution of precipitation in the vegetation canopy
Redistribution of precipitation during single events. The results of observations made between May 1988 and June 1991 are shown in Figure 1. Based on the measured data, the following relationships between throughfall ( Pt , mm), stemflow ( Ps , mm), canopy interception ( Pi , mm) and precipitation depth (P, mm) and precipitation duration (T, hour) were established
Pt = -0.99 + 0.900P r = 0.999 ...........(1)
Ps = -0.07 + 0’049P r = 0.940 ..........(2)
Pi = 2.1 [ l - exp(-O.O57P) ] + 0.06T r = 0.979 ..........(3)
Seasonal pattern of precipitation redistribution. Because many of the controlling factors vary with season, precipitation redistribution in the vegetation canopy also varies seasonally. The data for 1989 are shown in Figure 2.
In January and February, which were affected by snowfall, the interception ratios are obviously higher. In April, May and June, which were affected by leaf fall and high rainfall, the interception ratios are obviously lower, while in the same period the ratios for throughfall and stemflow are higher. Based on the results of other work (Ma Xuehua and Yang Guangying, 1990; Ma Xuehua et al., 1992), it can be seen that the annual interception ratio for moso-bamboo forest (11.1%) is lower than that for Chinese fir (16.3%) and slightly higher than that for Masson’s pine (10.3%). However, the annual stemflow ratio (4.4%) is much higher than the values for Chinese fir (1.1%) and Masson’s pine (14%) and this can be explained by the smooth trunk of the bamboo and the general shortage of bark.
Precipitation interception by the soil cover. Because of the intensive management and the rapid decay of the litter, only limited soil cover exists. The amounts of living soil cover and litter measured in October 1991 were only 5090 and 2134 kg/ha. The sum of their precipitation interception capacities for a single event measured using the soaking method is only 0.6mm. They are much lower than those for a Chinese fir plantation (15 860 kg/ha and 4.09 mm) and for a Masson’s pine plantation (25 470 kg/ha and 5.12 mm) (Ma Xuehua and Yang Guangying, 1990). The water retention capacity per unit weight of the soil cover on the moso-bamboo forest floor is, however, similar to that reported by Jiang Qiuyi (1989), who found that the water retention capacity of the living soil cover (930 kg/ha) and litter (2130 kg/ha) on a moso-bamboo forest floor totalled 0.289 mm.
Runoff ckavacteristics of the research watershed
The observations from the runoff plots show that surface runoff or rainfall excess occurs only rarely on the floor of the bamboo forest and that the amounts are also verj small. This can be explained by the high infiltration capacity of the moso-bamboo forest floor. An analysis of the mean intensity of each precipita- tion event (Ma Xuehua et al., 1992) shows that in general precipitation cannot exceed the infiltration ability, and quick runoff should therefore not often occur. It was, however, often observed in the water- shed. The first reason is the uneven distribution of infiltration properties over the entire watershed. For example, in the channel itself or near the channel, where there is water flow or where the surface will be covered by the increased water flow occurring during precipitation events. the infiltration capacities are often near zero or very low. When precipitation falls onto the saturated area or onto the water surface, it will be directly converted to runoff and it may be termed direct runoff. The generation of direct runoff reflects the influence of the forest to only a limited extent. The second reason is that the instantaneous pre- cipitation intensity can exceed the infiltration rate and therefore lead to instantaneous and local surface runoff.
Our observations showed that the quick runoff composed of direct runoff and surface runoff exhibits a rapid response to precipitation. The quick runoff often appears during the same day as the precipitation, although the amounts involved are small. The interflow within the upper soil layer, which is termed upper interflow, has a slower response to precipitation and often appears up to three days later. The precipitation in 1990 was close the mean annual value and the runoff for 1990 was selected and separated into different components (Table IV). Increased runoff occurring within the same day as the precipitation is assumed to be quick runoff, whereas increased runoff occurring within three days is assumed to be upper interflow and the rest is attributed to runoff composed of deeper interflow and underflow.
From Table IV and other observations the following conclusions can be drawn:
- Runoff components. The annual runoff in this research watershed is 901.4mm and therefore accounts for 54.8% of the annual precipitation of 1643.5 mm in 1990. The components comprising the total run- off are: quick runoff 13.1 mm (0.8%), upper interflow 246.0mm (15%) and slow runoff 642.3mm (39%)
- Variation of runoff components. The range of variation of the monthly runoff components (Xmax - Xmin) can be used as an index to evaluate the hydrological effects of the forest. In 1990 they were: precipitation 167.5 mm, total runoff 122.4mm, quick runoff 4.1 mm, upper interflow 52.9mm and slow runoff 68.7 mm. The quick flow ratio remained mainly in the range 0.4-0.7%, which is similar to the channel extent, and it can therefore be concluded that the quick runoff is only affected to a limited extent by precipitation variation and is therefore mainly composed of direct runoff. The slow runoff and upper interflow are greatly affected by precipitation variation. This can be explained by the high infiltration capacity of the moso-bamboo forest floor.
- Increasing drought period runoff and decreasing peak runoff. Because a high proportion (54%) of precipitation is converted into interflow and underflow and because the water retention capacity is so great, peak runoff rates are reduced and the drought period runoff is increased. Although the watershed area is limited (only 11.7ha) and the annual precipitation depth is not very high and its seasonal distribution is uneven, runoff from this watershed never stopped. In the research period the minimum observed runoff was 0.6mm/day. The maximum peak runoff observed during a rainfall event with a total depth of 331.4mm and a duration of five days (this represents an average intensity of 2.8 mm/h) was only 2.3 mm/h. By comparing the mean rainfall intensity and the peak runoff rate it is evident that the peak runoff is less affected by instantaneous precipitation intensity, and more affected by the total depth and the average intensity of precipitation.
Input-output budgets,for some nutrient elements
Because of the limited analytical facilities, only the concentrations of N, P, K, Ca, Mg and SiO2 were measured on a monthly basis. SiO2 was measured because the Si element is an important component of bamboo wood. In addition, the pH values of different water components were measured. The atmospheric pathways of element input include wet and dry deposition. Wet deposition can be easily measured by analysing the chemical components of gross precipitation. Measurements of dry deposition in forests is much more complicated and difficult because it is influenced by many factors and can reach the forest floor by many different pathways. For example, some could fall directly through the canopy onto the forest floor, whereas some must first be captured by the canopy and thereafter in several ways (washing, wind blowing, gravitation, etc.) transferred to the forest floor. In addition, dry deposition is not vertical and the velocities of differently sized particles may be very different. The dry deposition captured by the canopy can also be mixed with wet deposition and elements released from the canopy or even absorbed by the trees. In this research the element input by precipitation is regarded as wet deposition, the total element input by throughfall and stemflow is regarded as the total net input to the forest ecosystem and the element output by streamflow is regarded as the total output of the forest ecosystem.
Element concentrutions in difeerent linter components. The average element concentrations in different water components are shown in Table V. The order of magnitude of the element concentrations associated with gross precipitation is similar to that found by other researchers in this region. With the exception of the decrease in P, which often appears to be insufficient to meet the growth requirement, all other element concentrations in throughfall are increased due to their mixing with dry deposition and elements released from the vegetation. The fact that the concentrations of all elements in throughfall except K are nearly equal to those in stemflow differs from other forest types. This can be explained by the smooth bamboo trunk and the general shortage of bark.
The concentrations in surface runoff are much higher than those in other water components. This can be explained by the element-rich nature of the humus and topsoil. In contrast, because of the filter effect of the forest floor, the element concentrations in streamflow, which is mostly composed of interflow and under- flow, are much lower than those in surface runoff. However, compared with precipitation in the open all concentrations except N and P are increased in streamflow.
Seasonal variation of element concentrations. The seasonal variations of element concentrations in the precipitation in the open are not very regular, but there are obvious peak periods in summer and winter (Figure 3). The variations of concentrations in throughfall and stemflow appear to follow a similar pattern (Figures 4 and 5), whereas those in surface runoff and streamflow exhibit different patterns. There is only one peak period in summer for surface runoff (Figure 6) and this reflects the high temperature, humidity, organic matter decomposition rate and more erosive rainfall in this period. On the contrary, there is almost no seasonal variation of concentrations in streamflow and the magnitude of the variation of the element showing the greatest variation (Ca) is less than 4ppm (Figure 7). This can be explained by the filter effect and the decreased surface runoff from the forest floor.
Element input-output budgets. The annual input-output budget of each measured element in the research watershed is shown in Table VI. The precipitation under the canopy refers to the sum of throughfall and stemflow and the difference in its element input from that of precipitation in the open is termed net canopy input. When the element input associated with precipitation either in the open or under the canopy is viewed as the watershed input and compared with the watershed output (streamflow), it can be seen that in the case of N, P and K this watershed is in an accumulating state, whereas for Ca, Mg and SiO2 there is a net loss.
pH values of diferent water components. The observations showed that acid precipitation is not a problem in the study area and that the pH conditions do not affect tree growth. The pH values of precipitation in the open vary between 6.50 and 7.60, with a mean of 6.87; those of throughfall vary between 6-45 and 7.05, with a mean of 6.77; those of stemflow vary between 6.50 and 7.10, with a mean of 6.82; those of surface runoff in runoff plot vary between 6.60 and 7.55, with a mean of 7.02; and those of streamflow in the research watershed vary between 6.65 and 7.25, also with a mean of 7.02 (Figure 8)
The important hydrological benefits of moso-bamboo forest can be summarized as follows:
- Soil physical and hydrological properties. For the 0-60cm soil layer the bulk density is 1.00g/cm3, the total porosity is 71.74% and the non-capillary porosity is 5.81%, the water retention capacity is 430mm and the maximum effective water retention capacity is 313mm, which is 28% higher than those of a nearby Chinese fir plantation and a nearby natural broadleaved forest. The parameters in Horton’s infiltration equation measured under drought conditions are fo 29.10 mm/min, fc 8.28 mm/min and k 0.2391. The infiltration properties are much better than those of the nearby Chinese fir plantation.
- Redistribution of precipitation. The annual canopy interception ratio is 11.1% and the annual stem- flow ratio is 4.4%. The effect of soil cover on this redistribution is small and no greater than 0.6mm for a single precipitation event.
- Regulation of runoff. The annual runoff ratio is 54.8%. The ratio for quick runoff comprising both surface runoff and direct runoff is only 0.8%; the ratio for upper interflow is 15%; and the ratio for the slow runoff composed of deeper interflow and underflow is 39%. The quick runoff is com- posed mainly of direct runoff and is therefore not greatly affected by the seasonal variation of precipitation. The moso-bamboo forest is very effective in reducing peak runoff and increasing low flows.
- Input-output budgets of some nutrient elements. For N, P, Ca, Mg and SiO2, but not K, the differ- ences in concentration between throughfall and stemflow are small. Compared with precipitation in the open, the concentrations of all elements except P are increased in throughfall and stemflow. The concentrations of all elements are highest in surface runoff. After filtering by the forest soil, the con- centrations of N, P and K are decreased compared with those in throughfall and stemflow, whereas the concentrations of Ca, Mg and SiO2 are increased. The annual element inputs (kg/ha) by precipitation in the open are N 10.1, P 0.89, K 18.8, Ca 25.8, Mg 3.1 and SiO2 10.1, respectively, and those by throughfall and stemflow are N 17.7, P 0.38, K 56.5, Ca 31.4, Mg 4.8 and SiO2 26.2. This means that the inputs of all measured elements, except P, are increased by the canopy interception effect. The annual element outputs by streamflow are N 3-0, P 0.28, K 16-6, Ca 38.9, Mg 8.3 and SiO2 125.7, respectively. Compared with their inputs by throughfall and stemflow, the outputs of N, P and K are lower and the outputs of Ca, Mg and SiO2 are higher.
- The pH values of all water components exhibit only limited variation within the range 6.45-7.60. The average pH values are 6.87 for precipitation in the open, 6.77 for throughfall, 6.82 for stemflow and 7.02 for both surface runoff from runoff plots and for streamflow in watershed.
We acknowledge the valuable technical guidance of Professor Ma Xuehua and Professor Jiang Youxu of our Institute in this project and the kind assistance of Professor D. E. Walling of the University of Exeter for both revision of the English and advice on the content of our manuscript.
- Cao Qungen 1989. ‘Studies on the hydrological effect of Phyllostachys pubescens stands’, Bamhoo Res., (1982) 2, 24-44 [in Chinese].
- Jiang Qiuyi 1989. ‘Water capacity of aerial part biomass and its effects on hydrological characters in the forest land’, J. Zhejiung Forestry Colf., 6(2), 176- 181 [in Chinese].
- Ma Xuehua and Yang Guangying 1990. ‘A study on soil-physical properties and change of soil moisturc content in man-made forests of Cunninghamia lanceolata and pinus massoniana’, Foresr Res. 3( I), 64-88 [in Chinese].
- Ma Xuehua, Yang Maorui and Liu Yongmin 1992. ‘A study on characteristics of runoff in forest plantation of Cunninghamia lanceolata and pinus massoniana’, Forest Res., 5(3), 284-289 [in Chinese].
- Nanjing Institute of Soil Science. The Chinese Academy 1978. The Measuring Methods of Soil Physical Properties. The Scientific Publishing House, Beijing, 144 pp [in Chinese]
- Wang Yanhui 1990. ‘Hydrological and ecological effects of the soil in Phyllostachys pubescens platation', J. Bamboo Res 9(4), 40-49