EFFECT OF FORESTS ON WATER REGULATION AND SOIL EROSION CONTROL

Transcription

1 Forests & Water/REF/16 EFFECT OF FORESTS ON WATER REGULATION AND SOIL EROSION CONTROL I. INTRODUCTION: Dr. Vo Dai Hai Deputy Director General Forest Science Institute of Vietnam Dong Ngac Tu Liem Hanoi/Vietnam For nearly past three decades now due to many causes the area of forests in Vietnam has decreased remarkably. The forest cover ratio remains 33% and is unevenly distributed. In a number of upstream and large water reservoir watershed areas, the forest cover remains about 10%. The territory of Vietnam is not big but due to abruptly dissected topography, the rainfall distribution is much affected. In places the rainfall attains 3000 mm/year while rainfall in some places reach only about mm/year. Deforestation resulted in soil erosion, floods, quick sedimentation at rivers and lakes bottom, seriously effecting the life span of the irrigation and hydro-electric power works. The research on and establishment of watershed protective forests for water supply in Vietnam arised as an objective and urgent demand in recent years, especially since there was a requirement for safe and long-term operation of the irrigation and hydro-electric power works. At present of the 19 million ha of forests and forest lands, the planned area of specialized protective forests is 6 million ha (representing 31.6%) of which the area of protective forests in the upstream watershed areas is 5.55 million ha (representing 92.2%), but still now only about 3 million ha are under forests. It is actually shown now that many forests are not brought into full play their protective role, their stability and permanency are weak due to lack of proper structure and rational distribution. To have a stable environment strategy for the society there must be first of all the research on and establishment of a permanent and later on optimum forest asset in which three aspects must be determined: Rational forest cover for the whole country and each region. Locations where forests are needed. Rational forest structure at each place. One of the problems that must be solved is to determine on the rational structures for upstream protective forests aimed at bringing into full play the function of soil protection and water regulation, creating permanent and stable forest stands. This is an important direction and at the same time an effective and long-term approach in the environment strategy of Vietnam. 1

2 This paper focuses mainly on research results obtained so far in Vietnam on protective capability of various types of vegetation structure (water regulation and soil erosion control) so as to put forward models of watershed protective forests of best structures, laying foundation for feasibility study on protective forest areas in Vietnam as well as completion of technical indices for legal procedures already available for protective forests, setting orientation for tending and protecting of watershed forests. The research has been done with the financial support from International Foundation for Science (IFS). II. MATERIALS AND METHODS: The experiments are laid out in two different places with different forest structures. -Sopai Kon Ha Nung (Central Highland of Vietnam): 12 permanent plots. -Pleyku town Gia Lai (Central Highland of Vietnam): 4 permanent plots. Plot size is 10*20m=200m 2. The plots are laid out on slope Diagram of the run-off plot designing is shown in figure 1. 2

3 III. RESULTS AND DISCUSSION: 3.1. Effect of forest cover: To investigate the effect of vegetation cover on soil erosion and run-off we have carried out a study in three-storeyed evergreen broad-leaved mixed natural forests with two different cover degrees: and and bamboo forests 8-10 m high on slope gradient 15 o. Research results are shown in table 1. Table 1: Effect of forest cover on soil erosion and run-off Vegetation structural types -Rehabilitated 3-storeyed mixed natural forest -Depleted 3-storeyed, mixed natural forest. -Bamboo forest 6-10m high. -Bamboo forest 6-10m high. -Secondary natural forest -Secondary natural forest (Data from Bui Nganh 1977) Cover Soil erosion Run-off degree T/ha % M 3 /ha % Data in table 1 show that if the forest cover decreases from to , soil erosion will increase 42.2%, surface run-off increases 30.4% (in natural forests); soil erosion increases 27.1%, surface run-off increases 33.8% (in bamboo forests plots ); whereas in Nui Tien mountain (Huu Lung district) soil erosion increases 27.8%, run-off increases 49.8%. Through analysis of variance of one factor we find that soil erosion and run-off under and forest covers are quite varied (calculated F>F 0.05 from table). In natural conditions without interference, the general trend is that the forests are gradually rehabilitated and their protective role is brought into full play but with negative interference by man the forests will become exhausted, their cover is low and conditions are created for the acceleration of soil erosion and floods; causing serious consequences to the production and the people s life in the region. Thus with focused protective forest areas in the process of forest rejuvenation or minor forest product harvesting one must not reduce the forest cover degree to under 0.7 or create large openings in the forests. 3

4 3.2. Effect of forest strata on soil erosion and run-off: Together with forest cover and forest composition, forest strata also exert great influence on soil erosion and run-off. Experiments were carried out in evergreen broad-leaved mixed natural forests with different number of storeys. - Three-storeyed mixed natural forests, cover degree : These are the forests that have undergone rational logging and adequate rehabilitation time. - Two-storeyed natural forests, cover degree ; these are young forests undergoing rehabilitation period. - One-storeyed natural forests, cover degree ; these are forests their emergent storey was over-cutting, the ground cover vegetation with creepers has been clearly cut (forests that have undergone tending in the old way in Vietnam). Results of our experiments are shown in table 2. Table 2: Effect of forest strata on soil erosion and run-off Vegetation structural types 1. Mixed natural forest with cover degree Mixed natural forest with cover degree Mixed natural forest with cover degree Number of Soil erosion Run-off storeys t/ha % m 3 /ha % Thus it is seen that three-storeyed forests are the best in soil and water conservation (lowest soil erosion and run-off), the worst are one-storeyed forests (soil erosion is over thrice and run-off is 1.5 times). Through analysis of variance with one factor we found out that between two-storeyed and threestoreyed forests the amounts of soil erosion and run-off are not significant different (calculated F<F 0.05 from table); while between one-storeyed and twostoreyed (or three-storeyed) forests these amounts are markedly different (calculated F>F 05 from table). These results from the experiments show us clearly the especially important role of the ground cover vegetation in soil erosion and run-off control. The function of soil and water conversion of a ground cover vegetation combined with the canopy of a medium-tree storey above it is equal to that of a 3-storeyed forest. Thus in establishing protective forests for watershed management it is better to create and maintain multistoreyed forests and attention must be paid to the protection and maintenance of the ground cover vegetation under the forest canopy. 4

5 3.3. Effect of species composition on soil erosion and run-off of a number of forest plantation types: Research results in the world and home country show that apart from forest cover and strata, capability of soil and water conservation of tree species are also different. In the United States of America capability of soil erosion control of hundreds of agricultural crops have been made into coefficient in the Universal Soil Loss Equation by Wischmeier W.H-Smith D.D. (C coefficient). In Vietnam, research results of Nguyen Quang My in Tay Nguyen Research Programme II show that capabilities of soil erosion control of agricultural and industrial crops such as beans, maize, sweet potatoes, coffee,... are very different. Forest tree species in Vietnam have almost not been studied, therefore in general many difficulties are still met with in establishing protective forests. To partly solve the above-mentioned difficulties and lay a foundation for the selection of tree species for protective forests we have carried out research on effects of different forest tree species on soil erosion and run-off in different ecological zones. Research results of the experiment laid out in Soil Erosion Research Station in Pleiku township (Gia Lai province) are shown in table 3. Table 3: Soil erosion and run-off data in the forest plantations on basalt soil (slope gradient 10 o ), Pleiku-Gia Lai (rainfall 2250 mm/year) Forest plantation types Soil erosion Run-off t/ha % m 3 /ha % 1. Pinus kesiya forest A. auriculiformis mixed planted with Cinnamomum camphora 3. A. auriculiformis mixed planted with Pinus kesyia 4. Tectona grandis forest Results from experiments show that the protection capability is best with Pinus kesyia forest - coniferous forest (lowest soil erosion t/ha and run-off 1,262.7 m 3 /ha/year respectively). The protection capability is average with mixed plantation of Acacia auriculiformis and Cinnamomum camphora, Acacia auriculiformis and P. kesyia. With Teak plantation due to low planting density, low cover degree, the ground vegetation being frequently removed in the process of tending Hopea odorata that was planted under the canopy, soil erosion and runoff here are very great (soil erosion 2,3607 t/ha, surface run-off 1,557.0m3/ha). Apart from the types of forest plantation in Pleiku twonship (Gia Lai province), a number of other types of forest plantation in the Forest Experimental Research Station (Hoa Binh province) were also studied. Research results are shown in table 4. 5

6 Table 4: Capability of soil erosion and run-off control of a number of forest plantation types on 15 o slopes (Hoa Binh) Forest plantation types Soil erosion Run-off kg/ha % m 3 /ha % 1. Acacia auriculiformis Acacia mangium Bamboo forest Aleirites montana Data measured (table 4) at Binh Thanh (Hoa Binh province) from 1/July to 12/November/1993 with total rainfall of mm show that soil erosion was lowest in Acacia auriculiformis plot: kg/ha (100%), in bamboo plot kg/ha (117.13%) and soil erosion was highest at Aleirites montana plot: kg/ha and Acacia mangium plot kg/ha. As regards run-off the variation law is similar to that of soil erosion but there is a variation that the run-off at bamboo plot ranks third (higher than A. auriculifomis and A.mangium). Thus the function of soil conservation of bamboo forest is not inferior to those of other broad-leaved, thick-crowned species such as A.auriculiformis, A. mangium,... but its water conservation function is inferior to those of other species. This can be explained as follows: in bamboo forest there is a rather thick layer of fallen leaves hardly to decompose. This layer tightly cover the ground surface thus checking soil erosion. On the other hand in bamboo forest the surface soil layer is dry and hard, the roots of bamboos are small, shellow in the ground, therefore the water roaking capability of the soil is poor resulting in abundant surface run-off. These research results agree well with the law in bamboo forests in Kon Ha Nung (Tay Nguyen). In general the protection capability of forest plantations is far inferior to that of natural forests (especially in water regulation) because most of forest plantations in Vietnam are single-storeyed ones, ground vegetation and the litter being poor the former is frequently cut and fallen leaves are collected for fuel. In establishing protective forests in critical and very critical areas it is necessary to create multi-storeyed forests, attention must be paid to forest rehabilitation measure by regenerated native species under forest plantation canopy, converting forest plantations into multi-storeyed forests. 6

7 3.4. Effect of forest litter: Besides living trees and plants as main components of a forest, the litter (the falling twigs, leaves,...) in the forest plays no less important part in water regulation and soil erosion control. Under the forest canopy the litter tightly covers the ground surface, prevents rain-drops to strike directly the ground surface, absorbs and retains part of the rain water and at the same time acts as obstacle reducing the run-off speed, partly checking soil erosion and floods. Thank to the vegetation cover and the litter layer that the humidity of the surface soil layers (0-30 cm) in the forest in the sunny days is always 2-4 times higher than that of the soil in the open and bush land and savanna (see table 5). Table 5: Variation of soil humidity in a number of vegetation types (data collected in March, April, May and June 1993 and 1994) Vegetation type Humidity (%) at the soil depth (cm) Bare land Savanna +bushes Rehabilitated forest after slashand-burn cultivation, cover degree Three-storeyed forest, cover degree One-storeyed forest without ground vegetative, cover degree Tree-storeyed forest, cover degree Two-storeyed forest, cover degree Bamboo forest, cover degree Imperata cylindrica savanna The variation of humidity in the surface soil layers is very great but in deeper layers it is unremarkable. It must be noted that humidity in bamboo forest and Imperata cylindrica grassland is the lowest, the soil is compact. Data collected in 1993 show that the fallen twigs, leaves etc. in tropical broad-leaved evergreen mixed natural forests in really considerable (11.2 t/ha), the water absorption capability of the litter depends on the latter s status (see table 6). 7

8 Table 6: Water absorption capability of the forest litter Amount of water absorbed by the litter Litter status Dry weight (g) Weight (g) % as compared with litter dry weight 1. Coarse litter or not yet decomposed 2. Partly decomposed (30-40%) Data in table 6 show that the water amount (weight) absorbed by dry litter is 1.38 times that of the latter s weight while partly decomposed (30-40%) litter can absorb the amount (weight) of water 3.21 times its dry weight. Thus on one hectare of natural forest the layer of litter can absorb 35,840 litres of water equivalent to a 3.6 mm rainfall. This is remarkable contribution of the layer of litter to the run-off checking. Thus with the watershed protective forest areas the layer of litter should not be removed. It must be kept for natural decomposition and covering the soil. Due to the high capability in self-decomposition and humidity preservation of the litter, the soil under it is harboured by rich and abundant soil fauna of which the earth worm is the most important, making numerous holes and tunnels in the forest and form a dense network under the ground and after dying they leave the holes making the soil absorbing more water (see table 7), and by this the underground water is increased and the surface run-off reduced. Table 7: Speed of water absorption by basalt soil in a number of types of natural forest Vegetation structural types Water absorption speed (mm/min) 1 -Savanna +bushes Rehabilitated forest after slash-and-burn cultivation, cover degree One-storeyed forest devoid of ground vegetation, cover degree Depleted forest (three-storeyed, cover degree ) 5 -Two storeyed natural forest, cover degree Three-storeyed natural forest, cover degree

9 3.5. Capability of rainfall interception of a number of forest types: One of the very important roles of the forest is the keeping part of the rainfall from directly falling on the ground by its canopy, redistributing the rainfall and thus reducing the run-off and soil erosion. In Vietnam the studies on rainfall interception by forest canopy have been made only in 1970 s. From then on this subject has not been continued and completed including research method. Rainfall measurement under the forest canopy in rainfall interception is very complex, especially in case of mixed broad-leaved natural forest in Vietnam. That complexity is first of all due to the forest being composed of many trees and plant species, many stories, lianas,... That is why we may not apply the method of rainfall measurement in temperate forests to the natural forests in Vietnam. There must be an appropriate measurement method, ensuring the accuracy and suitable to practical conditions in Vietnam. Research method: To be worked out the method, we have laid out two permanent experimental plots, one under a three-storied natural forest with forest cover 70-80% (little to moderate exploited) and another under depleted forest (30-40% cover). In the plots rain gauges are laid out by randomized systematic method. Then we have carried out analysis and calculation of the data for combination of 3, 4, 5, 6,... rain gauges and the results show that with 16 rain gauges the error is less than 10%. Therefore 16 rain gauges will be used for through fall measurement. +On natural forest: We concentrate our research on the most popular structures of Vietnam s natural forests that differ in structure. Research results are shown in table 8. Table 8: Forest structural types 1. Forest after rational selective logging with enough rehabilitation time. Capability of rainfall interception of natural forest types Forest cover (%) Number of stories Rainfall intercepted by the canopy (%) Depleted forest Young forest with enough rehabilitation time 4. Young forest after tending by old way (total clearance of ground vegetation cover) 5. Forest rehabilitated after slash and burn cultivation

10 6. Bamboo forest Data from table 8 show that rainfall interception is best 11.67% and 10.34% with 3 and 2-storied forest respectively, next come the forest rehabilitated after slash and burn cultivation and bamboo forest (9.51% and 8.96%). The rainfall interception is lowest (6.91% and 5.72%) with one-storey and three-storeyd forests but the cover degree is low (30-40%). +On man-made forest: The research results are shown in table 9. Table 9: Capability of rainfall interception by a number of man-made forest types Forest type Rainfall interception by forest canopy (%) 1. Acacia auriculiformis mixed with Cinnamomum camphora Whole forest stand Individual trees A. auriculiformis C. camphora Tectona grandis T. grandis Pinus kesiya P. kesiya A. auriculiformis + P. kesiya 8.17 Results from table 9 show that the capability of rainfall interception by man-made forests is not very high as compared with that of natural broad-leaved mixed uneven-aged forest. This is understandable as man-made forest structures are simpler than those of natural forest: their canopy is composed of only one storey. The cover is usually low due to wide spacing of forests and sparse ground vegetation cover. Thus in establishing protection forests there must be created multi-storeyed ones with high cover Research on stem flows: Usually the stem flow in the forest is net very great but in many cases it is not too small to be overlooked (Moltranov A.A.). Forest trees regulate and redistribute the rainfall through their crown and stems, bringing part of the rainfall to run down along tree stems thus lessening the impact of the rain drops on the ground surface. This is another function of the forest vegetation in curbing soil erosion and run-off Effect of the crown of the forest trees on the stem flows: In the studying the stem flows in the forests many authors take stem diameter and height as basis unit to classify forest trees in stem diameter classes to calculate the stem flows for the whole forest stand. But it is practically shown that 10

11 apart from stem diameter and height of the trees, other factors also effect the stem flows. To further provide a scientific base for studying stem flow we have made study on the effect of tree crowns (crown diameter and length) on stem flows. Three tree species subjected to the study are: Acacia auriculiformis, Pinus kesiya and Anisoptera cochinchinensis. For each species two trees having similar stems and height but differ in crown size are selected to study. Data on studied trees are shown in table 10. Table 10: Growth data on studied trees Tree species D 1.3 (cm) H (m) Crown length (m) 1. Acacia auriculiformis 2. Acacia auriculiformis 1. Pinus kesiya 2. Pinus kesiya 1. Anisoptera cochinchinensis 2. Anisoptera cochinchinensis Crown diameter (m) Results of data analysis in over two years show that there is a significant difference in the amount of stem flows in all the couples of trees of the same species (F calculated > F 0.05 from table). This shows that although the trees are of the same stem diameter and height but differ in crown sizes, the amounts of stem flow are different Method of quantifying the stem flow: From the above research and in combination with previous studies, the way to quantify the amount of stem flow for a whole forest stand are given as follows: Step 1: Laying out representative plot, conducting full measurement and enumeration and classifying forest trees into diameter classes. Step 2: In each diameter class 3-4 trees of average tree height and crown (crown diameter and length) are selected to measure the amount of stems flows. The results obtained are divided to the number of stems to have average amount per tree. This average amount then is multiplied by the number of trees in the diameter class to obtain the amount of the whole diameter class. Step 3: Total amount of stem flows in all diameter classes is the amount of the whole forest stand Research result:

12 As species composition of natural forests is complex and budget is limited, research can not be conducted on the amount of tree stems flows for all tree species, the error related to species composition in the diameter classes studied thus must be overlooked. Forest trees are classified in stem diameter classes 10 cm apart (middle values of tree diameter classes studied are: 15, 25, 35, 45, 55 cm,... Results are shown in table 11. Table 11: Tree stem flow in a number of forest structure types TT Forest type Stem flows (percentage of rainfall) 1 Three-storeyed forest, cover degree 30-40% One-storeyed forest, cover degree 70-80% (without ground vegetation cover) Three-storeyed forest, cover degree 70-80% Two-storeyed forest, cover degree 70-80% 1.53 Data in table 11 show that the amount of tree stem flows in a forest is not very great (variation between %) of which attention must be paid to the fact that the stem flow is the greatest (3.44%) in three-storeyed natural forests. The stem flows are about the same in one-storeyed and two-storeyed forests (1.44 and 1.53% respectively). The lowest stem flow (1.17% is observed in depleted forests. This is understood because with multi-storeyed forest, forest cover degree and tree density per unit area are being high, forest canopy dense and continuous, much of the rainfall is thus intercepted, of which a large part does run along twigs, branches down to the stems Distribution of rainfall in the forest: Basing on the research results mentioned above, basic data on the distribution of rainfall in a number of forest types in Vietnam is given in table 12. It must be noted that the measured stem flows will take part in run-off. Table 12: Rain water distribution in the forest Forest type 1. Three-storeyed forest, cover 30-40% 2. One-storeyed forest, cover 70-80% (without Rainfall intercepted by the canopy (%) Stem flow (%) Runoff (%) Underground flows and others (%)

13 ground vegetation cover) 3. Three-storeyed forest, cover 70-80%. 4. Two-storeyed forest, cover 70-80% Data in table 12 show that due to specially effective regulating function of forest vegetation that surface run-off in all forest types is very small (from 1.33% to 1.88%). The rain water constituting underground flow and others (evaporation, soil moisture,...) is very great ( %). This important function of forest has been recognized by many people as Each tree is an irrigation work. One of the strategies for environment protection in Vietnam is to establish system of specialized protective forests in each region, each watershed as well as inter-regional forests. III. CONCLUSIONS: 1. Cover degree, strata and species composition are forest structure factors that greatly affect soil erosion and distribution of rainfall in the forest area. Multi-storeyed forest (2 and 3 storeys) of high cover degree (>70-80%) are the best structural models for protective forests against soil erosion and flood in which the ground cover vegetation and litter layer play a specially important role. Dense grass cover likes Imperata cylindrica, bamboos have good capability in soil erosion control but their water regulation is not so good. Therefore, where only soil erosion control is mainly required it needs only to create a tight cover for ground surface such as a vegetation cover of Imperata cylindrica or bamboos. 2. The rainfall intercepted by forest canopy varies from 5.72 to11.6% for natural broad-leaved evergreen forests) and from 4.39 to 9.8% for forest plantation. To measure the throughfall in Tay Nguyen with error less than 10% at least 16 rain gauges are used with randomized-systematic distribution in 10 m sided Latin squares. 3. The stem flow in the forest is not very great, varies from 1.17 to 3.44%. When select sample trees for determining stem flow there must be paid to the length and width of crown. 13