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Hydrological impacts of thinning in a deciduous forest ecosystem PRESENTED BY HZAL SERENGIL, Y., G Ö KBULAK, F., Ö ZHAN, S., HZAL , A., SENG Ö N Ü L, K., BALCI, A.N., Ö ZYUVACI, N ISTANBUL UNIVERSITY FACULTY OF FORESTRY DEPT. OF WATERSHED MANAGEMENT

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hydrological impacts of thinning in a deciduous forest ecosystem
Hydrological impacts of thinning in a deciduous forest ecosystem

PRESENTED BY HZAL

SERENGIL, Y., GÖKBULAK, F., ÖZHAN, S., HZAL, A., SENGÖNÜL, K., BALCI, A.N., ÖZYUVACI, N

ISTANBUL UNIVERSITY

FACULTY OF FORESTRY

DEPT. OF WATERSHED MANAGEMENT

hydrological impacts of thinning in a deciduous forest ecosystem2
Hydrological impacts of thinning in a deciduous forest ecosystem

NEED FOR THE STUDY

Turkey is located on a geography that

includes diverse ecological conditions.

hydrological impacts of thinning in a deciduous forest ecosystem3
Hydrological impacts of thinning in a deciduous forest ecosystem

The 72 millions of population is concentrated in larger cities which led to the implementation of huge water resources development projects.

Istanbul, the economic capital is one of the largest cities in Europe with a population of over eleven million

hydrological impacts of thinning in a deciduous forest ecosystem4
Hydrological impacts of thinning in a deciduous forest ecosystem

More than 15 water reservoirs are in operation around the city and new pipeline projects to carry all the water within a circle of 300 km in diameter are on the way. The annual supply of water resources doubled from 1994 to 2002 with the accelerated investments (ISKI, 2002).

hydrological impacts of thinning in a deciduous forest ecosystem5
Hydrological impacts of thinning in a deciduous forest ecosystem

BACKGROUND

The results of most paired studies have been summarized and discussed in a number of reviews starting with Hibbert (1967) and followed by Bosch and Hewlett (1982), Hornbeck et al. (1993), Stednick (1996), Sahin and Hall (1996), Vertessy (1999, 2000) and finally by Brown et al. (2005).

hydrological impacts of thinning in a deciduous forest ecosystem6
Hydrological impacts of thinning in a deciduous forest ecosystem
  • To highlight some of the conclusions based on these reviews;
  • the response to treatment is highly variable and, for the most part, unpredictable (Hibbert, 1967),
  • streamflow response to the treatment depends on both the mean annual precipitation of the watershed and on the precipitation for the year under treatment (Bosch and Hewlett, 1982),
  • infiltration and evapotranspiration (ET) plays a key role in determining what happens to the flow regime after treatment (Bruijnzeel, 1988),
  • changes in annual water yield from forest cover reductions of less than 20 % of the watershed could not be detected by streamflow measurements (Bosch and Hewlett, 1982; Stednick, 1996).
hydrological impacts of thinning in a deciduous forest ecosystem7
Hydrological impacts of thinning in a deciduous forest ecosystem
  • In large watersheds or the ones that are managed toward multiple uses including water production or recreation, intensive or sharp treatments might not be applicable.
  • Gotmark et al (2005) emphasized the benefits of partial cutting on the biological diversity in European perspective while Shao et al (2005) mentioned the new forestry policy in China requiring that forests be harvested with selective cutting methods.
  • In Turkey, the situation is similar. Clearcutting is performed in some situations but out of consideration in many cases, and thinning is the most preferred harvesting method in water producing watersheds and the ones assigned for recreation or aesthetics purposes.
hydrological impacts of thinning in a deciduous forest ecosystem8
Hydrological impacts of thinning in a deciduous forest ecosystem

HYPOTHESIS

The common knowledge that suggests a 20 percent lower limit for cutting treatments to be able to cause any detectible change on the streamflow is valid for Belgrad Forest ecological conditions. Therefore an 11 percent thinning is not expected to cause an alteration in water yield and regime.

hydrological impacts of thinning in a deciduous forest ecosystem9
Hydrological impacts of thinning in a deciduous forest ecosystem

EXPERIMENTAL WATERSHEDS IN BELGRAD FOREST

The experimental watersheds are located in the Belgrad Forest (41° N, 28° E) which has been preserved as the only old-growth oak-beech natural forest near Istanbul (Figure 1). The climate of the watersheds and surrounding area according to Thornthwaite classification system is humid, mesothermal oceanic with a moderate soil-water deficit in summer. Long term mean annual precipitation is 1050 mm and mainly fall from October to March.

Figure 1. Location of experimental watersheds.

hydrological impacts of thinning in a deciduous forest ecosystem10
Hydrological impacts of thinning in a deciduous forest ecosystem

Table 1. Mean annual precipitation, temperature and Thorntwaite potential evapotranspiration (PotET) during the calibration period (1979-1985).

Parent materials mainly consist of carboniferous clay schists and neogene loamy, gravelly deposits. The soils are usually shallow to deep, gravelly, loamy clay in texture, rich in organic matter with medium to good permeability rates and high erodibility potentials without carbonate reaction. The mull type forest floor has an average depth of 5 cm (Ozhan, 1977). Subsurface flow is the main mechanism to feed the streams in the watersheds (Balci et al., 1986).

hydrological impacts of thinning in a deciduous forest ecosystem11
Hydrological impacts of thinning in a deciduous forest ecosystem

Topography is generally gentle, and mean elevation is around 140 m. Both watersheds are on a southern aspect adjacent to the divide which is about 3-4 km from the Black Sea, under the influence of prevailing northern maritime winds during the rainy period.

Dominant vegetation includes oak (Q. frainetto Ten., Q. cerris L.) and beech (F. orientalis L.) tree species mixed with varying amounts of Carpinus betulus L., Castanea sativa Mill., Populus tremula L., Alnus glutinosa L., Acer trautvetteri Med., Acer campestre, Ulmus campestris L., and Sorbus torminalis Crantz. (Yaltirik, 1966) with a normal crown closure.

Table 2. Some properties of experimental watersheds.

hydrological impacts of thinning in a deciduous forest ecosystem12
Hydrological impacts of thinning in a deciduous forest ecosystem

Table 3. Forest stand properties in the watersheds before treatment

(derived from Belgrad Forest; forest management plan).

Species; O:Oak, H:Hornbeam, B:Beech, M:Minor broadleaved (<10%), Overstorey/Understorey

Diameter classes (cm); a: <7.9, b: 8-19.9, c: 20-35.9, d: >36

Crown closure (0 - 1.0); 1: 0.11-0.40, 2: 0.41-0.70, 3: >0.71

hydrological impacts of thinning in a deciduous forest ecosystem13
Hydrological impacts of thinning in a deciduous forest ecosystem

2.2. Field methods

Both streams draining Watershed-I (control) and Watershed-II (treatment) were instrumented with 90° and

120° concrete sharp-crested V-notch weirs

and automatic water level recorders.

Data collection has been started in 1979 in an

attempt to study the effects of timber

harvesting upon water quality and yield.

The paired watersheds were calibrated from

1979 to 1985. In February 1986 11 percent

of the standing timber volume was removed

from the treatment watershed (Watershed-II)

by employing a standard individual selective

cutting technique. The felled timber was taken

out of the watershed through horse dragging on forest roads.

hydrological impacts of thinning in a deciduous forest ecosystem14
Hydrological impacts of thinning in a deciduous forest ecosystem

2.3. Rainfall-Runoff Process

Mean annual precipitation was 1090.5 mm during the calibration period (1979-1985), while the following year (March 1986-February 1987) after treatment it was 1124.9 mm, little more than average. Runoff coefficient of W-II was mostly higher than the W-I in the long term (0.27>0.21) (Figure 2), both highly variable ranging from 0.02 to 0.63. Coefficients were higher in the rainy years and after rainy periods of 2-3 subsequent years. The precipitation in the previous 3 hydrologic years before 1985 was lower than average.

Figure 2. Runoff

coefficients

of the experimental

watersheds.

hydrological impacts of thinning in a deciduous forest ecosystem15
Hydrological impacts of thinning in a deciduous forest ecosystem

Figure 3. Annual hydrographs of the experimental watersheds during the calibration period (1979-1985).

The high flow period was from October to May (Table 1). The monthly precipitation was over 100 mm in the first 6 months of this period, including a nonsignificant amount of snowfall. The hydrographs reached to the peak in January, while the rainiest month was December. The general shape of the hydrographs reflected the soil water deficit in summer months, and the soil water replenishment, continued throughout the autumn.

hydrological impacts of thinning in a deciduous forest ecosystem16
Hydrological impacts of thinning in a deciduous forest ecosystem

The history of the rainfall-runoff process can be determined by correlograms (Muftuoglu, 1991; Salas, 1993). The monthly correlogram shows at least 2 months of strong, 2 months of weak linear history (r0=0.648, r1=0.644, r2=0.533, r3:=0.234, r4=0.058) which means that the runoff amount of the current month is significantly affected not only with the precipitation of this month but also the previous 4 months.

Table 4. The 0-8 lag cross correlations between monthly precipitation and streamflow.

hydrological impacts of thinning in a deciduous forest ecosystem17
Hydrological impacts of thinning in a deciduous forest ecosystem

Figure 4. The monthly variation of precipitation, streamflow (W1) and the change

in the correlation coefficient between the two experimental watersheds.

hydrological impacts of thinning in a deciduous forest ecosystem18
Hydrological impacts of thinning in a deciduous forest ecosystem

STATISTICAL METHODS

2 REGRESSION APPROACHES WERE APPLIED. DAILY AND MONTHLY.

We considered the monthly flows that run over the prediction interval as statistically significant. Because the frequency distribution of the daily flows, as mentioned in the flow regime section below, was far from being Gaussian, application of a t test procedure would not be adequate.

It is a representation of the range of values that an individual y might take on for a given xi. It incorporates the parameter uncertainty as well as the unexplained variability of y. The (1-) 100% prediction interval for a single response, given xi, is

yi  t s [1 +1/n + (xi-χ)2 /SSx]1/2

where

s = [(SSy-b1Sxy) / n-2]1/2

yi is the best estimation of Y according to X=xi, while t is the t value for n-2 degree of freedom and exceedence probability of 0.05. χ is the mean value of X, s is the standard error of the regression (Bayazit, 1996) and Hirsch et al., 1993).

The prediction interval bands are further from the best-fit line than the confidence bands, a lot further if you have many data points. The 95% prediction interval is the area in which you expect 95% of all data points to fall (Motulsky and Christopoulos, 2003).

hydrological impacts of thinning in a deciduous forest ecosystem19
Hydrological impacts of thinning in a deciduous forest ecosystem

Figure 5. The predictions of both regression approaches for the post treatment year.

The post treatment year water surplus calculated by regression equations (observed-predicted) based on monthly and daily data were 79.28 mm and 106.11 mm, respectively.

hydrological impacts of thinning in a deciduous forest ecosystem20
Hydrological impacts of thinning in a deciduous forest ecosystem

Figure 6. The observed and predicted monthly streamflow of W-II with prediction intervals (p= 0.05).

January was the only month that observed flow exceeded the prediction interval (p<0.05).

The difference was 7.76 mm which means that at least this amount of water was

achieved by thinning, statistically. It is 2.92 percent of the annual post treatment flow.

hydrological impacts of thinning in a deciduous forest ecosystem21
Hydrological impacts of thinning in a deciduous forest ecosystem

TREATMENT NOT EFFECTIVE ON FLOW REGIME

FLOW DURATION CURVES

Figure 8. FDC for the dry months

(April-August) after the treatment.

Figure 9. FDC for the rainy months

(October-February) after the treatment.

hydrological impacts of thinning in a deciduous forest ecosystem22
Hydrological impacts of thinning in a deciduous forest ecosystem

DISCUSSION and CONCLUSIONS

The climatic condition of a region is a significant factor to determine the results of a forestry treatment.

In this study the hydrologic response of the treatment watershed to the thinning was mostly concealed in the following summer months because of the high potential ET rate in the region and also possibly stimulated growth of the thinned stands.

In published studies based on the works in USA, the largest relative changes in streamflow were observed in summer months after removal of eastern deciduous forest and western conifer forest. However, the largest absolute streamflow increases occurred during wet winter months (Jones and Post, 2004).

In this study the situation was quite different, as the summer flows were very low due to limited summer precipitation in the region. The largest absolute streamflow increases were observed in the winter months.

hydrological impacts of thinning in a deciduous forest ecosystem23
Hydrological impacts of thinning in a deciduous forest ecosystem

The 11 percent thinning was not effective enough to change the flow regime of the streams draining watersheds covered with deciduous forests, but caused a statistically significant increase on the streamflow of January in the following year.

The mechanism to explain the water surplus in the winter months in this situation might be the quicker replenishment of soil water beneath the thinned stands in fall. Owing to the 4 months long rainfall runoff history of the watersheds, the difference in the recharging time of the soil moisture might have caused the difference on the water yield in the dormant winter months.

hydrological impacts of thinning in a deciduous forest ecosystem24
Hydrological impacts of thinning in a deciduous forest ecosystem

The additional water gained by the thinning treatment was around 3 percent of the annual flow in the first year while the annual precipitation was over the average. Consequently, it is not possible to suggest such a slight treatment as a tool to be used in water resources development works. However, it can be considered as a baseline which suggests that less than 20 percent thinning might also cause a statistically detectible change in water yield.