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Abstract "Heft 42"


Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: P. Trüby


Heft 42


Oscar Thiers

Roble (Nothafagus obliqua [Mirb.] Oerst.)-Sekundärwälder in Zentral- und Südchile:
Bestimmung der für die Bestandesproduktivität wichtigen Standortsfaktoren


Freiburg im Breisgau 2004

ISSN 0344-2691


Summary:

Approach to the problem and objectives

The expanse of Chile's secondary forest is dominated by Nothofagus obliqua covers ca. 1.6 million ha (CONAF et al. 1999). Due to their high productivity and their favorable silviculture potential, these forests have drawn a lot of attention from public and private forest owners in the last decade. The relationship between site factors and stand growth should be a considerable component in forest planning. The following study intends to comprehend the site variability of N.obliqua-forests. Furthermore, the relevant climatic and edaphic growth factors will also be examined taking varying spacial scales into consideration.

Study Area

The study area range from 35°04' to 41°08' southern latitude and is bordered in the north by the Curicó province (VII region) and in the south by the Llanquihue province (X region). The N. obliqua stands are located within the three largest geomorphological regions of Chile: Coastal Range (n=1), Central Depression (n=21), and Andes Range (n=12).

Materials and Methods

The data base used for this work was obtained from three projects, which were carried out at the Universidad Austral de Chile. These data sources have differing spacial relations, ranging from national (DONOSO et al. 1993) to regional (LARA et al. 1999) and local (THIERS 2000) scales. The climatic data are based on three different sources (SCHLATTER et al. 1997; SCHLATTER et al. 1995; SANTIBANEZ and URIBE 1993). For the description and the sampling of the soils, the same procedure was used at each scale level (SCHLATTER et al. 1981; SCHLATTER et al. 1989). For each scale level, the soil and climate data of the N. obliqua stands were analysed with the help of descriptive statistics. The variability of the site productivity for the N. obliqua stands was aquired with the help of principal component analysis (PCA) (ISEBRANDS and CROW 1975). By means of  correlation and regression data, the relationship and the productivity patterns within the chosen N. obliqua secondary forests could be represented using edaphic and climatic site variables.

Results

Characteristics of volcanic ash soils underneath N. obliqua- Forests

The soils from young and middle aged volcanic ash generally have high available soil moisture levels. Due to rapid litter decomposition in the upper soil, L-Mull (2-4 cm) is the predominant humus form. Underneath the organic layer an Ah-horizon has formed, which is normally underlain by two transition horizons (AB,BA), a B-horizon and a C-horizon. The B-horizon has already completed the process of brunification. In general, volcanic soils are developed deep to very deep (100 - 140 cm). The soils have no skeleton content. The dominating textures are sandy silt (Us) and typical silt (U). The bulk density is relatively homogenious throughout the entire soil profile with values between  0.40 and 0.60 g/cm3. The water storage capacity lies between 200 - 250 mm per 1 m soil depth.

The pH (H2O)-values lie between 5.5 - 6.0, which is evaluated predominantly as moderate acidic. The volcanic soils are rich in organic matter. In the topsoil the median values are between 80 - 90 mg C/g. For the most nutrients the supply is in a middle to high evaluation range. The main rooting zone of N. obliqua stands is in the topsoil horizon, in which 58 - 73% of the fine roots (< 2 mm) are found. However, for more then half of the Ah-horizons, a low to very slight available P-content (1 - 4 mg/kg) was found. The soils from volcanic ashes have a high to very high sorption potential for P. Thus, a highly significant negative relationship between extractable Al and the available P was found. The maximum absorption (Smax) of P (Langmuir-Adsorption isotherm) lies between 100 - 400 µg/g TS. In soils with values of extractable Al exceeding 1.500 mg/kg, the amount of available P is less 4 mg/kg.

The soils from middle aged volcanic ashes show stronger weathering intensity than soils from younger ashes. In middle aged ash soils exists no considerable difference between the pH-values measured in H2O and KCl.  In addtion, the nutrient content (P, K, Ca and Mg) in the subsoil developed on middle aged ashes is lower than in younger ash soils.  Despite these differences, no definite conclusion can be drawn concerning the weathering intensity of the soil ashes groups using these methods. 

Nutrition Conditions for  N. obliqua- Forests

Considering the results of leaf analysis, the nutrition condition of N. obliqua-stands lies within at favorable to very favorable range for the major elements (N,P,K,Ca,Mg) and the trace elements (Fe, Zn, Mn, Cu).  Whereas a slight to very slight plant available P content (1 - 4 mg/kg) was found in the soil, the P-content in the leaves lies within the high to very high range (1,5-3,0 mg/g). It can be presumed that the P-uptake is improved through the formation of mycorrhizal symbiosis.

Site Variability and Productivity for N. obliqua- Forests

Both, the site variability and the productivity of N. obliqua forests depend strongly on the climate in central and southern Chile. By considering primarily the water balance of the stand sites within the national scale, humidity indexes and evapotranspiration were recognised as important climactic parameters for the site's variability and productivity. The productivity can be explained predominantly (53% of the variance) by the humidity index during the growth period.

By varying the considered scales from the national to the regional level with similar macroclimatic characteristics,  the edaphic parameters grain size (portion of T, U) and the bulk density as well as the topography (elevation) were identified as most important site variables in describing productivity. However, both the edaphic and the topographic variables indicate a close relation to the climate of the study area. Due to of this relation, the influence of climate on the variablility of the stand sites can be clearly identified for the regional scale, too. 51% of the variance of the productivity could be explained by the factor site elevation. 

The studied sites of N. obliqua stands showed especially high productivity in the southern part of the Central Depression (39°00' bis 41°00' s lat. and 75°15' to 72°55' west long.). These sites lie between 200 and 300 meters above sea level and show a mean anual precipitation of 1,500 - 2,000 mm, a dry period below 2 months, and an evapotransporation below 600 mm.


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