Panel 1: The chilean case

Panel leader: Luis Zelada

THE GENUS PROSOPIS AND ITS MANAGEMENT AT THE TAMARUGAL PAMPA

Juan José Aguirre
Forester
Silviculture Division, Forestry Institute of Chile

Johannes Wrann
Forester
Silviculture Division, Forestry Institute of Chile

1. GENERAL BACKGROUND

The “Tamarugal Pampa” forest resource has been the subject of a number of research studies, on account of its undeniable importance for the present and future of the human activity in the area.

1.1 Location

The study area is located in Region I, and covers the Pintados and Bellavista Salt Flats. The geographic coordinates of the area are: 69° 31'–69° 45'8 West Longitude and 20° 17'–20° 50' South Latitude.

1.2 Access

The main access route to the aforementioned salt flats is the North Pan American Highway, which crosses both of them and the plantations from North to South. The secondary road going inland from the Pan American Highway through La Tirana to the townlet of Pica, is a ready access to the northern portions of the forest. It is open year round and has a very good tarmac cover.

2. SIZE OF THE RESOURCE

The Forestry Institute of Chile carried out a cartographic survey of the existing forest resources at the Bellavista and Pintados salt flats in 1964 and 1970.

The area covered by the tamarugo forest on the first survey was 7,371 ha, equivalent to 20.1% of the forest cover. The situation altered rapidly and, in 1970, the tamarugo area reached 16,482 ha, equivalent to 67.3% of the total forest resource.

In view of the lack of background data on the plantations established after 1970, a forest inventory was carried out to ascertain the present size, location, stock, composition, structure, and general condition of the tamarugo forests and associated species.

2.1 Methodology

As a first step, the following basic data regarding the study area were collected: a) general charts, Army Geographic Institute (IGM); existing subject charts, including: “Canchones Forest Area”, “Pampa del Tamarugal Forest Area”, and others; c) aerial photographs of the Pampa del Tamarugal, scales 1:60,000; 1:20,000; and 1:5,000.

2.1.1 Preliminary Map Preparation

A previous step to drawing of the preliminary map was interpretation of the aerial photo-grametric material, so as to gather the necessary data for the base map. The procedure used was to outline all forest stands equal to or larger than 3 ha in area, later to be incorporated into the preliminary map.

Once the photointerpretation was completed, all data collected were incorporated into the base map, at a scale of 1:70,000, containing the state of the resource as of 1970.

2.1.2 Sampling Procedure

The sampling procedure used was the “Random Stratified Method”, which has the advantage over the merely random one, using similar samples, of having smaller values for the variances required to estimate medians and totals.

The use of this design requires a prior stratification of the population to be sampled. In this case, the criteria used for the stratification were: typifying of species, and typifying of forest types.

Basing on the two criteria above, and taking stands larger than or equal to 3 ha in area, the following strata were obtained:

• Tamarugo Plantations
• Algarrobo Plantations
• Mixed Stands
• Tamarugo Natural Stands

As the stands less than 3 ha in area account for less than 1% of the resource, they were not sampled and the species making them up of were not identified.

The availability of data regarding time of plantation establishment and stocking made it possible to refine the stratification process and divide it into homogeneous substrata, which, in turn, were used to tailor the size of the sampling to each vegetational stratum.

The purpose of the sampling was to estimate the median of the main parameters of the stands for all the strata and substrata, with a significance level of 0.05.

The number of sampling units for the four aforementioned strata was 75, with tamarugo plantations accounting for 69.3% of the total.

Both the choice of stands assigned a sampling unit and the latter's location in the stand were totally randomized.

2.1.2.1 Stand parameter recording

The recording of the stand parameters was made both from a quantitave and qualitative standpoint.

Quantitative standpoints were:

1. Height and length of crown
2. Crown diameter
3. Main stem diameter 1 m above ground
4. Number of live trees
5. Spacing

Qualitative stand parameters measured were:

1. Exploitation indices
2. Seed tree aptitude

2.1.3 Final Map and Area Measurement

After photointerpretation was corrected by the field survey, it was incorporated into the 1:50,000 IGM charts, from which the final map was drawn. The area of each stand making up the stratum was measured on this map.

Table 1 shows the area of the various strata making up the surveyed forest resource.

TABLE 1
Total area and percentage share of each stratum Tamarugal Pampa

It is worth mentioning that the Pintados Salt Flat concentrates 90.1% of the existing forest area, while the Bellavista deposit has only 2,115 ha (9.9%). Furthermore, the former shows better conditions for the establishment of new plantations, as it accounts for 82% of the area considered suitable for Prosopis tamarugo.

2.1.4 Area Distribution per Stratum

Age was taken as the basis to chart the distribution of the areas covered by the various strata, and, when such data was unavailable, crown size was used as criterion.

2.1.4.1 Tamarugo plantations

As date of establishment is available for 98% of these plantations, this stratum was divided into two groups:

1. Tamarugo plantations of known age (8–49 years)
2. Tamarugo plantations of unknown age.

Figure 1. Fourteen-year-old tamarugo at the “La Pillalla” sector. Pintados Salt Flat.

TABLE 2
Tamarugo Plantation Area by Age Class (8 to 49 years) Tamarugal Pampa, 1981

For the plantations of unknown age, the criterion used was crown size. Table 3 shows the area covered by these plantations.

TABLE 3
Area by Crown Class of Tamarugo Plantations of Unknown Age Tamarugal Pampa

2.1.4.2 Algarrobo plantations

Algarrobo occurs in two sectors within the Tamarugal Pampa, La Guaica and El Algarrobal, the latter in the Refresco Farm.

As plantation date for this resource is not available, it was classified as per crown size class.

TABLE 4
Algarrobo Plantation Area by Crown Size Class Tamarugal Pampa

2.1.4.3 Mixed stands

Nearly all of the algarrobo and tamarugo mixed stands are located in the Refresco Farm. Although this resource only accounts for about 2% of the forest resource of the Pampa, it is relevant because of the considerable crown size observed in some individuals, and the homogeneity of the specimens both in strength and foliage thickness.

TABLE 5
Area of Mixed Plantations by Crown Size Class Pampa Tamarugal

2.1.4.4 Tamarugo natural forests

Natural tamarugo stands are located in the vicinity of La Tirana. Two types of forest are distinguished here: a low-stock one, possibly as an effect of human action, and a high-stock one with either very little apparent human intervention, or with good regeneration rates.

The low density stands are usually found in small forests and are affected by the wind blowing permanently in this region, which accumulates sand at the foot of the trees over the years.

TABLE 6
Area by Type of Stand - Natural Tamarugo Forests Tamarugal Pampa

2.1.5 Estimating Dasometric Parameters

2.1.5.1 General average per stratum

It is always convenient to know the mean values of the vegetational strata parameters found in the study area, as they make it possible to characterize the resource. The average value of a given parameter is useful, as it can be compared directly with the value it has in other strata, despite the limitations imposed by the high level of the aggregate.

TABLE 7
Stand parameter mean values by stratum Tamarugal Pampa

* No. of stumps under same crown
** Stem diameter 1 m above ground

From the data in Table 7, it can be drawn that the mixed stand stratum shows the greatest values for height, crown diameter, crown length and stem diameter of all the strata considered.

In view of the interest raised by the mixed stand stratum, these stands were broken down according to the species making them up.

TABLE 8
Mean Values, Stand Parameters by Species - Mixed Stands

2.1.6 Area Distribution According to Other Stand Parameters

2.1.6.1 Tamarugo plantations

According to the height classes defined, varying from less than 4 m to over 10 m, 50.7% of the area covered by this stratum corresponds to the 4–7 m height range.

If this resource is considered according to crown size class, which varies from less than 5 m to over 11 m, with a 3-m clearing in between, 52.5% of the plantation area falls in the bracket of 5–8 m.

As regards the number of stems per planting pit, varying from 4 to 8, 47.1% of the area has an average of 6 stems.

Last, if plantation density is taken as the distribution parameter, 63.3% of the area has been planted at a 15 × 15 m spacing, with spacing ranging from 6 × 6 m to 20 × 20 m.

2.1.6.2 Algarrobo plantations

67.4% of the area covered by this stratum has a mean height of 6.2 m.

54% of the total area planted with algarrobo shows an average of 9 stems per planting pit, the range varying from 5 to 16 stems.

As regards spacing, 58% of the algarrobo area is planted at 20 × 20 m.

2.1.6.3 Mixed plantations

The number of stands making up this stratum is the lowest of those considered in this study.

If mixed plantations are analized in terms of area distribution by crown size class, 98.7% of the area is found to belong to the large crown class. The same occurs for the greatest total height, greatest crown diameter, length of crown and average number of stems per plantation pit.

85.5% of the mixed plantations has been established at a 20 × 20 m spacing.

2.1.6.4 Natural tamarugo forests

The low-stock forest is made up by three stands of Substratum 1, and represent 79.9% of the total area covered by this stratum. The remaining 20.1% corresponds to high-stock forests, made up by two stands.

3. GROWTH, DEVELOPMENT AND PRODUCTIVITY OF THE TAMARUGO SPECIES

With the purpose of gathering some preliminary background data on this species, the Forestry Institute started studies on tamarugo plantations in 1964. In the course of this research project, some relationships for this tree were estimated in general terms, such as age-total height and age-crown diameter. The following regressions were also estimated:

• Crown diameter as a function of age

• Height as a function of age

• Crown area as a function of age

In 1968, as a follow up on research on tamarugo, INFOR developed a project on growth and fodder production of this species. 29 permanent plots were established in the different plantations existing at the time to carry out this study. Each plot included 25 trees.

The data from each plot was complemented with the charts of the Pampa drawn in 1970, which made it possible to locate each of them in the plantations map, adding the data on salinity and depth of water table collected by Castillo (1966).

In 1976 and 1981, the permanent plots —which had been set up in 1968— were controlled in order to measure some growth variables and see the growth pattern of this species through time. In the last control carried out, only 20 plots could be located, out of the 29 originally set up, or 69% of the total. The identifying signs had been removed on the remaining 31%, making it impossible to determine exactly which individuals to measure.

TABLE 9
Breakdown of Permanent Tamarugo Plots by Age, Depth and Salinity of Water Table. Pintados Salt Flat. August 1981.

* Plot Identification No.
** Plot No. 2 corresponds to a mixed tamarugo-algarrobo stand

3.1 Height Increase as a Function of Age

The small number of sample plots available to characterize the various site situations suggested to adopt a general average growth function for the sector (Pintados Salt Flat), as the age in each of these plots was uniform, making every individual cruve have little statistical consistence.

Data from 15 permanent plots were used to draw the height vs. age curve, eliminating those which showed biased values. As the number of samples was small, data from 37 sample plots for the forestry survey were included (Book II A Study of the species of the genus Prosopis at the Tamarugal Pampa, CORFO-INFOR 1981).

The regression used was of the type

y = b₀ + b₁ (In x)

where: y = total height in m
x = age (years)
In = natural logarithm

The values found for the above function are:

y = -1.2410 + 3.2548 (In x)

Figure 2 shows the general average growth curve for the height age ratio of tamarugo.

*   Plot 2 is mixed (tamarugo-algarrobo)
** S (dn-1) = Standard deviation for small samples

FIGURE 2. Average height growth curve (Age-Height Ratio) for tamarugo. Pintados Salt Flat.
Base: 52 plots.

3.2 Height Increase as a Function of Stem Diameter

This ratio was computed using as data base a set of 17 permanent plots, with a function of the same type as in the former case, where y = height in m and x = stem diameter.

Height and diameter values correspond to the average for each plot. To compute the mean diameter per plot, data relevant to each stem 1 m above ground were used. The values obtained for this function were:

y = -6.7106 + 5.7086 (In x), with R = 0.95

Figure 3 shows the average diameter at breast height (BHD)/height growth for tamarugo.

FIGURE 3. Average growth curve (BHD/Height) for tamarugo.
Pintados Salt Flat. y = b₀ + b₁ (In x)
Base: 17 plots.

3.3 Total Height vs. Crown Projection Area

A set of 39 plots, both permanent and sample, was used for the computation of height as a function of crown projection area. As in the former cases, height is the independent variable and crown projection area the dependent one.

The values obtained for the function were:

y = -6.7252 + 3.6411 (In x), with R = 0.87

The crown projection area is expressed in m.

Figure 4 shows the values found for this function in tamarugo.

FIGURE 4. Crown projection area vs. total height
y = b₀ + b₁ (In x)
        Base: 39 plots.

3.4 Mean Growth of Tamarugo

The values found for the various parameters of this species until 1981, in the 20 permanent plots surveyed, are shown in Table 10. A breakdown of the standard deviation for BHD, basal area, crown area and height is also included.

3.4.1 Mean Annual Height Increase

To determine the mean height increase, the data obtained from 20 permanent plots in 1968 and 1981 were used. From the findings it can be concluded that this species shows varying height increase according to age.

Table 11 below shows the values for the mean annual height increase for different ages of tamarugo.

TABLE 10
August survey data and results summary. Tamarugo

* Parcela 2 es mixta (tamarugo-algarrobo)
** S (dn-1) = Desviación estándar para muestras pequeñas

TABLE 11
Mean Annual Height Growth for Tamarugo Tamarugal Pampa, 1981

* The greatest height gain was achieved by the 16-year-old plot.