Abstract

The habitat of Copiapoa solaris is described. Four sites were studied and the average density per ha was I 75 plants. To estimate the aerial biomass the aerial volume was calculated mathematically as an ellipsoid partially protruding above ground. This aerial volume was calculated using the length, breadth and height values of the cushions. Making an allowance of 20% reduction for interstitial space between branches of the cushion, a standing aerial biomass of 30,85 m³/ ha was estimated.

Fig.1 Copiapoa solaris cushions growing in the desertic environment near El Cobre, Atacama Desert, Chile.

Introduction

Copiapoa solaris (Figure 1) is a succulent member of the Cactaceae with a cushion like growth form, occurring on the coastal slopes south of Antofagasta (Chile). In his book Ritter (1980) gives taxonomic information about this plant. Little information is given on habitat and none on density and biomass above ground. Some ecological information is available for C. haseltoniana from the locality of Paposo, south of Antofagasta (Mooney et al. 1977). The purpose of this paper is to add to our knowledge on the habitat, population characteristics and above ground biomass of this species. While doing morphometric studies on C solaris, the idea occurred to obtain the standing aerial biomass through mathematical-statistical means.

Methods

Four stands of C. solaris were studied at El Cobre, Atacama Coastal Desert, Chile (Figure 2). The density of C. solaris was obtained using the point centered quarter method (Cottam et al., 1953). The four sample sites yielded information on 160 plants. The volume was obtained by meas­uring the length, width and height of the plants and applying a formula developed by the second author. Eighty-two plants were used for volumetric measurements. Field work was done during November 1973.

Fig. 2 Map showing the position of El Cobre (arrow), where the research was done.

As the mathematical-statistical developments and details surpass the scope of this paper, interested reader can write directly to the second author.

Results
Notes about the habitat of Copiapoa solaris

As far as is known C. solaris occurs within a restricted area centred around the localities of Blanco Encalada and El Cobre. Both are coastal hamlets used mainly by transient fishermen, and miners who during our visits were operating a minor copper concentration plant at the locality of El Cobre. 'El Cobre‘ means 'The Copper‘, reflecting the presence of copper deposits in the area. The topography is characterised by the coastal mountain range ('cordillera de la costa') steeply emerging from the ocean and landwards, leading to the barren Atacama Desert. The study area was hilly, with an altitude between 550 and 700 metres.

The climate is arid and following Koeppen‘s classification, it corresponds to the BWn type, that is a desert climate with strong oceanic influence and high cloud cover. The nearest available climatic data corresponds to Antofagasta, revealing twelve arid months, no frost, an average temper­ature of 17.2°C and an annual rainfall of 4 mm.

These rainfall figures do not reflect the rare events of strong rainfalls linked to the 'El Niño' phenomenon. We had occasion to witness such a storm and it was of such magnitude that the road to Blanco Encalada was washed away through mud, sand and stone avalanches.

The extreme aridity is attenuated by the frequent, often dense, coastal fogs called ,,camanchaca“ by the locals. The fog tends to concentrate in the form of a cloud band at an estimated height of 500 to 850 metres. It shows a recurrent pattern; usually it is overcast in the early mornings, the clouds dissipating during the late morning and returning during the late afternoons. Winds from SW and S are prevalent.

Results on volume measurements

Mean values and standard deviations of the three basic size measurements taken on our sample are summarised in Table 1. With this information, and taking into account the relatively regular shape of the cushions, one could get a first approximation of areal volume that would give an indication of areal biomass.

Table 1. Basic information on size measurements (in cm)

Statistic	Major axis	Minor axis	height
Sample size	160 plants	160 plants	160 plants
Minimum	9.0	6.0	6.0
Mean	96.4	77.7	45.5
Maximum	230.0	190.0	90.0
Standard deviation	51.9	43.0	19.9

A model was developed by the second author which, as a first approximation, assumes the plants to be solid. It consists of an ellipsoid, pushed into the soil so that only the part representing the plant protrudes. A graphic representation of the model appears in Figure 3, using the dimensions of an average plant. The measurements of major axis, minor axis and height together with the depth of the modeled ellipsoid below the soil surface, determines a unique ellipsoid, and hence the volume of the protruding part.

Fig. 3 Eilipsoidal model of the cushions of Copiapoa solaris.

If the part of the ellipsoid protruding , above soil level (i.e. the height of the plant) is chosen to be a fraction somewhere between 1/3 and ½ of the vertical axis of the ellipsoid (two figures we thought to be reasonable choices) it appears that the volume of the protruding part is relatively insensitive to this choice. We thus settled for the average of these two figures (i.e. 5/12).  We also allowed a 20% reduction for intersti­tial space between the branches of the cacti. The final formula proposed for the volume of each plant then simplifies to a fraction of 0,3417 of the product of the major axis, minor axis and height. Average plant volumes are based on the data given in table 2.

Table 2. Average volume per plant (m³), the Standard Deviation of volume and statistics related to biomass, by site and pooled over sites, allowing 20% subjectively estimated reduction for interstitial space between branches.

Statistic	Site 1	Site 2	Site 3	Site 4	Pooled
N	20	28	21	13	82
Av. (Vol. m3)	0.1926	0.1883	0.1461	0.1684	0.1754
SD (Vol.)	0.2504	0.2016	0.1592	0.1842	0.1997
Density (plants/ha)	90.66	353.38	244.67	173.00	175.91
Biomass (m3/ha)	17.46	66.54	35.75	29.13	30.85

The values for an average plant were obtained with the data from 82 plants and were 0,1 754 m³ (SD 0,1997), and a computer representation of a cushion of Copiapoa solaris is given in Figure 4.

Fig. 4. 4 three-dimensional. idealized, computer.generated representation of a cushion of Copiapoa solaris

Density measurements

The density estimates by site for all data pooled are also given in Table 2. The method of estimation of the plant density (number of plants per hectare) based on distances to the nearest plant using maximum likelihood has been described by Rossouw (1984) and by Laubscher and Rossouw (1986). The average density per ha was 175.91 plants.

Biomass estimations

The above ground biomass of C. solaris (in units of m³/ha) was computed as the product of the density of plants and the average plant volume, and was, pooling the results of the four sites together, 30,85 m³/ha.

Conclusions and discussion

By using mathematical-geometrical methods it was possible to develop an easy, expedite method to obtain a first approximation of biomass values for plants which approximate a regular cushion shape. This was done by measuring length, height and width and information on density. In this way it was possible for the first time to give an estimate of the areal biomass for this species in this extreme environment.

Apart from other Cactaceae, plants with an ellipsoid cushion shape are also present in the Euphorbiaceae, and in these cases the same metho­dology could be applied to establish areal biomass.

Considering the climate and the conditions of 12 and months and an annual average rainfall of only 4 mm, survival and accumulation of biomass by C. solaris seems to be a remarkable achievement. C. solaris is by far the dominant vascular plant in this desert ecosystem. By storing water in its roots and body, it creates a water reservoir that may be tapped by insects and rodents. Lizards were observed sheltering under the cushions. From the nearby locality of Paposo it is known that lizards eat the flower parts of Copiapoa cinerea var. haseltoniana (Weisser et al. , 1975).

The accidental incineration of one dead cushion revealed a great quantity of insects and reptiles that became exposed when they fled from the flames. This is an indication of the importance of the spiny cushions in providing shelter.

C. solaris seems to be heavily predated by herbivores, probably snails of which shells were abundant in the dead cushions, and insect larvae. Injuries caused by these agents could facilitate bacterial and fungal infections that may lead to the death of the plant.

C. solaris is by far the dominant vascular plant over an extensive area, and therefore the basis of the eco­system. Compared with others, this ecosystem is relatively simple and therefore a good research object to study interrelations and interactions in an ecosystem. This scientific importance together with the very restricted distribution of this plant which was able to colonize such extreme environment, makes protection measures for at least part of this habitat highly recommendable.

Acknowledgements

We wish to acknowledge the support received from the Facultad de Ciencias, Universidad de Chile, Santiago Chile; the Convenio Chile - California The National Botanical Institute Pretoria, and for the help received from Professor H Mooney, Mrs Gulmon, Mrs J N Weisser, Miss S. van Eeden, Mr L Robres and Mr Lembcke.

References

Cottam, G., Curtis, J.T. & HaIe, BW. 1953. Some sampling characteristics of a population of randomly dispersed individuals. Ecology 34: 741-457. 

Laubscher, N.F. & Rossouw, 1. 1986. Paint a interval estimation of the Rayleigh Parameter censored data. Technical Report, Department Statistics, University of Port Elizabeth, Port Elizabeth

Mooney, H.A., Weisser, P.J. & Gulmon, S.L. 1977 Environmental adaptations of the Atacaman Desert cactus Copiapoa haseltoniana. Flora 166: 117-124

Ritter, F. 1980. Kakteen in Südamerika. Ergebnis meiner 20 jahrigen Feldforschungen, Band Spannenberg, Germany. Friedrich Ritter Selbstverlag

Rossouw, 1. 1984. Estimation of the parameter the Rayleigh Disthbution for censored data. M. 5 thesis, Department of Statistics, University of Port Elizabeth.

Walter, H. & Leith, H. 1960-1967. Klimadiagramn Weltatlas. VEB Gustav Fischer, Jena.

Weisser, P.J., Weisser, J.N. & Robres, L. 1975. Note on cactus and flower predation by lizards in the Atacama Desert. Aloe 13: 117-118. 

Pablo J Weisser Botany Department, University of Venda, Private Bag x 5050, Thohoyandou, Venda

Nico F Laubscher Company Statistician, 5 A Nylon Spinners (Pty) Ltd, P 0 Box 272, 7535 Bellville, South Africa