Rolando H. Braun Wilke
Plant ecologist and director, Argentine Institute for Research Arid Lands (IADIZA), Mendoza, 5550 Argentina
Plant communities present in arid zones have received scarce attention until recently. These communities can be: a) woodlands of semi-deserts, which include sparse trees—usually small—and/or shrubs, together with an undergrowth consisting of grasses and ephemeral forbs; b) shrublands, consisting almost completely of shrubs; c) almost pure grasslands.
The production of dry matter of woody species present in arid zones may have relative economical importance. Let us consider, for instance: a) timber and lumber furnished by some plant resources of the western (dry) Chaco in the northwestern area of Argentina (Aspidosperma quebracho blanco, Prosopis nigra, etc.); b) forage given by some components of the "algarrobal" (open forest of Prosopis flexuosa) in the flatlands of central western Argentina—50% of the livestock diet; or the one provided by the "nopaleras" (Opuntia spp) in the Chihuahuan Desert of Mexico; c) the industrial value of the populations of Jojoba Simmondsia chinensis) in the Sonoran a) Random distribution of plots of appropriate size, for nonhuahuan Desert. Plant resources may also have a fundamental value for soil conservation, refuge for animals, and so on.
This paper summarizes some general features concerning studies undertaken with the purpose of estimating phytomass (in most cases only for woody species) and net production of communities and populations present or introduced in the arid zones of Argentina and Mexico.
The method employed for the determination of phytomass has been the direct one by harvest. Ideas developed in other areas of the world, for similar conditions and materials, have been adapted. The basic common pattern consisted in the assessment of allometric relations between plant dimensions and dry weights of their fractions.
The normal general steps are as follows:
a) Random distribution of plots of appropriate size, for non-destructive, repeated, sampling, in representative sectors of each community or population. This sampling includes measurement of dimensions (height, diameter of crowns, basal diameter of stems); and other related aspects.b) Mapping of plants (sometimes repeated).
c) Phenological studies, repeated.
d) Destructive sampling (following non-destructive sampling) of selected plants outside control plots. The quantity of plants, was in accordance to the number of plants in each interval of size class of the population involved. Age estimations for each plant were made.
e) Dry weights, at 70°C, by plant fractions, for each species involved, were determined. In the case of trees, sample discs of branches and stems were dried. Also, in some, cases dry weights of litter harvested in certain areas were obtained.
f) Search for appropriate allometric relations, indicated by the adjustment for potential functions of the type y= a. xb (or other), was undertaken between the following parameters: 1) weight, basal diameter of stems, the volume expressed by a geometric figure which contains the aerial portion of the plant; and 2) total dry weight of the plant fractions (leaves, stems and, in some cases, roots). In addition, the relation between ages and parameters are indicated by a and b.
g) Calculation of phytomass (leaves, woody portions, etc.), based on the most appropriate relations obtained in (6), applied to dimensions of the non-destructive sampling performed in control plots.
h) Calculation of net primary production: a) using age estimations for size classes of plants (hence, for their weights); b) phytomass differences from one year to another.
Tables 1 and 2 summarize the information concerning seven studies performed in the framework of the Special Project N° 11 on "Arid and Semiarid Zones Development" of the Organization of American States.
a) In the "algarrobal" (Prosopis flexuosa community) the study was restricted to the above-ground portion of all components (including herbaceous ones). In the Chacoan forest, aerial and underground parts of the six most important species, according to their density, distribution and type, were studied.
The study of Larrea cuneifolia community of Mendoza's Piedmont includes other woody and herbaceous forage components. In the Mexican populations of Guayule (Parthenium argentatum) and Jojoba (Simmondsia chinensis), underground parts were partially studied.b) The harvest method is the most feasible one in most of the situations encountered in arid regions of Iberoamerica.
c) Generally, the best adjustment obtained, indicated by curvilinear r, has been for the correlation between a cylinder or a truncated cone, containing the plant, and the dry weight of plant fractions.
d) Age estimation usually has been made by taking into account branching and other different observations. In the case of Jojoba, this was done by studying plantations. This is a decisive and singular fact, so that sometimes one has to obtain phytomass differences from one year to the other, in order to estimate net primary production.
e) Underground biomass, of those species for which it was determined, amounts to approximately one-third of aerial biomass. But soil volumes penetrated by roots may be larger than those of crowns.
f) The studies also include, among others, gathering of data on physical environment, use, management, interferences, activities of wild fauna and domestic animals.
g) In studies 1 and 2, total nitrogen and carbon and calorific values were also determined; in study 7 bromatological analysis were performed.
Table 1. Research material
Study case
Location
Vegetation
Mesological conditions
studied species
Density (plants/ha)
1
Mendoza (Argentina) 34°02' S 67°58' W (572 m)
Open exrophytic forest and shrubland of Prosopis flexuosa ("Monte" formation') Disclimax to preclimax stage.
Eolic-alluvial flatland. Sandy to sandy loam soil. Temperate-warm climate; 260 mm/yr, mostly in summer.
Trees: 2 Shrubs: 9Small shrubs: 2
667
7.6101.228
2
La Rioja (Argentina) 30°30' S 66°15' W (500 m)
Chacoan forest of Prosopis nigra. Secondary succession after eight years of clearing.
Alluvial Piedmont plain. Sandy soil. Warm climate; 350 mm/yr, more than half in summer.
The six most important sp. (Two of them shrubs)
2084-4963(depending on the cor- responding field)
3
Mendoza (Argentina) 32°53° S 68°51° W (850 m)
Piedmont shrubland with net predominance of Larrea cuneifolia. (Monte formation). Disclimax
Piedmont plain. Stony area with coarse materials. Temperate-warm climate, 220 mm/yr, 70% in summer.
Dominant Larrea cunefolia (jarilla)
17.375
4
Zacatecas (Mexico) (1500 m)
Open shrubland: Opuntia, Yucca and Acacia (Chihuahuas desert). Disclimax.
Undulated and broken land. Sandy shallow soil, warm- temperate climate; less than300 mm/yr.
Opuntia streptacantha (nopal cardbn)
430
5
Zacatecas (Mexico) (2000 m)
Open shrubland (Chihuahuas desert). Disclimax.
Mountain slopes. Temperate-warm climate; than 300 mm/yr.
Parthenium argentatum (guayule)
19.200
6
Sonora (Mexico) 29° N 112° W (30-100 m)
Wide open community dominated by Prosopis juliora (Sonoran desert). Disclimax
Coastal bahada. Alluvional land Coarse. sand and gravel. Warm desert; about 100 mm/yr, irregular
Simmondsia chinensis (Jojoba)
641
7
La Rioja (Argentina) 30°30' S
Plantations in Chacoan forest clearings.
Alluvial Piedmont plain. Sandy soil. Warm climate; 350 mm/yr, mostly in summer
Opuntia ficus indica (tuna)
1.111
Table 2 Some details on studies and results
Study case
Control Plots,
Most satisfactory allometric relation (potential functions of the type y 3 y. xb).
Adjustments (curvilinear r)
Phytomass (kg/ha)
Production (kg/ha/yr)
Sizes
Quantity
1
10 × 25 m
10
Volume (of a cylinder containing aerial parts)-dry weight
Usually higher than 0,90
Trees: 7.083 Shrubs: 2.917 Small shrubs: 444
Trees: 684
Shrubs: 623
Small shrubs: 1802
5 × 20 m
6 + 6
Volume (of a cylinder containing aerial parts)-dry weight
Higher than 0,90 for wood; than 0,84 for leaf; than 0,86 for roots
Aerial: 4432 2.196 Subt.: 1568 984 (depending on the field)
Aerial: 289 749 Subt.: 128 204 (depending on the field)
3
4 × 5 m
24
Volume (cylinder)-dry weight
Volume (truncated cone)-dry weight0.94
0.95
4,700
1.2004
5 x50m
4
Volume (cylinder)-dry weight
Height-dry weight Stem diameter-dry weight0.99
0.97
0.95
12.734
1.565
5
5 × 5 m
4
Volume (cylinder)-dry weight
Height-dry weight (Old) stem diameter-dry weightHigher than
0.97
0.930.94
Aerial: 3.287
Subt.: 1.008
Aerial: 534
Subt. 1406
10 × 30 m
8
Volume (cylinder)-dry weight
Height-dry weight (Old) stem diameter-dry weight
Higher than
0.940.96
0.92
1.573
327
7
Non-destructive sampling in several plantations.
Volume (cylinder)-dry weight Height-dry weight
0.98
0.94
approx. 17.000
2400 + 200 (fruit)
Braun, W. et al (1979). 'Productividad Primaria Aérea Neta del Algarrobal de Nacunan (Mendoza)'. Deserta (5): 7–39.
Braun, W., R.H. y Colaboradores, (1980). 'Distribucion y Balance de Fitomasa y Nutrientes en dos Comunidades Vegetales de Desmonte, de Los Llanos'. La Rioja, Argentina (en prensa).
Passera, C.B. y R.H Braun, W. (1980). 'Productividad Primaria Neta de un Jarillal Pedemontano'. (inédito).
Braun W., R.H. y J.J. Lopez, G. (1979). 'Biomasa y Produccion Ecologica del Nopa Gardon (Opuntia streptacanta Lemaire)'. Deserta (5): 73–81
Braun, W.; R.H. y H. Gomez, C. (1979. 'Biomasa y Produccion Ecologica de Guayule (Parthenium argentatum Gray)'. Deserta (5): 83–94.
Braun, W. R.H. y M. Espericueta B. (1979). 'Biomasa y Produccion Ecologica de Jojoba (Simmondsia chinensis Link) e, el Desierto Costero de Sonora'. Deserta (5): 57–72.
Braun W. R.H. et al (1980). (Productividad Ecologica y Valor Forrajero de Tunales (Opuntia ficus-indica) de Los Llanos, Provincia de La Rioja'. Quaderno Tecnico 1–79: 29–37.
