4 Discussion and conclusion

The flora of the semi-arid region of North-East Brazil was described by Luetzelburg (1922-23). According to him, the dominant species in the caatinga were Caesalpinia ferrea, Caesalpinia pyramidalis, Caesalpinia echinata, Aspidosperma pyrifolium, Spondias tuberosa, Magonia pubescens, Hymenaea martiana, Mimosa verrucosa, Mimosa tenuiflora, Cereus squamosus, Cereus jamacaru, and Pilocereus gounellei. Recently, Gomez (1981) studied the vegetation of Cariris Velhos, Paraíba state, and reported that the dominant species in that region were Caesalpinia pyramidalis, Mimosa tenuiflora, Croton sonderianus, Combretum leprosum, Aspidosperma pyrifolium, Jatropha pholiana, Spondias tuberosa, Pithecellobium foliorosum, and Pilocereus gounellei. These floristic compositions are similar to my results in Campina Grande and Patos.

According to Whittaker (1970), the average biomass of shrubland formation is in the range of 2 to 20 kg per m², which includes the value of 2.94 kg m², thus bracketing our value of 2.94 kg m² obtained from stands used for firewood and ranching. The phytomass of the stands varies from site to site according to the intensity of human activities.

The area has been degraded by varying human impacts such as insufficient fallowing period within the shifting cultivation; charcoal production; felling wood for fuel used in brick production; demand for fuel wood by the bakeries in (Campina Grande and Patos; and fence construction to enclose goats (fig. 12.6).

According to Saito et al. (1988), productivity of cultivated crops for 1986 in the município of Patos was 1,050 kg per 9,683 ha, for perennial and annual cotton; 44 kg per 2,299 ha, for maccather bean; 600 kg per 1,962 ha, for maize; and 1,200 kg per 60 ha, for rice. Livestock figures in Patos in 1980 were 14,274 head of cattle, 1,762 head of sheep, and 293 head of horses for the total area of 59,499 ha. Nishizawa and Pinto (1988) reported that during 1981, 16,253 tons of charcoal and 1,569,612m³ of firewood were consumed in the region, including Seridó Paraibano, Piemonte da Borborema, Depressão do Alto Piranhas, Cariris Velhos, and Agreste da Borborema. In 1981, 1,890m³ of timber was produced for house and fence construction. Saito et al. (1988) noted that a single family consumes 100 kg of firewood in a week (five tons per year). Increasing demand for firewood by the bakeries in Patos was also reported by Nishizawa and Pinto (1988).

These human activities have resulted in a lot of bare ground within NorthEast Brazil, thus promoting land degradation. Once the vege tation has been removed, tropical rains create a sandy surface soil and vegetation recovery, including seed germination and seedling establishment, is limited by the severe condition of the soil surface (Hayashi 1981, 1988). We should, with proper use of the land, conserve plant cover to protect the soil against erosion. It is necessary, therefore, that the production of livestock, crops, and fuels be managed within the capabilities of the natural biological potential for the area.