How do Native Trees Establish on Degraded Caatinga Sites?

Abstract

This study evaluated tree establishment by seeding in manure-enriched substrate or by seedling planting in manure-enriched holes in two experiments carried out in Patos (PB), Brazil, in degraded Caatinga sites. Experiment one evaluated the effects of the joint seeding (absence and presence of joint seeding, but no singular seeding of any of species) of four tree species (Mimosa tenuiflora, Poincianela pyramidalis, Anadenanthera macrocarpa and Tabebuia aurea), at two levels of cattle manure (absence and presence) in a site protected from grazing. The 2x2 (absence and presence of joint seeding, and absence and presence of manure addition to the soil) factorial experiment was used with four treatments groups in two replicates to which five completely randomised blocks were assigned tentatively to control environmental heterogeneity and reduce the experimental error (e.g.: soil or fertility or erosion slope). Additional data set from six other similar adjacent blocks (size, number of treatments and plots, and level of environmental degradation), submitted to previous and current overgrazing by small ruminants (browsing goats and grazing sheep) was analysed independently. Experiment two, located in an overgrazed site currently protected from grazing, evaluated tree seedling establishment (natural and that resulting from planted Mimosa tenuiflora and Poincianella pyramidalis seedlings in manure-enriched holes according to a randomised completely blocked design with four treatments in two replicates (four controls: when no tree seedling planting occurred, and when seedling planting of one of the two tree species occurred) to which six blocks were assigned tentatively to control environmental heterogeneity and reduce the experimental error. Tree seedling establishment, natural or resulting from seeding, in plots enriched or not with manure and protected or not from grazing, was not observed during three years. Planting of M. tenuiflora accelerated the process of tree establishment, resulting in 36% survival rate and 177.5cm length growth at the end of the third year, while the other tree seeding or planting treatments resulted in no established plants. Observations should continue to determine the period suitable for making degraded Caatinga sites once again available for production.