Qualidade de mudas e estabelecimento em campo de espécies florestais de importância econômica em resposta ao sítio de plantio

Abstract

In the implantation of forest stands, the interaction quality of the seedling versus the planting site is decisive for the better establishment of the species in the field. However, for forest species native to the Amazon region, little is known about the direct effects of these factors on the survival and growth of plantations. In the present study, we intend to analyze whether the seedling quality standard influences the field performance of forest species of economic importance in response to the quality of the site. Seedlings of three native forest species of high economic value in the region were used: Bertholletia excelsa, Dipteryx odorata and Tachigali vulgaris. At the end of the nursery phase, the seedlings were grouped into four size classes according to the variation in the root collar diameter (rcd): B. excelsa and D. odorata (1: 3 ≤ rcd < 4 mm; 2: 4 ≤ rcd < 5 mm; 3: 5 ≤ rcd < 6 mm; 4: 6 ≤ rcd < 7 mm) and T. vulgaris (1: 2,5 ≤ rcd < 3,5 mm; 2: 3,5 ≤ rcd < 4,5 mm; 3: 4,5 ≤ rcd < 5,5 mm; 4: 5,5 ≤ rcd < 6,5 mm). Before planting in the field, morphological attributes of seedling quality were measured in 120 plants (10 per size class per species). The field experiment was arranged in a split-plot design inside three randomized blocks. Two main factors were tested for three forest species: site preparation methods (mechanical site preparation and manual holing preparation) and seedling size (according to four classes of root collar diameter). Each block of 30 x 42 m was divided into two plots of 15 x 42 m according to the site preparation methods (mechanical site preparation = subsoiling + harrowing and manual holing preparation). In each plot, each species was allocated separately (subplots), where the diameter classes were allocated in four planting rows (one per diameter class) in a 3 x 2 m spacing. This thesis is structured around three specific objectives: a) to evaluate the relationships between morphological attributes that evaluate seedling quality in the nursery and the short-term responses of the combined effect of seedling quality and planting site on field performance of three species native to the Amazon (manuscript 1); b) to evaluate the effect of time on the effects of silvicultural practices on soil quality and on the relationships between initial attributes and field performance of these three species in different site qualities (manuscript 2); and c) to evaluate the combined effect of initial seedling size and planting site on growth, biomass and stem quality for species of economic and ecological relevance in the Amazon region (manuscript 3). For all species, root collar diameter was a good predictor of destructive attributes that evaluated seedling quality, such as Dickson’s quality index. Site preparation methods resulted in different site qualities. Mechanical site preparation improved the total porosity (62.20%) and reduced the bulk density (0.92 g cm-3) and penetration resistance (0.91 MPa) compared to manual holing preparation (53.19% for total porosity, 1.12 g cm-3 for bulk density and 2.67MPa for penetration resistance). Survival after one year was not affected by initial attributes or site preparation methods. The initial attributes were poor predictors of field growth after one year for D. odorata and T. vulgaris. The field performance of B. excelsa seedlings was affected by initial attributes, site, and the combination of both. The initial size increases the final size in mechanical site preparation. The definition of a target plant concept cannot be generalized to all evaluated species and site conditions. For D. odorata and T. vulgaris, a size of 3 mm reached good performance, and for B. excelsa, the seedlings were larger (more than 5 mm root collar diameter and 40 cm height). The difference between sites decreased by half 43 months after intervention (from 2.9 times for 7 months after intervention to 1.5 times at 31 and 43 months). Silvicultural practices influenced changes in growth with time after planting. For B. excelsa, the initial size classes had changes with time that were more similar, while for T. vulgaris, site qualities had changes with time that were more similar. For all species, the relationships between seedling size and growth were affected by the site (B. excelsa and T. vulgaris) or the year (D. odorata) but not by the combination of both. For B. excelsa and D. odorata, the relationships were stronger in the first year, independent of the site condition, while for T. vulgaris (in mechanical site preparation), the relationships became stronger in the second and third years. These results are important to define the benefits of applying mechanical site preparation to improve the soil physical properties or the performance of the species in the short and medium term. Three years after planting, T. vulgaris had 8- and 6-fold higher stem volume and total biomass, respectively, than B. excelsa and D. odorata. B. excelsa allocated a high quantity of biomass to roots (root mass fraction), and D. odorata had the highest and lowest biomass allocated to stems and branches, respectively. Overall, the productivity, accumulation and partitioning of biomass of species were poorly explained by the initial seedling size and site quality, especially for D. odorata and T. vulgaris. For B. excelsa, the largest seedlings reached high values of increment in biomass and stem volume when planted in mechanical site preparation. The initial seedling size was not a strong predictor of final plant size for all species under different site conditions, despite moderate correlation values for B. excelsa in mechanical site preparation. Only site quality affected the quality of trees (forked stems) and only for T. vulgaris. Mechanical site preparation had 61.6% forked trees versus 21.3% manual holing preparation. These findings suggest species-specific responses of native species to silvicultural practices, which are important to the establishment of productive plantations in Amazonia.