A fast exploration of very deep soil layers by Eucalyptus seedlings and clones in Brazil

Abstract

Although pioneer studies showed several decades ago that deep rooting is common in tropical forests, direct measurements of fine root distributions over the entire soil profile explored by the roots are still scarce. Our study aimed to compare, 2 years after planting, fine root traits of Eucalyptus trees planted from cuttings and from seedlings in order to assess whether the propagation mode has an influence on the capacity of the trees to explore very deep soils. Soils cores were sampled down to a depth of 13.5 m at the peak of leaf area index (LAI), 2 years after planting, under three Eucalyptus clones (belonging to species E. saligna, E. grandis  E. urophylla, E. grandis  E. camaldulensis) and under E. grandis seedlings in the same Ferralsol soil. LAI was estimated using allometric equations based on destructive sampling of eight trees per genotype. All the genotypes exhibited fine root densities roughly constant between the depths of 0.25 and 6.00 m. Changes in fine root traits (diameter, specific root length and specific root area) were low between the topsoil and the root front. The ratios between mean tree height and root front depth ranged from 0.8 to 1.2 for the four genotypes. Although tree vertical extension was roughly symmetric above and belowground for all the genotypes, the depth of the root front ranged from 8.0 m for the seedlings and the E. grandis  E. urophylla clone to 11.5 m for the E. saligna clone. Soil water content profiles suggested that the four genotypes had the capacity to withdraw water down to a depth of 8-10 m over the first 2 years after planting. Total fine root length ranged from 3.3 to 6.0 km per m 2 of soil depending on the genotype. The root area/leaf area ratio ranged from 1.3 to 3.2 and was negatively correlated with LAI across the four genotypes. This pattern suggests that the genotypes more conservative for water use (with a low LAI) invest more in fine root area relative to leaf area than genotypes adapted to wet regions (with a high LAI). The velocity of downward movement of the root front might be a relevant criterion in the last stage of the breeding programs to select clones with a fast exploration of deep soil layers in drought prone regions.

Importance of deep water uptake in tropical eucalypt forest

“…In a second growth stage, after canopy closure, Eucalyptus trees take advantage of their dual root system (Canadell & Zedler 1995;Fan 2015), with the shallow lateral roots utilizing rain in the wet season, while in the dry season, the trees rely on their deep roots that take up the water stored in deep soil layers during the previous wet season, and the water stored in very deep soil layers during the first months after the harvest of the previous stand (water below 10 m, including the water-table). The exploration of very deep soil layers has already been reported for other Eucalyptus species but the root front displacement was slower than in our soil (Christina et al 2011;Pinheiro et al 2016): in Australian planted forests, Eucalyptus trees are able to take up water down to a depth of at least 8-10 m within 7 years after planting (Robinson, Harper & Smettem 2006). Similarly, soil water depletion down to 8 m was observed in 3-year-old South African Eucalyptus plantations (Dye 1996).…”

“…; Pinheiro et al . ): in Australian planted forests, Eucalyptus trees are able to take up water down to a depth of at least 8–10 m within 7 years after planting (Robinson, Harper & Smettem ). Similarly, soil water depletion down to 8 m was observed in 3‐year‐old South African Eucalyptus plantations (Dye ).…”

“…They have a dual rooting system with high densities of shallow roots and roots down to more than 18 m depth, this maximum depth being roughly proportional to canopy height (Christina et al 2011). Seedlings and clones of Eucalyptus genotype commonly planted in Brazil (mainly Eucalyptus grandis and hybrids) exhibit a similar pattern of very deep rooting in the field in the absence of any physical or chemical barrier to root growth (Pinheiro et al 2016). Eucalyptus plantations are therefore interesting forest ecosystems for disentangling the different mechanisms associated with deep root functions throughout tree development.…”

Soil compaction caused by harvesting, skidding and wood processing in eucalyptus forests on coarse‐textured tropical soils

“…This is very important in these soils because they typically have a denser and naturally compact layer in the upper B horizon (Santos et al., ). Additional compaction in this situation would be likely to severely hamper root growth, which for Eucalyptus may expand down to 8–10 m 2 years after planting in coarse‐textured soils (Pinheiro et al., ).…”

Relações entre parte aérea e raízes em povoamentos de teca

“…O maior crescimento do sítio florestal depende da rápida e eficiente aquisição de recursos acima e abaixo do solo e, portanto, as árvores de maior crescimento geralmente apresentam maiores área radicular específica (ARE) e área foliar específica (AFE) e, também, uma correlação positiva entre o índice de área radicular (IAR) e índice de área foliar (IAF) (Al Afas et al, 2008;Reich, 2014;Jo et al, 2015). Na predição do crescimento do sítio florestal, há que se lidar com a complexidade dos processos fisiológicos envolvidos na alocação de carbono (C) para os componentes das árvores, principalmente na formação de folhas e de raízes finas (diâmetro < 2 mm), as superfícies de aquisição dos recursos radiação solar, água e nutrientes minerais, respectivamente (Campoe et al, 2012;Pinheiro et al, 2016).…”