Variação entre-sítios na alometria e densidade da madeira de Goupia glabra Aubl. na Amazônia
Interspecific variation in allometry may be related to wood density because density correlates with mechanical support (King, 1981; Niklas, 1993; Putz et al., 1983). Light-demanding pioneer species in gaps need to invest more in growth to reach the canopy rapidly and to better compete with other tree species. In contrast, shade-tolerant species allocate more energy to survival through greater investment in mechanical support. Therefore, there is a tradeoff in biomass allocation (and, by inference, wood density) due to the different costs of support between these two extremes. Pioneer trees require less investment in wood tissue to sustain their crown, giving them more rapid growth and lower support costs, whereas shade-tolerant species invest more in building tissues (and thus have higher wood density), decreasing their susceptibility to physical damage, which ultimately results in a greater survival advantage (Putz et al., 1983; Thomas, 1996; King et al., 2006; Poorter et al., 2006).
Although the patterns of differentiation in architecture and wood density among functional groups of trees are widely recognized, studies assessing the variation in allometry and wood density of a single species growing in different environmental conditions are rare. Individual trees within a given species have been found to have lower wood density (Nogueira et al., 2007) and shorter height for any given diameter (Nogueira et al., 2008a) in southern Amazonia as compared to central Amazonia. To our knowledge, the present study is the first to develop species-specific allometric equations for populations of a species with parameters specific to each of these subregions. At the community level and at different spatial scales, several studies have shown significant correlations between wood density and environmental variables that are affected by variation in altitude, latitude and soils (Wiemann and Williamson, 2002; Muller-Landau, 2004; Baker et al., 2004; Wittmann et al., 2006; Nogueira et al., 2007; Swenson and Enquist, 2007; Slik et al., 2010).
The present study aimed to compare the architectural features and wood densities of Goupia glabra growing at two different sites in terra-firme (not seasonally flooded) lowland forest in Brazilian Amazonia, which differ mainly in terms of soil type and light condition. Our central hypothesis was that G. glabra would have distinct allometric relationships related to environmental site conditions, and that this may be reflected in its wood density.
At both sites the average annual temperature is about 27°C and the mean maximum and mean minimum are 31°C and 23°C, respectively, while annual precipitation ranges from 2200 to 2300 mm with two distinct seasons (INMET, 2009, for a 30-year historical series). Precipitation is concentrated between December and April, when the mean monthly rainfall is over 200 mm. However, NOlinda is more uniformly wet, whereas Apuí has a more pronounced decline in precipitation between June and August. Moreover, the sites differ in terms of vegetation, soils and geological formation. The predominant vegetation type in Apuí is open montane wet forest and the soils are mainly Acrisols (red-yellow latosol in the Brazilian system; Santos et al., 2011). In Nova Olinda do Norte, the dominant vegetation is classified as evergreen lowland tropical moist forest and the predominant soils are Xanthic Ferralsols (allic yellow latosol in the Brazilian system), which are heavily weathered, acidic, and very poor in nutrients such as P, Ca, and K.
G. glabra has a wide distribution, occurring from the tropical rainforests of Panama and Colombia to Amazonian terra firme forests. Trees can reach up to 40 m in height and up to 130 cm in diameter at breast height (DBH), growing preferentially either on clayey or well-drained sandy soils (Loureiro et al., 1979; Ferreira and Tonini, 2004). This species is closely associated with gaps (Loureiro et al., 1979; Boot, 1996) and is classified as a shade-intolerant pioneer species (sensuSwaine and Whitmore, 1988). Although its wood is heavy, with a density ranging from 0.7 to 0.9 g/cm3, G. glabra has relatively high growth rates in sites with direct sunlight (Loureiro et al., 1979).
For each sampled tree ≥ 10 cm DBH, the following data were recorded: DBH, stem height (Hs), total height (Ht), crown length (Ch), crown diameter (Cd), commercial volume (Vc), stem form factor (FF), and wood density (Dg). DBH was measured at 1.30 meters above ground level, or just above any buttresses. Hs is the vertical distancebetween the ground level and the height of the lowest living branch. Ch was defined as the difference between the first branch and final height of the crown, and Ht is the sum of Hs and Ch. Cd was estimated by measuring the horizontal distance from the trunk to the vertical projection of the crown edge in four compass directions 90o apart. Cd of a tree was calculated by the arithmetic mean of the two perpendicular directions of the crown. Vc was calculated by cubic scaling of the bole using the Smalian method for obtaining the areas of cross sections at 0.5 m, 1 m, 1.3 m, and 2 m and thereafter at 2-m intervals up to the first living branch. All measurements included the bark. FF was calculated as the ratio between the bole volume calculated by the Smalian formula and cylindrical volume with a cross-sectional area equal to the measured DBH.
Cross sections (5-cm thick) were cut with a chainsaw from each selected tree to determine wood density (Dg) at six positions: 0.5 m, 1 m, 1.3 m, 2 m, middle and top of the bole. The samples were divided into heartwood, sapwood and bark and were weighed in the field on a portable electronic balance. The volume of each sample was determined immediately after collection to avoid dehydration. The wood samples were fixed on a stylus and submerged in a container of water placed on a balance. According to the Archimedes principle, the volume of water displaced by the immersed sample without touching the container corresponds to the volume of the sample; the amount was recorded in grams, since the density of water is near 1 g/cm3 (ASTM, 2002). Subsequently, the samples were placed in paper bags and dried in an oven with forced air circulation at 105°C (ASTM, 2002). After a week, we began weighing each sample at intervals of 24 hours. The average basic wood density for each tree was calculated as the ratio between the dry weight and the volume of the fresh sample.
Exponential nonlinear models were tested to determine the relationship between tree size (DBH) and wood density for each site separately. Two-way analysis of variance was used to test the effects on wood density of site and of the position of the cross section along the length of the bole. The effect of position in this analysis was calculated for three positions: 1.30 m (the DBH measurement point), 50% (the midpoint of the bole) and the top of the bole. Because it is not appropriate to assume that samples are independent within the same bole, we randomly selected different trees to obtain the wood density of each bole position within each site.
There was a significant difference in stem form factor (FF) between sites (t = 2.66, p = 0.01, GL = 63). The FF in Apuí (0.83 ± 0.082, mean ± standard deviation) averaged 7.8% higher than in NOlinda (0.77 ± 0.079). Volume fitted as a function of DBH was highly significant for both sites (Apuí: r2 = 99.7%; NOlinda: r2 = 96.5%, p<0.0001 for both cases, Table 1), and the model parameters differed significantly, with both greater slope and intercept for Apuí (Table 1, Figure 2f).
Tree architecture within the same species may vary during ontogeny and depends on abiotic conditions (mainly light availability) in which individual trees grow. Rapid changes in the development of the crown may occur when a tree reaches the canopy due to an increase in light exposure (King and Maindonald, 1999; Sterck and Bongers, 2001; Osunkoya et al., 2007). Although G. glabra trees had similar Ht at both sites, this was not true for the relationship between DBH and Hs, indicating that trees in NOlinda invest more in stem elongation in the smaller size classes, but for larger trees the opposite pattern occurs: trees in Apuí invest more in stem elongation throughout tree ontogeny, which resulted in significantly higher regression slope and lower intercept at this site. Such high initial stem growth in NOlinda can be attributed to the fact that a tree growing in dense rain forest, where it is assumed that light incidence is lower when compared to the open forest in Apuí, needs to invest more in vertical growth at this stage to increase access to light.
The differences in growth in Cd and Ch corroborate the reduced growth found for Hs (in the initial phase) for trees in NOlinda, where they invest less in Ch and Cd, but have deeper and wider crowns when fully developed. In Apuí the relationship between DBH and Ch was not significant, indicating that Ch remains constant after stems reach a certain height, at least for trees ≥ 10 cm DBH. Because low light intensity implies a lower photosynthetic rate, perhaps it is a better strategy for a species growing in the dense rainforest to invest energy in early height growth to reach the canopy rapidly and, in later stages, to increase specific leaf area by expanding the canopy to intercept a larger quantity of light. In fact, the variation in bole volume between sites is a consequence of differences in taper (form factor) and Hs during ontogeny.
Increasing canopy height may result in higher self-shading of the leaves due to the packing of leaf layers (Sterck and Bongers, 2001). However, the opposite phyllotaxis and the small leaves of G. glabra can minimize the effect of canopy shading. In this case, the increase in canopy height could result in an increase in the number of layers of leaves and, by inference, the specific leaf area. Rather, the better light conditions experienced by trees in Apuí means that they do not need to invest large amounts of resources in increasing photosynthetic area. A deep and wide canopy does not necessarily imply a greater number of leaf layers, but the distribution and geometry (shape, size and orientation) of foliage can play a crucial role in the light interception (Poorter et al., 2006).
Allometric differences in this study corroborate the variation in wood density between sites. Wood density is directly related to mechanical structure, and by inference, the ability of a tree to support its own weight and to resist exogenous forces, such as wind and pushing over by other trees (King, 1981; Putz et al., 1983; Chave et al., 2009). However, recent studies have cast doubt on these ideas, showing that the same strength for supporting tree weight is achieved by a trunk with low wood density (with lower construction cost) than by one with higher wood density (Anten and Schieving, 2010). Larjavaara and Muller-Landau (2010) proposed that species with low wood density, such as pioneers, prioritize short-term gains over long-term costs, thereby not only evolving low wood density, but also low trunk and xylem resistance. Low light levels may induce tropical trees to reduce their diameter growth, thus resulting in higher wood density (Thomas, 1996). This also results in a greater investment in vertical growth and crown expansion at the expenses of stem thickness (Sterck et al., 2001; Osunkoya et al., 2007; Poorter et al., 2006). A thinner trunk with high wood density is more flexible than a thicker one with low wood density and equal strength, and flexibility can be advantageous in reducing sail area during short gusts of wind and in allowing the tree to bounce back after having been struck by falling trees or branches. In this sense, the greater crown expansion of G. glabra in NOlinda required more investment in strength and flexibility, and thus in higher wood density. In a rainforest in Borneo, Osunkoya et al. (2007) showed that the safety factor (a measure related directly to physical properties of the wood) had strong positive relationships with crown diameter and depth, indicating that trees (regardless of species) with greater horizontal or vertical crown expansion had higher safety margins against rupture (see also King, 1996; Sterck and Bongers, 1998; Alves and Santos, 2002).
The difference in taper between sites may also represent an adaptive strategy in terms of mechanical support because the increase in the stem base may improve support and fixation (Sposito and Santos, 2001). A larger taper in NOlinda can be related to decreasing risk of deflection of the stem as a result of the greater weight generated by the crown expansion. Therefore, we concluded that there are different adaptive strategies during the ontogeny of G. glabra acting on resource allocation and occupation of space mainly as a result of differences in light availability between sites. NOlinda trees need to invest heavily in initial height growth by allocating more carbon per unit volume of stem in this phase, so that an increase in the safety margin against breakage may be achieved when trees reach the canopy and start investing more in crown expansion. In contrast, trees in Apuí allocate less carbon per unit volume of stem during their ontogeny, as canopy expansion is significantly lower than in NOlinda. In fact, the non-significant relationship between DBH and wood density for NOlinda and the positive relationship for Apuí (Figure 4) demonstrate that carbon allocation to stems in NOlinda occurs equally throughout ontogeny, whereas for trees in Apuí the allocation of biomass changes as the tree grows, with an increase occurring at the end of ontogeny when horizontal crown expansion starts.
Among tropical tree species, tree growth is negatively related to wood density (Thomas, 1996; Muller-Landau, 2004; Nascimento et al., 2005; King et al., 2006). In addition, within the same species wood density is higher in slow-growing trees than in fast-growing trees (Koubaa et al., 2000), indicating that G. glabra may have faster growth in Apuí and, as a result, there must be a difference between sites in the age of trees of same diameter. However, such a difference should be evaluated empirically, and we suggest this approach for future studies.
The distribution of life-history strategies, and particularly the wood density of tropical tree species, may vary among sites depending on environmental features such as climate periodicity, soil conditions, disturbance regimes and competition for light (Muller-Landau, 2004; Nogueira et al., 2007; Patiño et al., 2009). Sites with high disturbance regimes on relatively fertile soils may favor fast-growing species that have low wood density, while sites with lower frequencies of disturbance and with nutrient-poor soils reduce tree growth (Baker et al., 2004; Muller-Landau, 2004). Patiño et al. (2009) have shown that branch xylem density varies considerably across Amazonia, and that 33% and 26% of this variation is attributable to phylogeny and environmental factors, respectively. The remaining 41% is attributable to intraspecific differences related to phenotypic plasticity in many tree species.
In the present study, the difference in wood density between sites is a straightforward consequence of the phenotypic plasticity of G. glabra in response to different environmental conditions. In seasonally flooded várzea(white water floodplain) forests in the Amazon, Wittmann et al. (2006) attributed the differences in wood density of two tree species, Tabebuia barbata and Hevea spruceana, to the level and duration of flooding, which leads to different water conditions and, consequently, to the reduction of tree growth during the aquatic phase, i.e., sites with higher levels and durations of flooding result in greater physiological stress that decreases tree growth and increases wood density. In the present study, differences in soil fertility and disturbance regimes between Apuí and NOlinda can also be considered as crucial factors for different strategies of G. glabra growth, and the subject is appropriate for future studies.
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