In recent decades, the global planted area of Eucalyptus
has increased significantly, covering approximately 10% of the planted forest area worldwide (approximately 12 million ha) [1
]. In Brazil, Eucalyptus
forests occupy 5.1 million ha, of which over 1.1 million are in São Paulo [2
]. The supply chain activities of the wood industry, including production, harvesting, and transportation, contribute to the social, economic, and environmental well-being of society. For example, approximately 0.97 ha of native forest is preserved per 1.0 ha of Eucalyptus
plantation in Brazil [2
]; thus, millions of hectares of native forests are preserved. Eucalyptus
is the most productive planted forest in Brazil, with an average productivity of 40 m3
]. This high productivity is linked to the availability of natural resources, genetic characteristics, and silvicultural management [4
]. Productivity in São Paulo State is limited by nutrient deficiencies in soils of low fertility, high nutrient export at harvest, and inefficient nutrient use. The lack of nutrients such as nitrogen, phosphorus, boron, potassium, and magnesium or the excess of nutrients such as calcium, magnesium, and copper limits productivity [5
Among the natural resources required for plant growth (i.e.
, water, light, and nutrients), nutrient availability is the resource most easily manipulated by foresters, throughout practices of soil preparation and conservation and applications of fertilizers. The efficiency of this management approach is site-specific for each type of nutrient applied [5
Although knowledge on tree nutrition has advanced, losses in timber productivity associated with nutritional problems continue at operational scales [6
]. Trees without equilibrated nutrition may lead to a heterogeneous population, compromising the site productivity [9
]. For industrial forests, understanding the value and spatial distribution of the fertilization response is essential for optimal investments in silviculture. A twin plots (TP) design is one approach to quantify the potential response to fertilization. This method establishes paired plots (control and treated plots) across a number of selected sites that represent the landscape [8
]. This approach differs from that of the classic experimental design of fertilization experiments because of a greater power of statistical inference across many stands, leading to a broader understanding of fertilization response by covering most of the environmental variability in a short period of time [6
]. The control plot may be a normal plot of a permanent inventory network used to measure the actual productivity of a forest under the traditional fertilization regime of a particular owner. The treated plot receives an intensive treatment (high levels of fertilization and weed control) for quantitative insights into the factors that limit operational productivity.
The relationship between resource availability and productivity is expressed in the following equation: Production = supply + resource capture efficiency + resource use efficiency
. Thus, productivity is strongly influenced by the resource supply and is dependent on the efficiency of the plant in capturing and using these resources [10
]. Resource use efficiency is an indication of how plants use resources (e.g., biomass produced per unit of resource consumed) and is a determining factor in wood growth. Growth efficiency, e.g., the amount of wood growth per unit of leaf area, also express how active are the leaves in converting carbon into biomass; this process is strongly influenced by leaf nutritional status [12
The use efficiency of different natural resources has been examined in several studies (i.e.
, water use efficiency-WUE; light use efficiency-LUE; and nutrient use efficiency-NUE; [10
]. With increases in the resource supply, e.g., through fertilization, plants use not only nutrients but also other resources more efficiently [10
]. For example, an increase in the availability of water can increase LUE, WUE, and nitrogen use efficiency [11
]. Soil fertility influences LUE by increasing the amount of light absorbed with an increase in leaf area index (LAI) or by increasing plant efficiency in using radiation absorbed with an increase in leaf retention, as occurs in response to potassium fertilization [14
Even though nutritional management of Eucalyptus
leads to significant increases in the productivity and sustainability of forest plantations in Brazil [15
], few studies have quantified light use and growth efficiency in response to fertilization. The objective of our study was to gain insight into fertilization response, light use efficiency, and growth efficiency of Eucalyptus
plantations along an edaphoclimatic gradient in southeastern Brazil. Moreover, this study addressed the following questions: What is the difference between actual and potential productivity of Eucalyptus
forests in southern Brazil? Is there variability in wood productivity on a temporal and spatial scale that can be adjusted by forestry management? Can the increases in productivity correlated with fertilization be explained by increases in light use and growth efficiencies?
The additional fertilization led to large increases in wood increment, with an average response in the annual increment of 5.3 Mg3
. Similar results were found in other studies, with responses of 4.8 Mg3
] and 4 Mg3
] in Eucalyptus
plantations in São Paulo State.
Region 3 was the most responsive to fertilization, which can be explained primarily by the climate, with no dry periods, and by the high clay content in the soils; consequently, the water retention capacity and organic matter content were high. Plant growth is controlled by the most limiting resource, according to Liebig’s law of the minimum [32
]. In tropical regions where water and light are not limiting resources, nutrition starts to be important and normally limits wood growth [29
]. For region 3, the negative correlation of FR with the sum of bases and the silt content, which is an indicator of the primary source of nutrients [33
], revealed nutritional aspects interfere more with FR than water availability.
In region 1, the climate has extended dry periods and the soils are sandy, which resulted in the lowest productivity (Table 4
). Soil physical properties, particularly the clay content, are directly correlated with wood quality (lignin and holocellulose content) and the productive capacity of a site [34
]. In this region, where water is a limiting factor, the extra addition of nutrients did not lead to an extra amount of wood, confirmed by the positive correlation between the FR and sand content.
The largest response to fertilization was in the region that also had the highest productivity (Region 3), which was different from the response observed by Ferreira and Stape [6
]. Therefore, the area with the highest nutrient limitations on growth was also the most productive. Thus, water, the primary growth factor [17
], was not limiting at this site, whereas in the other regions, particularly in region 1, the most limiting factor was water.
In the twin plots, LAI increased by 15% and GE increased by 10% compared with the normal plots. The increase in leaf area might explain the increase in productivity. Albaugh et al.
] found much larger gains, an increase of 101% in the LAI, in response to fertilization in loblolly pine. In Eucalyptus nitens
plantations, nitrogen fertilization increased the LAI by up to 3.1 units, i.e.
, 56% increment [36
Fertilization increased the LUE and the GE, which was also reported by Binkley et al.
] for Eucalyptus
and for other forest trees, including loblolly pine [38
] and Liquidambar styraciflua
]. These responses might be a result of higher photosynthetic rates with the addition of nutrients [40
] or caused by less carbon partitioning to shoots [10
Additional fertilization reduced nutrient limitations in the Eucalyptus
plantations and increased wood productivity, LAI, and LUE. Thus, we can identify and quantify opportunities to improve the current fertility management of these plantation populations (high probability of response in 85% of the experimental blocks), particularly in region 3. Moreover, with nutrient limitations, wood production was reduced by 5.3 Mg·ha−1
. We identified that for all sites, organic matter and clay content were related to FR. Additionally, for the intermediate and high fertility sites (region 2 and 3, respectively), potassium and magnesium, and the sum of bases and the silt content, respectively, were related to FR (Table 6
). All of these responses together indicate these variables can be used as diagnostic tools for managers to decide where to invest in fertilization.
Water availability was the primary element affecting productivity and potential response to fertilization. When water resources are available, nutrient limitations become more apparent as the primary difference among the three regions. For practical applications, forest managers can use these results to determine which regions should receive priority in analyses of fertilization investment. For example, on a regional scale, fertilization investments would have a much higher return in region 3 compared with the other regions because of the greater potential for response and generation of economic returns from the plantation forests. Our findings do not provide exact information of where the responses would occur, but the main drivers are described in Table 6
(quantity of organic matter, clay content, potassium, magnesium, sum of bases and silt content). Fisher and Binkley [42
] proposed a decision support method to minimize the risks. According to the authors, the risks are related to the average and variance of the response, being a high negative risk if the response is low and with a high variance. In our case, the high percentage of plots that had a positive FR (85%) could help managers to support a decision to apply fertilizer.
The aim of this study was to quantify the potential wood productivity and the proportional response to fertilization with a complete nutrient supplement when plants were grown across a wide range of soil types and climatic conditions. Future studies should focus on the separate effects of different nutrients following fertilization to determine the most limiting elements in each environment.