Extreme weather events are expected to increase in frequency and intensity in South America, likely causing decreased plant productivity and altering species distributions. Yerba mate (Ilex paraguariensis) is a tree species native to South America and has an ecological, social, and economic importance in several countries. Natural forests and cultivated areas of yerba mate will be endangered by climate change because of the expected reduction in water availability. Here we determined how clonal genotypes of yerba mate (BRS BLD Yari, BRS BLD Aupaba, BRS 408, and EC40) respond to four levels of soil water holding capacity (100%, 80%, 60%, and 40% WHC) over 60 days, by evaluating the plants’ morphophysiological and biochemical characteristics. We observed a reduction in plant height and biomass accumulation related to the decrease in water availability; physiological and biochemical parameters indicated that water-deficit stress reduced the plants’ C assimilation and increased their production of bioactive compounds. BRS BLD Yari had a higher tolerance to low water availability, with greater biomass accumulation and photosynthetic rates that indicate greater water use efficiency. Understanding how different yerba mate genotypes respond to water deficit is essential for species conservation and developing climate-adapted breeding programs.

Knowledge of the morphological and physiological responses of trees to planting density is important for adjusting plantation establishment practices to attain the desired size and characteristics of wood products. In this review, we provide an overview of how planting density is managed as a silvicultural practice and summarize recent advances, trends, and gaps to guide future research in this field. We applied a PRISMA methodology to select papers published between 1981 and 2020 that tested how monospecies planting density affects the morphological and physiological characteristics of planted trees; we observed an increasing trend in the number of relevant publications over this period. These studies focused mainly on species used for timber production (Eucalyptus and Pinus), and research was concentrated in the United States, China, and Brazil. In general, the 255 selected studies identified that planting density has a major impact on stem diameter and volume, resulting in greater stand productivity of high-density stands, especially for trees having a smaller individual volume. Studies evaluating the physiological response of trees to planting density also showed that species ecology, solar radiation, water requirements, and site-specific characteristics have a greater influence on plant growth than planting density. Considering the physiological aspects in high-density plantations, water relations based on transpiration and water use efficiency are the main aspect guiding plants growth rate. Despite major advances in intensive silviculture around the world, there remains a knowledge gap for tree species grown for their leaves, fruits, or seeds and a lack of data related to the physiological response of trees to planting density. The results of this literature survey can improve forest management decisions in regard to forest stand use and develop novel study avenues for silviculture.

The production of forest seedlings with adequate morphological and physiological characteristics is essential for the success of plantations. Substrates and irrigation are the major factors determining seedlings’ growth. Substrates made of urban and agricultural residues are a sustainable alternative to peat-based substrates. In this study, we evaluated how composted sewage sludge substrates affect the growth and gas exchange in seedlings of Cedrela fissilis Vell. Seedlings were produced under daily irrigation depths of 6, 9, and 12 mm, and on different substrates. The substrates were based on sewage sludge composted with Eucalyptus bark or sugarcane bagasse, and a commercial substrate based on peat, involving a double factorial design with 12 treatments (3 irrigation depths × 3 substrates). Both physical and chemical characteristics of substrates were analyzed, and morphological traits and gas exchanges of seedlings were measured. Sewage sludge-based substrates presented different characteristics according to the material it was mixed. Eucalyptus bark provided higher bulk density (0.19 g cm-3) and lower total porosity (75%) to the substrate, while sugarcane bagasse increased macroporosity up to 60%. Seedlings produced in sewage sludge-based substrates presented a height up to 17.8 cm and stem diameters of between 8.39-10.29 mm. Higher shoot and root dry mass was obtained in sewage sludge-based substrates with irrigation depth of 9 mm, which were 3.71 and 2.01 g, respectively. Photosynthetic carbon assimilation varied between 2.26 and 3.23 µmol CO2 m-2 s-1, and water use efficiency varied from 2.058 to 3.395 µmol CO2 (mol H2O)-1, with the highest values being obtained in seedlings produced in sewage sludge-based substrates with irrigation depth of 6 mm. Our results demonstrate that sewage sludge-based substrates are an efficient alternative to commercial peat-based substrates for seedling production.