Brazilian savanna is a seasonally dry biome, highly diverse and distributed mainly on nutrient-limited soils. Interactions between water stress and nutrient availability are important evolutionary filters in these environments. Previous evidence indicated that reducing the nutritional limitation increases growth rate, optimizes water transport and decreases stomatal conductance in woody plants. However, the anatomical mechanisms that explain such responses are not well-understood. We studied the effects of long-term (20 years) nutrient addition (N, NP and P) on soil chemistry and hydraulic morphological and anatomical traits in six dominant woody savanna species. Nutrient addition and decrease in soil moisture, probably related to changes in grass cover, influenced the plant hydraulic traits at the anatomical level, namely increasing the xylem vessels’ diameter. Consequently, the specific theoretical xylem conductivity (K tx) increased in five species under NP and P addition. Additionally, the stomatal pore index (SPI) decreased with species-specific responses regarding the intrinsic water use efficiency (iWUE). Four species had higher vulnerability to cavitation (l vul) under NP and P addition. Using community-weighted mean and structural equation modeling approaches, we observed that nutrient enrichment at the community level did not affect iWUE, while the vulnerability to cavitation (l vul) strongly increased. The K tx and SPI were positively and negatively affected by nutrient addition, respectively, but the effects were not as strong as expected due to contrasting species responses. These changes optimized water transport with a hydraulic safety cost and reduced water loss. In comparison with responses to N addition, the greater P-limitation in Cerrado vegetation explains the inter-specific convergence in the responses of P-fertilized individuals. We showed that long-term responses to increased nutrient availability in dystrophic soils include anatomical changes in savanna woody vegetation with relevant interactions with soil-plant–atmosphere water relations.

Woody plant encroachment in grassy ecosystems is a widely reported phenomenon associated with negative impacts on ecosystem functions. Most studies of this phenomenon have been carried out in arid and semi-arid grasslands. Therefore, studies in tropical regions, particularly savannas, which are composed of grassland and woodland mosaics, are needed. Our objective was to evaluate the accuracy of woody encroachment classification in the Brazilian Cerrado, a tropical savanna. We acquired dry and wet season unmanned aerial vehicle (UAV) images using RGB and multispectral cameras that were processed by the support vector machine (SVM), decision tree (DT), and random forest (RF) classifiers. We also compared two validation methods: the orthomosaic and in situ methods. We targeted two native woody species: Baccharis retusa and Trembleya parviflora. Identification of these two species was statistically (p < 0.05) most accurate in the wet season RGB images classified by the RF algorithm, with an overall accuracy (OA) of 92.7%. Relating to validation assessments, the in situ method was more susceptible to underfitting scenarios, especially using an RF classifier. The OA was higher in grassland than in woodland formations. Our results show that woody encroachment classification in a tropical savanna is possible using UAV images and field surveys and is suggested to be conducted during the wet season. It is challenging to classify UAV images in highly diverse ecosystems such as the Cerrado; therefore, whenever possible, researchers should use multiple accuracy assessment methods. In the case of using in situ accuracy assessment, we suggest a minimum of 40 training samples per class and to use multiple classifiers (e.g., RF and DT). Our findings contribute to the generation of tools that optimize time and cost for the monitoring and management of woody encroachment in tropical savannas.

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