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Introduction
In recent years, trees in urban areas have gained importance due to the multiple benefits they provide to the population (Houlden et al., 2021; Nesbitt et al., 2017). Therefore, the conservation and maintenance of green spaces has become a key objective in sustainable urban development plans (United Nations [UN], 2023). The urban environment often increases stress on trees and shortens their lifespan; thus, the periodic monitoring and evaluation of urban trees play a crucial role in decision-making, helping to develop effective management plans that maintain tree health and, in turn, provide greater benefits to the population (Fang et al., 2023; Mullaney et al., 2015).
Several methods have been proposed to assess the condition of trees, such as “The Guide for Plant Appraisal” (Council of Tree and Landscape Appraisers [CTLA], 2018), the i-Tree ECO program (USDA, Forest Service, 2013), the "Forest Inventory Analysis" (USDA Forest Service, 2016), and the “Urban Tree Health” method (Bond, 2021). In Mexico, forest health has been a topic of study primarily since 2012, when the National Forest and Soil Inventory (INFYS) and the Health Management Division of Mexico's National Forestry Commission (CONAFOR) agreed to begin measuring two forest health indicators: ‘crown condition’ and ‘tree damage’ (Alvarado-Rosales et al., 2021). These indicators have been used to determine the health condition of trees by means of quick, practical, cost-effective, and reliable assessments in urban areas (Saavedra-Romero et al., 2016; Zaragoza Hernández et al., 2015).
In Mexico, the Ficus genus is one of the most used for street trees in parks, gardens, and public spaces (Alanís-Rodríguez et al., 2023; Martínez-Trinidad et al., 2021). In Cuernavaca, Morelos, about 89 % of the trees are exotic species, and Ficus microcarpa L. f. is among the four species that make up 76 % of the urban tree population (Ramírez-Rodríguez et al., 2020). Given the abundance of F. microcarpa in the city of Cuernavaca, this study aims to assess the health condition of this species in the urban area using the indicators ‘crown condition’ and ‘tree damage.’
Materials and Methods
Study area
The study was carried out from July 2022 to February 2023 in the municipality of Cuernavaca, in the northwestern state of Morelos (19° 01’ 29” N y 99° 20’ 31” W).
A map of the municipality’s urban area was created (Figure 1), outlining its three characteristic climatic regions: north = C(w2) temperate subhumid; center = A(C)w2 warm subhumid (Group C); and south = A(C)w1 warm subhumid (Group C). The central region was selected as the study area because it had the highest number of F. microcarpa individuals. The map was divided into 1-ha square plots, and a survey was conducted using the Google Earth® georeferencing system to select plots containing at least one individual of the target species. A total of 3 436 plots were identified, of which 98 were evaluated, representing a sampling intensity of 2.85 %.
Tree measurement variables
Total tree height (m) and crown length (m) in the vertical direction were measured using a clinometer (Suunto PM-5). Diameter at breast height (DBH) of trees with DBH ≥ 7.5 cm (measured at 130 cm above ground level) was recorded using a diameter tape (Forestry Suppliers Model 283d). Crown condition and health categories (Table 1) were evaluated following the sampling procedures of Randolph and Bechtold (2018), Bechtold et al. (1992), and Schomaker et al. (2007).
Table 1 Variables and health categories of the crown condition indicator (Bechtold et al., 1992; Randolph & Bechtold, 2018) for natural forests in the southern United States.
| Variable | Health categories |
|---|---|
| Live Crown ratio | >50 % good; 31-50 % moderate; <30 % poor |
| Crown density | 0-20 % poor; 21-50 % average and 51-100 % good |
| Foliage transparency | 0-30 % standard; 31-50 % moderate and 51-100 % severe |
| Dieback | 0-5 % none; 6-20 % light; 21-50 % moderate; 51-100 % severe |
The measured variables were as follows: live crown ratio (LCR), crown density, foliage transparency, and dieback. LCR was measured as a percentage and determined as the coefficient between live crown length (measured vertically) to the total tree height. Crown density was assessed in 5 % intervals by estimating the quantity of branches, foliage, and reproductive structures that block light passing through the crown. Foliage transparency was measured based on the amount of light passing through the foliated living rate of the crown. Dieback was estimated in 5 % intervals, considering the recent mortality of branches, represented by fine twigs, starting at the terminal part of the tree towards the inner part of the tree. For the assessment of crown density and foliage transparency, the evaluation card proposed by Schomaker et al. (2007) was used (Figure 2).
Figure 2 Card proposed by Schomaker et al. (2007) for the calculation of crown density and crown transparency.
Tree damage
Trees from 98 randomly selected plots were evaluated by observing the main damages, taking as a guide the manual of the INFYS (CONAFOR, 2017); the record was adjusted to the most frequently observed damages. All those factors that negatively affect tree development in the short or long term were considered.
Data analysis
The data from 387 trees were recorded in a Microsoft Excel™ database, and the analysis was conducted using descriptive statistics. Due to the type of data, Kolmogorov-Smirnov and Lilliefors tests were performed at a significance level of 0.05 to determine whether the data followed a normal distribution. Based on the results of these tests, histograms were generated using Sturges' rule. These analyses were carried out using R Studio (R Core Team, 2020).
Results and Discussion
A total of 32 dead trees (8.26 %) were recorded out of 387 F. microcarpa trees. The significance values of all variables were lower than the established threshold (α = 0.05), indicating that the data were not normally distributed. Figure 3 shows the frequencies of tree measurement variables. The heights were distributed into 11 classes; the most frequent class, comprising 27 % of the trees, was between 3 and 5 m, while the least frequent class (1.12 %) corresponded to trees between 18 and 22 m. On the other hand, the diameters were divided into 10 classes; trees with diameters between 20.2 and 40.4 cm were the most frequent (39 %), while those with diameters greater than 60 cm exceeded 27 %. The classes 8 (141 to 162 cm) and 10 (182 to 202 cm) had the fewest individuals. Regarding crown length, 10 classes were obtained, with the first and second classes being the most frequent, with crown lengths of 1.8 m (14.7 %) and 3.63 m (37.8 %), respectively.
Figure 3 Tree measurement variables of Ficus microcarpa evaluated in the city of Cuernavaca, Morelos, for the years 2022-2023.
Tree heights in the city of Cuernavaca are similar to those recorded for trees in other urban areas of Mexico (Román-Guillén et al., 2019), where most trees have a height of around 5 m, due to pruning conducted to prevent interference with electrical wiring/overhead cables (Amer et al., 2023). It has also been reported that the average diameter of F. microcarpa trees in urban streets is 32.7 ± 15.6 cm, and the crown length is 4.59 ± 1.7 m (Amer et al., 2023), which matches the measurements of the trees in Cuernavaca. Height and diameter data mostly correspond to young to mature trees. Tree diameter is associated with age; smaller diameters correspond to younger trees (McPherson, 2016) and functional trees that provide more benefits to the population.
The strength and vitality of a tree are directly correlated with its stem and structure, so healthy trees are better able to withstand external damage and stress (Román-Guillén, 2019). The relationships between tree measurement variables such as total height, diameter, and crown length can help determine growth patterns and improve urban landscape planning and management practices (Chinchilla et al., 2021).
Crown conditions
Regarding the crown condition variables, Figure 4 shows that the trees were distributed into 10 classes. The most frequent category was class 9, with a live crown ratio ranging from 76.76 % to 86.35 %, classifying the trees as being in good health (Table 1; Bechtold et al., 1992; Randolph & Bechtold, 2018). Regarding crown density, 88 individuals fell into class 7, which includes values between 57.57 % and 67.16 %; these values indicate that the crowns are in good health (Table 1; Bechtold et al., 1992; Randolph & Bechtold, 2018). Concerning foliage transparency, most trees were in the range of 29.69 % to 39.5 %, with a total of 82 individuals, which classifies them as having moderate health (Table 1), according to Randolph and Bechtold (2018). Finally, dieback was considered incipient, as 195 trees fell within the 0 % to 9.8 % range (Table 1; Randolph & Bechtold 2018; Bechtold, 1992).
Figure 4 Crown condition variables of Ficus microcarpa in Cuernavaca, Morelos, for the period 2022-2023.
Foliage loss is one of the early indicators of tree stress (Schomaker et al., 2007). Crown condition indicators (such as foliage transparency, LCR, crown density, and dieback) are valuable tools for assessing tree health, primarily based on the degree of stress, which can be expressed in several ways and will depend on the stress factor and the response of the plant (Pontius & Hallett, 2014). In general, the crowns of F. microcarpa in Cuernavaca appear to be healthy; however, foliage transparency suggests that they are experiencing early stress. Moreover, the recorded mortality rate of 8.26 % indicates that while the trees seem healthy, they may be at risk of sudden death due to persistent and prolonged stress. Specifically, the continuous stress experienced by urban trees-mainly due to poor planting locations, limited growth space, and inadequate management (Tan & Shibata, 2022)-can increase their susceptibility to pests and diseases (Brace et al., 2020; Percival, 2023; Webb et al., 2023). It is important to note that certain insects are attracted to volatile compounds emitted by stressed plants (Atkinson, 2017). Likewise, some endophytic fungi can become pathogenic when trees are under stress (Hietala et al., 2018). Therefore, crown condition indicators are essential tools for detecting stress, which, as demonstrated, can significantly impact tree health in multiple ways.
Tree damage
Table 2 shows the ten damage agents affecting F. microcarpa. The most common issue was reduced growing space, affecting 55.21 % of trees. This category includes trees with limited root growth areas or those surrounded by pavement. The second most frequent damage was topiary, impacting 18 % of individuals. In third place, insect damage caused by species from the suborder Sternorrhyncha affected 15.7 % of trees. Only 1.1 % of the trees were considered free of damage.
Table 2 Tree damage in Ficus microcarpa in the municipality of Cuernavaca, Morelos.
| Damage agent | Trees with damage (%) |
|---|---|
| Healthy | 1.1 |
| Sternorrhyncha | 15.7 |
| Ganoderma | 0 |
| Parasitic plants | 2.5 |
| Topping | 1.9 |
| Topiary | 18.0 |
| Reduced living space | 55.2 |
| Exposed root | 1.4 |
| Spiders | 2.8 |
| Cankers | 1.9 |
The damages contrast with the 37 types reported by Saavedra-Romero et al. (2016) in the San Juan Aragón urban park and the 26 damages recorded by Zaragoza Hernández et al. (2015) in different parks across Mexico City. In both studies, root growth space was not considered a damage factor, whereas in this research, it was the most frequent issue. According to Cibrián et al. (2007), urban trees are primarily affected by abiotic damage, which aligns with the findings of this study.
It has been reported that limited root growth space and water scarcity impact stem diameter, as well as crown diameter and volume, particularly in F. microcarpa trees (Amer et al., 2023). Trees surrounded by pavement face significant constraints, including reduced water infiltration (Suárez-Islas & Mateo-Sánchez, 2023) and restricted root growth (Orman-Ligeza et al., 2018). Studies have shown that in the absence of water, abscisic acid is released, suppressing root growth (Mehra et al., 2022). Pavement also increases site temperature, affecting photosynthesis, transpiration, and respiration (Carrillo-Niquete et al., 2022; Chaves-Barrantes & Gutiérrez-Soto, 2016). Another critical factor influencing tree development is soil compaction, which leads to poor aeration, restricting growth and potentially causing hypoxia (Habibi et al., 2023). Additionally, it can promote the formation of shallow, lateral, and thick roots (Hernández-Castro et al., 2021), which may damage urban infrastructure.
Topiary ranked as the second most frequent type of damage, which agrees with the findings of Pino et al. (2022) for trees in Venezuela. This practice disrupts the natural architecture of trees, reduces their environmental services, and diminishes their ability to recover from adverse effects (Zaragoza Hernández et al., 2015). Additionally, it alters the source-demand relationship in carbohydrate translocation (Martínez-Trinidad et al., 2013) and affects other physiological processes. Furthermore, the wounds caused by frequent pruning delay the compartmentalization process (Kolařík et al., 2021), leaving trees vulnerable to harmful agents.
Conclusions
In the city of Cuernavaca, Ficus microcarpa trees predominantly range between 3 and 5 m in height and have diameters between 20 and 40 cm, classifying them as young-mature trees. Crown condition and tree damage indicators are used to evaluate the health condition of F. microcarpa. The crown condition variables suggest that the trees are generally in good health, except for foliage transparency, which indicates a moderate health condition. Foliage transparency is an early stress indicator, likely attributed to the identified damage agents, most of which are abiotic or anthropogenic in origin. It is essential to assess the set of factors that directly influence urban tree development, as city conditions often impose significant limitations on proper tree growth.
