Simulating Sustainable Forest Management Practices Using Crown Attributes: Insights for Araucaria angustifolia Trees in Southern Brazil

Abstract

Araucaria angustifolia (Bertol.) Kuntze, commonly known as Brazilian pine, is a significant tree species in the Brazilian flora that once covered an area of 200,000 km² in the Southern region. During the 1970s, high-quality timber logs from this conifer became the primary export product of the country. However, the species is endangered due to uncontrolled exploitation and is subject to a harvesting ban. It is crucial, therefore, to explore sustainable cultivation methods for this species, which necessitates urgent research and scientific insights. In this study, we present a simulation of a management strategy for in situ conservation by manipulating growth space and crown size dynamics. Forest inventory data and mixed forest regression equations were employed to describe the horizontal dimensions of average and maximum potential crown growth, resulting in two management scenarios. The results presented in management diagrams show that both approaches required logging numerous trees to ensure adequate space for healthy tree growth and provide soil coverage and forest protection. Therefore, the absence of effective forest management initiatives for Araucaria forests may have further implications for the structure, production, conservation, and overall development. To address these challenges, we propose two hypotheses: firstly, that tree diameter depends on crown dimensions, which are in turn influenced by tree growth space, and, secondly, that crown dimensions serve as a reliable indicator of existing competition and can be utilized to simulate forest management practices. We urge that implementing sustainable forest management initiatives for Araucaria angustifolia at selected locations can contribute to expanding natural forest areas, mitigate deterioration caused by high competition, discourage illegal logging, and prevent overexploitation of their edible seeds, which hinders regeneration. Our results underscore the significant implications of the lack of forest management initiatives in rural properties, potentially resulting in irreversible deterioration. The exact consequences of this deterioration remain unclear, emphasizing the need for further studies to understand its eventual effects on the growth reaction of trees of different diameters, ages, and crown conditions after the liberation of their crowns.

1. Introduction

Araucaria angustifolia (Bertol.) Kuntze (Brazilian pine) trees were important in the Brazilian economic matrix during the 1970s. They occupied, at that time, about 200,000 km², mainly in the three southern states of Brazil (Paraná, Santa Catarina, and Rio Grande do Sul). Prior to logging, this species was the dominant species in forests over much of its range. It is well-known for the high-quality wood density of its logs and the production of edible seeds by female trees. As a consequence of overexploitation and subsequent drastic loss of native forest areas, legal barriers to logging were imposed, which culminated in the total disinterest of landowners and local farmers in considering planting Araucaria, and instead, they opted for exotic tree species, especially Pinus taeda and P. elliottii [1,2]. As a result, this tree species is restricted to forest remnants and usually confined to legally protected reserves as a requirement of the Brazilian Forest Act [3]. However, as noticed in several rural properties, legal reserves do not assure integral protection, being still susceptible to illegal logging and overexploiting of edible seeds [4,5], which is one of the important causes of the reduced natural regeneration capacity of these forest remnants.

Araucaria forest’s actual situation, which, in a few decades, passed from being an abundant and valuable forest to restricted areas remnants, corroborates the lack of an efficient forest policy based on the sustainable principle, first described by von Carlowitz in 1713. Under this principle, economic, environmental, and social sustainability must be considered together; that is, forests must produce material and immaterial benefits for the actual and future population in an equal quantity and quality as today. A condition for this is the maintenance of large forest areas.

Land use changes are not only a Brazilian reality but are common in people’s history and continue to occur worldwide. The solution is not prohibiting forest use, but, rather, conservation through scientific and technical knowledge. Forests without management will be overstocked until a natural disaster such as wind, fire, herbivory, or diseases favors gaps opening, reducing the density and tree competition.

Unmanaged Araucaria forest presents high-density trees, with intra-specific and interspecific competition. Trees have crown size reduction for any stage of development, from young to old trees, which results in a reduction of effective crown coverage and leaf area, photosynthesis, growth, seed production, and early death of the plant, conditions that are easily verified in various forest dynamics studies [6,7,8]. Trees require differentiated growth space and nutrition conditions with distinct dynamics as they develop, as shown by the studies of morphometric relationships of free-growing trees and trees under competition [9,10,11,12], as well as in the studies of size, competition, and growth relationships [13,14], among others. Therefore, such studies allow quantifying the growth space needed for young and adult trees in pure and mixed forests [15].

Plant growth rates are driven by factors influencing the number of resources captured and resource use efficiency. In trees, the amount of light caught and the efficiency of light use strongly depend on crown characteristics and leaf traits [16]. For example, Araucaria trees have horizontal branches but direct them to the top of the canopy in search of light when under competition [12,13,17,18].

As competition increases, death and the fall of branches occur, resulting in short and irregularly shaped crowns. In contrast, trees with adequate growth space have horizontally distributed branches and long circular-shaped crowns. Trees that grow without competition have canopies with maximum dimensions for the environment providing the highest growth rates [19] and are a reference for the species’ potential [20].

Crown diameter and diameter at the breast height (dbh) ratios efficiently determine growth space, finding viable applications in pure and mixed forests [21,22,23], such as those remnants with Araucaria. Moreover, such studies allow for determining the effect of the intraspecific and interspecific competitors.

Other descriptors such as slenderness, stand density, and proportion of the basal area of the species can also be used as independent variables to develop allometric models with mixed effects to estimate the crown diameter [24]. In summary, the growth space and logging can be modeled and indicate an appropriate time for re-establishing trees with high vitality under a proper growth space.

Thus, due to the lack of management initiatives for the Araucaria forests, consequences in terms of structure, production, conservation, and development need to be considered, which also supports the proposed hypotheses: (i) the tree diameter depends on the crown dimension and this on trees’ growing space, and (ii) the crown dimension is a good reference element for the actual competition and to simulate management regimes.

This research aims to establish practical management guidelines that describe the growth space for forming trees with regular vital crowns under two forest management scenarios. The first one considers the size of the mean potential crown, while the second one the dimension of the maximum potential crown. In both cases, the space and number of trees per unit area that maintains productive, stable, and healthy forest growth was also considered.

2. Materials and Methods

2.1. Study Area Description

The data for describing growth space originated from the Long Term Ecological Project—PELD/CNPq “Conservation and Sustainable Management of Forest Ecosystems—Araucaria Biome and its Transitions” installed in a natural unmanaged forest area at the National Forest of São Francisco de Paula (29°24′ and 29°22′ S and 50°22′ and 50°25′ W, 900 m altitude), in the municipality of São Francisco de Paula, Rio Grande do Sul, Brazil. The vegetation is classified as Mixed Ombrophylous Forest (MOF) according to the phytogeographic classification [25]. The region’s climate is Cfb by Köppen classification, indicating a mesothermic and super-humid climate characterized by mild summers and cold winters [26].

2.2. Field Inventory

In this study, we used data measured in a sample plot of 0.5 ha, installed to monitor the forest dynamics. The sample plot was refined considering 50 subunits of 10 × 10 m and comprised all living trees numbered and located with UTM co-ordinates (east and north). The sampling unit was located within a forest remnant, with no border effect. However, to quantify the number of trees in the sample unit, trees whose crown projection exceeded the limit of the sample unit were systematically either included or excluded from the chosen sample unit.

To demonstrate the method, we considered only the competition established by the crown in selecting trees. Therefore, sanity and defects criteria were not considered, as they do not influence the method, although, in practical situations, they must be observed as a selection criterion.

All trees were identified at the family, gender, and species level, and their circumference at breast height (c) and total height (h) were also measured. The regeneration was also characterized by identifying tree species seedlings inside the sample plot. Finally, the c was converted to the diameter at the breast height (dbh).

In this research, all tree species other than Araucaria were disregarded since they occupied the great majority of the canopy’s lower strata. Instead, the tree species found in the lower strata of the forest were considered in other studies [14]. Thus, the adjustments and simulation of management models based on the canopy diameter and basal area were for Araucaria, since its canopy dominance, economic value by having good shape logs, and release with cuts will allow light entry, temperature increase, and better growth conditions for species that grow below the canopy [27,28,29,30].

Here, conifers were prioritized, assuming that releasing Araucaria crowns at a larger size and better quality allows a better growth condition for the remaining conifer and latifoliate that usually grows below the canopy.

2.3. Crown Diameter Estimation

We simulate crown diameter (Cd) using a regression model in which the dbh is the only input data (Equation (1)). The model was proposed by Costa et al. [31] and described the crown diameter of Araucaria trees without competition and dominant trees growing within the forest. This model used several field inventory data, including data from the same study area (Appendix A). The coefficient of determination (R²) was 0.84, and the root mean square error (RMSE%) percentage was 14.5.

Cd = 1.3886 + 0.2038 × dbh

(1)

The authors demonstrated the statistical coincidence of inclination and level of independent equations calculated for the two growth situations that were motivated by similar linear relationships between these variables described by Volkart [32], Longhi [33], Seitz [9], Wachtel [34], Chassot et al. [35], and Zanon [36]. These studies encompassed almost all the original occurrences of Araucaria in Southern Brazil.

For the determination of the maximum potential crown diameter (Cd pot. max.) the Equation (2) had the intersection coefficient recalculated to describe the values of maximum crown diameter found in the sampled set composed of 694 trees growing with some competition (9.9 > dbh > 97.0) and 175 trees growing without competition (14.0 > dbh > 59.0). Similar procedures as the mentioned above were successfully adopted by Grote [20], Pretzsch [37], Pretzsch et al. [38], and Costa et al. [31].

Cd pot. max. = 4.8601 + 0.2038 × dbh

(2)

The two equations developed at the individual tree level that express the crown’s dimensional relationships have an application in even age, uneven age, multi-species, and multi-layered forests [20,21,23,39,40].

Additionally, to reinforce the usage of Equations (1) and (2) in this study and to gather the main crown diameter equations for Araucaria for forest management purposes, an artificial neural network (ANN) was developed containing 27 valid regression equations coming from studies developed in different geographic regions of occurrence of the species (Appendix B). The study areas were from Misiones, Argentina, to different locations in the Brazilian states of Rio Grande do Sul (RS), Santa Catarina (SC), and Paraná (PR) (Appendix A and Appendix B).

2.4. Evaluated Scenarios

The spatial distribution of all trees was determined using ArcGIS 10.3 software. Then, based on the polar co-ordinates of the plot vertex (x, y), the position of each single Araucaria tree was obtained in relation to the origin with the polar co-ordinate (0, 0).

The distance between every single tree and the nearby trees was determined by comparing the crown radii plus the radius of the crown. When the sum of the crown radii was greater than the distance between the trees [(r_{crown i} + r_{crown target}) > dist. i, target], the tree was defined as a competitor and received coding for potential logging. The next non-concurrent tree was then selected as the target tree and the process restarted from the beginning.

The procedure was performed by establishing two density scenarios: (i) the average crown diameter [using Equation (1)] and (ii) the maximum potential crown diameter [using Equation (2)]. The number of remaining trees (N) and their respective basal areas (G) were then determined for each scenario.

Thus, the current dbh of a tree is the result of a growth stage with a compatible crown size. With increasing age or competition, successively smaller increments and canopy size reduction occur, keeping the tree alive with insignificant increments due to lower photosynthetic capacity (Figure 1).

Finally, the potential size of the crown, density, and basal area was used to build a forest management dendrogram [41,42]. The forest management dendrogram further identifies thinning intensity scenarios over the chosen study area, suggesting insights for forest managers. It is important to mention that trees with dbh ≥ 40 cm, broken crowns, and deformities were removed from the sample because, under these conditions, the current dbh cannot be explained.

According to Ishii et al. [41], as trees grow larger, the expansion of their crowns may not be accurately reflected in dbh measurements. This discrepancy contributes to the inaccuracy of estimating forest productivity. However, there is a feedback loop between the structure, environment, and tree growth, as Pretzsch [37,38] described. In mixed forests, the canopy structure is both influenced by and influences interspecific environmental conditions. Therefore, understanding the mechanisms behind diversity–productivity relationships requires considering both the crown plasticity promoted by the mixture and the variations in crown structural traits among co-existing species [42,43,44,45,46].

3. Results

3.1. Forest Inventory Results

The forest inventory showed 49 broadleaved species with a density of 912 trees·ha⁻¹ and presented an average dbh of 15.8 cm with a coefficient of variation (CV) of 151.3%. The mean h achieved 12.8 m with a CV of 35.2%. On the other hand, Araucaria showed a density of 288 trees·ha⁻¹, a dbh of 39.6 cm, and a CV of 148.5%. The mean h was 20.2 m, with a CV of 17.3%. The basal area reached 60.0 m²·ha⁻¹, of which 35.6 m²·ha⁻¹ (59.3%) belonged to Araucaria, a dominant tree species, confirming the high competition in the chosen uneven-aged, mixed forest remnant.

It is also important to mention that natural regeneration was not noticed in the study area due to the low light intensity near the soil. Most of the plants that establish themselves do not develop due to low luminosity, this being a factor in forest aging.

3.2. Forest Management Scenarios

In this research, 72 trees·ha⁻¹ were maintained with a tree density adjusted with the mean crown equation (Scenario 1) and 50 trees·ha⁻¹ with the maximum potential crown size equation (Scenario 2), respectively, as shown in

Table 1. Biophysical characteristics and number of trees to be logged and maintained according to the average and maximum crown diameter as a function of the current measured diameter breast height.

Type	Trees	N	G	CPA	Dist.
	Total	288	35.6
Mean crown diameter (Scenario I)	Remnants	72	3.0	38.5	13.3
	Selective logging	216	32.6
Maximum crown diameter (Scenario II)	Remnants	50	4.0	102.1	16.0
	Selective logging	238	31.6

Where: N = number of trees·ha⁻¹; G = basal area, m²·ha⁻¹; CPA = horizontal crown projection area, m²; Dist. = average distance between target trees, m.

.

In both management scenarios, the number of remaining trees ensures the preservation of protective effects on the environment, forest stock containing healthy trees, and conditions to establish and grow natural regeneration (Figure 2), a condition not usually found in unmanaged Araucaria forests, due to the legal barrier to using it.

In the simulation developed with the average crown (Equation (1)), the dbh of the remnant mean basal area reached 27.3 cm and 31.9 cm with the maximum potential crown (Equation (2)). The remaining trees in the forest do not show a regular distribution, which is why it is less than ideal, i.e., 150 to 180 trees, for the target dbh of 40 cm [9]. For this dbh, the average crown equation of this work resulted in 140 trees, proving the adequacy of the proposed management guidelines.

In contrast, after both simulations, the low number of tree remnants resulted from the high tree density and their irregular distribution, which brought many competitors that were suggested for logging. The distribution and dimensions of trees before simulation result from the original irregular location of trees, a common characteristic of uneven-aged mixed forests, and clarifies the absence of trees at some points of the simulated area (Figure 2). However, even in this condition, applying silvicultural techniques at an appropriate time, before the high competition was established, could provide a better distribution of target trees and, consequently, reduce the loss of forest increment due to the non-regular occupation of the soil. As a result, more trees would be well-distributed, with more significant individual increments, resulting in higher forest production.

As a comparison, if we consider a tree with an average basal area of 44.0 cm found in the forest prior to the simulation, theoretically, it would be possible to maintain a density of 114 trees per hectare with this diameter at breast height (dbh) if they were evenly distributed. This value is higher than the 72 trees per hectare reported in Table 1, which resulted from the management approach based on Equation (1).

The chosen simulation may generate heterogeneous openings in the canopy, known as gaps, due to irregular tree distribution and intense competition. Consequently, the number of trees that remain standing after logging is smaller compared to the potential number if the trees were evenly distributed across the terrain. This outcome is likely to facilitate the establishment and persistence of broadleaf species, which are currently not competitors of Araucaria. Interestingly, this condition may persist until the next revision of the forest management plan.

However, in addition to the irregular distribution of the trees in the study area, some remaining trees had larger dbh, reaching up to 97 cm, which requires a larger area for crown expansion and the consequent reduction in remaining standing trees. In this scenario, if the remaining trees in one hectare had grown without competition, the average canopy area would cover 5343.64 m² of the soil surface, leaving 4656.36 m²·ha⁻¹, which would be occupied by trees of natural regeneration or planted trees, when these were not satisfactorily established.

According to the adopted criteria, the forest manager can prioritize the release of trees based on their crowns and vitality and cut trees above a specific dbh or those with defective or diseased stems, among other factors. However, depending on the decision, it also implies the final spatial distribution of the remaining trees, dbh range, growth stock, and the stock removed from the forest. Therefore, establishing only a dbh criteria may not assure a more significant number of standing trees besides reducing the mean dbh of the remaining trees.

Thus, selecting one of the forest management scenarios should be preceded by a careful analysis of the general conditions of the forest, such as tree size, crown vitality, and the interest in reducing or increasing the period between successive logging interventions. In any case, forest management yields better results if it favors the growth of young trees with growth potential that does not require large gaps in the canopy, assuring a higher incidence of light onto the lower canopy layer, keeping more trees growing in the forest. Likewise, regular management actions at reduced intervals allow the formation of healthy trees that are well-distributed on the dimension classes [47].

3.3. Forest Management Dendrogram

Figure 3 presents the density of two (dashed line) and three (solid grey line) times the number of trees calculated for the average condition, i.e., conditions found in competing forests that indicate the potential for selective logging. By using the maximum potential crown size as a reference, the interval between logging interventions can be extended. This results in increased free space between the remaining trees, fostering optimal conditions for establishing and growing natural regeneration. Notably, the study area exhibited a complete absence of natural regeneration.

On the other hand, the crown of high vitality may indicate a more conservative scenario, maintaining a greater number of trees growing in the forest and reducing the period between logging interventions, as shown in Figure 3. The scenario that predicts the horizontal projection area of the maximum potential of the crown shows, respectively, between dbh 15 to 80 cm, crowns with 7.9 to 21.2 m, where the potential number of trees per hectare is between 204 and 28 trees, respectively.

In the same range of the dbh mentioned above, using the mean crown ratio, crowns would reach diameters from 4.4 m to 17.7 m, allowing the establishment of 657 trees and 41 trees, respectively. Interestingly, the region between the two proposed management densities in Figure 3 is considered to have the optimal density. In such conditions, the crowns show a more significant increment, and soil coverage and a greater volume of growing timber in the forest remain.

The density lines—2× (dashed line) and 3× (solid grey line)—in Figure 3, represent, respectively, the density of two and three times the number of trees calculated for the average condition found in competing forests that indicate the need for the application of release cut-offs.

4. Discussion

4.1. Forest Management Simulations

The Araucaria crown is composed of structural organs (i.e., branches) and photosynthetic organs (i.e., needles) that are vital constituents in determining the magnitude of light captured [48] and the ability to shade neighboring trees [49,50]. Therefore, crown attributes (i.e., size and architecture) are commonly used in modeling individual tree growth [51,52].

The reported results confirm the inferred hypothesis that the crown architecture and structure (shape and dimension) play an essential role in photosynthetic capacity and growth rate. Its development is influenced by the density, competition, and availability of space and resources, serving as an indicative element for planned interventions in forest management. For example, crown shape (defined as the ratio of crown length to crown diameter) results from the spatial organization of branches and foliage, which may be related to the efficiency of light utilization [16].

Thus, the results of the work (Figure 3) indicate the need for intervention in the forest, favoring the forest ecosystem, structure, stability, diversity, and production, since leaves (needles in the case of Araucaria) are of significant importance for tree productivity due to their fundamental role in carbon (C) assimilation through photosynthesis and transpiration [16,53].

Araucaria, under competition, promotes an initial loss of needles and branches in the lower part of the crown and then the lateral canopy (Figure 1), making it asymmetric and, consequently, reducing the growth capacity of the tree. In addition, the change in the competitive environment may lead to the growth of new branches near the trunk and end of the branches, a fact commonly observed in the Araucariaceae family [54], without ensuring effective plant growth, at least in a short period.

The natural and unmanaged forest sampled here has among the highest values mentioned in ten other studies where densities between 25.7 m²·ha⁻¹ to 46 m²·ha⁻¹ were reported [46]. In these high-density conditions, Araucaria would register a growth space of 6.6 m, which would allow reaching a maximum dbh of 25.0 cm. This competition promoted a substantial reduction in crown size, developing an irregular crown shape and losing needles and vitality (Figure 1), with a subsequent decrease in dbh increment rates. The estimation of crown size here did not seek to describe the actual crown size and shape, but the dimension that a tree of equal dbh would have while growing without competition.

On the one hand, the current diameter at breast height (dbh) of a tree is influenced by its available space and crown size before any competition occurs. Therefore, in order to determine the required growing space for its full development, it is essential to implement thinning regimes at specific stages of tree growth. On the other hand, the absence of such interventions may lead to crown size reduction and deformation, as observed in the selected study area (Figure 1).

The proposed management was carried out at the level of the trees, which were dispersed over the terrain occupying different sites, had different dimensions, and were found at variable distances from each other, as in uneven forests or thinned plantations. In this scenario, the plan was to harvest selected trees and not the forest, which provides, in each period, the necessary growth space for the target trees. Understocked areas and gaps can occur temporarily due to competitors’ harvesting or even the absence of a tree on the site. Consequently, the volume of the area is reduced, although the dbh of the remaining tree increases.

Interestingly, the relationships between growth space, dbh, number of trees, volumetric production, wood quality, health, stability of the trees, and natural regeneration are dynamic processes that change periodically, directly affected by the rate and intervals between cuts. Therefore, light and frequent interventions make it possible to reduce risks, maintain the stability and health of the trees, and reduce empty spaces. In this work, no considerations were made in this sense, which would make it theoretical and lengthy, since the heterogeneity and complexity of the forest, in its changing environmental aspects, require focus on the objectives established for the forest and require that a forester be aware of environmental, growth, and economic relationships. Thus, the described strategies must be considered as management guidelines, adaptable to the environment and the production objectives established for the location, observing the site’s capacity.

The greater growth space provided by cutting according to the maximum canopy size guideline reduces the number of trees remaining after cutting, with a consequent gain in tree size. Still, the lower density of trees will lead to a greater reduction in the total volume of wood.

Finally, the number of remaining trees will be greater if the average dimensions of the crowns are followed, generating fewer empty spaces on the land and a greater volume of wood produced. Thus, these guidelines do not oblige but indicate intervention possibilities that need to be evaluated for each tree, with considerations that transcend the available space, size, stability, and health of the tree. The selection of a specific guideline curve from Figure 3 and determining the appropriate time for its application are technical decisions that should be tailored to each forest environment, taking into account the established objectives. It is reasonable to expect that the optimal cutting intensity lies between the two guideline curves presented.

4.2. Thinning Intensity Issues

The resulting number of trees in both simulations, average and maximum, reported in Table 1, will undoubtedly cause heterogeneous openness in the canopy due to the irregular distribution of trees and the intense intra- and interspecific competition as expected for uneven-aged mixed forests. Thus, the number of trees that remained standing was smaller than the potential number of trees in the forest if the trees were regularly distributed over the terrain or the tree release process started earlier. As mentioned before, it also allowed the maintenance of broad-leaved species that did not compete with Araucaria, with permanence until the next revision of the forest management plan, as well as facilitating natural regeneration, leaving the forester to regulate the desired species.

Interestingly, our field observations, conducted over several years in the study area and Southern Brazil region, have demonstrated the ability of certain adult trees to exhibit restored height growth. This was observed, for instance, in a thinned population interplanted with Pinus taeda L., and as well as trees growing without competition (Figure 4).

It is easy to notice the crown limit, in a flat form that has grown until height stagnation (old crown), and the parable crown overlapping the previous (Figure 4, left). The image on the right shows the inner crown with a superposition of pseudo-whorls produced during stagnation. Stem elongations follow in increasing dimensions and also increase the distance of the pseudo-whorls in the young part of the tree.

Therefore, logging interventions would promote an increase in the growth space and allow the maintenance of regular growth rates. However, it is important to highlight that those silvicultural interventions, when established at the right time, avoid crown deformation and the reduction of dbh tree increment, which may sometimes be irreversible or might need a longer time to recover the crown shape and size and use the great free space among the remaining trees.

According to the current dbh, the management strategies described here provide the necessary growth space. Since the crown is responsible for photosynthesis and plant growth, it is accepted that trees with isolated growth present the highest growth efficiency for the site, which can be described by the increment ratio in volume and dbh, among others, in relation to the space occupied on the ground, expressed by the horizontal projection of the crown. In the same way, as highlighted by Costa et al. [40], the reduction of the growth space leads to a reduction in the dimension of the crown, deformation of the structure, reduction of the leaf area, and growth efficiency. Greater absorption of photosynthetically active radiation (APAR) is often proposed as a reason for greater productivity in mixed-species forests than in monocultures [37,55,56,57].

Although the crown dimension is reported to be a good reference element for the actual competition and can be used to simulate forest management regimes, it is important to consider that neglecting any silvicultural interventions is a clear sign that the crown growth could be affected in an irreversible way. In the near future, this is expected to lead to high levels of thinning intensity, which could have potential implications for the remaining trees’ wind resistance and timber properties. Therefore, these factors need to be carefully considered.

In addition, the scenarios were compatible with the space required for the growth of Araucaria trees with large dbh and crowns. These effects may improve resource acquisition or support higher resource-use efficiency [58].

Contrasting tree allometric relationships in mixtures compared with monocultures can also influence APAR. For a given tree diameter, the crown sizes (width, length, surface area, and leaf area), shapes, or heights of a given species can differ in mixtures compared with monocultures [37,59,60,61,62]. These allometric differences can add to the effects of the vertical foliage distribution when it allows crowns to expand sideways at different levels in the canopy or upwards or downwards away from other species [59].

Although Araucaria is able to withstand shading, it suffers a substantial increment reduction [63]. Young Araucaria trees that are established below the upper canopy strata of dominant trees, growing without lateral competitors, require between two and seven years to effectively grow (in diameter, height, and crown aspects) once the trees occupying the upper strata are logged.

4.3. Insights of Forest Management Initiatives

The selected study area shows some competition due to the deformed crowns of some Araucaria trees (Figure 1 and Figure 4). In these conditions, the positive effect of crown liberation and growth recovery must be considered individually. However, the genus Araucaria is genetically capable of restoring growth with the apex reiteration of branches and lateral twigs [54]. Seitz also reported such evidence, observing the cyclic growth in the height of adult trees for up to seven years and the establishment of natural regeneration after logging some adult trees [9].

The panorama vitality loss of Araucaria crowns and the reduction of growth capacity, coupled with the lack of management protocols currently imposed by the legislation, induces illegal logging. Previous studies reported illegal logging as one of the significant environmental threats besides deforestation and land conversion [47].

Other threats include the overexploitation of edible seeds, which also impacts regeneration. On the other hand, the proposed forest management promotes the formation and management of trees of great vitality and desired dimension, besides maintaining environmental benefits, adding value to forest products, and economically making the maintenance and increasing of forest remnants viable in rural properties. Furthermore, this approach could help alleviate the antagonism between landowners and the government, fostering a greater motivation to convert land currently used for cattle ranching, such as pastures, back into Araucaria forests. This would offer an intriguing opportunity for land use transformation and contribute to the restoration of forested areas.

Therefore, as Kjučukov et al. [64] mentioned, integrative forest management protocols are necessary to support biodiversity in forest remnants of rural properties that would be viable under active forest management initiatives. In their research conducted in the Czech Republic, the authors also highlighted the need to maintain some undisturbed areas, mainly within protected reserves, to ensure the presence of old-growth trees. Hence, when carried out with caution and consistent spatial and temporal practices, sustainable forest management yields benefits for the forest ecosystem in terms of environmental, social, and economic aspects. Therefore, the significance of this study can be justified by its potential to enhance the growth of Araucaria, ensure the continuity of natural regeneration, provide resources and space for growth, and indirectly contribute to carbon sequestration, plant diversity, soil protection, and climate regulation [65].

In the proposed method, the selection of Araucaria trees that remain after logging (target trees) should consider the distance and crown length, prioritizing long regular crown trees with high vitality and the qualitative characteristics of the stem. For example, larger crowns have a higher number of needles and a higher leaf area index than smaller crowns. Therefore, they are more efficient, as they have a greater photosynthetic area per m² of soil cover, providing a more significant production [65], and can be used as a competition descriptor [4,23]. Furthermore, it is important to avoid maintaining poor-quality trees in the forest. Not only do they have lower commercial value, but they are also more susceptible to diseases, which can hinder efficient forest renewal and the dispersal of seeds [66].

Interestingly, the growth efficiency (stem wood volume growth per unit leaf area) is considered a measure of tree vigor, where leaf area is the measure of occupied growing space [67,68]. It has been assumed that the growth of stem wood volume is the best measure of the efficiency with which growing space is occupied because allocation to stem wood generally has a lower priority than allocation to shoots, roots, biochemical defensive, and storage compounds [69,70]. Furthermore, differences in growth and the crown should be explainable within a species regarding individual trees’ capture and utilization of growth-limiting resources and overall stand dynamics [71].

Therefore, the growth of a forest can be influenced by high-density cuts, which can improve space and resource conditions with minimal intervention rates. This approach aims to maintain species conservation and provide ecosystem services. The crown structure is a crucial variable influencing stand productivity, although its response to different stand factors has been reported to vary [71].

Shape and dimension are influenced by factors such as density, and resource and space availability. In mixed stands, using the crown dimension as a variable in the management modeling may better predict the changes in the ecosystem than just the dbh [72].

Araucaria’s crown shape and length depend highly on density intra- and interspecific competition. However, this study confirmed the proposed hypotheses indicating that moderate interventions may contribute to increasing diversity and maintaining growth rates, structure, stability, natural regeneration, and production. Closed canopy forests limit the growth of codominant and dominated trees of old-growth species. In addition, the absence of light in the sub-canopy decreases the temperature and does not break the seeds’ dormancy.

4.4. Future Research Perspectives

Adding variables such as crown attributes and dbh showed some perspectives on current and forthcoming forest management initiatives with Araucaria at selected rural properties where the abundance of Araucaria trees is representative. This also raises additional concerns for environmental agencies, as they have to review forest management plans and ensure the accurate monitoring of the number of trees being logged. Interestingly, previous studies showed how remotely sensed systems can be used to detect individuals or groups of trees [73,74,75,76] using high spatial resolution images derived from remotely piloted aircraft (RPAs). Such approaches can retrieve these attributes to support ground measurements and policy monitoring initiatives. This aspect becomes even more critical when we consider that management practices in MOF have not been adopted for more than four decades, and this non-intervention, as reported in this study, has apparently not improved the conservation status of the tree species.

Finally, many challenges remain to be faced when it comes to the forest management of Araucaria. For this reason, advancements in managing mixed forests are welcome and require additional studies to make sustainable management viable in selected rural properties of Southern Brazil, where an abundance of Araucaria trees is evident. The fall of legal barriers to the use of mixed forests can return the interests of rural owners to Araucaria, as a product with high value that can be continuously offered when the sustainability concept is used in forest management.

Aspects related to the context of mixed forest management, especially those related to the ecology of systems, are beyond the scope of this research. However, they should be included in future studies to ensure the effective, sustainable forest management implementation of this tree species.

5. Conclusions

The study area is very characteristic of this forest typology and represents the reality in several legal reserves existing in rural properties in Southern Brazil. In summary, the occurrence of trees with reduced crown size, irregular distribution, or partially absent crowns, despite having a relatively large diameter at breast height (dbh), confirms the presence of high local competition. It suggests that the actual tree diameter results from competition conditions established before the current time. It further supports the notion that crown dimensions influence dbh, which, in turn, depends on the available growing space for the trees. The results reported in this research evidence that there is a need to regulate the crown structure by logging some trees under competition and to promote the gap opening for better conditions for the remaining trees to grow and promote natural regeneration.

The crown size is demonstrated to be a valuable source of information for accurately simulating Araucaria’s forest management without affecting the forest ecosystem’s conditions. Such interventions may ensure greater vitality in the remaining trees and improve their structure and productivity. Therefore, we argue that the crown dimension is a good reference element for the actual competition and can be used to simulate forest management regimes at selected rural properties. However, neglecting silvicultural interventions might affect crown growth in an irreversible way, which could cause high levels of thinning intensity in the near future, whose implications for wind resistance and timber properties of the remaining trees remain unknown.

Furthermore, the proposed method promotes and encourages additional studies and simulations. It is also strongly suggested that one considers the species’ auto-ecology, wildlife interaction, and some additional peculiarities of the study area not investigated here. However, such factors should be addressed in the future to ensure the proper implementation of the forest management of Araucaria in the southern region of Brazil.