Initialization
To create the biophysical aspect of the model, the climate, soil, road, river, plot, and tree entities are initialized at the start of the simulation. The values for climate, soil, and road remain static throughout the simulation because they represent fixed baseline conditions: soil pH is not expected to change significantly over the 17-year rotation period; roads are treated as permanent infrastructure with fixed spatial locations; and monthly climate inputs (e.g., precipitation, temperature, and evapotranspiration) are based on historical climatological averages rather than dynamic weather events, reflecting long-term environmental patterns rather than short-term variability. Dynamic climate projections were not included in this version to isolate social–ecological interactions without introducing additional uncertainty; future versions may incorporate climate change scenarios as data availability improves. The climate entities were initialized using raster files representing temperature, precipitation, and evapotranspiration values characteristic of the study region. Temperature and precipitation data were taken from 2010–2018 records provided by WorldClim, based on CRU-TS 4.06 [
33] and downscaled using WorldClim 2.1 [
34], while for evapotranspiration, we used the global map of monthly reference evapotranspiration at approximately 1 km resolution developed by [
35]. The soil entity attributes, including pH values, were derived from maps provided by the Bureau of Soils and Water Management [
36], while hydrologic soil group classifications were based on data from [
37]. Finally, the road and plot entities were constructed from vector files extracted from topographic and elevation raster data of the case study site provided by UPLB LGMO; these datasets are not publicly available.
Building on this initialization, the tree entities were generated based on field inventory data and additional modeling assumptions. The individual attributes of the tree agents were derived from a 2019 tree inventory conducted in the northern part of the UP LLG site. In the absence of a dedicated inventory for the ITP section and due to the lack of publicly available, nationally aggregated data on tree densities in logged secondary forests in the Philippines, we based the initial population estimate on the modeling parameter used by [
38], whose work focused on simulating tree growth and carbon accumulation in the Philippine context using an assumed density of 400 trees per hectare. The original inventory contained 2565 trees, and to approximate the density across the entire ITP area (1411 hectares), we replicated this dataset 50 times, yielding a total of 128,250 trees—roughly one-quarter of the estimated tree density per hectare—and randomly positioned them within the ITP area, as illustrated in
Figure 5. This relatively low figure reflects the logged condition of the area and the presence of mixed land uses, such as agroforestry, roads, and zones with intermittent human habitation.
Overall, the initial model environment consists of a landscape comprising 1369 parcels—excluding those intersected by rivers—each representing a 1-hectare plot of land (see
Figure 6). Across this landscape, 127,652 tree stands are randomly distributed, with some parcels also containing roads designated for timber transport.
The model also includes social agents: the market, labor agents, the university agent, investor agents, and special police agents. Each of them was assigned a random location outside the environment during initialization, and the number of labor, investor, and special police agents was determined by parameter values associated with each agent type.
Table 2 lists the parameter variables along with their descriptions and their respective recommended value ranges.
Submodels
To operationalize interactions within the ITP system, LUNTIAN integrates five interdependent submodels. Their design was informed by a combination of field observations, stakeholder input, and the relevant literature to ensure scientific validity and contextual relevance. Together, these simulate the ecological, operational, economic, labor, and governance processes introduced in the Process Overview and Scheduling Section. The following provides a detailed description of each submodel.
Stand Growth Submodel: It manages tree development by simulating the physical growth of tree agents, their fruit production, and mortality at each simulation step. This submodel is based on the framework for single-tree models proposed by [
41]. The growth process involves three core components: (1) diameter increment using species-specific growth equations; (2) mortality prediction based on probabilistic assessments; and (3) recruitment, which captures processes of maturation, fruiting, and seedling survival. Tree life stages are defined by diameter at breast height (DBH), progressing from seedling to sapling, then pole, and finally adult. The equations used in this submodel were based on the established literature (see
Appendix A) and thus had already been calibrated.
Forest Operation Submodel: It introduces human-driven activities such as harvesting, planting, and nursery management. The harvesting and planting routines are adapted from the GAMA Forest Model described in [
42], incorporating techniques such as selective logging [
43] and enrichment planting [
44], which are executed only in plots under active investment agreements. Harvesting takes place twice—at the beginning and end of the investment cycle—while enrichment planting is triggered when either basal area or tree density falls below a defined threshold. During harvests, the university agent consults the market agent to determine roundwood prices (in board feet), after which profits are computed and allocated between the university and investors based on a tunable profit-sharing ratio.
Nursery management is tightly integrated with planting and harvesting to support the sustainability of enrichment efforts. This process, led by nursery laborers, begins with collecting seedlings from the ITP area and cultivating them in designated nursery plots; once mature, saplings—prioritizing native species—are transferred to investment plots, and if the supply is insufficient, the university supplements it by purchasing additional seedlings to meet planting targets. This system ensures a steady pipeline of native tree species, reinforcing the plantation’s long-term regenerative capacity.
Although specific silvicultural treatments such as pruning, thinning, and fertilization are not modeled as discrete actions, they are implicitly accounted for through labor and maintenance costs. These aggregated costs reflect typical plantation upkeep activities that enhance tree growth and productivity. However, the model considers only the revenues from final harvests when calculating economic returns, while intermediate income streams—such as those from pruned biomass—are excluded to maintain the focus on maximizing net returns from full harvest cycles.
Investment Process Submodel: It facilitates the evaluation and allocation of investment opportunities and is activated once a set number of nurseries are established to ensure that the ITP can adequately support incoming investment requests. Investable plots are then offered to potential investors categorized by risk preference: risk averse (investing with a 0.25 probability if projected gains are <50%), risk neutral (0.50 probability), and risk seeking (0.75 probability if gains are ≥ 50%), following [
45]. These probabilities determine the likelihood of transitioning from an “interested-passive” state—where investors are aware but uncommitted—to an active investment state. After completing an investment, each investor evaluates the outcome; if the result is deemed unsatisfactory, the investor may transition into a “not-interested passive” state, where the probability of the investor remaining disengaged (
) or returning to the “interested-passive” state (
) is governed by their risk profile.
Figure 7 illustrates these state transitions.
The design of the investment process submodel was guided by stakeholder input from a meeting with the Land Grant Management Office (LGMO), which played a key role in shaping the intended role of investors in the planned ITP implementation. Based on this meeting, the ITP is conceptualized as a program to engage potential investors—particularly university alumni—by inviting them to finance one-hectare parcels, guiding the representation of investor agents as key initiators of plantation activity.
Employment Dynamics Submodel: It governs the allocation of labor resources and captures transitions between formal employment and informal harvesting. This submodel operates in three phases, initial harvesting and planting, mid-cycle maintenance, and final harvesting and replanting, and community members and university workers form the labor pool.
To address labor shortfalls, the university issues open hiring calls whenever demand exceeds supply. Workers evaluate job opportunities by sensing the presence of other laborers and comparing expected wages with potential income from independent harvesting. If a community member remains unemployed for several months, they may eventually shift to informal harvesting—particularly in fringe parcels at the plantation’s border. This mechanism reflects field reports of labor competition observed at the study site and is contextualized by broader historical precedent. Ref. [
1] documents how as early as the 1970s, an estimated 200,000 upland families across the country were already engaged in shifting cultivation; forestry experts at the time expressed concern that providing more efficient harvesting tools might unintentionally expand unauthorized timber extraction. These insights support the submodel’s assumption that labor scarcity can trigger transitions from formal employment to informal, unsanctioned forest use.
The employment dynamics submodel, therefore, captures the evolving tension between cooperative labor systems and opportunistic resource use. The full progression of laborer engagement is illustrated in
Figure 8.
ITP Policing Submodel: It introduces a spatial enforcement mechanism to maintain order and discourage unauthorized activities—particularly illegal harvesting—within the plantation. Policing agents represent special police officers who patrol the ITP area, with only laborers and these agents being permitted to enter the plantation in the model; while real-world access could include external actors, such complexity is beyond the current model’s scope. Patrols are repositioned every three months to simulate unpredictability, and enforcement occurs when a community member enters a police agent’s observable range, triggering a “catch” event. This results in the laborer forfeiting any financial gain, becoming inactive, and accumulating a violation count. These penalties are designed to reinforce rule compliance and deter repeated infractions.
The design of this submodel was informed by a site visit in January 2023, during which the researchers were escorted by special police assigned to monitor the site. Informal interviews conducted during the visit revealed that their duties included regular patrolling, discouraging unauthorized activities, and intercepting informal harvesters; the officers also shared that patrol teams operate in multiple groups and rotate locations periodically—patterns directly reflected in the modeled agent behaviors, including spatial movement and group deployment.