Background and objectives: Understanding how plant species respond to environmental gradients is one of the most important topics in plant ecology and plays a fundamental role in explaining distribution patterns, defining ecological niches, and supporting conservation planning. Under current conditions, where climate change and environmental stresses increasingly affect the structure and dynamics of ecosystems, investigating species-environment relationships has gained particular importance. The genus Crataegus is one of the valuable components of the Hyrcanian forests and is of considerable ecological, conservation, and medicinal importance. However, limited information is available regarding the environmental tolerance ranges and ecological differences among its species and subspecies. Therefore, the aim of this study was to model and compare the response curves of Crataegus microphylla Koch and two subspecies, Crataegus pentagyna subsp. pentagyna Waldst. & Kit. and Crataegus pentagyna subsp. pseudomelanocarpa (Popov ex Pojark.) K.I. Chr., along bioclimatic, edaphic, and topographic gradients in the Hyrcanian forests using Huisman–Olff–Fresco (HOF) models.
Methodology: Distribution data for the studied taxa were compiled from authoritative floristic sources, including Flora Iranica, Flora of Iran, and herbarium specimens preserved in the Herbarium of Iran (TARI). After verifying the taxonomic identification of specimens, geographic coordinates were extracted and a species occurrence database was established. Environmental variables, including bioclimatic variables, soil characteristics, topographic indices, and solar radiation, were obtained from the WorldClim, SoilGrids, and EarthEnv databases. To reduce multicollinearity among predictor variables, correlation analysis was performed and highly correlated variables were excluded. Species and subspecies response curves along environmental gradients were then fitted using HOF models, and the best-fitting model was selected based on the Akaike Information Criterion (AIC). In addition, to determine the relative importance of environmental variables, ensemble modeling based on Generalized Linear Models (GLM), Generalized Additive Models (GAM), Random Forest (RF), and Gradient Boosting Machine (GBM) algorithms was applied.Results: The results indicated that climatic variables exerted a greater influence on the distribution of the studied taxa than soil and topographic factors. Among the examined variables, Temperature Annual Range (Bio07), Mean Temperature of Driest Quarter (Bio09), Precipitation Seasonality (Bio15), and Precipitation of Warmest Quarter (Bio18) were identified as the most important factors controlling species distribution. The response curves revealed significant differences in tolerance ranges and response patterns of the taxa along environmental gradients. C. pentagyna subsp. pentagyna exhibited a narrower tolerance range to climatic variables, whereas C. pentagyna subsp. pseudomelanocarpa demonstrated greater ecological flexibility. Likewise, C. microphylla showed broader tolerance ranges and more balanced responses across many temperature and precipitation gradients. These observed differences indicate ecological niche differentiation among the taxa and their varying capacities to adapt to changing environmental conditions.
Conclusion: The results of this study confirmed the effectiveness of HOF models in identifying non-linear species responses, determining ecological thresholds, and estimating optimal environmental ranges. The findings further demonstrated that the studied taxa differ considerably in their tolerance and sensitivity to climatic factors and, consequently, are unlikely to respond uniformly to future climate change. These results can contribute to predicting future distributional changes, identifying stable habitats, setting conservation priorities, managing genetic resources, and planning restoration projects in the Hyrcanian forests. Moreover, the findings provide a scientific basis for developing climate change adaptation strategies and promoting the sustainable management of forest ecosystems in northern Iran.