Human-induced global warming has led to extreme weather events and unprecedented climate records worldwide, resulting in the emergence of what has been described as a new global climate domain (Harvey et al., 2023). Changes in temperature and precipitation patterns and the increased frequency of extreme weather events are expected to impact ecosystem functioning in multiple ways. Species with greater adaptability are likely to cope more effectively with these changing conditions. In this respect, the brown bear (Ursus arctos) is very mobile and has a broad trophic niche (Bojarska and Selva, 2012; Martin et al., 2012). This dietary flexibility may enhance its capacity to adapt to the consequences of climate change (Navarro et al., 2021). Nevertheless, during the hyperphagia period (late summer and autumn, before hibernation) brown bears are particularly sensitive to food availability, as they must accumulate sufficient fat reserves to support reproduction in the case of adult females (Costello et al., 2003; Di Domenico et al., 2012; López-Alfaro et al., 2013) and ensure survival in winter season (Ryan et al., 2007; Schwartz et al., 2006). During this period, bears across central-southern Europe and the Middle East feed on different nut-producing species such as beech (Fagus sylvatica), various oak species (Quercus sp.) and sweet chestnut (Castanea sativa) (e.g., Ambarlı, 2016; Bojarska and Selva, 2012; Ciucci et al., 2014; De Angelis et al., 2021; Naves et al., 2006; Paralikidis et al., 2010; Rodríguez et al., 2007), some of which are threatened by the effects of climate change (del Río et al., 2021; Dyderski et al., 2025, 2017; Kasper et al., 2022; López-Tirado et al., 2018; Rubio-Cuadrado et al., 2018; Urli et al., 2015).

Climate change severely impacts forest ecosystems, as prolonged droughts, heatwaves and heavy rainfall compromise primary productivity (Nussbaumer et al., 2018), a key factor in the fruiting success of nut-producing species (Journé et al., 2021). Climate change alters the phenology of nut-producing trees, affecting the timing of phenological events (e.g., flowering and fruiting); it lengthens the growing season, which favors increased productivity but also makes irregular yields more likely due to extreme weather events. Thus, phenology has been suggested to be a key factor in determining future species distribution, as climate-driven shifts in timing can lead to decline or expansion of suitable habitats for many plant species (Piao et al., 2019; Vitasse et al., 2011). Several nut-producing species, such as beech and oaks, show remarkable interannual variability in fruit production inherent to the species, although mast years (Gea-Izquierdo et al., 2006; Nussbaumer et al., 2018, 2016) may also be associated with the effects of climate change (Hacket-Pain and Bogdziewicz, 2021). For example, Clevenger et al. (1992) observed fruiting failure in Quercus spp. and F. sylvatica in northern Spain for up to 4 consecutive years, in some cases overlapping among species in the same year.

Bears relying on nut-producing trees for food during hyperphagia have to adapt to the aforementioned changes, such as in the case of bears in the Cantabrian Mountains (northern Spain) (Lucas et al., 2025; Navarro et al., 2021; Pérez-Girón et al., 2022). Here, the long-term climate-related changes in the availability of food resources, such as a decrease in the availability of Vaccinium fruits, have already been linked to an increase in the consumption of nuts by Cantabrian bears (Rodríguez et al., 2007). The non-territorial character of bears (Swenson et al., 2023) allows them to increase their use of space or to carry out long-distance movements searching for food resources across the landscape (De Angelis et al., 2021). Indeed, bears can aggregate, sometimes in remarkable numbers, at clustered food sources during hyperphagia (Ballesteros et al., 2018; Ciucci et al., 2014). For example, during the 2017 hyperphagia period, Ballesteros et al. (2018) reported exceptional bear aggregations linked to a large variation in the availability of nuts; at least 31 different bears were feeding on oak acorns in an area of ca. 3 km² (with the main concentration observed in about 50 ha) where good acorn production occurred for orographic and microclimatic reasons, in a year when there was generalized fruiting failure in nut-producing species caused by heavy frosts in spring throughout most of the bear range. The dependency on nut-producing trees could become even greater, as warming conditions may lead to a further reduction in the hibernation period in addition to that already observed (e.g., Pigeon et al., 2016; Planella et al., 2019), increasing energy demands and intensifying reliance on stable, year-round food sources. Additionally, if these resources become scarce, bears may increasingly turn to anthropogenic food sources, potentially intensifying human-wildlife conflicts (Kurth et al., 2024).

Under the current scenario of climate change, the distribution and productivity of species such as thermophilic oaks (Q. faginea, Q. ilex, and Q. suber) and sweet chestnut may increase (Benito Garzón et al., 2008; Pérez-Girón et al., 2020) as a result of a better tolerance, adaptation and competitive capacity of these tree species, compared to beech or Atlantic oaks (Duque-Lazo et al., 2018; Dyderski et al., 2025; Urli et al., 2015). Although a shift in dominant species may ultimately occur, hybridization among oak species (Sánchez De Dios et al., 2006) and the gradual development of mixed stands are also likely. Mixed stands are up to 30% more productive in biomass than pure stands (Pretzsch et al., 2013), thus potentially increasing available resources to bears. Dietary plasticity is expected to help bears to cope with the effect of interannual variability in nut production among tree species (Ambarlı, 2016; Bojarska and Selva, 2012; Ciucci et al., 2014; De Angelis et al., 2021; Naves et al., 2006; Paralikidis et al., 2010; Rodríguez et al., 2007). However, as some species are more constant in producing nuts (e.g., sweet chestnut; Petit and Larue, 2022), the potential impact of climate change on the distribution of nut-producing species could generate new scenarios of food availability and shortage for bears during hyperphagia (Navarro et al., 2021). In such scenarios, spatial complementarity among nut-producing species may be essential to ensure a minimum availability of food to bears at a landscape level. Spatial complementarity in our case is understood as the spatial and temporal availability of multiple nut-producing species, which can collectively support the dietary needs of brown bears during hyperphagia. However, to date, the different responses of these nut-producing species and their complementary provision of food for the brown bear under different climatic scenarios has not been evaluated.

Assessing the potential effects of climate change on nut-producing species during hyperphagia is key to understand the adaptation of Cantabrian brown bears to changing environmental conditions, and its potential impacts on its recovery (Lucas et al., 2025). However, despite growing evidence of climate-driven changes in forest composition, little is known about how these shifts may affect the availability and spatial distribution of key trophic resources for the Cantabrian brown bear during critical stages such as hyperphagia. In particular, the spatial complementarity of nut-producing species, where the presence of multiple species may offset local deficits, remains understudied. The Cantabrian Mountains offer a particularly suitable setting for investigating these dynamics, given the region’s ecological complexity and the heightened sensitivity of mountain ecosystems to climate change. Projected climate change impacts are expected to drive rapid ecological shifts in these habitats, including a significant decline in suitable habitat and upward shifts in ecological niches (Dyderski et al., 2025, 2017; Lenoir et al., 2008; Pérez-Girón et al., 2024).

In this context, the objectives of this study are: (i) to evaluate the impact of climate change on nut-producing species that are important to the diet of the brown bear (Ursus arctos) during the hyperphagia period; (ii) to analyze functional complementarity among species through species richness; and (iii) to identify vulnerable areas to better understand the challenges faced by the brown bear in adapting to changing environmental conditions. Additionally, conservation strategies are proposed to safeguard key trophic resources critical for the survival and well-being of the brown bear. Together, this work aims to provide a comprehensive understanding of the interactions between climate change, food resource availability, and the ecological requirements of the brown bear during hyperphagia, ultimately contributing to its long-term conservation under increasing environmental pressure.

Understanding the potential impacts of climate change on nut-producing species distributions requires acknowledging the limitations inherent to modeling approaches used in this study. Our SDMs assume static ecological niches over time, thereby not accounting for potential adaptive responses, or ecological plasticity of tree species under changing climatic conditions. Additionally, we did not incorporate dynamic vegetation feedbacks, such as species interactions, interspecific competition, ecological succession, or natural disturbances (e.g., wildfires, pest), all of which can significantly alter community composition and influence future species assemblages (Hao et al., 2025; Harvey et al., 2023; Wisz et al., 2013). Our models also do not consider species dispersal limitations, potentially overestimating the availability of suitable habitat in regions that may not be colonized within the projected timeframe. Finally, projections are subject to uncertainties inherent in climate models (GCMs) and emission scenarios (SSPs), which, although we have attempted to minimize through median ensemble approaches, may still influence the robustness of future predictions. For these reasons, our results should be interpreted as plausible trajectories under a specific set of assumptions, rather than precise forecasts, and are best used to inform adaptive and precautionary conservation strategies under future climate uncertainty.

The projected changes in habitat suitability for the main nut-producing species in the Cantabrian Mountains, influenced by climate change, may have implications for forest composition under different scenarios (SSP3-7.0 and SSP5-8.5) and for the availability of food for bears. Our findings, which are consistent with those of Benito Garzón et al. (2008); del Río et al. (2021); Dyderski et al. (2025, 2017) and López-Tirado et al. (2018), suggest that the Atlantic species located at the margins of their distribution, such as F. sylvatica, Atlantic oaks (Q. petraea) and C. avellana, are projected to experience substantial shrinkage of their area of distribution as a shift in the ecological niche shifts to higher elevations or more northerly latitudes is not feasible (Lenoir et al., 2008). As an aggravating factor, some of these species may be considered more resistant or tolerant than others to certain climate factors such as drought. For instance, Q. petraea would be better able to tolerate drier conditions than F. sylvatica (Kunz et al., 2018), possibly indicating a loss of competitiveness of the latter (Kasper et al., 2022; Rubio-Cuadrado et al., 2018). By contrast, the suitable habitat for thermophilic and Mediterranean nut-producing species, such as C. sativa or Q. ilex, is expected to undergo expansion in the Cantabrian Mountains (Álvarez-Álvarez et al., 2025; Pérez-Girón et al., 2020). These species may be more tolerant, adaptable and competitive under changing climatic conditions. Consequently, they are likely benefit from the decline in Atlantic species (Duque-Lazo et al., 2018; Dyderski et al., 2025, 2017; Urli et al., 2015). Consequently, a gradual shift in forest composition toward thermophilic and Mediterranean species is expected, accompanied by a reduction in the richness of nut-producing species, decreasing the diversity of nut resources for bears over time, which would have only two species (or less) available across wide areas within the estimated bear range. The effects of climate change are expected to occur gradually, leading species to first reduce or cease fruiting and eventually enter a survival phase, ultimately resulting in their replacement by other species.

These changes may, to some extent, reflect the scenario that bears have historically encountered in most of peninsular Spain, where Mediterranean acorns probably constituted a primary food source (Nores and Naves, 1993; Viana et al., 2022). Indeed, holm oaks are already present in many parts of the Cantabrian Mountains, and bears already feed on their fruits in their current range (Navarro et al., 2021). Both, Atlantic and Mediterranean oaks show a high variability in nut production both between individuals and within individuals between years, as well as synchronous nut production within species, even in mast years (Gea-Izquierdo et al., 2006; Nussbaumer et al., 2018, 2016), which could increase the risk of nutritional stress for bears relying solely on acorns in years of production failure (Ciucci et al., 2014). Such risks have been highlighted by observations of exceptional bear aggregations linked to a high level of variation in food availability, as reported by Ballesteros et al. (2018). The primary concern arising from these limitations lies in their implications for reproduction: female bears must accumulate substantial fat reserves, estimated at a minimum of 19% body fat, to support successful female bear reproduction (López-Alfaro et al., 2013; Welch et al., 1997). Additionally, although the approach of Cantabrian brown bears to humanized areas has been primarily attributed to intraspecific competition rather than a direct shortage of food (Blanco et al., 2021), it cannot be ruled out that increasing scarcity and unpredictability of food sources may intensify such competition. This, in turn, could lead to a greater reliance on anthropogenic food sources, thereby elevating the risk of human–wildlife conflict (Kurth et al., 2024).

By contrast, asynchronous nut production across different oak species increases the likelihood that at least one species is producing at any given time, helping to buffer interannual fluctuations in food availability (Gea-Izquierdo et al., 2006). This temporal complementarity, combined with the higher overall productivity typically observed in mixed-species stands (Pretzsch et al., 2013), may help moderate the nutritional concerns associated with mast failure years by smoothing fruiting variability across the landscape. In this regard, the projected stability or slight increase in the habitat suitability of Q. pyrenaica could favor the formation of such mixed stands. This fact not only contributes to structural forest diversity but also provides potential refuge areas for bears, especially in landscapes where acorn production from Mediterranean oaks is expected to be low and scattered (Quintano et al., 2016). Furthermore, brown bears have been shown to preferentially use large and diverse highly aggregated mixed deciduous forest stands to feed on acorns (Pérez-Girón et al., 2022), and the presence of mixed stands could therefore reduce the risk of food shortages.

Sweet chestnut emerge as an important food source for bears during the hyperphagia and winter time, when other nuts are scarce (Navarro et al., 2021; Pérez-Girón et al., 2022; Rodríguez et al., 2007). Our results suggest that the habitat suitable of sweet chestnut may increase toward areas in the northwest and southwest of the Cantabrian Mountains, coinciding with the current bear range and with a large part of the projected expansion areas. Chestnuts have the advantage of relatively consistent fruit production (Petit and Larue, 2022) compensating for variations in other nut-producing trees during the hyperphagia while possibly benefiting from climate change (Álvarez-Álvarez et al., 2025; Pérez-Girón et al., 2020). However, chestnut stands are currently strongly linked to human settlements, which influences bear foraging behavior: for instance, bears tend to spend significantly less time foraging in chestnut orchards than in other forest ecosystems, presumably due to higher human activities in chestnut stands (Pérez-Girón et al., 2022). A wider availability of chestnut trees apart from human settlements may favor the accessibility for bears to sweet chestnuts, especially during years when other food sources, such as acorns and beech nuts, are less abundant.

From a conservation standpoint, the high mobility and the wide trophic niche of brown bears (Ambarlı, 2016; Ambarlı and Bilgin, 2012; Bojarska and Selva, 2012; Ciucci et al., 2014; De Angelis et al., 2021; Gavrilov et al., 2015; Huber and Roth, 1993; Lucas et al., 2025; Martin et al., 2012; Mertzanis et al., 2005; Naves et al., 2006; Paralikidis et al., 2010; Rodríguez et al., 2007) are expected to help bears to cope with food shortages. However, special attention should be paid to areas characterized by high species richness, as reflected in Figure 4, because they constitute biodiversity hotspots that require specific sustainable management and conservation efforts (Ramachandra et al., 2018). The establishment of chestnut trees in areas of both low and high species richness areas may enhance available nut production during hyperphagia. In addition, for Pyrenean oak trees, for which habitat suitability is projected to be stable or showing a slight increase, actions to enhance fruiting and thereby improve food availability could benefit bears. For example, silvicultural management that increases light availability by reducing stand density, which would promote fruiting and thereby improve food availability across these ecosystems (Gea-Izquierdo et al., 2006; Martiník et al., 2013).

In this context, the recovery of the Cantabrian brown bear in Spain stands as a remarkable conservation success, but it also brings new challenges for coexistence with local communities. As bear populations expand and their range increases, the likelihood of encounters with humans, livestock, and agricultural activities inevitably grows (Bombieri et al., 2019). Effective management strategies must balance the conservation of this iconic species with the needs of rural communities. Restoring and managing forests, particularly those with chestnut and other native species, as part of ecological connectivity networks (Díaz-Varela et al., In Press) and area-based conservation measures (Díaz-Varela et al., 2018) can strategically reduce human-bear conflicts. Such habitat management serves as a proactive approach, complementing financial compensation for damages and public awareness campaigns (Bautista et al., 2019). Ultimately, long-term success will depend on fostering a culture of coexistence, where both bears and people thrive in shared landscapes. Sustainable chestnut forest management and ecosystem restoration could play a pivotal role in achieving this balance. This is especially important taking into account that the conservation status of chestnut forests (Habitat 9260) in Spain is currently assessed as “Unfavourable-Inadequate” in the Atlantic region and “Unfavourable-Bad” in both the Mediterranean and Alpine regions, highlighting the urgent need for targeted restoration and sustainable management efforts (European Environment Agency, 2023).

Our results suggest that climate change will lead to marked shifts in the distribution of nut-producing tree species in the Cantabrian Mountains, with important ecological implications for forest composition and brown bear conservation. Specifically, thermophilic and Mediterranean species such as C. sativa and Q. ilex are projected to expand, while Atlantic species such as F. sylvatica, C. avellana, and Q. petraea are likely to contract significantly due to their limited climatic tolerance and geographic constraints. These transitions could result in reduced nut species richness across large portions of the brown bear’s current and future range, potentially increasing the risk of nutritional stress and food shortages, particularly during years of mast failure.

Despite these risks, our projections also highlight opportunities for proactive management. For instance, promoting sweet chestnut plantations in areas of both low and high species richness, particularly on mid-elevation slopes with minimal human disturbance in the western regions, could enhance the availability of reliable nut resources during hyperphagia. Within ecological corridors and connectivity networks where chestnut populations are present, prioritizing the conservation, restoration, and promotion of chestnut forest is essential. Moreover, in regions where Q. pyrenaica is projected to persist or expand, implementing targeted silvicultural interventions, such as thinning to increase light availability, may boost acorn production. This would not only improve seasonal food supply but also help maintain structurally diverse forest habitats preferred by bears. Therefore, collectively, these measures have the potential to enhance food availability, mitigate human-bear conflicts, and contribute to reversing the unfavorable conservation status of Castanea habitats throughout Spain.

Finally, it is important to acknowledge that our predictions carry inherent uncertainty due to modeling assumptions and climate change projections. Therefore, they should be interpreted as plausible trajectories rather than deterministic forecasts. Further research aimed at refining species distribution models, incorporating potential adaptive responses, integrating dynamic vegetation feedbacks, and synchronizing bear presence with the phenology of nut-producing species, will be essential to improve the predictive capacity and ecological relevance of future scenarios.