…Zheng et al. introduce the cold tolerance index (CTI), which projects the thermal conditions of a treeline onto the species' thermal niche breadth to quantify how close a treeline is to the lower limits of a species' realized thermal niche. In their study, Zheng et al. introduce the cold tolerance index (CTI), which projects the thermal conditions of a treeline onto the species' thermal niche breadth to quantify how close a treeline is to the lower limits of a species' realized thermal niche. This index measures the relative distance between the growing season temperature at the trees' upper boundary (e.g. the alpine treeline) and the cold limit of tree species' thermal niche (Fig. 1a). A lower CTI indicates that a population is living at the limit of its cold tolerance, whereas a higher CTI suggests that it is less constrained by temperature. Zheng et al. found that the average CTI across 66 treeline shift records from nine tree species was 0.21, indicating that treelines are typically located in the lower half of the species' thermal niche. This result is similar to the findings of Xie et al. (2025), which indicate that treelines typically occur at relative positions where heat conditions are c. 35% below the genus-level thermal optima within the ecological niche range. The CTI framework moves beyond traditional metrics, which often rely solely on temperature change, by adding a dimension of sensitivity based on species-specific ecological thresholds. Zheng et al. hypothesize that range boundaries situated near the cold limit of a species' thermal niche are likely to respond more rapidly to climatic warming (Fig. 1b). This is confirmed by the strong negative exponential relationship between the treeline migration rate and CTI. The results suggest that species whose range edges are near their cold tolerance limits are more sensitive to temperature changes and therefore more likely to exhibit faster upslope shifts (Fig. 1b). Conversely, species with more buffered thermal limits may not respond as rapidly, even when temperatures rise at a similar magnitude. This discovery challenges the conventional view that warming linearly shifts treelines upward. The CTI framework represents a significant advancement in applying niche theory to the study of species' range shifts. Traditionally, ecological boundaries have been viewed through the lens of environmental tolerance, with temperature being considered the primary factor driving range limits (Körner, 2021). By demonstrating that the CTI accounts for c. 48% of the variation in treeline shift rates, a figure more than double the 21% explained by local temperature changes alone, Zheng et al. highlight that populations experiencing stronger cold stress are more likely to respond rapidly to warming. This finding shifts the paradigm from viewing temperature as the sole driver of range shifts to understanding that the position of a species within its thermal niche plays a crucial role in its response to climate change. Meanwhile, this confirms that warming does not affect all range boundaries equally, and the asynchronous shift of treeline and isotherm is theoretically supported (Zheng et al.). Populations at the cold edge of their thermal niche are more likely to exhibit rapid upward movement in response to warming, as this alleviates a primary physiological constraint. In such cases, the release from cold stress leads to faster growth, improved recruitment, and greater potential for range expansion. Furthermore, the closer a community is to the thermal niche edge, the more fragile its steady state becomes, as it moves further from the optimal niche. Climate change is more likely to disrupt the balance (Angert et al., 2020); even small temperature changes may trigger rapid shifts and treeline fluctuations. However, the rate of treeline shift can be much slower for populations at warmer conditions or those constrained by nonthermal factors, such as moisture availability or competition (Liang et al., 2016; Xie et al., 2024). This variability suggests that the concept of niche breadth is a critical determinant of how species respond to climate warming. It is not enough to simply know the temperature change; one must also understand the ecological constraints acting on a population. A larger CTI indicates that the geographical boundary marked by an observed treeline may not necessarily represent the boundary of the thermal niche. Treeline populations at high latitudes or altitudes may face additional limits on establishment due to soil moisture availability, competition with other species, or nonnatural factors such as human disturbances (Lu et al., 2025). This insight adds a layer of complexity to the simple model of warming-induced treeline shift and underscores the importance of considering multiple ecological axes in range shift predictions. Zheng et al.'s work has profound implications for both basic ecological science and conservation. The CTI framework provides a more mechanistic and predictive approach for understanding species' treeline shifts under climate change. By incorporating the sensitivity of populations at their thermal edges, CTI allows for more accurate, process-based forecasts, moving beyond simple correlative models. This enables ecologists to predict how different populations within a species' range might respond to warming. For example, populations with a smaller CTI will likely experience faster shifts than those at warmer conditions (Fig. 1a,b). Such predictions are crucial for conservation, as they allow for more targeted interventions. The CTI framework also enhances our understanding of species' vulnerability to climate change. Not all range-edge populations are equally vulnerable. Cold-edge populations, which are more sensitive to warming, may act as early indicators of environmental change, potentially driving shifts in ecosystems (Hargreaves & Eckert, 2019). However, other populations, already living beyond their thermal optimum but still located at the geographic edge of the species' distribution, are more likely to be constrained by changes in factors beyond temperature, such as precipitation, nutrient cycles, or biotic interactions. By identifying these vulnerable populations, conservation efforts can be better prioritized. Integrating CTI with climate models can help identify biogeographic hotspots of sensitivity, guiding the design of more effective conservation strategies, such as the establishment of protected areas, migration corridors, or assisted migration initiatives. The CTI framework also has the potential for broader application beyond alpine ecosystems. Future studies could test the applicability of CTI to other life forms, such as shrubs, herbaceous plants, and animals, and explore its relevance for other types of range boundaries, including latitudinal shifts. This would allow researchers to apply the CTI across a wide range of ecosystems, improving predictions of species' responses to climate change at multiple scales. Moreover, while Zheng et al. used the realized niche based on occurrence records, future research could refine the CTI by incorporating direct physiological measurements of cold tolerance or modeling the fundamental niche. This would help clarify whether populations with a high CTI are genuinely not cold-limited or whether they are simply failing to occupy their full thermal potential due to factors such as dispersal limitations or biotic interactions, as highlighted by Laughlin & McGill (2024). These future improvements could further enhance the robustness of the CTI and its applicability to different species and ecological contexts, allowing for better predictions of species' range shifts under climate change. In conclusion, the work by Zheng et al. represents a major advancement in understanding the drivers of species' range shifts under climate change. By integrating the CTI into the study of treeline dynamics, they provide a more nuanced, process-based approach to predicting range shifts, one that considers the sensitivity of populations at their thermal edges. Their findings not only challenge traditional views of treeline movement but also offer new insights for conservation planning, species vulnerability assessments, and the future study of ecological niches. Future research should build on this framework by incorporating additional ecological factors, expanding its application across species and regions, and refining the CTI through physiological measurements. Ultimately, this approach promises to improve our ability to predict and manage the ecological impacts of climate change. WW thanks the National Natural Science Foundation of China (42330508 and 42571078) for the funding support. The New Phytologist Foundation remains neutral with regard to jurisdictional claims in maps and in any institutional affiliations.
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