1. Climate warming is expected to increase upslope shifts of alpine treelines globally. However, the ecological filters controlling tree recruitment in the alpine belt remain poorly quantified, limiting accurate predictions of treeline responses to climate change. 2. To fill this gap, we conducted a decade‑long, factorial field experiment at abrupt (Ordesa) and diffuse (Tessó) Pinus uncinata treelines located in the Spanish Pyrenees. Our experimental treatments manipulated seed addition, herbivory exclusion, shrub competition, and soil scarification. We monitored seedling presence and abundance annually and analyzed their interactions with climate variables, specifically growing-season temperature and snow depth.3. Seedling recruitment at alpine treelines was strongly filter-limited and varied between sites. Seed addition enhanced emergence at both treeline types, with a steeper, density-dependent response at the diffuse treeline. Herbivore exclusion (1-mm mesh) consistently increased densities across cohorts, underscoring herbivory as a critical biotic filter. Climatic and biotic factors interacted to shape establishment: at the abrupt treeline, warmer growing-season maxima and dense shrub cover suppressed recruitment, while snow depth exerted contrasting effects across cohorts, from protective to limiting. At the diffuse treeline, year-one seedlings peaked at intermediate growing-season maximum temperatures under low shrub cover, whereas older cohorts showed more variable responses, occasionally persisting under dense shrubs when cooler growing season and deeper snow provided facilitative microclimatic conditions. These patterns highlight that both propagule supply and the interplay of climatic stress and biotic interactions jointly determine early recruitment above treeline.4. Synthesis. Treeline advance under warming occurs only when multiple filters align—adequate seed supply, favourable microclimatic windows, moderated herbivory, and shrub effects that remain facilitative rather than competitive. Because these filters are context- and life-stage dependent, forecasts must move beyond climate envelopes to integrate fine-scale propagule dynamics, episodic heat/snow extremes, and density-dependent biotic interactions. Models coupling these processes with long-term observations will better predict forest–tundra change.

1. Climate warming is expected to increase upslope shifts of alpine treelines globally. However, the ecological filters controlling tree recruitment in the alpine belt remain poorly quantified, limiting accurate predictions of treeline responses to climate change. 2. To fill this gap, we conducted a decade‑long, factorial field experiment at abrupt (Ordesa) and diffuse (Tessó) Pinus uncinata treelines located in the Spanish Pyrenees. Our experimental treatments manipulated seed addition, herbivory exclusion, shrub competition, and soil scarification. We monitored seedling presence and abundance annually and analyzed their interactions with climate variables, specifically growing-season temperature and snow depth.3. Seedling recruitment at alpine treelines was strongly filter-limited and varied between sites. Seed addition enhanced emergence at both treeline types, with a steeper, density-dependent response at the diffuse treeline. Herbivore exclusion (1-mm mesh) consistently increased densities across cohorts, underscoring herbivory as a critical biotic filter. Climatic and biotic factors interacted to shape establishment: at the abrupt treeline, warmer growing-season maxima and dense shrub cover suppressed recruitment, while snow depth exerted contrasting effects across cohorts, from protective to limiting. At the diffuse treeline, year-one seedlings peaked at intermediate growing-season maximum temperatures under low shrub cover, whereas older cohorts showed more variable responses, occasionally persisting under dense shrubs when cooler growing season and deeper snow provided facilitative microclimatic conditions. These patterns highlight that both propagule supply and the interplay of climatic stress and biotic interactions jointly determine early recruitment above treeline.4. Synthesis. Treeline advance under warming occurs only when multiple filters align—adequate seed supply, favourable microclimatic windows, moderated herbivory, and shrub effects that remain facilitative rather than competitive. Because these filters are context- and life-stage dependent, forecasts must move beyond climate envelopes to integrate fine-scale propagule dynamics, episodic heat/snow extremes, and density-dependent biotic interactions. Models coupling these processes with long-term observations will better predict forest–tundra change.

This systematic study investigates the flow, heat transfer, and thermal cracking behavior of n-undecane fuel under supercritical pressures, emphasizing the impact of pressure on the dynamics of fuel heat transfer and the kinetics of thermal cracking. Integrating insights from supercritical fluid research – such as the role of local specific heat capacity in heat transfer and pressure effects on microchannel flow – experiments quantify heat absorption, cracking conversion, and product distribution at 2.5–4.5 MPa. Results show that pressure negligibly affects physical heat absorption but modulates chemical heat absorption via pressure-dependent reaction pathways. At fixed conditions (outlet temperature: 650°C; flow rate: 40 mL/min), increasing pressure from 2.5 to 4.5 MPa enhances cracking conversion by 2.55% and gas production by 2.33%. Elevated pressure (3.5→4.5 MPa) promotes the formation of higher-molecular-weight liquid fractions and alkane-dominated gases, suppresses cracking acceleration, and enhances radical reactions (e.g. hydrogen transfer, cyclization). At 620°C, increasing the pressure to 4.5 MPa enhances the heat release of gaseous products, which is attributed to higher gas yields. These findings establish pressure as a critical parameter for optimizing the thermochemical balance between heat transfer and cracking in hydrocarbon fuel systems.

This systematic study investigates the flow, heat transfer, and thermal cracking behavior of n-undecane fuel under supercritical pressures, emphasizing the impact of pressure on the dynamics of fuel heat transfer and the kinetics of thermal cracking. Integrating insights from supercritical fluid research – such as the role of local specific heat capacity in heat transfer and pressure effects on microchannel flow – experiments quantify heat absorption, cracking conversion, and product distribution at 2.5–4.5 MPa. Results show that pressure negligibly affects physical heat absorption but modulates chemical heat absorption via pressure-dependent reaction pathways. At fixed conditions (outlet temperature: 650°C; flow rate: 40 mL/min), increasing pressure from 2.5 to 4.5 MPa enhances cracking conversion by 2.55% and gas production by 2.33%. Elevated pressure (3.5→4.5 MPa) promotes the formation of higher-molecular-weight liquid fractions and alkane-dominated gases, suppresses cracking acceleration, and enhances radical reactions (e.g. hydrogen transfer, cyclization). At 620°C, increasing the pressure to 4.5 MPa enhances the heat release of gaseous products, which is attributed to higher gas yields. These findings establish pressure as a critical parameter for optimizing the thermochemical balance between heat transfer and cracking in hydrocarbon fuel systems.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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