• Energy Research

Climate warming is expected to enhance tree growth at alpine treelines. A higher growth rate is forecasted as temperatures rise and growth becomes less dependent on the temperature rise. Since radial growth is just one component of treeline dynamics those forecasts do not necessarily apply to treeline elevation or latitude; treelines can shift upward or poleward or remain stable.

Significance Forest trees can live for hundreds to thousands of years, and they play a critical role in mitigating global warming by fixing approximately 15% of anthropogenic CO 2 emissions annually by wood formation. However, the environmental factors triggering wood formation onset in springtime and the cellular mechanisms underlying this onset remain poorly understood, since wood forms beneath the bark and is difficult to monitor. We report that the onset of wood formation in Northern Hemisphere conifers is driven primarily by photoperiod and mean annual temperature. Understanding the unique relationships between exogenous factors and wood formation could aid in predicting how forest ecosystems respond and adapt to climate warming, while improving the assessment of long-term and high-resolution observations of global biogeochemical cycles.

Species with wide geographical ranges exhibit specific adaptations to local climates, which may result in diverging responses among populations to changing conditions. Climate change has advanced spring phenology worldwide, but questions of whether and how the phenological responses to warming differ among individuals across the natural range of a species remain. We conducted two experiments in January and April 2019, and performed daily observations of the timings of bud break in 1-year-old seedlings of sugar maple (Acer saccharum Marshall) from 25 Canadian provenances at two thermal conditions (14/10 and 18/14 °C day/night temperature) in a controlled environment. Overall, bud break started 6 days from the beginning of the experiments and finished after 125 days. The earlier events were observed in seedlings originating from the colder sites. Bud break was delayed by 4.8 days per additional degree Celsius in the mean annual temperature at the origin site. Warming advanced the timing of bud break by 17–27 days in January and by 3–8 days in April. Similar advancements in bud break were observed among provenances under warming conditions, which rejected our hypothesis that sugar maple populations have different phenological responses to warming. Our findings confirm the differentiation in ecotypes for the process of bud break in sugar maple. In cases of homogenous spring warming across the native range of sugar maple, similar advancements in bud phenology can be expected in different populations.

Chir pine (Pinus roxburghii Sarg.) is a common tree species with ecological and economic importance across the subtropical forests of the central Himalayas. However, little is known about its growth response to the recent warming and drying trends observed in this region. Here, we developed a 268-year-long ring-width chronology (1743–2010) from western Nepal to investigate its growth response to climate. Based on nearby available meteorological records, growth was positively correlated with winter (November to February; r = 0.39, p < 0.05) as well as March to April (r = 0.67, p < 0.001) precipitation. Growth also showed a strong positive correlation with the sum of precipitation from November of the previous year to April of the current year (r = 0.65, p < 0.001). In contrast, a negative relationship with the mean temperature in March to April (r = −0.48, p < 0.05) suggests the influence of warming-induced evapotranspiration on tree growth. Spring droughts lasting 4–6 months constrain Chir pine growth. These results are supported by the synchronization between droughts and very narrow or locally missing rings. Warming and drying tendencies during winter and spring will reduce forest growth and resilience and make Chir pine forests more vulnerable and at higher risk of growth decline and dieback.

Drought is the driver for ecosystem production in semi-arid areas. However, the response mechanism of ecosystem productivity to drought remains largely unknown. In particular, it is still unclear whether drought limits the production via photosynthetic capacity or phenological process. Herein, we assess the effects of maximum seasonal photosynthesis, growing season length, and climate on the annual gross primary productivity (GPP) in vegetation areas of the Loess Plateau using multi-source remote sensing and climate data from 2001 to 2021. We found that maximum seasonal photosynthesis rather than growing season length dominates annual GPP, with above 90 % of the study area showing significant and positive correlation. GPP and maximum seasonal photosynthesis were positively correlated with self-calibrating Palmer Drought Severity Index (scPDSI), standardized precipitation and evapotranspiration index (SPEI) in >95 % of the study area. Structural equation model demonstrated that both drought indices contributed to the annual GPP by promoting the maximum seasonal photosynthesis. Total annual precipitation had a positive and significant effect on two drought indices, whereas the effects of temperature and radiation were not significant. Evidence from wood formation data also confirmed that low precipitation inhibited long-term carbon sequestration by decreasing the maximum growth rate in forests. Our findings suggest that drought limits ecosystem carbon sequestration by inhibiting vegetation photosynthetic capacity rather than phenology, providing a support for assessing the future dynamics of the terrestrial carbon cycle and guiding landscape management in semi-arid ecosystems.

En tant qu'indicateur important de l'aridité atmosphérique, le déficit de pression de vapeur (DPV) fait l'objet d'une attention croissante dans les recherches sur le changement global au cours des dernières décennies. Cependant, les données proxy disponibles ayant la capacité de représenter la MPV sont encore rares dans le monde. Ici, nous avons démontré une indication « cachée » dépendante de la fréquence de l'isotope de l'oxygène de la cellulose de l'anneau des arbres (δ18OTRC) à l'aridité atmosphérique dans les montagnes occidentales de Kunlun, en Asie centrale. En appliquant l'analyse de corrélation et de corrélation partielle, des facteurs climatiques dépendant de la fréquence déclenchant les valeurs δ18OTRC ont été détectés. Pour les données brutes avec variabilité inter-décennale, les variations δ18OTRC ont principalement indiqué les précipitations de mai à août (Local : r = -0,59, p < 0,001 ; Source d'humidité : r = -0,53, p < 0,001), l'évapotranspiration (ET) (Local : r = -0,57, p < 0,001 ; Source d'humidité : r = -0,51, p < 0,001). Après un filtre passe-haut de 5 ans, la série δ18OTRC a révélé une corrélation très significative avec le DPV de mai-août (Local : r = 0,82, p < 0,001 ; Source d'humidité : r = 0,80, p < 0,001). La corrélation partielle a en outre confirmé les liens étroits avec le DPV à haute fréquence lorsque l'impact de toute variable éco-climatologique supplémentaire (précipitations, température, ET, humidité du sol 0–40 cm) a été exclu. En revanche, si l'effet de la MPV de mai à août était exclu, les relations entre δ18OTRC et d'autres variables devenaient insignifiantes. Collectivement, il a été conclu que δ18OTRC des montagnes occidentales de Kunlun peut indiquer un TPV à des échelles de temps < 5 ans. Nos résultats suggèrent que les significations climatiques dépendantes de la fréquence doivent être prises en compte lors de l'interprétation du δ18OTRC déterminé multi-processus comme un proxy climatique. Como un indicador importante de la aridez atmosférica, el déficit de presión de vapor (VPD) se está prestando cada vez más atención en las investigaciones de cambio global durante las últimas décadas. Sin embargo, los datos proxy disponibles con la capacidad de representar VPD todavía son escasos en el mundo. Aquí demostramos una indicación "oculta" dependiente de la frecuencia del isótopo de oxígeno de celulosa de los anillos de los árboles (δ18OTRC) a la aridez atmosférica en las montañas occidentales de Kunlun, Asia Central. Al aplicar la correlación y el análisis de correlación parcial, se detectaron impulsores climáticos dependientes de la frecuencia que activan los valores de δ18OTRC. Para los datos brutos con variabilidad interdecadal, las variaciones de δ18OTRC indicaron principalmente precipitación mayo-agosto (Local: r = –0.59, p < 0.001; Fuente de humedad: r = –0.53, p < 0.001), evapotranspiración (ET) (Local: r = –0.57, p < 0.001; Fuente de humedad: r = –0.51, p < 0.001). Después de un filtro de paso alto de 5 años, la serie δ18OTRC reveló una correlación altamente significativa con la VPD de mayo-agosto (Local: r = 0.82, p < 0.001; Fuente de humedad: r = 0.80, p < 0.001). La correlación parcial confirmó además los fuertes vínculos con la VPD de alta frecuencia cuando se excluyó el impacto de cualquier variable ecoclimatológica adicional (precipitación, temperatura, ET, humedad del suelo 0–40 cm). Por el contrario, si se excluyó el efecto de la VPD de mayo-agosto, las relaciones entre δ18OTRC y otras variables se volvieron insignificantes. En conjunto, se concluyó que δ18OTRC de las montañas occidentales de Kunlun puede indicar VPD en escalas de tiempo < 5 años. Nuestros resultados sugieren que las significaciones climáticas dependientes de la frecuencia deben considerarse al interpretar el δ18OTRC determinado por múltiples procesos como un indicador climático. As an important indicator of atmospheric aridity, the vapor pressure deficit (VPD) is being paid increasing attention in global change researches during recent decades. However, available proxy data with the capability to represent VPD is still scarce in the globe. Herein we demonstrated a "hidden" frequency-dependent indication of tree-ring cellulose oxygen isotope (δ18OTRC) to atmospheric aridity in the western Kunlun Mountains, Central Asia. In applying correlation and partial correlation analysis, frequency-dependent climatic drivers triggering the δ18OTRC values were detected. For the raw data with inter-decadal variability, δ18OTRC variations mainly indicated May–August precipitation (Local: r = –0.59, p < 0.001; Moisture source: r = –0.53, p < 0.001), evapotranspiration (ET) (Local: r = –0.57, p < 0.001; Moisture source: r = –0.51, p < 0.001). After a 5-year high-pass filter, the δ18OTRC series revealed highly significant correlation with May–August VPD (Local: r = 0.82, p < 0.001; Moisture source: r = 0.80, p < 0.001). The partial correlation further confirmed the strong linkages with high-frequency VPD when the impact of any additional eco-climatological variable (precipitation, temperature, ET, soil moisture 0–40 cm) was excluded. In contrast, if the effect of May–August VPD was excluded, relationships between δ18OTRC and other variables turned insignificant. Collectively, it was concluded that δ18OTRC from the western Kunlun Mountains may indicate VPD at timescales < 5 years. Our results suggest that frequency-dependent climatic significances should be considered when interpreting multi-process determined δ18OTRC as a climate-proxy. كمؤشر مهم للجفاف الجوي، يتم إيلاء اهتمام متزايد لعجز ضغط البخار (VPD) في أبحاث التغير العالمي خلال العقود الأخيرة. ومع ذلك، لا تزال البيانات البديلة المتاحة مع القدرة على تمثيل الأمراض التي يمكن الوقاية منها باللقاحات نادرة في العالم. هنا أظهرنا إشارة "خفية" تعتمد على التردد لنظير أكسجين السليلوز ذي الحلقة الشجرية (δ 18OTRC) إلى الجفاف الجوي في جبال كونلون الغربية، آسيا الوسطى. عند تطبيق تحليل الارتباط والارتباط الجزئي، تم الكشف عن المحركات المناخية المعتمدة على التردد التي تحفز قيم δ 18OTRC. بالنسبة للبيانات الأولية ذات التباين بين العقدين، أشارت اختلافات δ 18OTRC بشكل أساسي إلى هطول الأمطار في شهري مايو وأغسطس (محلي: r = –0.59، p < 0.001 ؛ مصدر الرطوبة: r = -0.53، p < 0.001)، التبخر والنتح (ET) (محلي: r = -0.57، p < 0.001 ؛ مصدر الرطوبة: r = –0.51، p < 0.001). بعد مرشح التمرير العالي لمدة 5 سنوات، كشفت سلسلة δ 18OTRC عن ارتباط كبير للغاية مع VPD من مايو إلى أغسطس (محلي: r = 0.82، p < 0.001 ؛ مصدر الرطوبة: r = 0.80، p < 0.001). كما أكد الارتباط الجزئي الروابط القوية مع VPD عالي التردد عند استبعاد تأثير أي متغير مناخي بيئي إضافي (هطول الأمطار، درجة الحرارة، ET، رطوبة التربة 0–40 سم). على النقيض من ذلك، إذا تم استبعاد تأثير اضطراب ما قبل الولادة في مايو وأغسطس، فإن العلاقات بين δ 18OTRC والمتغيرات الأخرى أصبحت ضئيلة. بشكل جماعي، تم استنتاج أن δ 18OTRC من جبال كونلون الغربية قد تشير إلى VPD في الجداول الزمنية < 5 سنوات. تشير نتائجنا إلى أنه يجب مراعاة الدلالات المناخية المعتمدة على التردد عند تفسير العمليات المتعددة المحددة δ 18OTRC على أنها وكيل مناخي.

AbstractMountain treelines are thought to be sensitive to climate change. However, how climate impacts mountain treelines is not yet fully understood as treelines may also be affected by other human activities. Here, we focus on “closed‐loop” mountain treelines (CLMT) that completely encircle a mountain and are less likely to have been influenced by human land‐use change. We detect a total length of ~916,425 km of CLMT across 243 mountain ranges globally and reveal a bimodal latitudinal distribution of treeline elevations with higher treeline elevations occurring at greater distances from the coast. Spatially, we find that temperature is the main climatic driver of treeline elevation in boreal and tropical regions, whereas precipitation drives CLMT position in temperate zones. Temporally, we show that 70% of CLMT have moved upward, with a mean shift rate of 1.2 m/year over the first decade of the 21st century. CLMT are shifting fastest in the tropics (mean of 3.1 m/year), but with greater variability. Our work provides a new mountain treeline database that isolates climate impacts from other anthropogenic pressures, and has important implications for biodiversity, natural resources, and ecosystem adaptation in a changing climate.