Abstract:

Climate warming in the Arctic, with surface temperatures frequently reaching record highs, has led to an increase in heat waves and extreme high-temperature events that have severely impacted the health of polar ecosystems. In Nordic Polar Regions, characterized by fragile ecological environments and low biodiversity, heatwave events may exceed the physiological thresholds of vegetation, causing plant mortality and, in the long term, leading to transformations in vegetation types. Owing to the significant influence of heatwaves on surface energy balance and their substantial contribution to climate change, changes in polar vegetation and the disturbances they may cause have become a new research focus. This study utilized temperature observation data from 77 stations and identified heat-wave events using the relative threshold method. Four indices-heat wave number (HWN), heat wave frequency (HWF), heat wave duration (HWD), and heat wave magnitude index (HWMI)—were used to quantify these events. Field observation data were selected to identify the occurrence, duration, intensity, and spatial distribution of heat waves in northern Europe. Additionally, the relative changes in gross primary productivity (GPP) across different land cover types under the influence of heatwaves were analyzed to further understand the spatial and temporal relationship between heatwave events and vegetation growth. The results indicated that the frequency, duration, and intensity of heatwaves in the Nordic region increased, and the frequency of heatwave events from 2000 to 2020 was significantly higher than that from 1980 to 2000. Compared to 1980, the duration and intensity of heatwaves at each station increased by 1.1 and 1.2 times, respectively, by 2020, while the average number of occurrences rose to 2.8 times. In 2020, 22 stations experienced more than 15 heat waves, with a maximum of 18. The cumulative number of high-temperature days tripled, reaching a maximum of 86 days, threatening the normal physiological state of vegetation and causing widespread productivity decline. Simultaneously, the impacts of high temperatures extend to other seasons, with the annual duration of heatwave events increasing by approximately four days per year. By 2020, the average heatwave duration at the stations was 3.4 times higher than that in 1980. Approximately 60% of the stations experienced an earlier onset of the first heatwave, with 30 stations recording heatwaves occurring earlier than early spring in 2020. At approximately 97.4% of the stations, the last heat wave ended later, extending from the summer to autumn. The spatial distribution of heat wave events showed a pattern of increasing intensity and frequency with decreasing latitude, with southern Finland having stations with the highest number, longest duration, and highest intensity of heatwaves. These stations also experienced the greatest declines in productivity. Following the heat waves, 80.5% of the stations showed a decrease in GPP, primarily in Finland. The analysis confirmed that the magnitude of the GPP decline increased with increasing heatwave intensity. Except for wetland and ice-covered areas, the decline in GPP was more pronounced in other land cover types. Mixed forests exhibited the greatest decline in GPP (10.9%). In contrast, wetland and snow/ice-covered areas showed slight increases in GPP (0.2% and 4.1%, respectively), likely because climate warming provides more sunlight and melting snow and ice, offering additional moisture to vegetation, thereby promoting growth to some extent. The effects of heatwaves on vegetation productivity varied across seasons. Most summer and autumn heatwave events caused a decline in GPP, with 72.5% of summer heatwaves and 95% of autumn heatwaves resulting in reductions. Conversely, spring heatwaves largely contributed to increased productivity, with 57.2% of spring heatwave events leading to GPP increases. These seasonal dynamics align with vegetation growth cycles; spring heatwaves coincide with the early growth phase, thus enhancing productivity, whereas autumn heatwaves coincide with the end of the growing season, causing substantial declines in productivity. The magnitude of GPP changes caused by heat waves also differed by season. The largest decreases in GPP due to heat waves occurred in the spring (56.7%), summer (73.3%), and autumn (86.4%). Notably, productivity losses from spring heatwaves were more easily restored to pre-heatwave levels, especially for heatwave events lasting less than five days. Approximately 50% of these recovery periods took no more than 16 days and 90.5% were completed within 30 days. In contrast, vegetation productivity recovered longer in summer and autumn. For 85.0% of summer heatwave-induced productivity losses, recovery required more than 60 days, a proportion that increased to 98.8% during autumn heatwaves. This indicates that summer heatwaves in the Nordic region are more extreme and cause more severe vegetation damage. Additionally, we found that 69.4% of normal and moderate heatwave events led to a decline in GPP, whereas 71.8% of severe and extreme heatwave events caused a GPP reduction. As the intensity of the heatwaves increased, the number of events causing GPP also decreased along with the magnitude of the decline. Furthermore, longer individual heatwave durations exhibited similar patterns of productivity decline. Our findings reveal the response of vegetation productivity to heatwave events in the Nordic region, and provide a scientific basis for understanding and mitigating extreme climate events, studying vegetation dynamics, and advancing conservation efforts.

Key words: northern Europe, polar vegetation, heat wave, vegetation productivity, temporal-spatial difference

摘要：

全球变暖背景下北极温度屡破纪录，热浪灾害持续发生，严重影响着极地生态系统的健康。热浪事件对当地植被的扰动及植被对其响应的特征规律的变化已成为新的研究热点。使用台站观测识别热浪发生和影响程度，通过分析热浪的特征参数与植被总初级生产力的时空特征关联，分类探讨了强度、持续时间、发生季节不同的热浪事件对植被生产力的影响。结果表明，北欧地区热浪发生频率、持续日数和强度均呈现上升的趋势，且2000－2020年各热浪指数增长速率明显大于1980－2000年，年度发生热浪最多达18次，最大热浪累积日数达86 d，对植被正常的生理状态造成了威胁，导致了其生产力普遍下降。同时，热浪影响不断向其他季节扩张，其年度发生期跨度平均每年延长约4 d。除湿地与冰雪覆盖区域外，热浪事件均导致不同土地类型的植被总初级生产力下降，下降的比例占热浪事件总数的80.5%，其中下降幅度最大的是混交林，GPP平均降低了10.9%。热浪对植被生产力的影响存在显著的季节差异，其中夏秋季热浪事件对植被GPP下降的影响占主导作用，且秋季下降幅度也最大，而春季热浪与植被萌发同步，并未影响植被生产力的增长趋势。

关键词: 北欧, 极地植被, 热浪, 植被生产力, 时空差异