Abstract：
Historical temperature records reveal that the boreal forest has been subjected to a significant lengthening of the thermal growing season since the middle of the last century, and climate models predict that this lengthening will continue in the future. Nevertheless, the potential phenological response of trees to changes in growing season length remains relatively undocumented, particularly for evergreen boreal tree species growing in cold environments. Here, we used the recently defined zero growth (ZG) concept to extract and characterize the metrics of seasonal radial growth dynamics for 12 balsam fir trees subjected to a 12-year soil warming experiment using high resolution radius dendrometer measurements. The ZG concept provides an accurate determination of growth seasonality (onset, cessation, duration, growth rates, and total growth) for these slow-growing trees characterized by significant shrinkage in tree diameter due to dehydration in the winter. Our analysis revealed that, on average, growth onset starts at day 152 ± 7 (±1 SE, 31 May-1 June) and ceases at day 244 ± 27 (31 August-1 September), for a growing season duration of about 3 months (93 ± 26 days) over a 12-year period. Growing season duration is mainly determined by growth cessation, while growth onset varies little between years. A large part (80%) of the total growth occurs in the first 50 days of the growing season. Given the dynamics of growth, early growth cessation (shorter growing season) results in a higher average seasonal growth rate, meaning that longer growing seasons are not necessarily associated with greater tree growth. Soil warming induces earlier growth cessation, but increases the mean tree growth rate by 18.1% and the total annual growth by 9.1%, on average, as compared to the control trees. Our results suggest that a higher soil temperature for warmed trees contributes to providing better growth conditions and higher growth rates in the early growing season, when the soil temperature is low and the soil water content is elevated because of snowmelt. Attaining a critical soil temperature earlier, coupled with lower soil water content, may have contributed to the earlier growth cessation and shorter growing season of warmed trees.
摘要：
历史温度记录显示，自上世纪中叶以来，北方森林的热生长季节已大大延长，和气候模型预测，这种延长将在未来继续。然而，树木对生长季节长度变化的潜在物候响应仍然相对没有记载，特别是在寒冷环境中生长的常绿北方树种。这里,我们使用最近定义的零增长（ZG）概念来提取和表征12种苦瓜杉树的季节性径向生长动态指标，这些指标经过12年的土壤增温实验，使用高分辨率半径树枝计测量。ZG概念提供了生长季节性的准确测定(发病，停止,持续时间,增长率，和总生长）对于这些生长缓慢的树木，其特征是由于冬季脱水而导致树木直径显着收缩。我们的分析显示,平均而言,生长开始于第152±7天(±1SE，5月31日至6月1日)，并在第244±27天(8月31日至9月1日)停止，在12年内，生长季节持续约3个月（93±26天）。生长季节持续时间主要由生长停止决定，而生长开始在几年之间变化不大。总生长的大部分(80%)发生在生长季节的前50天。鉴于增长的动力，早期停止生长(较短的生长季节)导致较高的平均季节性增长率，这意味着更长的生长季节不一定与更大的树木生长有关。土壤变暖导致更早的生长停止，但使平均树木生长率提高了18.1%，年总生长率提高了9.1%，平均而言,与对照树相比。我们的结果表明，温暖树木的较高土壤温度有助于在早期生长季节提供更好的生长条件和更高的生长速率。当土壤温度较低且由于融雪而使土壤含水量升高时。较早达到临界土壤温度，再加上较低的土壤含水量，可能导致温暖的树木更早的生长停止和更短的生长季节。
