Amplified climate warming in high latitudes is expected to affect growing season timing of the vast boreal biome. It is unclear whether the presence of permafrost (perennially frozen ground) might have an influence on changes in growing season timing. This study examined how different environmental variables explained, either directly or indirectly, the variation in growing season timing of boreal forest stands with and without permafrost. We expected that environmental variables explaining the variation in growing season timing differed or had different explanatory power depending on permafrost presence or absence. The growing season was delineated from daily gross primary productivity (GPP) time series derived from 40 site-year data of net ecosystem carbon dioxide exchange measured with eddy covariance techniques over five black spruce (Picea mariana [Mill.])-dominated boreal forest stands in North America. In permafrost-free forest stands, a combination of start in canopy ‘green-up’ in spring and the timing of air and soil temperature increasing above freezing explained the start-of-season (SOS_GPP). Results from commonality analysis and structural equation modeling suggest that canopy ‘green-up’ and air temperature directly affected SOS_GPP in permafrost-free forest stands. In addition, soil temperature acted as mediator for an indirect effect of air temperature on SOS_GPP. In contrast, none of the environmental variables, or their combination, explained the variation in SOS_GPP in forest stands with permafrost. The explanatory power of environmental variables was more consistent regarding the end-of-season (EOS_GPP). In both, forest stands with and without permafrost, EOS_GPP was directly explained by mean soil water content in the fall and the first day of continuous snowpack formation. A better understanding how environmental variables control SOS_GPP and EOS_GPP in forest stands with and without permafrost will help to refine parameterizations of the boreal biome in Earth system models.

预计高纬度地区气候变暖的加剧将影响广阔的北方生物群落的生长季节时间。目前尚不清楚永久冻土（常年冻土）的存在是否会影响生长季节时间的变化。本研究考察了不同的环境变量如何直接或间接地解释有和没有永久冻土的北方森林林分生长季节时间的变化。我们预计，解释生长季节时间变化的环境变量会因永久冻土的存在与否而有所不同或具有不同的解释力。生长季节由每日总初级生产力 (GPP) 时间序列描绘，该时间序列源自 40 个站点年净生态系统二氧化碳交换数据，采用涡流协方差技术测量五棵黑云杉 (Picea mariana [Mill.]) 为主的北方森林矗立在北美。在无多年冻土的林分中，春季冠层“绿化”的开始以及气温和土壤温度升高到冰点以上的时间解释了季节的开始（SOS _GPP）。共性分析和结构方程模型的结果表明，树冠“绿化”和气温直接影响无多年冻土林分的SOS _{GPP 。}此外，土壤温度在气温对 SOS _GPP的间接影响中起中介作用。相比之下，没有一个环境变量或其组合可以解释 SOS _GPP的变化在森林中矗立着多年冻土。关于季末（EOS _GPP），环境变量的解释力更加一致。在有和没有永久冻土的森林林分中，EOS _GPP直接用秋季的平均土壤含水量和连续积雪形成的第一天来解释。更好地了解环境变量如何控制有和没有永久冻土的森林中的 SOS _GPP和 EOS _GPP将有助于改进地球系统模型中北方生物群落的参数化。