Deciduous tree cover is expected to increase in North American boreal forests with climate warming and wildfire. This shift in composition has the potential to generate biophysical cooling via increased land surface albedo. Here we use Landsat-derived maps of continuous tree canopy cover and deciduous fractional composition to assess albedo change over recent decades. We find, on average, a small net decrease in deciduous fraction from 2000 to 2015 across boreal North America and from 1992 to 2015 across Canada, despite extensive fire disturbance that locally increased deciduous vegetation. We further find near-neutral net biophysical change in radiative forcing associated with albedo when aggregated across the domain. Thus, while there have been widespread changes in forest composition over the past several decades, the net changes in composition and associated post-fire radiative forcing have not induced systematic negative feedbacks to climate warming over the spatial and temporal scope of our study.

随着气候变暖和野火，北美北方森林的落叶树覆盖率预计将增加。这种成分的变化有可能通过增加地表反照率来产生生物物理冷却。在这里，我们使用陆地卫星衍生的连续树冠覆盖和落叶分数组成的地图来评估近几十年来的反照率变化。我们发现，平均而言，从 2000 年到 2015 年，北美北部地区以及从 1992 年到 2015 年，加拿大各地的落叶植物比例略有净减少，尽管大范围的火灾干扰导致局部落叶植被增加。我们进一步发现，当跨域聚合时，与反照率相关的辐射强迫的近中性净生物物理变化。因此，虽然过去几十年森林成分发生了广泛的变化，但成分的净变化和相关的火灾后辐射强迫并没有在我们研究的时空范围内对气候变暖产生系统性的负反馈。