Permafrost melting due to climate warming in recent decades has produced significant effects on forest ecosystems, especially the boreal biome at its southernmost limit in Asia. How this warming affects wood formation of trees at intra-annual resolution is unclear, yet is crucial for assessing the impact of permafrost melting on boreal forest growth. In this study, we compared the radial growth and intra-annual wood density fluctuations (IADFs) of Dahurian larch ( Larix gmelinii Rupr.) at a permafrost (PF) and a non -permafrost (NPF) site at the southernmost permafrost limit in northeast China and quantified their relationships with climate factors. Drought in early summer was the main factor limiting growth of Dahurian larch. The basal area increment (BAI) of trees at both sites increased initially and then decreased in the 1980s, probably in response to warm -dry climate conditions. Earlywood IADFs (IADF-E) occurred in 14.0% and 9.3% of dated rings at the NPF and PF sites, while the frequency of latewood IADFs (IADF-L) was 6.8% and 2.7% at these two sites. The frequency of IADF-E in trees at both sites was positively and negatively related to June temperatures (and vapor pressure deficit) and precipitation, respectively, suggesting drought stress in June triggered the formation of IADF-E. The IADF-Ls were probably formed in response to warm temperatures in the late growing season. A higher BAI and a lower frequency of IADF-Es of trees at the PF site than at the NPF site indicated that permafrost melting could alleviate drought stress in early summer and promote radial growth of Dahurian larch. This greatly improved forest carbon sequestration and wood quality of some northeastern Asian boreal forests may offset to some extent the adverse effects of warming -drying climates at some sites of northeast Asia. Larch IADF-Es recorded extreme droughts in early summer, giving us a new sight for reconstructing high -frequency extreme climate events. If climate warming continues, the benefits of permafrost melting will gradually disappear and even turn negative due to warmer -dryer climate conditions. Our findings provide valuable information for boreal forest management and conservation under future global warming.

Boreal forests are facing profound changes in their growth environment, including warming-induced water deficits, extended growing seasons, accelerated snowmelt, and permafrost thaw. The influence of warming on trees varies regionally, but in most boreal forests studied to date, tree growth has been found to be negatively affected by increasing temperatures. Here, we used a network of Pinus sylvestris tree-ring collections spanning a wide climate gradient the southern end of the boreal forest in Asia to assess their response to climate change for the period 1958-2014. Contrary to findings in other boreal regions, we found that previously negative effects of temperature on tree growth turned positive in the northern portion of the study network after the onset of rapid warming. Trees in the drier portion did not show this reversal in their climatic response during the period of rapid warming. Abundant water availability during the growing season, particularly in the early to mid-growing season (May-July), is key to the reversal of tree sensitivity to climate. Advancement in the onset of growth appears to allow trees to take advantage of snowmelt water, such that tree growth increases with increasing temperatures during the rapidly warming period. The region's monsoonal climate delivers limited precipitation during the early growing season, and thus snowmelt likely covers the water deficit so trees are less stressed from the onset of earlier growth. Our results indicate that the growth response of P. sylvestris to increasing temperatures strongly related to increased early season water availability. Hence, boreal forests with sufficient water available during crucial parts of the growing season might be more able to withstand or even increase growth during periods of rising temperatures. We suspect that other regions of the boreal forest may be affected by similar dynamics.

Permafrost degradation due to rapid increase in ground surface temperature (GST) in recent years may strongly affect boreal forest ecosystems. However, comparatively few studies have explored how changes in permafrost affect tree growth dynamics in boreal forest. Here, we used a tree ring network of 12 Dahurian larch (Larix gmelinii) sites across permafrost regions in northeast China. We observed an increase in L. gmelinii growth over the past decade, seemingly linked to a shift in their climatic limitations, where winter GST has become the most strongly limiting factor for L. gmelinii growth. The recent increase in growth was particularly strong in older trees (>300years), which could be related to older trees having a more developed root system. GST was the main limiting factor for tree growth. While summer GST had a somewhat consistently positive correlation with tree growth, winter GST has shifted from a negative to a strongly positive correlation with growth in the last decade, coincidental with a sharp increase in winter GST since 2004. Winter GST is also strongly correlated with the rapidly thawing permafrost dynamics. Overall, our results suggest a link between recent changes in the permafrost and shifts in climate-growth correlations for one of the main boreal tree species. As a result, L. gmelinii has experienced an important increase in radial growth that may indicate that, unlike what has been reported for other boreal species, it may temporally benefit from warming climate in the continuous permafrost region of the Asian boreal forests. Plain Language Summary Dahurian larch (Larix gmelinii) is widely distributed in northeastern Asian boreal forests and the only tree species that can endure the extreme cold climate in the permafrost plains of Asian boreal forests. Permafrost regions have degraded greatly with the rapid warming in the recent decade, posing a serious threat to the survival of larch. We sampled a tree ring network of Dahurian larch from the southern edge of its natural distribution to detect how climate warming has influenced larch growth in the permafrost regions. Our results showed that soil temperature is the dominant climatic factor limiting tree growth. With increasing soil temperatures, larch showed a rapid increase in growth in the last 10years, especially pronounced in older trees. This unusual phenomenon was linked to a sharp increase in winter soil temperature, which determines the soil freezing depth of the permafrost. We also found that the larch grew more quickly in the region where the soil freezing depth is shallow. We concluded that warmer winter soil temperatures have caused an increase in larch growth in recent decades.