Expanding the Yukon tree ring Blue Intensity network to assess divergence effects and enhance climate reconstructions

The Divergence Problem (DP) – classically defined as a breakdown in the relationship between tree growth and temperature in recent decades – poses a major challenge to developing robust tree-ring based climate reconstructions. This is particularly relevant in northwestern North America (NWNA), a climatically sensitive yet underrepresented region in dendroclimatological networks. To address this gap and the implications of the DP, we developed a network of 34 white spruce sites across southern and central Yukon, using latewood Blue Intensity (LWBI) as a high-resolution proxy for maximum summer temperatures. Sites span a broad elevational range, allowing assessment of how proximity to local treeline influences climate sensitivity. LWBI chronologies from sites within 100 m of upper treeline exhibit the strongest ( r 2 = 0.46) and most temporally stable relationship with June-August (JJA) temperatures. In contrast, lower elevation chronologies display weaker coherence and declining sensitivity in the recent period that is consistent with the DP. Ring width chronologies express poor correlations and temporal stability and are not recommended for temperature reconstructions in this region. Despite near-treeline LWBI sites presenting the best option for optimising reconstructions, there is slight evidence of a non-linear relationship between LWBI and JJA temperatures, leading to the underrepresentation of extreme warm summer temperatures. The results herein offer a strategy for minimising the DP and enhancing the reliability of temperature reconstructions in NWNA through targeted site selection. However, in a warming world, a progressive weakening in temperature limitation – even at optimal sites – will likely lead to a degradation in calibration accuracy in future reconstructions.