Fig. 6

Correlation coefficients (Pearson’s) between lodgepole pine annual height growth at each site and standardized precipitation–evapotranspiration index (SPEI) values for months prior to the year of growth (July to Dec) and through the growing season. (Only these months are shown to facilitate readability and because very few correlations emerged before the prior July [seven months] and after the concurrent September [no months]. See Additional file 3 for a matrix of 24 months). To ensure adequate sampling depth, only 14 sites were considered in this analysis, and they are ordered with the lowest elevation site on the bottom of the matrix and the highest elevation site at the top. Color intensity indicates the magnitude of the correlation with dark blue indicating a strong positive correlation (r = 1) and dark red indicating a strong negative correlation (r = −1). More negative SPEI values are indicative of droughty conditions while more positive SPEI values are indicative of non-droughty conditions. Thus, a positive correlation between growth and SPEI (blue squares) corresponds to more growth during less droughty conditions, while a negative correlation between growth and SPEI (red squares) corresponds to more growth during more droughty conditions. Despite a seemingly discernible pattern with elevation, correlation coefficients for any given month were not significantly related to elevation per linear regression. See Additional file 3 for correlation matrices of precipitation and temperature. Correlations are based on data collected in 2016 on the Tripod Fire in the Okanogan-Wenatchee National Forest, Washington, USA