Pre-commercial thinning could mitigate drought stress of black spruce stands

• Boreal forests experience the effects of climate change faster than any other biome. • Thinning increases residual tree growth, particularly on moisture-limited sites. • On warm-dry sites, greater water availability increases radial tree growth. • On wet sites, greater water availability decreases the radial growth. • Black spruce growth may benefit from future warming on relatively wet sites. Rising temperatures are likely to increase the risk of drought across the globe over the next century. Boreal forests are particularly vulnerable to drought because temperatures within these biomes are projected to warm the fastest. Warm and dry conditions can reduce tree growth, particularly in regions that are already moisture-limited, which may reduce forest productivity. Forest stand density management, such as pre-commercial thinning (PCT), can reduce moisture stress for residual trees by reducing canopy evaporation and increasing soil water availability. PCT is applied 15–20 years after clearcut and removes smaller stems to increase resource availability to residual trees, thereby increasing diameter growth and wood value. How PCT can mitigate climate change impact in boreal forests is still unclear, partly due to heterogeneity in moisture availability and growth response. This study tests the relative effect of PCT on radial growth of black spruce ( Picea mariana (Mill.) B.S.P.) on sites with varying water availability to inform forest managers about PCT’s potential to mitigate anticipated effects of drought on tree growth. Tree discs from PCT and non-thinned plots were harvested from three contrasting site types in eastern Canada (warm-dry, warm-wet and cool wet). Using dendrochronology, the relationship between annual ring width index (RWI) and standardized-evapotranspiration index (SPEI) was explored in the time since PCT and during known climate anomalies. RWI showed a positive growth response to increasing SPEI on warm-dry sites, but a negative growth response on warm-wet and cool-wet sites. PCT provided a greater benefit to radial growth on warm-dry site types in the 15 years since treatment but provided no additional benefits during years of climate anomalies. Results suggest that PCT will remain an important forest management practice on moisture-limited sites in order to maintain black spruce productivity, whereas tree growth may benefit from future warming on relatively wet sites.