Local adaptation of balsam fir seedlings improves growth resilience to heat stress

Increasing frequencies of heat waves and drought are expected to shift the range and growth of balsam fir (Abies balsamea (L.) Mill.), a widely distributed cold-adapted boreal species. However, our ability to predict this species response to these climate anomalies remains limited, especially when considering how trees can exhibit delayed and persistant growth responses to these stressors, or legacy effects. Here, we assess the growth response of balsam fir seedlings from four populations following 60 treatment combinations of temperature and water deficit in the previous year. Although we observed moderate water deficit legacy effects on growth, there were no resilience or recovery responses. We did, however, observe considerable negative legacy effects on growth proportional to the level of warming, with average legacy growth declines reaching 45% under the highest warming treatment. Furthermore, the southern populations displayed a 28% higher average growth resilience to temperature stress compared with the northern populations, indicating a higher tolerance to warming. When comparing legacy effects on balsam fir populations at moderate warming conditions relative to the current local baseline climate, we report limited growth declines for southern populations and growth increases for the northern populations. While our results highlight the importance of legacy effects from heat stress in seedlings, they also provide evidence that careful selection of warm-adapted genotypes for reforestation efforts may help offset some of these legacy effects.