Epsilon Archive for Student Projects

Abstract

Fire was once a significant disturbance in southern Sweden, maintaining historic open-canopy forests dominated by fire-adapted species such as pedunculate oak (Quercus robur) and Scots pine (Pinus sylvestris). Fire exclusion associated with intensive forestry and suppression policies in the eighteenth and nineteenth centuries altered forest structure and composition, favoring dense, even-aged stands. Modern conservation policies reintroduce prescribed fire to restore biodiversity, reduce wildfire risk, and increase forest resilience, but a better quantification of post-fire growth impacts on surviving trees is needed. We examined the effects of low- to mixed-severity prescribed burns on post-fire growth of oak and pine in southeastern Sweden using a stem analysis method and reconstructed annual whole-stem volume increment from increment cores collected at multiple stem heights. Growth response was quantified as the ratio of cumulative observed post-fire growth to growth projected from pre-fire trajectories over the recovery period. Prescribed burning did not consistently reduce tree productivity or increase drought sensitivity, although individual responses varied among individual trees, species, and sites.

Oak age was the only individual tree attributed to significantly predicted post-fire growth response, with younger oaks showing more positive responses than older individuals. In Scots pine, no tree attribute consistently predicted growth response, although crown base height and scorch height showed stronger associations than age, potentially reflecting the fire-adapted morphology of the mature trees. Comparison of basal area increment (BAI) and stem analysis (StemA) derived estimates revealed no systematic mean bias, supporting BAI as a broadly useful proxy for whole-tree growth response. However, disagreement increased with the magnitude of the growth response, suggesting that changes in stem allometry and the vertical distribution of radial growth can limit the proportional representation of whole-stem growth by measurements at breast height. Prescribed-fire exposure did not significantly modify growth responses to the severe 2018 drought, although oaks maintained higher growth relative to projected baseline levels than pines.