Legacy effects under an emerging novel disturbance regime: A memory‐based framework to quantify tree growth responses

Abstract Moderate‐severity disturbances are becoming more frequent under climate change. These disturbances often leave legacy effects on surviving trees that shape future forest dynamics. Thus, understanding how these disturbance legacies impact tree growth is a pressing challenge. In this study, we applied a memory‐based framework to quantify the legacy effect of drought and intentional fire on tree growth, using tree cores collected from two conifer forests (Blodgett and Teakettle) in the western United States. We conceptualized the legacy effect as a shift in any component of the ecological memory (sensitivity, length or temporal pattern). We measured the drought legacy effect as shifted climate sensitivity and the fire legacy effect as an emergent fire memory. Focusing on the co‐existing dominant species in California mixed forests, we tested the hypothesis that the disturbance‐induced memory shifts (i.e. characteristics of legacy effect) could be predicted by species‐specific differences in drought‐ and fire‐tolerance. This study revealed divergent drought and fire response in co‐existing conifers. Drought‐intolerant species showed increased climate sensitivity following drought, while drought‐tolerant species did not. In contrast, fire‐intolerant species experienced post‐fire growth suppression. The length of fire legacy effects ranged from <2 years in sugar pine and incense‐cedar to >4 years in white fir and Douglas‐fir. Fire‐resistant species like ponderosa pine and Jeffrey pine did not exhibit a fire legacy effect. Between 2001 and 2020, ignoring both drought and fire legacy effects led to a 4.2% overestimate of total growth in white fir (least tolerant) and a 0.5% underestimate in ponderosa pine (most tolerant). Synthesis . In ecosystems subject to frequent drought or recurring fire, species tolerant of these disturbances exhibited weaker legacy effects with negligible impacts on post‐disturbance growth. Thus, these species may augment their competitive advantage under future disturbance regimes. The length of the legacy is a critical feature given the potential for increasingly frequent disturbances to outpace species' recovery capacity and thereby elevate mortality risks.