War fires promote short-term alien plant colonization in eastern Ukraine's pine forests

Russian full-scale military invasion has driven multiple ecosystem disturbances in the arid zone of eastern Ukraine, including fires of varying severity. Local forests, largely dominated by Scots pine (Pinus sylvestris L.), were planted as uniform monocultures in a region where low-density woodlands had historically prevailed. However, little is known about how military-related fires affect successional trajectories in disturbed pine forests. We conducted fully digital surveys of vascular plant composition using under-canopy imagery collected by an unmanned aerial vehicle. Transects were established in 29 pine stands with low to medium soil moisture content that had burned three years before the survey. We constructed linear models to examine short-term effects of fire severity on the proportion of alien flora, vascular plant diversity, and the success of tree regeneration. In line with our expectations, higher levels of stand damage were positively correlated with an increasing proportion of non-native plant species. However, this pattern was largely driven by Canadian horseweed (Erigeron canadensis L.), a widespread ruderal species that commonly appears as a short-term colonizer of disturbed forest sites. By contrast, we found no significant effects of burn severity on tree regeneration, which was poor across most transects, or on overall vascular plant diversity. We therefore discuss how war-altered disturbance regimes, including the potential for repeated fires, may interact with short-term ruderal colonization in artificial pine forests, particularly in relation to landscape-level fire risks and future successional pathways.