Tree stand structure drives hemeroby and naturalness, jointly shaping the functional diversity of the herb layer in urban parks

Urban parks play a vital role in providing essential ecosystem services, including the maintenance of biodiversity and the e n hancement of urban well-being. We analysed how variations in tree stand structure influence microclimate, hemeroby, natura l ness, and the functional diversity of the herbaceous layer in two parks in Dnipro, Ukraine. Denser and more diverse tree stands with a higher leaf area index created cooler and moister understory conditions with reduced light, whereas open and homogen e ous stands produced hotter, drier, and more brightly lit patches. Tree stand structure drove microclimatic contrasts that unde r pinned the hemeroby–naturalnes s gradients of the herb layer: S haded, humid interiors supported communities dominated by native perennial species of higher naturalness, while exposed, warm, and dry sites beneath open canopies were occupied by ruderal and adventive species indicative of high hemeroby. In this framework, hemeroby reflects the degree of anthropogenic transformation of the herb layer, whereas naturalness captures the retained memory of its near-natural state. Hemeroby and nat u ralness were found to jointly shape the functiona l diversity of the herb layer: D isturbed sites favoured short-lived, light-demanding species with high dispersal potential, elevated leaf nitrogen content, and persistent seed banks, whereas more natural sites were characterised by taller perennials with more conservative leaf traits. Using S tructural E quation M odelling, we demo n strate that these effects can be described by four main gradients linking stand density/thinning, stand diversity, and cooling versus heating potential to herb layer functional richness, evenness, and divergence. Our results show that tree stands do not merely “matter” in a general sense but actively engineer the microclimate , degree of anthropogenic transformation, and functional co m position of the herbaceous layer. This understanding provides a biologically grounded basis for park management that utilises stand structure to enhance biodiversity and stabilise ecosystem services in urban green spaces.