Relationships between leaf structural and functional traits of urban landscape plant species: implications for sustainable landscape planning

Urban planting decisions increasingly use trait-based methodologies; yet they frequently rely on labor-intensive physiological assessments. The use of visible leaf structural traits is an effective alternative. Nevertheless, their capacity for predicting functional performance has rarely been evaluated in a comparative, multi-species urban context. This study conducted a preliminary systematic evaluation to determine if two easily identifiable structural traits, i.e., phyllotaxy (spiral, opposite, decussate, rosette) and blade morphology (simple versus compound) can function as proxies for essential functional traits across 29 prevalent urban landscape species in the field. The relationship between two leaf structural traits, i.e., phyllotaxy (spiral, opposite, decussate, or rosette) and leaf blade morphology (simple or compound), and six leaf functional traits [relative chlorophyll index (SPAD), BET-specific surface area (SSA), moisture, and iron (Fe), magnesium (Mg) and copper (Cu) concentration in leaves] was examined. Leaf samples were collected from healthy, mature plants and target traits were examined using conventional portable meters and laboratory tests. Functional traits were standardized using z-scores, and Euclidean distances were calculated. Metric multidimensional scaling (Principal Coordinates Analysis - PCoA, 1–3 dimensions) was employed and assessed with Kruskal’s stress-1 and R-squared coefficient of determination. Species structure was investigated through k-means clustering, and pairwise trait associations were examined using two-tailed Pearson correlations (α = 0.05). Phyllotaxy exhibited stronger and consistent correlations with functional traits than blade type. Opposite/decussate arrangements frequently demonstrated higher SPAD and distinct SSA, although spiral phyllotaxy correlated with increased Mg and Cu levels. Complex blades had more moisture content, while simple blades typically displayed higher SPAD values. Pairwise relationships identified two functional axes, i.e., moisture–nutrient and pigment–micronutrient. Magnesium had a significant correlation with moisture (r ≈ 0.48, p < 0.01), whereas SPAD was associated with Cu (r ≈ 0.40, p < 0.05). Most other pairings showed slight correlations, suggesting low collinearity. Unsupervised analysis (k-means, metric MDS/PCoA) identified coherent groups corresponding to these gradients. Rosette clustered at extreme moisture–SSA, opposite/decussate taxa gravitated towards elevated SPAD, and compound-bladed species demonstrated increased moisture levels. The consistent trends support the use of identifiable structural traits to predict functional performance in the selection of urban plants. Two readily identifiable structural traits—particularly phyllotaxy—offer valuable insights into functional diversity in urban vegetation, facilitating more effective, on-site trait-based plant selection. Understanding these relationships might improve the selection of plant species for urban green spaces. Although the findings derive from a single season in a Mediterranean location, they encourage additional investigation across diverse climates and seasons to enhance guidelines for sustainable landscape planning.