<p>Phyllosphere microorganisms promote plant health, facilitate plant growth, and support ecosystem function. In this study, we compared the effects of leaf anatomy, physiological properties, and chemical composition on the diversity and abundance of epiphytic microorganisms across four forage species: wheat (<i>Triticum aestivum</i>), rye (<i>Secale cereale</i>), barley (<i>Hordeum vulgare</i>), and Italian ryegrass (<i>Lolium multiflorum</i>). The results showed that crop type significantly influenced microbial abundances on leaf surfaces and in whole leaves (<i>P</i> &lt; 0.05). Specifically, wheat exhibited higher abundances of aerobic bacteria, lactic acid bacteria, molds, and yeasts in whole leaves and on leaf surfaces than those of the other three forage species. Microbial abundance on leaf surfaces was lower than that in whole leaves among the four crops. The stomatal density on the abaxial leaf surface was significantly higher than that on the adaxial surface (<i>P</i> &lt; 0.0001) among the four crops. The main drivers of whole-leaf microbial abundance included soluble sugars, stomatal density, intercellular CO₂ concentration, and total water vapor conductance. Conversely, the key factors influencing surface microbial abundance were reducing sugars (affecting lactic acid bacteria and molds) and stomatal density on the adaxial surface (affecting yeasts). In conclusion, the morphology, physiology, and chemical composition of forage leaves collectively shape the colonization patterns and abundance of epiphytic microorganisms. Wheat exhibited larger microbial numbers than those of the other three forages. Soluble sugars and stomatal density emerged as key determinants of microbial community structure, whereas epidermal structure influenced the formation of specific functional microbial communities through a dual mechanism of physical selection and microenvironmental regulation.</p>