We also evaluated the presence/absence of bundle-sheath extensions (BSEs), which contribute to the mechanical support of leaves and enhance photosynthetic ability by improving hydraulic properties. Higher LMA, increased toughness, and higher C/N ratios prolonged leaf lifespan in evergreen species. Evergreen species accounted for a large proportion of understory woody plants (44%), whereas most small (87%) and canopy (100%) trees were deciduous. The plants were divided into three life forms based on the maximum height of adult plants: understory (<1 m), small (≥1 m to <5 m), and canopy (≥5m). We measured the toughness, mass per unit area (LMA), nitrogen content, and δ15N levels of the leaves of different life forms of 39 broadleaf woody plants in interior Alaska. The morphological and physicochemical traits of leaves are important in terms of plant adaptation to various growth environments, because such traits play central roles in various functions including photosynthesis. In addition, the results of this study provide insight into species coexistence and community assembly in temperate deciduous forests. Our results reveal differences in leaf phenological traits among life forms and indicate that plant functional traits can provide insight into the mechanisms underlying such differences. In addition, shrubs had a greater leaf and C:N ratio and a longer leaf emergence duration. Among all woody species, species with a small SLA and leaf area and species with thick leaves have a longer leaf emergence duration and LLS, which may be related to material energy reserves. Shrubs have a longer LLS compared with trees because they have a greater leaf thickness than trees (to prevent early spring frost), leaf phosphorus content (leaf, related to cell division), and C:N ratio and stem density (to prevent mechanical damage). Shrubs obtain a longer leaf life span (LLS) through early leaf emergence and later leaf senescence. In this study, we investigated leaf phenological and functional traits of 52 woody species (324 individuals) from a natural forest in the Loess Plateau in China. Leaf phenological traits vary greatly with plant species, are thought to relate to leaf functional traits, and play an important role in plant growth. This study also possesses practical value for plants with variant green periods in landscaping. Furthermore, the dormant deciduous daylily is more freezing-tolerant than the ever-growing evergreen species. Correlation analyses between the PGL and other indices indicated that the winter shoot growth probably maintained the green leaves of the evergreen daylilies, while a strong LFT might facilitate the defoliation of deciduous species. The leaf freezing tolerance (LFT) and relevant morphological, physiological and biochemical variations in leaf tissues in addition to the shoot growth status were determined during Experiments 2 and 3 when comparing natural cold acclimation to de-acclimation. The results showed that their foliar habits were consistent with those of previous reports but could be influenced dramatically by the air temperature. Experiment 1 was focused on the changes in the percentage of green leaves (PGL) between evergreen daylilies (Hemerocallis aurantiaca Baker) and the deciduous species (Hemerocallis fulva L.). Three independent experiments were conducted in the present study. A comparative study on evergreen and deciduous species with close relationships could help to uncover the factors that determine foliar habits when sibling mutants are absent. However, less attention has been paid to the cause of foliar habits, especially in herbaceous perennials. Evergreen ornamental plants can supply year-round aesthetic value and strong ecological benefits in comparison to deciduous species.
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