ADAPTATION MECHANISMS OF PLANTS OF DECIDUOUS SAL AND EVERGREEN HILL FOREST STANDS IN BANGLADESH

Abstract

Forests are fundamental sources of biological diversity and play important role in constituting terrestrial ecosystem on Earth. Plants, which are important for primary production, govern the structure of forest ecosystems. They play immense role in conserving biodiversity, ecosystem stability, natural resource sustainability, climate regulation, hydrologic cycling as well as providing habitat for plants and animals. Understanding the structure of vegetation and adaption mechanisms of forest plants is important for better management and conservation of forest ecosystems. In plant ecology, recent studies on the adaptation of plants to drought condition through changes in their leaf characteristics have received considerable interests although less is known about how tropical deciduous and evergreen forest plants adapt with their environment. The specific objectives of this study, therefore, were to (1) compare the vegetation structure (diversity and composition) of the selected study areas of Madhupur Sal forest and the Sitakunda Ecopark, (2) examine the relationships of plant vegetation structure with the soil properties of the study areas and (3) investigate the leaf morphological, physiological, and anatomical traits in relation to environmental variables such as soil properties and climatic factors to reveal the underlying adaption mechanisms of deciduous and evergreen forest plants. To study the vegetation of the two forests, a nested design with quadrats of 10 m × 10 m, 5 m x 5 m, and 1 m x 1 m was chosen. A total of 72 quadrats were studied, with 12 for each of the three different habits (herbs, shrubs and trees) under two different forest types. Soil samples were taken from 0-10 cm depth in the center of the quadrats for the analysis of physico-chemical properties. In three consecutive years 2017, 2018 and 2019, leaves were sampled from three individual plants for each of the 5 selected plant species from each forest types during summer (June) and winter (December). A total of 22 leaf parameters (8 morphological, 4 physiological, and 10 anatomical) were investigated and the results were averaged across three years to provide mean values for each season. Data revealed that the composition of herbs, shrubs and tree species differed between two forests. Difference in dominant and rare species between two forest types was revealed by the Importance Value Index. Madhupur Sal forest showed a significantly (P = 0.05) higher number of species (15.5) per quadrat than Sitakunda Ecopark (13.33). The 'productivity versus diversity' hypothesis was tested by regression analysis, which revealed significant negative correlations of tree (R2 = 0.29, P = 0.02) and shrub (R2 = 0.18, P = 0.05) species richness with soil P content across forest types. Plant density had a significant negative correlation (R2 = 0.39, P = 0.03) with plant DBH (diameter at breast height) in the Madhupur Sal Forest, indicating a negative interaction between plant growth and density, though this relationship was stronger in Madhupur Sal Forest than in the Sitakunda Ecopark. Based on both physical and chemical properties of soil, Principal Component Analysis (PCA) revealed that the two forests were separated from one another. Based on the species score, Non-Metric Dimensional Scaling (NMDS) analysis revealed that plots of the two selected forests were divided into two groups, with the Madhupur Sal forest being characterized by highly dominant species of Shorea robusta, Grewia nervosa, Mallotus philipensis, Glycosmis pentaphylla, Adina cordifolia, Litsea glutinosa, Antidesma acidum and Ficus hispida and that of the Sitakunda Ecopark was characterized by that of Castenopsis tribuloides, Erioglossum rubiginosum, Holarrhena antidysenterica, Phyllanthus emblica, Streblus asper and Suregada multiflora. Data also revealed that soil factors such as P (R2 = 0.53, P = 0.01), K (R2 = 0.27, P = 0.01), available N (R2 = 0.42, P = 0.01), C (R2 = 0.28, P = 0.01), clay (R2 = 0.28, P = 0.01), and sand (R2 = 0.25, P = 0.01) were significantly correlated with NMDS axes, implying that soil properties played role in structuring forest vegetation. In both seasons, nested ANOVA statistics demonstrated that forest types had stronger influence on the majority of leaf functional attributes than species identity, indicating the importance of forest types in understanding forest plant adaptation to drought conditions. The Madhupur Sal forest had significantly larger leaf fresh weight, leaf turgid weight, leaf area, specific leaf area and leaf dry matter content. The Sitakunda Ecopark had higher leaf water content, chlorophyll content, and stomatal density. All of these changes in leaf attributes between these two forest types followed similar patterns throughout the summer and winter seasons demonstrating that the functional types had a considerable influence on plant leaf traits. Leaf morpho-physiological properties, which indicate leaf economic traits, were used to differentiate the Madhupur Sal Forest from Sitakunda Ecopark between seasons using PCA based on the correlations between the leaf traits of the two forests plants. Madhupur Sal forest had much higher leaf dry matter content, leaf fresh weight, and leaf turgid weight content than Sitakunda Ecopark implying that the plants in this forest were more water-use efficient than that in Sitakunda Ecopark. Plants in Madhupur Sal forests had a higher specific leaf area than those in Sitakunda Ecopark suggesting that they compensated for the loss of photosynthetic product due to leaf fall during the winter by increasing the specific leaf area to improve photosynthetic potential for the rest of the year. Despite having higher chlorophyll content than deciduous plants in Madhupur forests, higher stomatal pore index values and open stomata might be responsible for greater transpiration and water loss in plants of Sitakunda Ecopark. Overall, the findings of this study demonstrated that soil conditions played a significant role on the diversity of plant species and vegetation composition in the Madhupur Sal Forest and the Sitakunda Ecopark. Data also revealed that plants in the Madhupur Sal Forest and Sitakunda Ecopark responded to drought stress by using avoidance and tolerance strategies, respectively.