Acclimatization of Tropical Palm Species Associated with Leaf Morpho-Physiological Traits to the Understorey Environment of Hevea Rubber Farms

Hevea rubber farm is viable for agroforestry systems since its canopy lessens extreme weather conditions and contributes to the adaptation of shade-tolerant plants. However, some limitations in the availability of soil water and shades vary with the age of rubber trees and affect the understorey plants’ acclimatization. Tropical palms are potentially associated plants for the rubber-based agroforestry systems because they are rainforest species adaptable to understorey environments. Two rubber farms, ages 12 and 25 years, intercropped with tropical palms were selected to investigate the acclimatization of the palms to the seasonal abiotic variations in the mature rubber farms. The studied palm species were Chrysalidocarpus lutescens and Rhapis excelsa in the 12-year-old rubber farm and Livistona speciosa and Licuala spinosa in the 25-year-old rubber farm, respectively. Leaf area, stomatal conductance, photosynthesis pigments, and leaf nitrogen content were identified as the palms’ morpho-physiological traits. The 12-year-old rubber farm had a marked soil water deficit in all soil depths at the beginning of the rainy season, reaching around 200 kPa at the 80 cm soil depth, while the 25-year-old rubber farm received greater light transmissions, ranging between 37 and 46% in the late dry season. All palms adjusted leaf area to balance the photosynthetic capacity. The Rhapis palm had greater acclimatization with significant responses of stomatal conductance. Other than the Licuala palm, all palms exhibited the allocation of chlorophyll pigments and nitrogen content significantly in their leaves in response to the different intensities of abiotic stresses in the understorey of the rubber farms.