Different genotypes of Dalbergia sissoo trees modified microclimate dynamics differently on understory crop cowpea (Vigna unguiculata) as assessed through ecophysiological and spectral traits in agroforestry system

Abstract

Microclimate being a crucial and challenging issue for the growth, survivability and ecological relevance of understory crops in agroforestry system, major aim of the present study was to determine that how the micro-environment was modulated by the canopies of different genotypes of a same tree species. Field experiments were conducted at Central Agroforestry Research Institute, Jhansi, during kharif (rainy) season located in a semi-arid region of Central India. Cowpea [Vigna unguiculata (L.) Walp. variety Gomti] was grown under three genotypes namely Bundel-2 (PT-2), Bundel-6 (PT-6) and one local of Dalbergia sissoo Roxb. tree. Cowpea was also grown in adjacent open field (treeless area) for comparison. Microclimate variables as incident photosynthetic photon flux density (PPFD), air temperature, leaf temperature, canopy temperature depression and soil surface temperature were monitored for understanding microclimate dynamics with reference to the understory crop. Several ecophysiological and leaf spectral traits were also evaluated for assessing the efficiency of the understory crop keeping relevance with microclimate modulation and their use as determinants. Light interception was remarkably different under the canopies of three different genotypes of D. sissoo trees. Intercepted PPFD (iPPFD) was higher by PT-2 and PT-6 than the local. Higher iPPFD was related to the higher leaf area index (LAI) of the corresponding tree genotypes indicating the larger canopy of the improved genotypes captured more light than the local one and thus a strong linear correlation was obtained between iPPFD and LAI. Microclimate variables such as air temperature, canopy temperature, canopy temperature depression, soil surface temperature and relative humidity (RH) were conspicuously different in the areas under the different tree genotypes in the field. There was clear vertical gradient of temperature at the top, middle and bottom layer of the tree canopies. Similarly, RH of the spaces in between the tree-rows showed a vertical gradient. Vertical profile of temperature was different depending on the tree genotypes and RH was much higher inside canopy than in the open field. Intensity of shade varied depending upon the tree genotypes and it was strongly associated with the LAI and iPPFD. Deep shade (50–60%) was observed under the PT-2 and PT-6, whereas moderate shade (up to 30%) was observed under the local genotype. Grain yield of cowpea was relatively less under the PT-2 and PT-6.Thus yield was found associated with the iPPFD by the respective tree canopies and the modulation of the microclimate dynamics. Various ecophysiological traits namely CO2 assimilation, electron transport rate across PS-II (ETR), transpiration, stomatal conductance, reflectance based leaf spectral traits like CCI, NDVI and PRI were also observed to be associated with differential responses of the understory crop. Our results highlighted about the relevance and significance of the ecophysiological and spectral traits evaluated as indicators towards better understanding of the microclimate modulation by the tree canopies of different genotypes and efficiency of the understory crop in agroforestry. Overall, these results demonstrated that the tree canopies of a same species played critical role not only in controlling the efficiency of understory crop, but also in modulating the microclimate which has significance for various ecosystem services perspectives.