Intercepted Photosynthetically Active Radiation (PAR) and Spatial and Temporal Distribution of Transmitted PAR under High-Density and Super High-Density Olive Orchards

Dario Mantovani,Luigi Ponti,Damiano Marchionni,Adolfo Rosati,Franco Famiani

doi:10.3390/agriculture11040351

Abstract

We quantified the photosynthetically active radiation (PAR) interception in a high-density (HD) and a super high-density (SHD) or hedgerow olive system, by measuring the PAR transmitted under the canopy along transects at increasing distance from the tree rows. Transmitted PAR was measured every minute, then cumulated over the day and the season. The frequencies of the different PAR levels occurring during the day were calculated. SHD intercepted significantly but slightly less overall PAR than HD (0.57 ± 0.002 vs. 0.62 ± 0.03 of the PAR incident above the canopy) but had a much greater spatial variability of transmitted PAR (0.21 under the tree row, up to 0.59 in the alley center), compared to HD (range: 0.34–0.43). This corresponded to greater variability in the frequencies of daily PAR values, with the more shaded positions receiving greater frequencies of low PAR values. The much lower PAR level under the tree row in SHD, compared to any position in HD, implies greater self-shading in lower-canopy layers, despite similar overall interception. Therefore, knowing overall PAR interception does not allow an understanding of differences in PAR distribution on the ground and within the canopy and their possible effects on canopy radiation use efficiency (RUE) and performance, between different architectural systems.

Highlights

The photosynthetically active radiation (PAR) intercepted by a crop canopy is one of the main factors determining biomass production, being the source of energy for the process of photosynthesis [1]
In the absence of other limitations, like water, nutrients or temperature limitations, the crop dry matter production can be modeled as a function of the cumulated PAR incident on the crop, the fraction of this that is intercepted by the canopy and its use efficiency [9], where radiation use efficiency (RUE) is the ratio of the dry matter produced per unit of radiation intercepted
Data show asymmetry between the two sides: the average TPARseasonal for all positions on the west side vs. those on the east side was 3704 mol m−2/0.43 vs. 4512 mol m−2/0.52 respectively. This is due to the tree row orientation which was NNE–SSW, exposing the east side of the rows, slightly oriented towards south, to greater irradiance than the west side, slightly oriented towards north, in agreement with model predictions [14,24,28]

Summary

Introduction

The photosynthetically active radiation (PAR) intercepted by a crop canopy is one of the main factors determining biomass production, being the source of energy for the process of photosynthesis [1]. Tree crops, including olive orchards, have discontinuous canopies, making the assessment of PAR interception and transmission difficult to both model and measure [5,11,12,13]. Due to these difficulties, only few authors have worked on either modeling or measuring PAR interception in different olive orchards, ranging from extensive systems to high-density ones. Mariscal et al [11], reported fractions of PAR transmittance ranging between about 0.05–0.85 (i.e., interception of 0.95–0.15, respectively) in orchards ranging from 0.1–0.7 of canopy ground cover These values were used to validate a model of PAR interception in olive groves. This can be achieved by designing the orchard with adequate canopy height, slope, width and alley width [14,26]

Methods

Results

Conclusion