Allometric Equations for Estimating Tree Biomass in Agricultural Landscapes in Western Kenya

Abstract

Much of the quantification of carbon (C) in trees usually relies on allometric equations, which relate different measureable properties of organisms to each other. Because biomass inventories are generally conducted to survey standing trees and since standing trees cannot be weighed to gauge their mass, allometric equations provide one of the only ways for estimating C stocks in standing trees. Assessment of tree biomass in agricultural landscapes has remained a great challenge because accurate, reliable and cost effective methods for monitoring C storage in trees were lacking. The high heterogeneity and diverse management effects on trees in agricultural landscapes limit the use of standard allometric equations developed for forests. This study aimed to develop allometric equations to establish a valid basis of up-scaling of landscape biomass C in agricultural landscapes. Ninety seven trees (diameter at breast height, DBH: 3-102 cm) randomly selected across three 100 km2 benchmark sites in Western Kenya were destructively sampled to determine aboveground (AGB) and belowground (BGB) biomass. Allometric equations were developed using standard destructive sampling methods and functional branch analysis (FBA). Crown cover and effective leaf area index (LAIe) were estimated from hemispherical photographs. The equations developed fit the data well with over 95% of the observed variation in biomass explained by DBH. Diameter alone provided reliable estimation of biomass with about 90% accuracy. Published equations that could otherwise be considered appropriate for xxii Western Kenya misjudged AGB and BGB by between 11-22% and 21-35%, respectively. FBA-derived allometry for trees with comparable branching form produced results similar to estimates from an allometry built through standard destructive sampling techniques. Agricultural mosaics of Western Kenya were estimated to hold about 22 t C ha-1 in standing trees; with 17 (standard error, SE 0.02) t C ha-1 in AGB of which leaves, branches and stem constitute 4, 39 and 57%, respectively; while the root system stock about 5 (SE 0.01) t C ha-1. The mean canopy cover (and standard deviation, SD) for the three sites evaluated was 45.3% (SD 18.8), 55.1% (SD 23.6) and 40.8% (SD 18.6) while mean LAIe was 0.65 (SD 0.83), 1.00 (SD 0.83), and 0.59 (SD 0.53) for Lower, Middle and Upper Yala, respectively. The study confirms that DBH serves as a more straightforward and robust proxy for estimating tree biomass in complex agricultural landscape mosaics. Advanced FBA techniques have great potential as a non-destructive approach for generating species-specific allometric equations for estimating tree C stocks and thus reducing measurement effort and impact. Equations developed will find greater application for programs interested in measurement, monitoring, management and up-scaling of tree C stocks to a landscape level. Keywords: Agricultural landscapes, Allometric equations, Carbon stocks, Functional branch analysis, Hemispherical photography