Andrzej Klasa1 and Doug Karlen2

department of Agricultural Chemistry and Environmental Protection, Warmia and Mazury
University in Olsztyn, Poland

2National Laboratory for Agriculture and the Environment, USDA Agricultural
Research Service, U. S.A.

11.1 Introduction

Populus consists of 25-35 species and among them hybridization is common. The genus itself has a large genetic diversity with some species growing 50 m tall with trunks of up to 2.5 m in diameter. All Populus species from family Saliceae are common in temperate climate zones but they are limited in tropical zones because the maximum temperature they can tolerate is approximately 30°C. Among the various species, Populus alba grows primarily in southern and central Europe, P. tremula in Europe and Asia (mainly in India), and P. tremuloides in North America with their northern border being Alaska. There are some reports concerning Populus deltoides growing in India [1], and although some Populus genotypes can be successfully grown on saline-sodic and alkaline soils, some tested clones could not survive those soil and climatic conditions in Uttar Pradesh province. In an age of globalization there is an increasing tendency for farmers, foresters, and owners of recreation areas to introduce different poplar species in non-native environments. Therefore, numerous Populus species are often found outside their natural borders. In Europe, for example, the most frequently grown poplar is hybrid Populus x euroamericana (Populus deltoides x P. nigra).

Berndes et al. [2] identified poplar short-rotation plantations (SRPs) as one of the most important sources of biomass for energy purposes, pointing out their energy and environ­mental soundness. In the United States, poplar species have been grown commercially for more than 100 years [3] and although, to date, business conditions have restricted their

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

cultivation to specific geographic locations, those developments have provided proof of concept for growing short-rotation trees using intensive agricultural techniques. Therefore, looking to the future and to the great opportunity to develop a viable biomass energy feedstock supply, the poplar industry in many countries is well positioned to play a signif­icant role in meeting global energy needs. Poplar species and their numerous hybrids are very productive genotypes due to their rapid foliar production, very high leaf area index and high photosynthetic rates. It was stressed [4] that the Populus genus, which contains more than 30 species worldwide, has remarkable clonal variation for both biomass pro­duction and distribution. Therefore, some species and clones are much more suitable for lignocellulose biomass production than others because they can allocate more biomass to aboveground plant parts than to roots. By understanding genetic variability among poplar species, Wullschleger et al. [4] pointed out that there is a starting point for future breed­ing programs aimed at obtaining clones of high and reliable yield level with a desirable distribution of assimilates in plant biomass.

Poplar is also one of the so-called energy crops that can, theoretically, improve relations between agriculture and environment. For example, studies on 12 poplar genotypes grown on marginal and agricultural soil in Hungary [5] confirmed wide genetic variability with average biomass yields of 27 Mg ha-1 ODM (oven-dry matter) and average energy yields of 309 GJ ha-1. With regard to carbon, the authors reported that the balance was always positive (i. e. the amount of carbon emitted to the atmosphere as carbon dioxide during combustion was always less than the amount of carbon sequestrated in SRP poplar biomass) because of the high content of carbon in roots, litter and stools. Therefore, under Hungarian conditions, short-rotation poplar can be an effective carbon sink and source of renewable biofuel. Many other environmental benefits associated with growing poplar for energy have been presented from a United States perspective [6], with the prediction that in the United States growing poplar in short-rotation systems will link phytoremediation of contaminated soils with bioenergy production.