Piantagioni da bioenergia: sostenibilita’ produttiva ed ambientale di nuovi cloni ibridi di pioppo (Populus spp.)
Author(s)
Mareschi, Leonardo
Date Issued
April 23, 2008
Type
Doctoral Thesis
Abstract
Short rotation forestry (SRF) systems imply the use of fast growing hardwoods, with high
planting density and harvesting cycle less than 10 years; this innovative system of plantation
forestry is catalyzing a great attention as alternative to surplus food crops and to produce
renewable energy in form of solid biofuels (wood chips and pellets) with higher sustainability,
in terms of energetic and CO2 balances, in comparison to liquid biofuels (bioethanol and
biodiesel). The most outstanding commercial applications of the systems exist in USA, with
poplars and willows, Sweden, UK and Poland with willows. In Italy, 6000 ha of SRF hybrid
poplar plantations were established by farmers with public grants (Rural Development Plans
2001-2007). In the near future, profitability of SRF will depend on high yield and low cultural
inputs, and on the ability of SRF plantations to provide environmental services such as
reduction of carbon emission to the atmosphere, and biofiltering of polluted sites/waters
(phytoremediation). The energetic efficiency of woody biofuels is strongly affected by N
fertilization, making use of chemical fertilizers with very high energy production cost.
The aims of this study were: i) comparing survival, growth and yield of new cultivars of
hybrid poplars clones according to two short harvesting cycles (1 versus 2 years) and to site
fertility across northern Italy; ii) comparing Nitrogen (N) removal in the harvested biomass
and its N use efficiency (NUE) among clones, sites and coppicing cycles.
Growth and above-ground dendromass yield of newly selected clones of hybrid poplars,
specific for SRF, were evaluated on four years within a network of experimental fields
established in spring 2003 across northern Italy. Annual rotation was applied in two
experimental fields: Zinasco and Bigarello with a planting density of 14.000 trees ha-1.
Biannual rotation (planting density of 6000 trees ha-1) was evaluated on three sites of low
(Bigarello), medium (Vinovo) and high fertility (Mira). Soil preparation, weed control and
plant water status were homogeneous across all sites. Nitrogen fertilization was applied only
in Mira (200 and 100 Kg N ha-1 in the third and fourth year, respectively). Each experimental
plantation was mechanically harvested after every one or two years.
In all sites rooting of stem cuttings, at the end of the first season, was very high (more than
90%) for all clones, with the exception of Lux clone. Shoot survival after coppicing was also
very high (more than 90%) for all clones and sites, with the exception for Lux clone and
Zinasco, where annual coppicing without fertlization decreased dramatically stump vitality.
Coppicing increased dramatically shoot density in the remaining sites, with a parallel decrease
of stem dimensions.
iii
The network of experimental plantations showed a strong yield variability in relation to the
soil fertility, to fertilization management and rotation cycle, with a range of SRF newly
selected clones of hybrid poplar performing better than traditional ones in medium and high
site conditions.
In Bigarello, with limiting soil conditions, there were not significant difference among clone
yields which varied from 5 (first rotation) to 7.5 (second rotation) Mg ha-1 year-1 in dry matter
(dm). In medium and high fertility site of Vinovo and Mira, there was significant difference in
clones yields, with the newly selected clones performing better than control clones (I214 and
Lux). In Mira, mean yield increased from first to second biennial rotation cycle from 15 to 20
Mg dm ha-1 year-1. The three best clones, AF8, Monviso and 832.148.044, had an annual yield
of 24, 23.5 and 22 Mg dm ha-1, respectively.
N removal, and N percent concentration (%N) in the harvested dendromass were studied in
the two biannual plantations of Bigarello and Vinovo, at the end of the first and second
coppice rotations. A third plantation was studied in central Italy (Terni). We found that poplar
shoot %N was negatively affected by stem diameter (R2=0.713**), being small shoots richer
in %N than larger ones. N removal was strongly affected by dendromass yield and %N. This
was higher in the branches than in the stem. Clone branchiness, the percentage of
aboveground dendromass allocated to branches, and %N were positively correlated
(R2=0.813*) in the most fertile site for the first biannual rotation. %N strongly increased from
the first to the second rotation in both sites of northern Italy; this was due to the dramatic
decrease of shoot dimension, as a consequence of the very strong increase of the shoot density
after coppicing. N removal per each Mg of dry dendromass passed from 6 to 10 kg N from the
first to the second rotation. In Vinovo more than 125 kg N ha-1year-1 was removed by the
most productive clone during the second rotation. These values highlight the importance of
judicious N fertilization in SRF poplar plantations, but also interesting potentiality of using
them as biofilters for the disposal of nutrient rich wastewaters (phytoremediation). Linking
SRF and phytoremediation can be a winning strategy for improving both the profitability and
sustainability of using woody energy crops as appreciable sources of renewable bioenergy.
Additional information
Dottorato di ricerca in Ecologia forestale
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