1. Introduction
Eucalypts are the world’s most valuable and widely planted hardwoods (up to 21.7 million ha in 61 countries by 2030 [
1]) and have numerous potential applications as short rotation woody crops (SRWCs) [
2,
3]. Several
Eucalyptus planting stocks have promise as SRWCs in Florida [
4,
5], including
E. grandis x
E. urophylla cultivars such as EH1. After four generations of
E. grandis genetic improvement for Florida’s unique climatic and edaphic conditions starting in the 1960s and clonal testing initiated in the 1980s across a wide range of site/soil types, the University of Florida released five
G Series cultivars in 2009 for commercial planting [
4,
5]. Although G1 is no longer commercially viable due to susceptibility to blue gum chalcid (
Leptocybe invasa), G2 through G5 have shown resilience to damaging freezes, tolerance to infertile soils, exceptional stem form, improved coppicing ability, chalcid resistance, and varying degrees of windfirmness.
G Series planting density trials established on former citrus lands and phosphate mined clay settling areas in central and south Florida demonstrated maximum mean annual increments (MAI
max) as high as 75.3 to 78.2 green mt/ha/year at 4304 and 5066 trees/ha, respectively [
6]. Economic analyses using current stumpage prices, high silvicultural management costs, and expected coppice yields, have shown that
G Series cultivars can generate internal rates of return greater than 10% [
6].
Most soils in Florida are sandy, with >90% of soil particles as sand, and have low nutrient and moisture holding capacities. Fertilization is necessary to sustain desired crop yield and quality. However, fertilizers are readily leached if not taken up by crop plants and consequently result in environmental pollution such as eutrophication. Applying biochar, a fine-grained, highly porous “charcoal” produced through pyrolysis (burning in a nearly oxygen-free environment) or gasification of numerous feedstocks, improves the physicochemical properties of soils, including bulk density, porosity, cation exchange capacity (CEC), and pH. It also increases soil water and nutrient holding capacities and consequently influences crop production while reducing leaching [
7]. Productivity of many crops significantly increased after soils were amended with biochar [
8,
9]. Sandy soils are more responsive to biochar than clayey soils [
10] due to their low water and nutrient holding capacities [
11].
Biochar, an ancient soil-building amendment, today has wide ranging applications [
12]. The International Biochar Institute (
www.biochar-international.org) has identified more than 50 uses for biochar, and worldwide interest in and demand for biochar are growing quickly. Current demand estimates suggest that biochar is a billion dollar plus industry worldwide with the two largest markets being North America and Europe [
13]. Depending on its particular properties, effective biochar uses include soil and crop improvement plus environmental benefits such as carbon sequestration, retention of nutrients and water, reduced leaching, and water purification, all of which are important in Florida.
Using experience in Florida, USA, we describe eucalypts’ potential for maximizing SRWC productivity through site amendment and genetic improvement, document their suitability for biochar production, and assess biochar’s potential for improving soil properties, tree nutrition, and eucalypts’ growth.
2. Materials and Methods
2.1. EH1 Planting Density Demonstration
An 8.1 ha, intensively fertilized, herbicided, and irrigated
E. grandis x
E. urophylla cultivar EH1 planting density demonstration was established in May 2011 near Hobe Sound, FL, as five rows of trees on 19.8m-wide former citrus beds. Planting densities of 2071 trees/ha (3.1 × 1.2 m spacing) and 1181 trees/ha (3.1 × 2.1 m) were monitored in 21 permanent 19.8 × 12.2 m plots through harvest in December 2017. To model EH1 yield at these two densities and estimate productivity at 2471 trees/ha, 18-, 30-, 36-, 42-, 48-, 65-, and/or 81-month data were fit to stand density, dominant height and basal area development functions in
E. grandis stand-level growth, and yield model equations used by Plessis and Kotze [
14]. Yields were the same for high and low management strategies because
E. grandis productivity under low management on citrus beds in central Florida generated similar yields to the EH1 under high management at Hobe Sound, FL [
6]. Silvicultural and other forest management costs were provided by agricultural companies exploring
Eucalyptus options in central and southern Florida. Stumpage prices were based on local
Eucalyptus mulchwood stumpage prices reported by the same companies and a forestry consulting firm familiar with the local markets.
Maximum net present values (NPV
max) calculated for two management strategies (
Table 1), two planting densities (2071 and 1181 trees/ha), two real discount rates (6% and 8%), and two stumpage prices (
$11.02 and 22.05/green mt) assumed three stages (two coppice stages following the original planting) in one planting cycle. Based on young coppice in Evans Properties’ EH1 plantation near Ft. Pierce, FL, expected coppice yields for stages 2 and 3 (first and second coppice, respectively) were assumed to be 90% and 80% of observed stage 1 yields, respectively. The optimum stage lengths were reported to the nearest 1/10th year; therefore, the annual interest rate was converted to an effective periodic rate.
2.2. EH1 Fertilizer x Planting Density Study
Cultivar EH1 was also planted in a fertilizer x planting density study in June 2015 on a former pasture at the Indian River Research and Education Center (IRREC) near Ft Pierce, FL. Five fertilizers (control, Green Edge (GE) 6–4–0 + micronutrients at 112, 224, and 336 kg of N/ha rates, and diammonium phosphate equivalent to 336 kg of N/ha) were applied as five treatment plots of three contiguous rows 3.1 m apart, for a total of 15 rows of 26 trees. Within the 26-tree rows, 5-tree row plots were systematically assigned one of three planting densities (3588, 1794, and 1196 trees/ha; 3.1 × 0.9 m, 1.8 m, and 2.7 m, respectively) such that 1794 and 1196 trees/ha were replicated twice, 3588 trees/ha once. The interior three trees of each fertilizer x planting density plot were periodically measured through November 2018. Analyses of variance and Duncan’s Multiple Range Tests of fertilizer and planting density means were conducted using SAS® (SAS Institute, Cary, NC, USA).
2.3. Biochar Tests
Five test trees were used for preliminary biochar evaluations in 2017–2018. One tree in the EH1 Planting Density Demonstration, one E. grandis cultivar G2 in a 2012 commercial plantation near Ft Pierce, FL, one Corymbia torelliana tree in an adjacent progeny test, one E. amplifolia in a progeny test near Old Town, FL, and one Quercus virginiana in a nearby natural forest each provided ~23 green kg of stemwood for testing by a lab in California at different pyrolysis temperatures to determine optimal charring temperatures for the different feedstocks. Their biochar physical and chemical properties were further tested by Celignis Analytical, Ireland, to guide the processing of biochar and as a comparative benchmark.
Biochar produced in Europe by Green Carbon Solutions’ (GCS’) Polchar, which specializes in pyrolysis and carbonization of different feedstocks, served as a comparison for the five Florida trees. Hardwood monoculture roundwood logs only were cut to size, split, and pre-dried. Pyrolysis involved a vertical retort operating through a range of temperatures up to a maximum of ~630 °C. Post production, the biochar was sampled and tested for physical and chemical properties. Polchar’s biochar was also used for the biochar–fertilizer study described below.
2.4. Biochar–Fertilizer Study
A two-row windbreak study of three E. grandis cultivars in one row and four C. torelliana progenies in an adjacent row offset 1.2 m away was established at the IRREC in July 2017 as a randomized complete block design with four complete replications of cultivars G3, G4, and G5 in 17 to 28-tree single row plots at 1.8 m within row spacing and one incomplete replication of cultivar G5 in a 13-tree single row plot. In February 2018, all four complete replications received an organic fertilizer (GE 6–4–0 + micronutrients at 336 kg of N/ha), and the two interior replications also received GCS’ Polchar biochar (11.2 mt/ha) by rotovating the two treatments into the soil to a 20 cm depth between and within 1.2 m of the two rows. The incomplete replication served as a control. The cultivars in this resulting biochar–fertilizer study were measured at 5, 11, and 16 months.
To monitor soil and foliage responses, 13 trees (one in the middle of each of the 13 cultivar plots) were resampled at biochar–fertilizer treatment ages of 0, 5, and 11 months (tree ages 5, 11, and 16 months, respectively). At each time, four soil samples were collected from a 0–20 cm depth within 1.2 m around each sample tree, and recently matured foliage was taken from four representative branches around the crown of each sample tree. The collected soils were combined by tree, air dried, and ground to pass through a 2-mm sieve prior to analysis for relevant properties. The tree leaf samples were combined by tree, oven dried at 75 °C to constant weight, and powdered to pass through a 1-mm sieve prior to analysis for nutrient concentration.
Soil pH was measured using a pH/conductivity meter (AB 200, Fisher Scientific, Pittsburgh, PA, USA) at the soil to water ratio of 1:1. Electrical conductivity (EC) of soil samples was determined at the solid to water ratio of 1:2 using the pH/conductivity meter. Available soil P was determined using the method of Kuo [
15]. Available metals in soil were measured by extracting the samples with Mehlich 3 (M3) solution at a solid to solution ratio of 1:10 [
16]. The extracts were filtered through a 0.45-μm membrane. Subsamples of the filtrate were acidified and analyzed for the concentrations of dissolved P, K, Ca, Mg, Fe, Mn, Cu, Zn, B, and Mo using inductively coupled plasma–optical emission spectrometry (ICP–OES) (Ultima, JY Horiba Group, Edison, NJ). Portions of the plant leaf samples (0.2 g each) were digested with 6 mL of concentrated nitric acid (HNO
3)/hydrogen peroxide (H
2O
2) and diluted to 100 mL. The concentrations of P, K, Ca, Mg, Fe, Mn, Cu, Zn, B, and Mo in the digested samples were determined using ICP–OES.
Analyses of variance and Tukey–Kramer tests of cultivar tree size, soil nutrient, and tree leaf nutrient means were conducted using SAS®. Changes between soil properties and leaf nutrients from 0–5, 5–11, and 0–11 months were also analyzed.
4. Discussion
Eucalypts can be very productive and economically feasible when intensively grown as SRWCs, even under our preliminary assumptions. As timber markets and forestry labor are not well established in central and southern Florida, our assumed silvicultural and other forest management costs for a start-up Eucalyptus operation are higher than for conventional forest plantations in the South. Stumpage prices may also change as local markets develop. Deployment of elite advanced-generation E. grandis families may further increase profitability of SRWCs in Florida primarily due to lower seedling costs (~$0.25/seedling versus $0.70/propagule) and economic feasibility of high-yield management regimes (>2471 trees/ha).
Even under high plantation establishment and management costs, low stumpage prices, and expected coppice yields, cultivar deployment can yield positive cash flows at real discount rates greater than 10%. Under current market conditions and management costs, low-density regimes (~1181–1483 trees/ha) are the most profitable for clonal forestry on average sites (e.g., citrus lands and flatwoods). Lower propagule costs could increase financially optimum planting densities. With proper mechanical site preparation, eucalypt plantations on clay settling areas could produce higher NPVs compared to average sites [
6].
The goal of our financial analysis was to demonstrate the profitability of Eucalyptus plantations under moderate to high discount rates and high operational costs in central and southern Florida’s developing forestry markets. Since most landowners were interested in NPV and IRR, we used NPV rather than land expectation value (LEV, also known as bare land value), even though our analysis of the EH1 planting density demonstration had unequal rotation/cycle lengths. Further background on the use of LEV for Eucalyptus in Florida is available (6,17).
The fertilizer and planting density differences observed in the IRREC fertilizer x planting density and biochar–fertilizer studies are consistent with previously observed influences of fertilizer and planting density on eucalypt productivity in Florida [
17,
18,
19,
20] and worldwide [
21,
22,
23,
24]. While inorganic fertilizers have been necessary for rapid growth of eucalypts on Florida’s infertile sandy soils, the observed response here to a slow release organic fertilizer, and its apparently beneficial coupling with BC, is encouraging for sustainable eucalypt management. Planting density effects were evident early, with, for example, the 3588 trees/ha in the fertilizer × planting density having the tallest trees at 9 months and the largest stand basal area but smallest tree DBH at subsequent ages. Similar effects of planting density have been noted for
E. dunnii seedlings and clones [
25]. Planting density trade-offs between harvest tree size, rotation length, establishment costs, and stand productivity impact plantation economics.
While our preliminary evaluation of cultivars G2 and EH1,
C. torelliana,
E. amplifolia, and
Q. virginiana suggests that all appear suitable for commercial BC production in Florida, evaluations of BCs made from various woods and other feedstocks have identified that feedstock and pyrolysis condition influence properties important for using BC as a soil amendment [
26,
27]. GCS’ new BC production facility near Ft. Pierce, FL, scheduled to begin pilot scale testing in mid-2019, will preferably be using the cultivars G2 and EH1 and other eucalypts grown in nearby plantations. Since key objectives in BC production include minimizing the combustion of carbon, maximizing carbon content, and minimizing ash, it is imperative to ensure consistency of feedstock and the production operating environment. Known biomass characteristics, such as for the
G Series cultivars [
28], are likely to be factors in the selection of future eucalypt feedstocks.
Research on BC impact on SRWCs and forest trees in general outside Florida has generated mixed results for aspects ranging from environmental impacts to tree growth responses. When broadcast in a temperate hardwood stand in Ontario, Canada, the major short-term BC impact was an increase in limiting soil P and Ca [
29]. One review of BC application in forest ecosystems found general improvements in soil physical, chemical, and microbial properties that were, however, BC-, soil-, and plant-specific [
30]. A BC made from
E. marginata decreased soil microbial carbon in a coarse soil [
31], and BC added to a sandy desert soil did not significantly change soil physical properties [
32]. Two BC types had different impacts on growth of young
Pinus elliottii in subtropical China [
33]. Varying doses of macadamia BC combined with two fertilizer rates had contrasting results on soil nutrients and ambiguous trends in the growth of young
E. nitens [
34]. BC did not enhance survival or growth of a
Eucalyptus hybrid on degraded soils in southern Amazonia [
35]. Compost and BC–compost mixes did not improve the performance of poplar, willow, and alder SRWCs [
36]. As evidenced by presentations at a 2018 international SRWC conference [
37], the biochar–fertilizer study reported here appears to be unique.
BC enhanced the soil properties of inherently poor Florida soils as well as the nutrient status of E. grandis, especially when applied together with organic amendments such as GE and/or chemical fertilizers. BC has a large cation exchange capacity, which facilitates retention of nutrients, particularly Ca, Mg, K, Fe, and Mn against leaching loss and thus enhances their efficient use by trees. In addition, BC has a large water holding capacity and thus improves water availability, which is especially important for Florida’s sandy soils during the dry season. Due to high temperature and humidity, decomposition of organic materials in Florida’s sandy soils is very rapid, and consequently these soils generally have a low organic matter content. BC can be a good organic amendment for these sandy soils, because it can stay in soil much longer than other organic materials, such as crop residues or manures.
Other potential
Eucalyptus bioproducts may be classified as naturally occurring, generated by biochemical processes, or as the result of thermochemical processes [
3,
38]. Naturally occurring
Eucalyptus bioproducts include wood products, terpenoids, phenolics, formylated phloroglucinol compounds, insecticides, repellants, antimicrobials, antifungals, and anticancers. Biorefineries such as a phosphoric lignocellulosic biorefinery [
39] can produce the biochemicals lactate with parenteral and dialysis applications, succinate potentially leading to acrylic, lactic, muconic, and fumaric acids, alanine for supplements, seasonings, and antibiotics, and cellulose nanocrystals and nanofibrils for polymer nanocomposites. Sulfite paper mills and the Sulfite Pretreatment to Overcome Recalcitrant Lignin process [
40] may produce jet fuel and graphene for products such as orthopedic medical implants. Thermochemical
Eucalyptus bioproducts include biochar, syngas, and biomaterials whose carbon fiber may yield surgical implants, fabrics, filters, orthotics, chairs, beds, etc., and graphene for surgical implants, drug delivery, cancer therapy, imaging, detection of toxins, pollution, etc., graphene oxide, and batteries. These bioproducts have a broad and exciting range of applications for enhancing the value of SRWCs.
