Management of the Hybrid Poplar Plantation

Since short rotation intensive culture hybrid poplar plantations are in many ways more farm crops than forests, much of the management of the stand from preparation to harvest is more agricultural in nature than typical forestry. Hansen (1983) defined short rotation intensive culture (SRIC) as being "the application of agronomic principles and practices for the attainment of forestry objectives." In other words, SRIC is essentially the farming of trees. Debate over whether a SRIC stand is actually a forest continues, with some voices contending that any tree-dominated landscape qualifies as forest, and with others insisting that SRIC stands lack the biodiversity and spatial complexity that characterize natural forests.

Hansen (1983) defined several elements of SRIC:

a) site preparation: weed control, mechanical planting b) short rotation (in the case of hybrid poplar, 8-11 years c) close spacing (often on the order of 1m by 1 m) d) fast growing hybrids e) fertilization necessary for sustained production f) irrigation (not always necessary in the Pacific Northwest g) insect and disease management h) irrigation

Many of these techniques are found in technical forestry, but are not utilized to the degree that they are in SRIC. Four of these, site preparation, very short rotation, fertilization, and irrigation, are linked to or affect soil characteristics, and the intensity of these treatments will depend on the characteristics of the site.

Soil Depth and Texture

Poplars favor loamy soils, but have been planted in soils ranging from sandy to clay loam (Jobling 1990). Sandy soils can be used as a substrate for a poplar stand, but only if there is a sufficient amount of moisture. Too high of a clay content can inhibit growth, as water potential is decreased below the uptake capacity of the poplar. Loam soils often exhibit the right combination of permeability, water potential, and drainage to sustain the water demands of poplar. Deep soils are optimal for poplars, as they are often well drained and offer greater stability for poplar roots. Soils used for poplar cultivation in the Pacific Northwest are often 60 or more inches deep.

Soil pH

Research conducted in Great Britain indicates that base-rich, loamy soil in a sheltered location are optimal for development (Jobling, 1990). Optimum pH is a very important factor in the success of a hybrid poplar plantation. Soil pH values of less than 5.0 or greater than 7.0 may mean that the stand may experience suboptimal productivity or increased mortality. Any vegetation indicative of soil acidity, such as rhododendrons or many ericaceous shrubs, may mean that the site is unsuitable for the culture of poplars.

Drainage

The natural history of poplars indicate that they prefer higher soil moisture availability. Poplars are common in riparian zones, and often are limited to them in more arid regions. Although hygrophilous, they are not wetland species; any soil where the water table is less than 50 cm from the surface is a poor choice for poplar cultivation.
Wetland vegetation, such as sedges and hardhack (Spirea spp.) can indicate that site is too poorly drained for poplar cultivation. However, this can be mitigated by artificial drainage. This is a common practice on flood plains in the Pacific Northwest (Cellier 1987), where drainage ditches are dug to encourage site drainage for cultivation.

Nutrient Management

One of the most important topics in maintaining the sustainability of any plantation is the nutrient supply in the soil. Nutrient removals can be reduced with longer rotations. Nitrogen removals, especially, are minimized with increased rotation interval. The reason for this is that young, quickly growing poplars have a high proportion of photosynthetic area and rapidly dividing tissue, both of which require high nutrient concentrations. For example, N concentrations in above-ground biomass in a 9-year old poplar clone (NC-5260) was only one-third that of 2-year old trees of the same clone (Hansen and Basker 1979).
Continuous monoculture of any crop, whether food or fiber, can deplete the soil of vital nutrients (Dawson 1983). Nitrogen is often the nutrient in question. The lack of nitrogen in a stand can be dealt with in two ways: fertilization or the plantation of a nitrogen-fixing species along with the primary crop. Fertilization is the artificial addition of nutrients or nutrient-rich material to a system. The value of fertilization has long been recognized, even in primitive cultures. Fertilization can be accomplished in two ways: the addition of inorganic, often synthesized, materials such as ammonium nitrate, and the application of biosolids, ranging from treated sewage to pulp and paper waste.
Refer also to to this basic diagram of nutrient pathways in a hybrid poplar stand.

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