El Ninio and management of plantations under drought condition

The El Ni?io weather event occurs when the equatorial Pacific Ocean gets up to 3°C warmer than usual, and it normally has a global impact on weather patterns. El Ni?o has officially started and is expected to become strong by summer.

This year’s event is predicted to be a big one, having a dramatic effect on weather patterns around the world. It’s likely to set new heat records, energise rainfall in South America, fuel drought in Africa, and disrupt the global economy. It may already be responsible for early-season heat waves in Asia and Europe this year resulting in record high global temperatures. New record daily global temperatures have already been set on three separate occasions.

South Africa is a relatively dry country and even in the wetter regions (Forestry areas) water is generally the single most limiting factor to tree growth. It is therefore important to manage all timber compartments based on trying to reduce water stress on trees. This includes all aspects of Forest Management: from the initial choice of species to plant, to the implementation of silviculture operations and the protection of soils during harvesting and extraction. Soils are the reservoir that store water and nutrients to drive tree growth, they should be protected and enhanced at every opportunity!

SITE-SPECIES MATCHING

Soil, rainfall, temperature, and topographical information are considered when selecting a species suited to an area. Site species matching is particularly important on drought prone sites. Lessons learn from past dry periods include:

? Soils with an Effective Rooting Depth (ERD) of less than 40 cm should not be planted (except for high rainfall mist-belt areas). Hydromorphic soils (wet) should not be planted.

? In coastal areas (below 400m) plant hardy species or hybrids on marginal sites (C. henryi hybrids, E. grandis x E. longirosrata and some hardy E. grandis x E. urophylla hybrids).

? On marginal mid-altitude sites plant A. mearnsii, E. dunnii or hardy hybrids.

? Marginal sites between 1100m and 1400m can be planted to E. dunnii, E. badgensis (above 1200m).

? High altitude marginal sites can be planted to E. badgensis.

? Clonal hybrids of E. grandis can be deployed on marginal E. grandis sites but it is important to consult with the nursery supplying the material to ensure that you are planting a ‘drought hardy’ clone.

? Black Wattle: Black wattle is a relatively drought hardy species and is often planted on shallow,

dryer sites, it is thus prone to damage during extended dry periods. Even in better forestry areas this tree is usually planted on the worst sites (Shallow, north facing, and rocky) and therefore may show signs of damage before compartments planted to other species. On clay/ loam to clay soils this tree can grow successfully on soils as shallow as 35 to 40 cm, however on sandier soils depths of 50 to 60cm are required.

? Soil texture plays an important role in determining ERD, soils with higher clay contents have a better moisture holding capacity.

BRUSH MANAGEMENT & SILVICULTURE

Plantation residues can be used to buffer the effects of dry periods. A mulch acts as an insulator keeping soils cooler during hot spells and reducing evaporation from the soil. Because surface soils remain cooler and moister, roots grow up to the surface and into the mulch thus increasing the volume of soil and moisture available to roots. Brush retention also results in increases in soil organic matter which helps to retain moisture and nutrients in the soil. Ideally residues should be mulched to form a compact layer of organic matter that can protect soils. However, this is an expensive operation and spreading brush, or even stacking it in brush piles, will improve soil protection and moisture retention.

SITE PREPARATION & PLANTING TECHNIQUE

Correct site preparation and planting technique can help to minimise seedling mortality. On regeneration sites, small well prepared (no clods and minimum depth of 30cm) pits are sufficient (quality rather than quantity). Intensive site preparation will result in soils drying out. At planting a litre of water should be poured into the planting pit, mixed with mineral soil to form a porridge (remove any clods), and the seedling planted into the porridge. Dry soil (or mulch) should then be scraped back to cover the wet soil and break the capillary action between the soil moisture and surface air. Alternatively, seedling can be planted into hydrogels using the same principle. Hydrogels are particularly effective in sandy soils that rapidly dry out. In dry times seedlings should be planted deeper at a depth of four to eight centimetres (to the top of the root plug). WEED CONTROL

Weeds compete with plantation trees for resources. Competition control becomes critical during times of water shortage. This means spring, autumn and drought periods are times when weeds will have the biggest impact on soil water reserves. Woody weeds should be prioritised on drought prone sites.

SPACING AND THINNING

Wider spacing of seedlings will produce larger diameter trees but the total timber volume production per hectare will also reduce. However, if stocking is high the diameter of trees is smaller resulting in higher harvesting and extraction costs. A lower stocking will result in less intra-specific competition and therefore less stress on trees.

As a rule, on good forestry sites used for pulp-wood plantations about 1500 to 1600 trees per hectare should be standing at rotation age. For average sites this can be reduced to about 1400 to 1500 stems per hectare and on poor sites stocking can be reduced to as low as 1200 trees per hectare.

Thinning of plantations can be used as an effective tool for reducing stress in dry times. However, the thinning would need to be implemented timorously.

Spacing and Thinning of Wattle: Wattle thinning is primarily done to reduce stand density at various stages in the rotation to reduce competition and remove inferior trees (damaged, suppressed, diseased, forked etc). Thinning can be adjusted to favour either bark or timber production: Lower stocking =bigger trees and better-quality bark. Higher stocking=higher timber tonnage but poorer quality bark.

Number, timing, and intensity of thinning can be manipulated to influence the end crop and to reduce stress in the plantation. On marginal sites heavier and earlier thinning can be used to reduce stress. For example, in seedling established plantations a single thinning operation could be implemented when the trees reach four to five meters, and the stand could be reduced to 1200 stems per hectare.

SALVAGE OF DROUGHT-DAMAGED COMPARTMENTS

Drought damaged compartments generally consist of a mixture of dead trees; trees with dead tops and epicormic shoots on the stem; and healthy trees. The relative proportion of these three categories of trees, the distribution of damaged trees and the age of the trees will determine the method of salvaging the timber.

A few general guidelines can be used to assist in making decisions relating to salvage of damaged compartments:

? If there are more than 1000 (or 60%) healthy trees per hectare, then the compartment can be grown to rotation age. But you need to assess individual compartments (How is damage distributed, how severe is the damage etc).

? If most trees have a diameter that is greater than 100mm then the compartment can be salvaged for pulpwood. Smaller diameter trees are not worth stripping and can be sold for charcoal production, firewood, or small diameter poles.

? Most trees with epicormic shoots up the stem will strip to a certain extent (approximately half of the logs).

? Dead trees and trees with damaged bark/ difficult to strip, can be felled and stacked for four to six months and the bark chipped off once dry. Logs need to be stacked in such a way to ensure maximum air movement through stacks (small gap between logs). Only stack logs with a diameter lager that 100mm, the cost to strip logs with a smaller diameter exceeds the species income.

Southern Africa is a dry region and drought is a regular occurrence. A timber crop will most probably experience an extended dry period at some time in the rotation. It is therefor important to plan for these droughts at time of planting. By selecting the correct species/clone for a site and implementing sound silviculture practices, drought induced mortality can be minimized.