Site Requirements Of Eucalypts

SITE REQUIREMENTS OF EUCALYPTS

SITE REQUIREMENTS OF COMMERCIAL EUCALYPTUS AND CORYMBIA SPECIES IN SOUTHERN AFRICA Martin Herbert Fractal Forest, P O Box 3075, Assagay 3624, South Africa email: [email protected]

INTRODUCTION Afforestation in southern Africa is practised in a sub-continent of generally low to very low rainfall. Those regions where sufficient precipitation falls to ensure the survival and vigorous growth of plantations are largely confined to a long and discontinuous belt along the eastern sea-board and adjacent escarpment, as well as portions of the adjoining highland interior. This encompasses a very wide range of growing conditions with regard to latitude, altitude, climate, lithology, soils, topography and biotic factors. Accordingly, land holdings can only be optimally and sustainably managed by uniquely integrating layers of detailed site data for individual management operations. The great diversity of site conditions and the presence of forestry into more marginal areas are the main reasons for the employment of a number of commercial Eucalyptus and Corymbia species as well as clonal hybrids within the industry. In addition, timber properties may be strongly influenced by genotype-site interactions, which is of major importance for an industry focused on both quality than quantity. As a result, sites need to be carefully evaluated for site-species/genotype matching. This in turn requires a detailed geographic information system for a range of site factors at a scale of approximately 1: 10 000 (i.e. compartment level). This section presents the optimum requirements of the principal commercial Eucalyptus and Corymbia species for such parameters.

SITE FACTORS When investigating the site’s ability to support tree growth, there are three main criteria for consideration, viz. climate, soils/lithology and pests/diseases. These constitute a dynamic set of interacting and partly compensating factors, and must thus be jointly considered and evaluated. For example, the importance of soils increases as precipitation decreases, and the impacts of pests and diseases tend to become more pronounced as tree growing conditions become more marginal. The most important climatic factor is ambient air temperature, as each genotype has a particular optimum range of physiological activity for fast and continuous growth, while resistance to frost/snow and diseases is also temperature related. Within a region, the thermic regime may be inferred from mean annual temperature (MAT), but it is also important to take note of the full range of expected mean monthly maximum and minimum temperatures, particularly those of January and July respectively. Increasing warmth promotes tree physiological activity but, in conjunction with high humidity, also susceptibility to disease; conversely winter coldness not only increases the risks of frost and/or snow damage, but also markedly reduces seasonal growth rates, unless species are specifically adapted to these conditions. Temperature conditions per production unit can be estimated using local models based on altitude, latitude and aspect, but, due to global climate change, only temperature recent data from the past 15 – 20 years should be considered. In general, temperatures decrease with altitude, exposure and latitude, and on southern as opposed to northern slopes. Where a tree species or genotype is planted in a suitable thermic regime, mean annual precipitation (MAP) largely determines its growth rate. Within a region, MAP not only defines the expected amount of precipitation, but also infers its monthly distribution. The summer rainfall areas of southern Africa are all characterised by a marked summer peak in precipitation with relatively dry winters of between 10 and 40 mm precipitation only per month. A decrease in MAP is also associated with a drier winter; thus MAP may be used as a basis for estimating the trees water requirements, as seasonal rainfall distributions are relatively similar. The MAP limits for the summer rainfall regions listed in

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Table 1 vary between 657 and 1 298 mm, depending on MAT (more evapotranspiration required) and the demand for water by the different species (fast-growing and drought-sensitive species require more water than low-yielding and drought-hardy species). The Cape winter and constant rainfall regions need to be considered somewhat differently, as the high winter precipitation occurs outside during the period of greatest evaporative demand (summer). MAP per production unit can be broadly estimated using models based on altitude, latitude and/or coastal distance, with local adjustments made according to aspect and/or relative topographic position (rain shadows); however, only data from the most recent 30 – 35 years should be used, due to shifts in precipitation patterns. In general, MAP increase with coastal proximity, altitude and on slopes facing orographic events. The restricted distribution of extensive regions of high rainfall in southern Africa results in the use of a large number of species with differing water requirements and drought tolerances. As the demand for water increases with temperature, MAP alone cannot be used in site-species matching; thus classes of effective precipitation (EP) have been developed, which may be used to meaningfully compare the water demands of trees relative to MAT (see Table 1). It is important to note that although the absolute value of MAP may not always be known at compartment level to the nearest millimetre, Table 1 reflects the importance of small differences in MAP on tree growth, and the site-specificity with which successful forestry operations need to be managed. The minimum EP class required by a species will rise or fall as available soil water holding capacity decreases or increases respectively. TABLE 1. MAT (°C)

Minimum MAP (mm) requirements per Effective Precipitation Class and MAT Effective Precipitation Class

Very low 657 675

Low 689 709

Moderate 722 743

High 754 776

Very high 787 810

Extra high 819 844

Ultra high 852 878

Moist 884 911

14.0 14.5

693

728

763

798

833

868

903

938

711

747

783

819

856

892

928

964

15.0 15.5

728 744

765 783

803 822

840 860

878 899

915 938

953 977

990 1 015

16.0 16.5

760

800

840

880

920

960

1 000

1 040

776

817

858

899

941

982

1 023

1 064

17.0 17.5

791 805

833 849

876 893

918 936

961 980

1 003 1 024

1 046 1 068

1 088 1 111

18.0 18.5

819

864

909

954

999

1 044

1 089

1 134

833

879

925

971

1 018

1 064

1 110

1 156

19.0 19.5

846 858

893 907

941 956

988 1 004

1 036 1 053

1 083 1 102

1 131 1 151

1 178 1 199

20.0 20.5

870

920

970

1 020

1 070

1 120

1 170

1 220

882

933

984

1 035

1 087

1 138

1 189

1 240

21.0 21.5

893 903

945 957

998 1 011

1 050 1 064

1 103 1 118

1 155 1 172

1 208 1 226

1 260 1 279

22.0

913

968

1023

1 078

1 133

1 188

1 243

1 298

13.0 13.5

Edaphic factors for consideration include soil effective rooting depth, texture, structure, drainage, fertility, stones and lithology. The effective rooting depth (ERD) of soils is defined as the horizon depth at which meaningful root penetration and/or activity ceases. Such limitations include hard/compacted rock, dense stone-lines, excessive soil firmness/hardness caused by strong structure or massiveness, and poorly-drained or water-logged horizons. Soil constitutes the medium for the physiological activity of roots, including nutrient and water uptake and respiration. These factors are strongly influenced by soil texture, structure and clay type. Texture allows the total available moisture (TAM) of trees to be estimated (Schulze, Huston and Cass, 1985), and these relationships have been used to compare the ERDs of the main textural classes of soils (see Table 2). Specific information on tree rooting conditions requires intensive field surveys (1: 10 000 scale) and a spatial data base

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SITE REQUIREMENTS OF EUCALYPTS

capable of detailing and interpreting the entire rooting profile (soil, saprolite and lithology). Table 2 ranks soil depths per textural class in terms of TAM (-10kPa); it is divided into classes of equivalent soil depth (ESD) from very shallow to unrestricted. Tree species have been rated in terms of their minimum ESD requirements, the faster-growing and more site-demanding requiring deep to very deep ESD classes and vice versa. Table 2 shows that the coarser textured soils demand a greater ERD to store an equivalent quantity of water. As with the MAP limits for each precipitation class in Table 1, the relatively small differences in ERD between the finer textured soils underline the large differences in the site’s ability to store water and act as a buffering system against variation in rainfall amounts and distribution. The clay loam soil texture class has been used as a reference texture for the earlier general ERD guidelines used in site-species matching (Schönau and Grey, 1987). TABLE 2.

Minimum ERD(cm) limits per ESD Class as influenced by soil texture

Equivalent Soil Depth Class Very Shallow Moderate Deep Very Extra Ultra Unshallow deep deep deep restricted 20 30 40 50 60 70 80 90 Silt 21 32 43 53 64 75 85 96 Silty clay 21 32 43 53 64 75 85 96 Silty clay loam 24 36 48 60 73 85 97 109 Clay 26 39 52 64 77 90 103 116 Silt loam 30 45 60 75 90 105 120 135 Clay loam 33 49 65 81 98 114 130 147 Loam 37 56 74 93 112 130 148 167 Sandy clay 41 62 82 103 123 144 165 185 Sandy clay loam 46 69 92 115 139 162 185 208 Sandy loam 62 93 124 155 185 216 247 278 Loamy sand 69 104 139 173 208 243 278 313 Sand 85 127 170 213 255 298 340 382 Pure sand Note 1: Soils with an ERD <30cm must be underlain by fissured rock and well-weathered saprolite. Note 2: Increase ERD by 1/3 for weak to moderately fine structured soils, and by 2/3 for moderately to strongly (< coarse) structured soils Soil textural class

Table 2 applies to apedal to weakly structured soils only; its ERD limits need to be adapted for more structured soils, as defined in the South African soil classification system (Soil Classification Working Group, 1991). The greater the degree of structure is, the less the effective soil volume available for root colonisation. Thus sub-angular/fine structure requires an additional ERD of 33% compared to apedal soils, while this figure is increased to an additional 67% for angular/medium to coarse structured soils. In using Table 2 it is important to note that no soil should have an ERD less than 30cm, with the exception that it is underlain by strongly weathered and fissured rock or saprolite, and that it does not occur in strongly water-shedding (convex) landscape positions (very droughtprone). Furthermore, such shallow soils are not suitable for species with large and highly active crowns, nor in aspects exposed to strong winds (high risk of wind-throw).

RECOMMENDATIONS Although a number of important site factors are identified in this section, it is important to view them holistically within an ecosystem (Herbert, 1993). With regard to the key components of MAT, MAP and ESD, it is significant to note that they have been optimised so as to largely account for the other site factors (except for wind exposure and fire damage). They should not be viewed in isolation, and by moving beyond the limits set in Table 3, it will generally be found that acute problems will arise concerning tree stress/die-back/mortality, poor yields and disease. However, where the minimum soil depth is marginal for a particular species, this may be off-set by an increase in MAP above the minimum recommended. A convenient “rule of thumb” is to compensate a lowering of ESD of one class by increasing the EP by one class (more if the winter drought is highly pronounced). Similarly,

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MAP may be relaxed somewhat in climes with a moderate to high effective precipitation, if the site is in a water accumulating landscape position (foot slope) and/or the soils are deep and underlain by a well-drained and friable sub-stratum. Such off-sets are also required in extending into or slightly beyond the warmer limits for MAT, where a more than adequate summer water supply may help compensate for a (periodically) over-warm site. Recommendations for the minimum commercial site requirements of tree species are given in Table 3. It should be noted that these refer to those site conditions under which each species grows at its relative best. Although a species may be grown outside of these conditions, growth will taper off strongly and the risk of stand failure and/or poor yields increases exponentially. Categories per key site factor in Table 3 are described as follows: Effective precipitation (EP): Frost resistance: Snow resistance: Equivalent soil depth (ESD): Soil drainage status (SDS):

Very low (VL), low (L), moderate (M), high (H), very high (VH), extra high (EH) Nil (N), light (L), moderate (M), heavy (H), severe (S) Nil (N), light (L), moderate (M), heavy (H), severe (S) Very shallow (VS), shallow (S), moderate (M), deep (D) Poor (P), moderate (M), imperfect (I), good (G), very good (VG)

Table 3 also includes a number of clonal hybrids of E. grandis with E. camaldulensis, E. longirostrata, E. nitens and E. urophylla. While particular individual genotypes may be highly site specific, many are “generalists” designed to exploit a range of site conditions within a forestry zone. The site requirements of the four hybrid groups listed in Table 3 are thus by definition broad, and apply only to local genotypes developed in southern Africa. The guidelines for suitable site conditions are provided mainly to indicate each groups potential for inclusion in planting programmes, but for optimum results, production units will still need to be individually matched with specific genotypes.

REFERENCES Herbert, M.A. (1993). Site requirements of exotic hardwood species, ICFR Bulletin Series 2/93. Schönau, A.P.G., and Grey, D.C. (1987). Site requirements of exotic tree species, In: Forestry Handbook, Southern African Institute of Forestry, Pretoria, p. 82-94. Schulze, R.E., Huston, J.L., and Cass. A. (1985). Hydrological characteristics and properties of soils in southern Africa, Water SA, Vol.11, No.3, p.129-136. Soil Classification Working Group (1991). Soil classification: A taxonomic system for South Africa, Memoirs on the Agricultural Natural Resources of South Africa No. 15, Pretoria.. Webb, D.B., Wood, P.J., and Smith, J. (1980). A guide to species selection for tropical and subtropical plantations. Tropical Forestry Papers 15, Comm. For. Inst., Oxford. pp.342.

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TABLE 3.

Key site requirements of commercial Eucalyptus and Corymbia species and clonal hybrids

Species C. henryi

MAT (°C) range Min. Max. 19.0 22.5

Mean Jan. max. temp. (°C)

EP 1

Frost Snow ESD 2 Soil resistance resistance drainage

31.5

L

N

N

M

M

E. badjensis

14.5

17.0

26.5

M

H

M

S

G

E. benthamii

14.5

18.0

26.0

L

S

L

M

I-M

E. dunnii

15.0

18.0

27.5

M

H

L

S

M

E. elata

15.0

18.0

27.0

M

M

L-M

D

G

E. grandis

16.5

20.5

29.0

H

N

N

M

I

E. longirostrata

19.5

23.0

32.0

VL

N

N

S

M-P

E. macarthurii

14.0

18.0

26.0

L

S

L

S

I-M

E. nitens

13.5

15.5

24.5

VH

S

H

D

VG

E. saligna

15.5

19.0

28.0

M

L

L

M

I-M

E. smithii

15.0

17.5

26.5

H

M

L-M

M

VG

Eucalypt hybrids E. grandis x camaldulensis

18.0

22.0

31.0 L - M

L

N

VS - M

I-M

E. grandis x longirostrata

20.0

23.0

32.5 VL - L

N

N

VS - M

M-P

E. grandis x nitens

15.0

17.5

27.0 H - VH

H

M

M-D

VG - I

E. grandis x urophylla

19.0

22.5

31.5 H - EH

N

N

S-D

I-M

1

EP: Effective precipitation (see Table 1) .

2

Comments Stem kino pockets and exudation may occur; rapid loss of vigour on cooler sites (< 19 °C); “new” species with good breeding potential. Moderate susceptibility to snout beetle; low susceptibility to Phytophthora, but increases on warmer sites; tolerates minor hydromorphy only. Low susceptibility to snout beetle; competitive on rocky/stony, frosty and/or high wind-chill sites. High susceptibility to snout beetle; competitive on rocky/stony sites and/or base rich lithologies; prefers cooler climes; vigorous root system. High susceptibility to termites; low susceptibility to snout beetle; avoid sites underlain by un-weathered rock; tolerates minor hydromorphy only. Highly susceptible to Cryphonectria and Coniothyrium cankers on hot and humid sites – NB temperature limits; sensitive to termites. High drought and disease resistance; rapid loss of vigour on cooler sites (< 19.5 °C); “new” species with good breeding and hybrid potential. Moderate susceptibility to snout beetle; Botryosphaeria canker problematic on very droughty sites. Juvenile stage highly susceptible to Mycosphaerella leaf blotch on warm (≥ 16 °C) sites; Botryosphaeria and Endothia cankers on off-sites; highly susceptible to termites; avoid droughty sites. Sensitive to termites; somewhat more cold and drought tolerant than E. grandis. Highly sensitive to Phytophthora and Pythium, especially on warmer sites (> 17.5 °C); avoid all hydromorphy, especially on mudstone, siltstone and shale sediments; avoid droughty sites. Broad site adaptability; limited adult crown and girth development; relatively modest yields, especially on well-watered sites. Broad site adaptability and competitiveness; good disease resistance; requires further genotype–site matching development. Very high leaf area index potential; highly competitive on recommended sites; tip die-back on droughty sites; avoid severe frost pockets; avoid high wind-chill. Good disease resistance and competitive on well-watered sites; less competitive on droughty sites.

ESD: Equivalent soil depth (see Table 2).

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