- Restoration of natural vegetation in degraded Imperata cylindrica grassland - 205

Journal of Vegetation Science 6: 205-210, 1995© IAVS; Opulus Press Uppsala. Printed in Sweden

Restoration of natural vegetation in degraded Imperata cylindricagrassland: understorey development in forest plantations

Kuusipalo, Jussi1*, Ådjers, Göran1, Jafarsidik, Yusuf1, Otsamo, Antti1, Tuomela, Kari1

& Vuokko, Risto2

Reforestation and Tropical Forest Management Project c/o Reforestation Technology Center, P.O. Box 65 (Jl. SeiUlin 28 B), 70711 Banjarbaru, Kal-Sel, Indonesia; 2Enso Forest Development Oy Ltd., Kuparintie 47, FIN-55100

Imatra, Finland; *Correspondence: Jl. P. Suriansyah 56, 70711 Banjarbaru, Kal-Sel, Indonesia; Fax + 62 511 93222

Abstract. Reclamation of former, degraded forest lands occu-pied by Imperata cylindrica is one of the crucial environmen-tal and forestry issues in the humid tropics, notably SoutheastAsia. We suggest that it is possible to gradually restore theoriginal natural forest cover with the help of a sacrifice fallowcrop of fast-growing exotic tree species. Recently, a set ofsuitable fast-growing plantation tree species has been identi-fied and stand establishment methods developed for this pur-pose. We assessed the regeneration of natural vegetation instands of different plantation tree species and evaluated theecological impact of species composition in the plantationunderstorey. PCA ordination, regression analysis and analysisof covariance were applied at different stages of the study. Wefound a marked vegetational resemblance between standsdominated by Acacia mangium: they had the highest numberof indigenous trees in their understorey, whereas stands ofother plantation trees supported more diverse grass and herbvegetation. A high proportion of evergreen woody vegetationreduces the risk of fire and grass competition and enhancessecondary succession towards natural forest.

Keywords: Acacia mangium; Natural regeneration; Refor-estation; Restoration ecology; Forest management.

Nomenclature: George (ed.) (1981); Anon. (1982).

Introduction

Along with the increasing population pressure andneed of new agricultural land, natural forests of south-east Asia are being decimated. During 1978 - 1988, thedeforestation rate has been 16 000 km2/yr. However, theforest area is actually diminishing twice as fast as thepermanent agricultural land is increasing (Anon. 1992).The difference may be accounted for by unsustainableland use practices, including repeated logging of valu-able timber particularly Dipterocarpaceae trees, whichoften predominate the upper canopy layers (Kostermans1992). Logging is usuall followed by illegal cutting ofthe residual timber, and subsequent shifting cultivation

with a short fallow period (see e.g. Eussen & Wirjahardja1973; Dela Cruz 1986 and references therein).

Abandoned shifting-cultivation areas are almost al-ways invaded by one of the most noxious colonizers ofthe degraded humid tropical forest soils, Imperata cylin-drica (alang-alang), a rhizomatous perennial pantropicalgrass which occurs over a wide range of soil conditionsand can take advantage of the alternation of moist andnot too long and severe dry seasons, typical for South-east Asia. Due to its high growth rate (8 - 20 t/ha/yr),I. cylindrica is a strong competitor for water, light andnutrients, it is also known to have allelopathic effects ontree seeds and other plants (Eussen & Wirjahardja 1973;Soerianegara 1980; Ohta 1992). Natural regeneration oftree vegetation on alang-alang grasslands is thereforeretarded or impossible. Regular fires over vast areascovered with dry Imperata shoots hamper any such suc-cession (Eussen & Wirjahardja 1973; Dela Cruz 1986).

The largest alang-alang grasslands are found in theinterior of the island of Borneo, on former dipterocarpforest soils. A frequently cited, though not accuratefigure for Indonesia alone is 200 000 km2 with an annualincrease of 150 000 ha (Soerjani 1970; Anon. 1990).Skerman & Riveros (1990) estimated that the total areaof Imperata grasslands throughout the tropics, naturalgrasslands included, is 2 000 000 km2.

I. cylindrica is known as a nuisance weed in agricul-ture. It suppresses the growth of annual and perennialcrops, and most farmers have little interest in reclama-tion of alang-alang lands for arable crops but preferslash-and-burn cultivation in primary or secondary for-ests instead. Large-scale animal husbandry is virtuallyabsent in Southeast Asia; moreover, the fodder value ofI. cylindrica is poor, except for a short period in thebeginning of the rainy season (Soerjani 1970). All in all,Southeast Asian alang-alang grasslands are largely con-sidered unproductive wastelands. For the benefit oflocal population and environment another type of landuse would be more valuable (Townson 1991).

206 Kuusipalo, J. et al.

Development of alang-alang grassland into tropical(preferably dipterocarp) rain forest is difficult due toannual fires and the lack of standing trees and a soil seedbank. Despite years of trial plantings, no feasible large-scale method has been found for restoring the originalvegetation. Planting of dipterocarp or other indigenoustree seedlings in the middle of grassland, even aftercomplete cultivation and regular spraying with herbi-cides, has had very limited success. A more suitablemethod is to establish a shade cover crop of fast-grow-ing plantation trees to gradually suppress the grass(Otsamo et al. in press). A special cover crop, like therubber tree, is too expensive to be used on a large scale.The most promising plantation trees to suppressI. cylindrica include Australasian Acacia spp., espe-cially A. mangium (Anon. 1983).

The rationale for the hypothesis that natural forestvegetation can be reclaimed with the help of a sacrificefallow crop of fast-growing plantation trees is that (a)physical properties and nutrient status of the top soil areimproved; (b) cover tree crop favours woody under-growth at the expense of more light-demanding grass-dominated vegetation because light, instead of nutri-ents, becomes a limiting factor for the understorey, and(c) biological diversity increases due to the more hetero-geneous and limited light availability, created by thecrowns of plantation trees (Tilman 1982).

Our hypotheses were that (1) the diversity of thenatural primary or secondary trees and (2) improvementof soil conditions, should be highest under trees formingthe densest canopies, which most effectively suppressgrasses and herbs.

Study area

The Riam Kiwa plantation area is located in SouthKalimantan, Indonesia, 3° 30' S, 115° E, at 100 - 200 ma.s.l. The topography is undulating with red-yellowpodzolic soils (ultisols), deeply weathered and heavilytextured, and partially replaced by oxisols (Burnham1984). The pH of non-forested grassland is 3.9 - 5.0,total nitrogen 0.025 - 0.33 % and clay content 54 - 58 %.Rainfall is ca. 2200 mm/yr, with a dry season from Mayto September, with only 20 % of the annual total.

Imperata cylindrica grasslands cover the area sinceWorld War II. In 1983, the Reforestation and NaturalForest Management Project, part of the Indonesia-Fin-land Forestry Programme, established a trial area inRiam Kiwa in order to study methods of reforestation.Today, 60 % of the 1000-ha trial area has been refor-ested. Since 1983, more than 100 species with variousprovenances have been tested; 10 - 15 species were foundpromising for grassland reforestation and different plan-tation methods have been tested (Otsamo et al. in press).

Sampling and statistical analysis

17 compartments out of the total of 40 were sam-pled randomly for the inventory. The compartmentsmeasured from 0.21 - 4.51 ha. 13 compartments wereplantations with one or two species planted 3 - 6 yrbefore the inventory; one was a 0.5-ha natural stand ofVitex pubescens; three, 0.5ha each, represented the origi-nal Imperata grasslands (see Table 1).

The inventory was carried out systematically alongthe planting lines - 2 m apart, including all occurringvascular plant species, representing the life forms trees,shrubs, climbers, herbs, grasses and ferns. The Vitexpubescens stand and Imperata sites were sampled simi-larly. Species occurrence was expressed as frequency.

Due to the varying size of the compartments, theeffect of sample area on the number of species waschecked with curve-fitting regression analysis.

Floristic relations between the compartment under-stories were described through Principal ComponentAnalysis, based on the Jaccard similarity index (Jongmanet al. 1987). Compartments were grouped for analysis ofcovariance according to the results of the PCA.

The two hypotheses mentioned in the Introductionwere tested with Covariance Analysis.

Results

151 vascular plant species, 89 trees, 19 shrubs, 13climbers, 11 herbs, 9 grasses and 10 ferns, were foundgrowing spontaneously in the 5 to 6-yr old plantations.Complete data are available upon request. Tree specieswith a frequency > 50 % in the plantations includeGlochidion capitatum, Vitex pubescens, Alstonia angusti-loba, Buchanania arborescens and Ficus grossularioi-des; the most frequent herbaceous species were Impera-ta cylindrica and Eupatorium pallescens. Both speciesproduce much above-ground biomass, particularly un-der open canopies. 18 of the tree species found in theplantations, belonging to 11 families, were also found inthe Imperata cylindrica grasslands. They are apparentlyadapted to the frequent occurrence of fires. Grassesaccompanying the dominant I. cylindrica include Saccha-rum spontaneum, Bromus insignis and Scleria spp.

Species-area relationships

The number of species appeared to be independentof compartment size (Y = 27.12 + 5.262 log X; r2 = 0.14).However, the number of tree species (Fig. 1) and thearcsin-transformed ratio: tree species / total number ofall vascular plant species (Fig. 2) were significantlyrelated to log compartment size.

- Restoration of natural vegetation in degraded Imperata cylindrica grassland - 207

Table 1. Data on the 17 plantation compartments and naturalstands, with year of planting, tree species, area (ha), numberof tree species, NT, and other vascular plant species, VT.

No. Yr Species Area NT VT

A 1987 Acacia mangium + Swietenia macrophylla0.91 13 1B 1987 Paraserianthes falcataria 2.75 16 21C 1986 A. mangium 0.84 10 6D 1986 A. mangium 1.98 29 5E 1991 Gmelina arborea 0.25 6 18F 1986 A. mangium 4.51 24 3G 1986 A. mangium 2.57 35 2H - Vitex pubescens (natural) 0.20 4 8I 1986 A. mangium + Shorea spp. 1.41 16 6J 1986 A. mangium 0.50 16 22

K 1986 P. falcataria 1.27 25 31L 1986 Species trial 2.31 14 3

M 1986 Eucalyptus deglupta 0.37 3 6N 1987 A. mangium + P. falcataria 0.85 13 2615 - Natural Imperata cylindrica (slope) 0.50 4 1016 - Natural I. cylindrica (valley) 0.50 9 1217 - Natural I. cylindrica (crest) 0.50 10 7

Principal component analysis

The approach was considered justified since thetotal number of species appeared to be statistically inde-pendent of sample area.

The results are shown in Fig. 3. No verbal interpreta-tions are given to the principal components; instead, ourinterest was in the configuration of different plantationcompartments in the floristic ordination. Three groupswere recognized.1. Most of the pure and one-mixed Acacia mangiumcompartments: D, F, G, I and J, are grouped together,whereas the A. mangium compartment C, trial compart-ment L - where A. mangium has currently taken over asa predominant species - and one of the Paraserianthesfalcataria trials (K) are in the vicinity, grouped togetherwith a mixed compartment of A. mangium and S.macrophylla (A) and a P. falcataria compartment (B).2. This group consists of a Eucalyptus deglupta stand(M) and a mixed stand of A. mangium and P. falcataria(N).3. This group comprises the pure Gmelina arboreastand (E) and a natural Vitex pubescens stand (H9 whichare clearly separated from the others. The Vitex andGmelina stands resemble each other in their very lownumbers of understorey species. In this group A. mangiumis missing.

Analysis of covariance

The arcsin- transformed percentage of the number oftree species out of the total number of all understoreyvascular plant species was compared in the threefloristically different subsets of samples, pure Acaciamangium stands, mixed A. mangium stands and stands

Fig. 3. Diagram of the PCA of compartments A - N (see Table1) for the first two principal components. Variance explainedby Axis 1 = 16.9 %, by Axis 2 = 14.5 %.

Fig. 2. Regression of the ratio: number of tree species / totalnumber of vascular plant species in the area of the compart-ment. Regression coefficient is significant (p = 0.01).

Fig. 1. Regression of the number of tree species and the area ofthe compartment. Regression coefficient is significant (p =0.01).

208 Kuusipalo, J. et al.

proved with the help of tree plantations; native speciesor agricultural crops can be interplanted later within therows of plantation trees (Parrotta 1992; Lugo 1993).This is an important option for land rehabilitation pro-grammes (e.g. Anon. 1991).

An interesting phenomenon is the rapid spontaneousemergence of indigenous trees and other plant speciesunder the plantations of exotics. The propagules of theseplants must have been dispersed into the plantations byanimal vectors or by wind from the narrow strips ofnatural vegetation along the creeks in the trial area orfrom the larger forests in the mountains several kmaway. The plants found in the plantations include trees,shrubs, climbers, herbs, ferns and grasses, and they maybe pioneer, secondary or primary forest species.

The plantations create microclimate and soil condi-tions which improve the germination and survival ofspecies unable to germinate in the open or withinI. cylindrica grass cover. Most of the tree species spreadnaturally to the plantation area are fruit-bearing plants,with animal vectors, notably birds and bats as the princi-pal means of dispersal. This suggests that plantationcanopy promotes the regeneration process by providingroosting habitats for seed-dispersing animals. Othershave light-weight seeds dispersed easily by wind over along distance. Plants characterized by a heavy-weightseed with no animal vectors, such as most of the diptero-carps, are absent in the spontaneous undergrowth(Whitmore 1972-1978). Our results thus give furthersupport to the findings from Central America presentede.g. by Lugo et al. (1993) and Parrotta (1993).

The majority of the tree species dispersed spontane-ously to the plantations belong to the pioneer or second-ary forest species characterized by effective means ofrapid dispersal. However, there were also 17 tree spe-cies typical of primary forests, including Buchananiaarborescens with a frequency of 57 %, Diospyros buxi-folia, Litsea firma, Dacryodes rostrata, Gluta wallichii,Sandoricum borneense and Stergulia rubiginosa. Thesespecies occur frequently in the nearby patches of naturalforest vegetation still persisting on hilltops and deepgullies along the brooks and rivers (Whitmore 1972-1978).

The proportion of trees was largest in pure or mixedstands of Acacia mangium. This tree is known as a veryeffective species in suppressing light-demanding grassand herb vegetation, mainly due to its dense crown,which also creates a moister and cooler microclimate,and abundant litter fall (Anon. 1983; Otsamo et al. inpress). It is evergreen even during a prolonged dryseason in climatic conditions typical of humid tropics(Anon. 1983). A. mangium is a nitrogen-fixing legume;nitrogen-rich litter increases the amount of organic in-put into the soil maintaining its biological activity and

where A. mangium was not planted. Due to the depend-ency of the dependent variable on the size of the sampleplot (see Figs. 1 and 2), compartment size was used as acovariate in the analysis (Table 2).

The result is quite straightforward. Both stand typeand compartment size have a reasonably significanteffect with a statistical risk level of about 1/20 on theratio between the number of tree species and the numberof all species. Multiple a posteriori comparison of means(LSD) reveals that both pure and mixed A. mangiumstands differ significantly from all other stand types butnot from each other. In other words, the proportion oftree species is significantly higher in pure or mixed A.mangium stands than in the rest of the stands, wheregrasses and other herbaceous vegetation prevail.

Discussion

Reforestation of Imperata cylindrica grasslands isextremely difficult due to compact and nutrient-defi-cient soil, hydrologic instability, large variation in sur-face temperatures of the soil, grass competition andallelopathy, and high fire susceptibility of the grass(Soerianegara 1980; Ohta 1992; Parrotta 1993). Naturalsuccession processes are prevented by annual fires,which, together with other conditions listed above, de-stroy plant propagules and seedlings, as well as fungalroot symbionts.

Problems of reforestation have already been studiedintensively in the Indonesian-Finnish trial area of RiamKiwa for more than ten years. More than 100 specieshave been tested, including fast-growing exotic planta-tion species already mentioned above, but also easilypropagated indigenous trees have been planted and evalu-ated in the area, including dipterocarps such as Shoreaspp., Hopea spp. and Vatica spp., and others, e.g. Duriospp., Peronema canescens, Artocarpus spp., Macarangaspp. and Agathis borneensis.

Several exotic species of pioneer tree character haveshown promising reforestation results in terms of growthas well as in suppressing I. cylindrica. However, asuccessful establishment of these species requires ratherheavy site preparation, which turns over the rhizomes ofI. cylindrica and improves the rooting of tree seedlings(Otsamo et al. in press). On the other hand, reforestationwith native climax species directly on grassland hasproved impossible or at least very laborious and expen-sive (e.g. Appanah & Weinland 1993). Low survival ofthese most probably results from shallow rooting un-able to penetrate into compact soil, and from the lack ofadequate fungal root symbionts (e.g. ectomycorrhizaeof dipterocarps). Rehabilitation of original vegetationseems feasible only if environmental conditions are im-

- Restoration of natural vegetation in degraded Imperata cylindrica grassland - 209

Table 2. Analysis of covariance and a posteriori comparisonof means (LSD) of the Riam Kiwa data. Legend to variables:

ASNREL (Dependent variable): Ratio between number of tree speciesand total number of vascular plant species; AREA (Covariate): Areaof compartment (ha); EXTRA (Independent class variable):1. Stands without Acacia mangium (n = 5)2. Pure A. mangium stands (n = 5)3. Mixed A. mangium stands (n = 4)

Analysis of covarianceSource Sum-of squares DF Mean-square F-ratio P

Extra 0.445 2 0.223 3.901 0.056Area 0.284 1 0.284 4.976 0.050Error 0.571 10 0.057

Fisher’s least-significant difference test1 2 3

1. Stands without A. mangium 1.0002. Pure A. mangium stands 0.045 1.0003. Mixed A. mangium stands 0.031 0.867 1.000

improving the nutrient status (Sanchez & Miller 1986).Phosphorus uptake is efficient due to the symbiosis withcompatible vesicular-arbuscular (VA) mycorrhizal fungi(Malloch et al. 1980). All in all, A. mangium seems tofavour shade-tolerant, relatively nutrient- and moisture-demanding tree species at the expense of light-demand-ing herbs and grasses in its understorey.

Inside the Eucalyptus deglupta plantation the lightintensity was seemingly much higher than in A. mangiumplantations of the same age, and the amount of I.cylindrica grass consequently much larger. Poor shad-ing and vigorous competition by grass may have inhib-ited the germination and survival of the indigenous plantseedlings. Plantations of Paraserianthes falcataria pro-vided better shading than E. deglupta, and I. cylindricahad been mostly replaced by other plants. However, thestand was light enough to allow the weed Eupatoriumpallescens to become dominant, followed by Clibadiumspp. as the second commonest shrub. The shrubby under-storey of P. falcataria is also pyrogenic, making thestand very susceptible to fire during the dry season.

Some of the promising tree species were perhapsunderrated in this study due to a limited sample ofstands; e.g. Gmelina arborea was represented just byone recently established compartment, although it isknown to have a good capability to suppress the grass(Otsamo et al. in press).

Successful plantations seem to be able to act asinitial steps in the secondary succession of I. cylindricagrasslands back to natural forests by allowing manyindigenous forest tree species to colonize the site withina relatively short period of time. Acacia mangium is aparticularly suitable species because it turns the re-source supply ratio into a more favourable one forwoody perennial undergrowth: light becomes a morelimiting resource than nitrogen, moisture and other soilresources. Grasses and herbs cannot take advantagefrom improved soil conditions due to their generallyhigher light demand compared to woody perennialswhich, in turn, can only establish on an improved soil(Tilman 1982).

The more woody, evergreen undergrowth, the lessersusceptibility to fire damage. This is a rule of thumbwhich has, for a long time, been appreciated by forestersand forest scientists working in Imperata areas, as wellas elsewhere in the tropics (Lugo et al. 1993; Parrotta1993). A. mangium seems to provide an alternativewhich creates suitable conditions for an abundance ofunderstorey trees. The results of this study supportsuggestions made by Parrotta (1993), i.e. that forestplantations on degraded tropical lands can be used notonly for production of wood and other forest commodi-ties but also for soil improvement and acceleration ofsecondary forest succession. Species and provenance

selection, together with plantation design and silviculturalpractices, play a key role in realizing these objectives(Otsamo et al. in press). Nitrogen-fixing species such asAlbizia lebbek used by Parrotta (1993) in Costa Rica, orAcacia mangium in the present study, can have a dra-matic effect on soil fertility through their production ofdecomposable, nutrient-rich litter and turnover of fineroots and nodules.

The influence of the proximity of natural forests onseed recruitment into plantations poses a special prob-lem when using fast-growing tree plantations as ‘fosterecosystems’ (Parrotta 1993). Understanding of seed ecol-ogy, including dispersal characteristics and habitat re-quirements for germination and seedling developmentis of particular importance in restoration plantation de-sign. Many important species, e.g. most of the dipterocarptrees predominating Southeast Asian moist forests haveheavy, seeds and, subsequently, spread very slowly intoareas where mother trees have completely disappeared(Appanah & Weinland 1993). Planting of dipterocarpsand other primary forest species characterized by a slowdispersal within the shelter of plantation tree canopiesand spontaneously grown natural trees would, therefore,speed up the restoration process considerably.

Acknowledgements. We thank Mr. A.P.S. Sagala and twoanonymous referees for their invaluable comments, and Dr.John A. Parrotta for sending us literature we needed for thecompletion of the study at our remote base. The study wascarried out under the auspices of the Reforestation and Tropi-cal Forest Management Project financed by the Finnish Inter-national Development Agency (FINNIDA) and implementedas a joint effort of the Ministry of Forestry, Indonesia, andEnso Forest Development Ltd., Finland.

210 Kuusipalo, J. et al.

viron. 41: 115-133.Parrotta, J.A. 1993. Secondary forest regeneration on de-

graded tropical lands. The role of plantations as ‘fosterecosystems’. In: Lieth, H. & Lohmann, M. (eds.) Restora-tion of Tropical Forest Ecosystems, pp. 63-73. KluwerAcademic Publishers, Dordrecht.

Sanchez, P.A. & Miller, R.H. 1986. Organic matter and soilfertility management in acid soils of the tropics. Trans. 18Int. Congr. Soil Sci. 6: 609-625.

Skerman, P.J. & Riveros, F. 1990. Tropical grasses. FAOPlant Prod. Prot. Ser. 23: 1-832. Rome.

Soerianegara, I. 1980. The alang-alang, (Imperata cylindrica(L.) Beauv.) problem in forestry. BIOTROP Special Pub-lication 5: 237-242.

Soerjani, M. 1970. Alang-alang (Imperata cylindrica (L.)Beauv.). Pattern and growth as related to its problem ofcontrol. BIOTROP Bulletin No. 1.

Tilman, D. 1982. Resource competition and community struc-ture. Princeton University Press, Princeton, NJ.

Townson, J.K. 1991. Imperata cylindrica and its control.Weed Abstracts 40: 457-468.

Vandenbelt, R.J. (ed.) 1993. Imperata grasslands in SoutheastAsia: Summary reports from the Philippines, Malaysiaand Indonesia. Multipurpose Tree Species Network Re-search Series, Report Nr. 18. F/FRED Project, WinrockInternational and USAID.

Whitmore, T.C. 1972, 1973, 1978. Tree Flora of Malaya.Manual for the Forester. Vols. I, II and III. Longman,Singapore, London.

Received 1 July 1994;Revision received 3 November 1994;

Accepted 14 November 1994.

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