Litterfall and nutrient return in tropical rainforest in ... · 59 % and 71 % of total amounts in CCAF and Malanga stand respectively. As stated by O'Connell [S], leaves are usually

Litterfall and nutrient return in tropical rainforest in the Chaillu area (southwest Congo)

J . J . Loumeto University of Brazzaville (Marien Ngouabi), Laboratoire de Botanique et ~ c o l o ~ i e , Brazzaville, Congo

Abstract

The amount of litterfall and nutrient return in two stands of tropical forest were studied over one year in the Chaillu region (southwest Congo). The aim was to investigate the validation of generalizations regarding litterfall and nutrients return in tropical rain forests and to quantify the ecological characteristics of experimental plots. Litter production was measured during 19 months in Malanga plots and 16 months in CCAF plots. The annual input was 6.1 and 10.7 tlha in Malanga stand (primary forest) and CCAF stand (secondary forest) respectively. Leaf litter accounted for at least 59 % of the total amount in both plots. The annual return of elements through the litter of both plots were different : 88.1 - 1 57.8 kgha for nitrogen, 3.7 - 6.2 kglha for phosphorus, 1.7 1 - 38.7 kglha for potassium, 28.8 - 70.1 kgha for calcium, and 21.2 - 45.0 kdha for magnesium. The amounts of litter biomass and nutrients input were lower in Malanga stand. These amounts are low, compared to the values generally quoted in tropical area in Africa, due to a long dry season in the Congo. Seasonnal variations of litterfall and most nutrient concentrations were irregular. Mean nutrients concentration of each litterfall component are indicated. They can be arranged as follow : N > Ca > K > Mg > P. Aiicolrmeu kluinmat~a is an important tree species of this forest. Its litterfall and nutrient contents are underlined.

Transactions on Ecology and the Environment vol 64, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541

Situation geographique de Ngoua 2 D! ,g.. 1

Figure 1: Geographic localization of the study site.

Figure 2: Monthly rainfall variation at Ngouha 2 in the Congo (1982-1983 period; units: mm).

1 Introduction

Litterfall has a key hnction for energy and nutrients transfert in plant-soil interface. Litter production and nutrient inputs by litter in natural forest ecosystems are widely studied in tropical forest. However few studies are concerned with the Congo or with the countries in the vicinity of the Congo (Gabon, Cameroon and Za~re).

The Congolese forests cover more than 60 % of the national territory corresponding to 9.9 % of the closed forests of the African continent, and 12.3 % of those of Central Africa. The forests of southwestern Congo, which occupy


at least 5 millions ha, are the natural expansion area of Limba (Terminalia superha) and Okoume (Alrcolmen klainemma). These species have a high economic value (Loumeto [ l ] ) . Primary congolese forests are estimated to cover 8.0 million ha (Locatelli [Z]).

The aim of our work is to contribute to the validation of generalizations regarding litter production and nutrient inputs in tropical rainforests. Okoume (Aucoumea kluitrema) is an important economical tree in Central Africa and one of the most important timber ressources of the Congo. Almost no data are available about the litter system of this species. We provide some informations about it. As stated by Guo & Sims [3], the amount of litterfall and nutrient, and any seasonal variations need to be clearly understood in order to manage nutrient and achieve long-term sustainable land use.

2 Materials and methods

2.1 Site

The study was carried out at Ngoua 2 ( Latitude = 2%" S, Longitude =

12"25" E, and Altitude = 250-600 m) as shown in figure 1 . The climate is sub-equatorial, characterized by the alternancc of two seasons (figure 2): the rainy season (october - may) and the dry season (june - september). Mean annual rainfall is 1644 mm and mean temperature is 25°C. Natural vegetation is dominated by some plant families such as Leguminosae (Caesalpiniaceae and Mimosaceae), Irvingiaceae, Meliaceae and Burseraceae. Among the characterisic species of secondary forest stands, there are : Alstonia hotmci, At~trocar-yotm rnicr.s/er, Aucoumeu kluitmemma, (.h.t~urizrm schweinfir~hii, Ceiba perlkuldru, Milicio exccl.s~r, F~mrtumia nJi.icaticr, Hur~~tlgmc~ n~c~d~rgcr.sccrrieisis, Holoptelccr ptmrlis, K h q a mthuthec~r, Mzrsat~g~r cecropioides, I'etztackerhra nmcrophylln. I'eter.sint~ih~r.s t71naocar-prrs. I'rerygotcr macrocurper, Pycticrtithw nr1goletmsis. Ricimdetldrotl he~rdelotii, liema guit~eet~.si.s, Ibrminnlicr superba and Xylopia ue~hiopicu (Teillier (41). Soils are ferrallitic according to the french classification. They are sandy, acid and have a low exchangeable capacity (Table 1).

Two forest sites were selected in the experimental plots of the ITTO Project PD77/90(F). They differed both in structure and species composition. Mnlarmgn stctt~d is a primary forest and CCAF forest is secondary forest according to Brown & Lugo [5] definition. Understory ground cover and abundance-dominance of non-woody plant groups of the studied stands are indicated in table 2

2.2 Methods

2.2.1 Litterfall measurement Fine litterfall defined as leaves, reproductive parts and twigs (Lips and Duivenvoorden, [ 6 ] ) was measured in 10 traps for each stand. Each trap was a

wooden frame of 1 m2 area (1 m X 1 m) with a 2 mm mesh. Collections of fallen material were made every 2 weeks. Litterfall was oven-dried at 65°C. Collections have been made during 19 months in Malanga forest and 16 months in CCAF forest.


Table 1: Some characteristics of the upper soil layer (0-5 cm) of the studied sites (C = organic carbon; N = total nitrogen; P = total phosphorus; E.C. = exchangeable cations in meq).

Malanga CCAF Forest lorcst

Loam (S)

Sandy (%)

1 Total E.C. i [ 0.73 1 0.33 1

2.2.2 Nutrient in litterfall One-month leaf litterfall was gathered for chemical analysis. Woody parts were lumped together into one sample for each stand. Annual nutrient input by leaf litterfall was calculated according to the following steps: i) monthly nutrient concentration (MNC) ii) monthly nutrient amount (MNA) = MNC X monthly biomass iii) mean of MNA iv) annual nutrient amount (ANA) = mean of MNA X 12. Chemical analysis for plants and soils were made at IRD (ORSTOM) Laboratory (french Research Center) at Pointe-Noire (Congo).


Table 2: Understory ground cover (9%) and abundance-dominance coefficient (Braun Blanquet coefficient) of the main non-woody plants groups.

2.2.3 Statistical analysis

Statistical analysis were performed on the basis of litterfall per month taking into account the whole sampling time. The Fisher test was used for correlations between rainfall and litterfall, while ANOVA was practiced for comparisons.

3 Results and discussion

3.1 Litter production

3.1.1 Annual litterfall

Annual litterfall for both study stands is indicated in table 3. Total litterfall for one year (October 97 - October 98) amounted to 6.1

tha and 10.7 tlha respectively for Malanga stand et CCAF stand. Litter production was higher in CCAF stand, due to its secondary

forest characteristics. Lo~ging was carried out in this area during 1960 -1975 (Loumeto et al. [7]). It is known that secondary forests are fast growing ecosystems, and their rate of net primary production exceed that of primary forests by a factor 2 (Brown & Lugo [S]). A secondary forest has more species than the mature one it replaces, and it develops maximum leaf biomass early in their development (Brown & Lugo [5]). Species richness also can be important and it was higher in CCAF stand.


Figure 3: Monthly litterfail variation (unit: g/m2) in Malanga forest.

Okoume leaves Others leaves

80 - 60 -

40 - 20 -

r-

- 80

- 60

- 40

- 20

i5 2;2?% i8r.;';3z-

T

O 25 23 28 29 27 31 28 7 13 14 At197 S 0 N F98 Jt At 0 N D98 At197 S 0 N F98 Jt At 0 N D98


l , 0 k o y - 1 , 160, , , Others , , leaves , , , , , ,

Figure 4: Monthly litterfall variation (unit: g/m2) in CCAF forest.

Woody parts , ,Regro~ucf ive, paps , ,

-

80 - - 80- - 60- - 40- - 20-

I

24 28 28 29 30 31 30 14 15 o 2 2 z .26* S97 0 N97Jn97 Jt At S N D98 S97 0 N97Jn97 Jt At S N D98


1244 Ecosvstems and Sustainable Development IV

Table 3: Litter production in Malanga and CCAF forest (October 97-October 98, kgha with standard error).

Malanga ] CCAF ~ k o u m c leaves 1

314 + 63 408 + 12 Others leaves

M~scaellenous

-- ...... - ..........-.-.. """"

The contribution (% of total litter) of Okoume leaves (3.7 - 3.8 %) and woody parts (25.3 - 27.8%) were similar in both forests. Leaf litter represented 59 % and 71 % of total amounts in CCAF and Malanga stand respectively. As stated by O'Connell [S], leaves are usually the main contributor to litterfall. The proportion of leaves in litterfall is affected by age and by the density of trees (Guo & Sims [3]). The high amount of miscallenous in CCAF site was due to long interval collections (April 98).

Amounts obtained in these stands lie within the low values recorded in other tropical rainforest (5-15 t/ha Proctor [9], Vitousek & Sanford [10]; in Africa, 8- 15 t/ha, Mosango [ l 1,121, Mosango et Lejoly [13]). However, they can be approched to those obtained in another congolese rainforest (Mayornbe) studied by Schwartz [14].

3.1.2 Monthly litterfall variations

Variations of litterfall amount are presented in figures 3 and 4 for Malanga stand and CCAF stand respectively. Litter was produced during the whole year whith significant differences between months, for all fractions, especially for non-okoume leaves, woody part and total amount in Malanga forest (Table 4). For okoume leaves, the lower amount was recorded during the dry season (June- August : Figures 3 and 4).


Table 4: ANOVA parameters for the between-month comparison of litterfall (F = anova coefficient, p = error probability, NS = not significant).

No correlations were obvious between rainfall and litterfall in either stands. The higher correlation was noted in Malanga forest for okoume leaves (r = 0.627 , p = 0.05). Netherveless, the resumption of precipitation, after the dry season, coincided with the higher amount of most litter categories (October - November). This period was that of the highest rainfall.

Many studies demonstrated the seasonal and annual variations of litterfall in tropical forests, where a relation with weather conditions such as temperature and precipitation was ment io~ed (Yamashita et al. [l 51). However, some authors noticed that rainfall did not appear to be a major determinant of litterfall (Herbohn & Congdon 1161, Lips & Duivenvoorden [6]), soil nutrient status being probably more important.

3.2 Nutrients

3.2.1 Monthly variations No clear seasonnal variation was noted for nutrient contents in leaf litter (okoume leaves or non-okoume leaves), neither in Malanga stand nor in CCAF stand. Many authors have also reported the absence of a significant effect of season on nutrient concentrations in litterfall in tropical rainforest. The presence in variable proportions of organs from different species which constitute the litter, and especially leaves, can be the main reason of this variability (Franken et al. 1979 in Smith et al. [ l 71). I'he intraspecific variations of Okoume leaves might result from fluctuations in withdrawal before the abscission, or from leaf age. The periods of high contents might correspond to the fall of non-senescent leaves (wind, insects, etc.).

3.2.2 Annual input Annual nutrient input by litterfall and mean concentration are indicated in table 5. Nutrient contents were higher in CCAF forest in most cases; the contrary was observed for nitrogen in leaves (okoume and non-okoume leaves) and phosphorus in woody part. Total amounts showed that nitrogen followed by calcium were both the most important nutrients. Phosphorus showed the lowest amounts. Most nutrients were brought by leaves. Total nutrient amounts and concentration in any litterfall fraction showed that nutrients may be arranged as follow: N > C a > M g > K > P .


1 246 E c o ~ y ~ t c r n ~ atzd S~c~rarnabIe De~dopmct~r /V

Table 5: Annual nutrient input to the forest floor bv litterfall (C = concentration, units: %: A = arnoint, unit: kgka).

[ Woodvpart I A (kglha) 19.2 ................ 0.4 ...................................... " d . - .......................... f I

Total I i l .. ( k ~ f h a 107.3 3.7 .- .................................................. "- .....s, ................................

CCAF [Okorrrrre lrnves C (%) I 1.34 L...... 0.57 .. ......................... I ................................. ........................... L ,.L., -.. .....*.. ...- l Okoume leaves A Jk~-.i- ~/ha( 5.9 1 Z O I.-.--.~----- --.-......- i Ollrc~rs lecrves C I ) 1 .87 0.51 . ...-..-...-M- .......................... j-. ---.W.-

t

1 Toval 1 A (kglha) [ 95.0 1 5.9

Nutrient amounts were higher in CCAF stand, consequently to litterfall mass. Brown and Lugo [S] mentionned that secondary forest return more nutrients to the forest floor than do primary forest.

Secondary forest has a higher capacity of production and nutrients recycling than primary forest due to the average age of trees. A lot of young trees are in their highest growth period. The soil of the Malanga stand had a higher concentration in exchangeable cations, and the turnover was higher in the secondary forest corresponding to the higher needs of the vegetation.


4 Conclusion Litterfall and nutrient inputs were higher in CCAF stand which was a secondary forest, except for phosphorus input. There was a seasonal pattern for most fractions, but not clearly linked to rainfall, except during the resumption of precipitation (Octobre-November).

The litter annual input was 6.1. and 10 7 tlha in Malanga stand (primary forest) and CCAF stand (secondary forest) respectively. Leaf litter represented at least 59 % of the total amount. The annual return of elements through the litter of both plots was different : 88.1 - 157.8 kdha for nitrogen, 3.7 - 6.2 kglha for phosphorus, 1.7 1 - 38 7 kslha for potassium, 28.8 - 70.1 kgha for calcium, and 21.2 - 45.0 kglha for magnesium. Compared to others data for tropical rainforest especially in Africa from literature, the values recorded in the congolese forests were among the lowest.

The data provided here should contribute to a better management of congolese forests to guarantee their sustainable production. The logging occuring in southwestern congolese forests since 1950 results in the decreasing production, or in decreasing the high grading in this area, for instance for Okoume (Azrcoumea klarrmma) which is commonly used for plywood (Franzini [lSl).

This work is a component part of an ecological study g,oing with a project supported by ITTO (PD 77/90 (F) which carried out a silvicultural experiment in tropical closed forest by girdling low economic~value species in order to favor the growth of high grade trees. On the other side, we wanted to emphasize the difference concerning hnctioning between primary and secondary forests The area of secondary is increasing over the world (Brown & Lug0 [5]), such as in the southwest Congo where unmanaged logging dated at least 50 years ago

Then as pointed out by by Vitousek and Sanford 1101, it makcs no more sense to describe a "typical" tropical forest than to describe a "typical" temperate forest. For instance, patterns of nutrient cycling in tropical forest are diverse.

Acknowledgements

Thanks must be given to : - International Foundation of Science (I.F.S.) for financial support ; - ITTO Project PD 77/90 (F) Ngouha 2 - sud for access to an

experimental field ; - C.P.A.L. (Centre Pilote d7Afforestation en Limba) for climate data

and facilities ; - Madam F. Reversat for review of the manuscript ; and, - local personal (5. Mouanda Nazo, P. Kama and Mantinou Goma) for

technical assistance.


References

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Litterfall and nutrient return in tropical rainforest in ... · 59 % and 71 % of total amounts in CCAF and Malanga stand respectively. As stated by O'Connell [S], leaves are usually