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Is there a difference
between the white water floodplain forests (várzea)
and black water floodplain forests (igapó) in relation
to number of species and density?
"Short communication"
Leandro Valle Ferreira
Projeto Dinâmica Biológica
de Fragmentos Florestais (PDBFF) - Instituto Nacional de
Pesquisas da Amazônia (INPA) - Manaus, Amazonas, BRASIL
RESUMO
Na Amazônia Central
existem dois tipos de florestas que são periodicamente inundadas
pela variação do nível dos rios. Àquelas inundadas por rios
de água branca, são denominadas de florestas de várzea,
enquanto aquelas inundadas por rios de água preta, são chamadas
de florestas de igapó. O objetivo desta nota foi comparar
os resultados dos inventários florísticos realizados nos
dois tipos de florestas inundadas, em relação ao número
de espécies e densidade. Os resultados obtidos mostraram
que não existe diferença significativa no número de espécies
e de árvores em relação ao tipo de floresta inundada.
Palavras-Chaves:
Amazonia, florestas inundadas, período de inundação, riqueza
de espécies
INTRODUCTION
Yearly water level fluctuations
in Central Amazonian floodplain forests reach 14 meters,
and the period of inundation can last up to 270 days (9).
Duration and predictability of the flooding period are an
important selective factor, influencing ecological processes
such as number of species, diversity, plant distribution,
seed and fruit dispersal, and reproductive phenologies (6,
7, 8, 9, 10, 14).
In the Amazon region
there are seven main types of floodplain forests (15). Of
these, the most representative are those periodically flooded
by white water rivers (várzea) and by black water
rivers (igapó). Várzea and igapó forests have unique
structural and floristic composition which result from differences
in sediment load and nutrient content of the water (10).
Várzea forests occur in geological formations of the quaternary
period, more recent and rich in sediment and with higher
primary production. Alternatively, igapó forests occur in
older formations of the tertiary period, which have low
levels of sediments and normally with lower primary production
(1).
The general view is
that forests of white water (várzea) river system have higher
biomass than forests of black water (igapó) river systems
(13, 16). However, some authors have also suggested that
várzea forests have higher number of species than igapó
forest (12), while other authors report the opposite (13).
There are two important problems in the comparisons carried
out so far. Firstly, the great variation in sampling methodology
-- such as minimum DBH, sampled area, and plot shape --
renders direct comparison between different floristic inventories
somewhat problematic, and secondly, a large variation in
the physical characteristics of the floristic inventory
sites, such as flooding period, topography, water quality,
and soil type.
The effect of flooding
duration, a critical factor in the determination of richness
and diversity patterns in floodplain forests (7, 9), has
been ignored by most authors when comparing floristic inventories.
A re-examination of
the floristic inventories performed in Amazonian floodplain
forests as cited in the literature, suggests that there
is no difference in the number of species between the two
floodplain forest types.
Seven floristic inventories
exist for igapó floodplain forests of Central Amazonia:
four in the Rio Negro (11, 14, 18, 19), two in the Rio Tarumã-Mirim
(6, 20), and one in the Rio Jaú (7). A further two igapó
inventories exist for Eastern and Western Amazonia: one
in the Rio Xingú (3), and another in the Rio Japurá (1).
In várzea floodplain forests, six floristic inventories
exist: two in Eastern Amazonia (2, 15), two in Western Amazonia
(1, 5), and two in Central Amazonia (17, 21).
The purpose of this
note is to compare number of species and density between
igapó and várzea forests from these published data. Because
of the differences in methodologies between the inventories,
some form of standardized comparison was necessary. To control
for minimum diameter at breast height (DBH) of trees in
the inventory, I only considered trees with a minimum DBH
of 10 cm. I controlled for differences in sampled area by
Analysis of COVARIANCE using inventory area as a covariate.
This resulted in a comparison of 5 várzea inventories and
5 igapó inventories for number of species and trees per
ha (Table 1). For both variables, I tested for the assumption
of homogeneity of slopes between the two forest types using
ANCOVA. In the absence of a significant interaction term,
I then tested for the main effect of difference between
forest types. No significant interactions were
found for any of the
variables tested (P=0.111 for number of species; P= 0.06
for number of trees). Likewise, there were no differences
between forest types for either of the variables once corrected
for area (P=0.406 for number of species, P=0.610 for number
of trees).
Table 1 - Comparison
of total or mean (*) number of species, trees, location,
and sampling methods in the floristic inventories performed
in várzea and igapó floodplain forests in the Amazon region.
Study |
Location |
Number
|
Sampled |
DBH |
Number
of |
Number
of |
|
|
Of
plots |
area
(ha) |
limit
(cm) |
species |
trees |
Várzea
forest |
|
|
|
|
|
|
Black
et al. 1950 |
Rio
Guamá |
1
|
1.0
|
10.0
|
60.0
|
423
|
Pires
& Koury 1958 |
Rio
Guamá |
1
|
3.8
|
10.0
|
107
* |
612
* |
Rankin
de-Merona 1988 |
Rio
Amazonas |
2
|
0.4
|
10.0
|
34.0
* |
217
* |
Campbell
et al. 1992 |
Rio
Juruá |
3
|
3
|
10.0
|
66.0
* |
573
* |
Ayres
1993 |
Rio
Japurá |
2
|
1
|
10.0
|
122.0
* |
498
* |
|
|
|
|
|
|
|
Igapó
forest |
|
|
|
|
|
|
Piedade
1985 |
Rio
Negro |
1
|
0.5
|
10.0
|
64.0
|
460
|
Campbell
et al. 1986 |
Rio
Xingú |
1
|
0.5
|
10.0
|
40.0
|
220
|
Ferreira
1991 |
Rio
Tarumã-Mirim |
1
|
1
|
10.0
|
90.0
|
451
|
Ayres
1993 |
Rio
Japurá |
1
|
1
|
10.0
|
119.0
|
546
|
Ferreira
in press. |
Rio
Jaú |
3
|
3
|
10.0
|
109.0
* |
943
* |
These results contest
the assumption that there are significant differences in
the number of species and trees between the two floodplain
forest types.
As a result of this
comparison of floristic inventories, I would suggest that
to gain a better understanding of Amazonian floodplain forests,
floristic inventories are standardized regarding location,
methods, data analyses, and flooding condition.
The sampled area should
be at least one hectare, with replication in different rivers.
Plot shape should be a square of 100 x 100m, divided in
25 quadrats of 20 x 20m. This shape have many advantages
because they sample more-compact and homogeneous areas than
belts or rectangular samples, square plots are usually superior
for correlating plant communities or populations with local
environmental variables. Square plots are also easier to
map and establish than other shapes and are less influenced
by edge effects (4). Minimum DBH recommended is 10cm, which
appears to be a the norm already used in floristic inventories
in Amazonia (4). Most important, all quadrats of the plot
must be identified as to the specific flooding condition
due to importance of this factor to species richness, diversity
and plant distribution (6,7).
ACKNOWLEDGMENTS
I thank Dr. Claude Gascon,
Susan G. Laurance, Glenn Shepard, and Maria N. F. da Silva
for helpful comments. INPA/MAX-PLANCK Project invited me
to explore the interesting Amazonian floodplain forests.
This note represents publication 004 in the Jaú National
Park technical series.
REFERENCES
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- Black, G.A., Dobzhansky, T.; Pavan,
C. Some attempts to estimate species diversity and population
density of trees in Amazonian forests. Bot. Gaz. 111:
13-425, 1950.
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