Supplementary Discussion 2. Bimodal flowering and bimodal rainfall seasonality
near the equator.
Latitude North 12
Figure 4 Rainfall distribution (filled bars) in Colombia
between the equator and 10 oN, bimodal flowering of
Tabebuia rosea in Cali (hatched bars) and location of the
Intertropical Convergence Zone.
The Intertropical Convergence Zone (ITCZ) is an area
8
of low pressure that forms where the Northeast Trade
Winds meet the Southeast Trade Winds near the
earth's equator. As these winds converge, moist air is
forced upward and cooled, causing a band of heavy
Tabebuia rosea
precipitation around the globe26. This band moves
4
seasonally with the area of most intense heating
Intertropical
Intertropical
caused by the zenithal position of the sun (‘summer
Convergence
ConvergenceZone
rains’) and determines the timing of the wet and dry
seasons at low latitudes (Figs. 4, 5b). At the ‘climatic
equator’ there may be two wet and dry seasons (Fig. 4,
Ibagué).
0
The ITCZ, and hence the climatic equator, is not
Equator
centered strictly at the geographic equator and, due to
variation in land temperatures, over land it ventures
farther north or south than over the oceans. In Colombia, the climatic equator is centered
at ~ 4.5 oN (Fig. 4). In northeastern South America, the ITCZ runs from SW to NE
through Guyana, as indicated by the temporal shift in the dry seasons (Fig. 5b).
The flowering periodicity of many tropical tree species is determined by rainfall
seasonality6. In most species of Tabebuia, resting flower buds open soon after leaf
shedding during the dry season. Thus, T. serrata flowers February – April in Venezuela,
north of the ITCZ, July to November in French Guiana and Amazonia, Brazil, south of
the ITCZ, and during both these periods in Guyana (Fig. 5c). Similarly, in Colombia, T.
rosea has a bimodal flowering pattern near the ICTZ (Fig. 4; Z. Calle, unpubl.), but, as in
Central America, a single flowering period in February-March in the north6. The
flowering phenology of Lecythis corrugata is similar to that of T. serrata (Fig. 5d). In
coffee (Coffea arabica), flowering is induced by the first rains of the wet season and
fruits are harvested 7-8 mo later. Consequently, from northern (10 oN) to southern
Colombia (1 oN; Fig. 4) the time of the principal coffee harvest changes from Nov/Dec to
May/July, respectively, and near the ICTZ there are two harvests27,28.
Bimodal flowering induced by bimodal rainfall is characterized by:
 Flowering during or immediately after a distinct dry season with 2-3 mo of rainfall
below 50 mm/mo (Figs. 4, 5b).
 Relatively long flowering periods caused by large inter-annual variation of rainfall
and flowering (Fig. 5c, d).
 Bimodal flowering is observed near the climatic equator, which in South America is
centered at 4-5 oN (Figs. 4, 5a). In the Guyana’s, the shift from ‘northern’ to
‘southern’ rainfall and flowering patterns occurs at 3.5 - 4.5 oN, with little change in
latitude along a West-East gradient crossed by the ITCZ (Fig. 5).
1
Figure 5 a – location of
the Intertropical Convergence Zone in northeast
South America. b-d –
Changes in rainfall
seasonality (b) and
flowering periodicity (c, d)
from N to S of the ITCZ.
Flowering data for
Tabebuia and Lecythis
were obtained from
herbarium collections at
the New York Botanical
Garden.
a
b
Flowering (%)
50
c
Tabebuia serrata
Lecythis corrugata
40
Venezuela
30
Roraima
Guyana
d
French Guiana
Amazonia
20
10
0
J
F M A M J
J
A S O N D J F M A M J J A S O N D J
In contrast, bimodal flowering induced by seasonal variation in sunset time (Fig. 2 )
occurs near the equator in Amazonian rainforests without a regular dry season and
only exceptional months with <100 mm rain.
References
24. Nieuwolt, S. Tropical climatology: an introduction to the climates of low latitudes
(Wiley, London, 1977).
25. Trojer, H. The phenological equator for coffee planting in Colombia. Pp. 107-113 in
Agroclimatological methods (UNESCO, Paris, 1968).
26. Müller, L.E. Der Wasserhaushalt als begrenzender Faktor beim Anbau tropischer
Nutzpflanzen. Ber. Dt. Bot. Ges. 80:53-60 (1967)
2