Aldáz, Iván

Tye, Alan

English

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Noticias de Galápagos Decel11ber 1999No. 60
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EFFECTS OF THE 1997-98 EL NIÑO EVENT ON THE VEGETATION
OF GALÁPAGOS
Alan Tye and Iván Aldaz
INTRODUCTION
During the last large El Niño event, in 1982-83,
heavy rain fell on Galápagos for nine months, forming
rivers and eroding channels in many places, induding
normally arid areas. The vegetational changes that
took place during that El Niño event are summarized
by Hamann (1985).
Similar changes occurred during the most recent El
Niño, which began in early 1997 with a heavier-than-
normal rainy season and continued to the end of the
wet season in 1998. During 1997, there was no garúa
season, merely a drier period about
September-Octobel~ and the vegetation did not dry up
as in normal years. In some areas with friable soils,
such as southern Santa Cruz ]sland and Alcedo Volcano
on Isabela Island, the heavy rains produced powerful
temporary watercourses, which eroded deep channels
and removed all surface vegetation, induding large
trees.
EFFECTS ON THE VEGETATION OF THE
SEMI-ARID ZONES
Herbaceous vegetatiol1
In the Littoral, Arid, and Transition zones of the
larger islands and throughout the lower islands, the
general appearance during July-December 1997 was
mllch greener than is usual for this normally dry part of
the year. The herbaceous understorey [ayer, dominated
by grasses, annual herbs, and creepers, was unusually
dense. Among herbaceolls species, Melltzelia aspera and
Cassia tora became extremely common, each in places
forming dense monospecific stands of height about
1-1.5 m. ]n many places, creepers (both native and
introduced species) covered the bushes and trees in a
dense carpet. Among the native species of creeper,
Passiflora ¡oetida did particular!y well. Rhynehosiamillima
also beca me very abundant, apparentiy throughout the
archipelago (induding Genovesa, Santiago, San
Cristóbal, Santa Cruz, and ]sabela islands, among others),
as in 1982-83; (Hamann 1985). Of the introdllced
species, Momordica eharalltia and Cueumis dipsaeeus
became much more widely distributed than in normal
years. On Floreana Island, Cuel/mis was fOllnd covering
hllge areas of shrllbland and woodland in the Arid and
Transition zones. It has since died back, and it is
undear whether El Niño has allowed it to extend its
distribution or whether it was already widely distrib-
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uted but was formerly, in normal years, much less con-
SpiCllOllS. Cl/eumis was first recorded on Española
Island during the 1982-83 El Niño. It became very com-
mon again on that island during the recent event.
Momordica beca me a very obvious component of the
Arid Zone vegetation on several islands. ]n all of these
cases, it is hard to know whether this apparent change
represents a real extension of distribution, or jllSt a tem-
porary in crease in abundance in areas where the plants
were already present.
Cactaceae
The Cactaceae both benefitted and suffered from
the effects of El Niño. Their rate of growth and flower
production were greater, this being especially obvious
in species of Opl/Iltia, but they absorbed so much water
that many larger °pulltia trees fell over under their own
weight, as happened in the previolls El Niño (Hamann
1985, Luong and Toro 1985). The plants ha ve a rela-
tively weak and shallow root system, which is not capa-
ble of sllpporting such a weight increase, combined
with the stronger winds that are experienced during El
Niño. Despite this mortality of adults, in many places,
regeneration of young plants both by seed and vegeta-
tively, was great during the Niilo periodo Large nllm-
bers of YOllng cacti were seen in many places in the
months following El Niilo. Thesc indude not only indi-
viduals growing from seed that germinated during El
Niño, but, jlldging from their size, there are also many
young plants that had germinated in preceding years.
These may have experienced a slldden growth spurt,
having been "waiting" for the favourable conditions for
growth that were provided by EJ Niño. In contrast to
the reports for the 1982-83 El NiílO (Hamann 1985), the
°pulltia flowered well twice: at the beginning of the
event in 1997, and again at thc beginning of the rainy
sea son following El Niilo, in early 1999.
Lowlal1d Scalesia specics
Effects on the low1and species of Scalesia were
mixed. Although no formal monitoring was carried
out, these species are so conspicuollS that effects on
them are comparatively easy to judge. In addition, two
projects begun during El NiílO allowed us to collect
some data on their populations. One project is to mon-
itor selected rare species for growth and reproduction,
and the other intends to bring up to date our knowl-
edge of the status and distribution of endemic plant
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December 1999No. 60
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Noticias de Galápagos
species of Galapagos. Both include Scalesia species
among their subjects. These observations revealed that
during El Niño many adult plants of lowland zone
Scalesia species died, perhaps beca use of root rot or tem-
porary flooding. One or both of these probably caused
the death of plants of Scalesia helleri in the demonstra-
tion gardens at CDRS, where nearly all of the plants
that had been cultivated over the previous three years
died. Mortality was also observed in adults of this
species in wild populations on southern Santa Cruz, as
was the case among older adults of Scalesia crockeri
(northern Santa Cruz), S. incisa, and S. divisa (both of
San Cristobal). On the other hand, all of these species
showed dramatically increased regeneration, with vir-
tually every adult of S. helleri, S. incísa, and S. divisa sur-
rounded by a circle of young plants. In the case of S.
crockeri, the population on the south coast of Baltra
Island increased enormously, spreading from a few iso-
lated patches, to a more or less continuous fringe along
the coastal cliff. Similarly, the growth of Scalesia helleri
on Santa Fe Island has been impressive, as in 1982-83
(Hamann 1985). It is hard to generalize from one group
of plants to another, but it seems likely that many plants
that share habitat requirements and growth characteris-
tics with the lowland Scalesia species would have been
similarly affected.
Effects on the rarest plant of Galápagos, Scalesia
atractyloides, were unclear. The species was believed
extinct until, in 1995, a population of S. a. darwinii was
discovered, consisting of five adult plants growing on
the vertical wall of a crater. Two of the five died in early
1997 from unknown causes; they might simply have
been old, or heavy rain might have affected them
adversely. The crater was enclosed by a fence in
December 1997, but the other three adults died at some
time during mid-1998. During 1997, several seedlings
were found below the adult plants. Although most of
these disappeared (probably eaten by goats) during the
year, a few survived until the fence was constructed.
Unfortunately, two goats entered the crater through a
gap in the fence (now closed) and most of the seedlings
disappeared. To date, two remain. A drought due to La
Niña might reduce the chance of more plants growing
from seed during 1999. In November 1998, the other
variety of the species, S. a. atractyloides, was rediscov-
ered, after having not been seen since the 1980s, at a site
on the west coast of Santiago. Only two adults are still
alive at the site, and no seedlings have been seen.
Again, although the site has now been fenced (in
December 1998), regenerabon from seed could be
delayed by a Niña drought.
Growth and phenology
Many species developed leaves much larger than in
normal years, in some cases to such an extent that they
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appeared to be a different species. This was true of
many species of shrub and tree of the Arid and
Transition zones, including Cordia lutea, Bursera grave-
olens, Piscidia carthagenensis, Croton scouleri, and
Cryptocarpus pyriformis. In some cases, flowers and
inflorescences were also larger than normal. Many
bushes grew higher than normal, including especially
Cryptocarpus. There was also a greater rate of growth in
height and trunk diameter of many shrub and tree
species (see data fram Alcedo in Aldaz and Tye, this
issue). Bursera graveolens, which is normally leafless for
most of the year, retained its lea ves for longer, and
individual trees lost their usual synchrony in the timing
of leaf production and leaf fall.
New records
Few new record s of intraductions to islands were
made during the 1997-98 El Niño, although it is proba-
ble that some have so far gone unnoticed. The new
island records that were reported are unlikely to have
been the result of spread facilitated by El Niño itself.
EFFECTS ON THE VEGETATION OF
THE HUMID ZONES
Vegetation communities
In the higher parts of the larger islands, the ferns,
herbs, grasses, and creepers grew tremendously, form-
ing dense, impenetrable masses. Both native and intro-
duced species flourished. Certain introduced species,
including Rubus niveus, Lantana camara, and Ricinus
communis, spread noticeably during the extended wet
periodo
Regeneration of native species was good in areas of
the highlands that are badly overgrazed by goats and
other feral ungulates. On both Alcedo Volcano and
Santiago Island, the areas of the highlands that have
been con verted in recent years to open pasture or bare
graund were covered with denser grawth of grasses.
Among the grass, regeneration of shrub and tree
species was excellent. There seem to have been two
reasons for this. The goats and other animals had large-
ly dispersed to the lower parts of the islands, where the
green vegetation was unusually succulent. Combined
with this, the heavy rains led to increased germination
and growth of young plants or resprouts. In the high-
lands of Santiago, the grassy areas, which were formerly
mixed Scalesia pedunculata - Zanthoxylum fagara -
Tournefortia rufosericea forest, developed abundant
regeneration, especially of Tournefortia. Unfortunately,
as El Niño ended, the goats moved back to the
highlands and ha ve already, within months, destroyed
much of this new growth. In some species, this
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December 1999Noticias de Galápagos
may have depleted a precious seedbank without
contributing to the production of replacement seeds.
Regeneration of Scalesia pedunculata on Santiago was
mainly limited to the small patches wi th remaining
adult trees, reflecting its relatively short-distance seed
dispersal mechanism (Hamann 1979). Many young
plants, up to 2.5 m height, were found in such patches.
Unfortunately, as the goats moved back to the
highlands, these saplings were their prime targets, and
most were immediately killed by having their
bark eaten.
Further observations from the humid zones of
Alcedo are presented by Aldaz and Tye (this issue).
Miconia robinsoniana
Effects on certain vegetation communities were
marked. Miconia robinsoniana, the dominant shrub of
the Miconia Zone on Santa Cruz, grew very well during
El Niño, with bushes reaching over 3 m in height.
Regeneration was good in areas where the species had
been rare in preceding years and which had been do m-
inated recently by a fern-sedge community.
Regeneration was similarly good in areas heavily
invaded by Cinchona succirubra, where the Cinchona was
cleared by the Galapagos National Park Service
(GNPS), such as on the outer slopes of Media Luna.
Unfortunately, the Cinchona also did well during El
Niño, and the density of young plants is now very high,
across the majority of the area of the Miconia and Fern-
sedge zones.
Scalesia pedunculata
The forests of Scalesia pedunculata in the highlands
of Santa Cruz suffered a mas s dieback during and after
El Niño of 1982-83. Causes were thought to be root rot
and high winds during El Niño rains, combined with
the succeeding La Niña drought (Hamann 1985). The
dieback resulted in strikingly even-aged stand s of the
species, which date from the subsequent regeneration.
This phenomenon has not been repeated during the
current evento Some wind-throw of adult trees was
experienced, but no mass die-off of entire stands.
Wind-throw also affected other trees in the highlands,
including Cinchona succirubra, where many adult trees
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were seen with branches torn off, although relatively
few trees were blown overo It remains to be seen
whether a Niña drought continuing through the rainy
season of 1999 will contribute to increased mortality of
the Scalesia, but so far the effectsdo not seem as severe as
in 1982-83. Hamann (1985) suggested that the normal
lifespan of Scalesia pedunculatais about 15 years. If that
is true, then the majority of the trees in the highland
forests, at least on Santa Cruz, are near the end of their
lives, since they germinated about 1982-84. Mass
dieback now might be facilitated by the poor resistance
of such older trees to unfavorable environmental
conditions such as drought. Species undergoing
periodic dieback and regeneration cycles, such as
Scalesia pedunculataare vulnerable to invasion by species
with a more continuous growth and regeneration
pattern. Invasive species such as Cinchona succirubra
and Pennisetum purpureum have become much more
common since the 1982-83 evento A worry now is that
a mas s dieback of Scalesia pedunculata could be followed
by invasion of its habitat by aggressive alien species
of plant, which could prevent the regeneration of
the Scalesia forests and thereby replace them. It remains
to be seen whether this will take place in the
coming year.
LITERATURE CITED
Hamann, O. 1979. Dynamics of a stand of Scalesia pedunculata
Hooker fiL, Santa Cruz Island, Galápagos. Bot. J. Linn.
Soco 78: 67-84.
Hamann, O. 1985. The El Niño influence on the Galapagos
vegetation. In G. Robinson and E. M. del Piño, eds., El
Niño en las islas Galápagos: El evento de 1982-1983, pp.
299-330, Charles Darwin Foundation for the Galápagos
Islands, Quito.
Luong, T.T. and B. Toro. 1985. Cambios en la vegetación de
las Islas Galápagos durante "El Niño" 1982-1983. In G.
Robinson and E. M. del Piño, eds., El Niño en las islas
Galápagos: El evento de 1982-1983, pp. 331-342, Charles
Darwin Foundation for the Galápagos lslands, Quito.
Alan Tye, Charles Darwin Research Station,
Galápagos, Ecuador (e-mail: atye@fcdarwin.org.ec).
Iván Aldaz, Charles Darwin Research Station,
Galápagos, Ecuador (e-mail: ivan@fcdarwin.org.ec).
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Effects of the 1997-98 El Niño event on the vegetation of Galápagos

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Effects of the 1997-98 El Niño event on the vegetation of Galápagos

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