Vine Ecology
Francis E. Putz
Department of Botany
University of Florida
Gainesville, Florida, USA
Note:
This online review is updated and revised continuously, as soon as results of
new scientific research become available. It therefore presents
state-of-the-art information on the topic it covers.
Herbaceous and woody vines, the latter also
known as lianas and bush ropes, climb by using other plants for support. This
characteristic of not being self-supporting allows vine stems to be narrow,
flexible and capable of phenomenal rates of growth in height or length. Vines have long attracted naturalists and story-tellers, yet despite the
contributions of Darwin (1867) and other 19th Century biologists to the study of
vines, it wasn’t until fairly recently that ecologists turned their attention to
this important group of plants. While there is still much to learn about this
long-neglected group of plants, the diversity and ecological importance of vines
is now widely recognized, thanks to the efforts of researchers around the world.
Nevertheless, there are still many aspects of vine ecology that await
investigation. See Photo 1 (Dilleniaceae lianas).
Vine Evolution and Distribution
The climbing habit has apparently evolved
numerous times in the plant kingdom. There are vines among such diverse taxa as
ferns (e.g., Lygodium), gymnosperms (e.g., Gnetum), various
palm lineages (e.g., Calamus and Desmoncus), and other
monocotyledons such as the pandans (e.g., Freycinetia). Flowering plant
families that are particularly rich in climbing species include the Bigononiaceae, Vitaceae, Leguminosae, Menispermaceae, and Hippocrateaceae. All
of the species of some genera are vines (e.g., Serjania) whereas others
include species of vines, shrubs, and trees (e.g., Bauhinia). There are
also numerous species that grow as vines when crowded, but are free-standing
shrubs or trees if they fail to encounter mechanical supports (e.g., species of
Croton; Gallenmúller et al. 2001).
Vines are found in forests from the tropics
to the boreal zones of both the northern and southern hemispheres, and in deserts as well as
rainforests. However, they are most diverse near the equator (Gentry 1991). Vine abundance
generally increases with forest disturbance, but also varies with other, less
understood factors. In the Amazon Basin, for example, woody vines seem most
abundant in seasonally dry forests, but this may be because these areas were subject to more human interventions during
pre-Columbian times when
Amerindian populations were much higher than they are today. In North America,
native vines (e.g., Vitis spp.) and exotic species (e.g., Celastrus
scandens, Dioscorea bulbifera, and Pueraria lobata) can be
exceedingly abundant, but the factors promoting this abundance are not clear. During forest succession after disturbance, lianas typically increase at first and
then decrease in abundance, but due to growth of the individuals that persist,
liana biomass tends to remain a constant fraction of the total forest biomass (Dewalt
et al. 2000). Of course, there also are instances of vines blanketing an area so
thoroughly that succession is impeded for several decades (Photo
2).
How Vines Function
Although the lianas dangling from the
forest canopy understandably draw our attention, the regeneration phase in their life history
also deserves
our close consideration. Most lianas regenerate either from seed or as vegetative
offshoots from the roots or fallen stems of established individuals. Liana
seedlings often escape notice, even from experienced ecologists, because they
are self-supporting and otherwise resemble tree seedlings. People with the
ability to recognize
all the tree species in a forest, a daunting task in diverse tropical forests,
often assume that the unidentified seedlings are lianas. Given the radical
differences in leaf and stem morphology between liana seedlings and adults, as well as the fact that up to 30% of the woody plant species in a
forest can be lianas, learning to identify all of them is a big challenge. Only after liana seedlings reach free-standing heights of 0.5 meters to 3 or 4 meters, do
they start to climb. Most liana root sprouts are also initially free-standing
and therefore difficult to recognize as climbing plants, but it is a bit easier
to recognize lianas that emerge from fallen liana stems. Vegetative propagation
is extremely important in many species of lianas, which makes counting
“individuals” challenging.
Herbaceous vines and lianas display a large
diversity of climbing mechanisms. Some species climb with the aid of
adventitious roots that emerge from stems, while the stems of other species twine
around their supports. Many species have specialized structures for grabbing
supports, sometimes referred to as “prehensile apparati.” The most familiar
prehensile apparatus is the tendril, but tendrils come in various sizes and
evolutionarily are derived from a variety of structures. Tendrils can be
modified leaves, leaflets, stipules, inflorescences, branches (e.g., Hippocrateaceae), or stems (e.g., Omphalea in the neotropics and many
Leguminosae in the paleotropics). Species with stem-derived tendrils tend to be
able to successfully grasp larger diameter trellises than those with other sorts
of tendrils, but all vines except the root climbers are limited to climbing
fairly narrow supports.
The scarcity of suitable supports that reach
from near the ground up to the canopy is a major limitation for most climbing
plants. Failure to locate and attach to trellises is the fate of most vine
stems, other than root climbers, which are rare among tropical lianas. In the
dense vegetation of forest edges, potential supports are plentiful, which helps
explain why vines are so abundant there. The likelihood of encountering supports
is increased somewhat by the growth-induced circular movements of stems and
tendrils called circumnutation spirals (e.g., Baillaud 1962). Using time-lapse
photography in still air, researchers have revealed that circumnutation spirals
of vine stems can be 50 cm in diameter and that tendrils can circumnutate over
distances nearly twice as large. Even more surprising is the fact that these spirals
apparently become elongated towards potential supports (e.g., Tronchet 1945),
which further increases the probability of finding a support. A similar
phenomenon of directed foraging for supports has been described for root
climbing herbaceous vines (Strong & Ray 1975). To increase their likelihood of
encountering a large tree trunk to ascend, these vines grow along the forest
floor in the direction of the darkest part of the horizon, which is often the
trunk of a large tree. One possible mechanism behind these directed “foraging”
behaviors involves growth responses to minor differences in concentrations
of ethylene near and away from stems of other plants, but this hypothesis awaits
testing.
When a climbing plant reaches the top of its
host, further height growth requires the location of a taller support of the
appropriate diameter. Searcher shoots of vines emerging from the crowns of understory trees are often quite obvious. If they encounter a suitable support
and successfully attach to it, their progress towards the canopy continues. Although searcher shoots of some lianas can extend upwards for as much as 2 meters
above their last support, if they fail to find a support, they fall over and are
replaced by another shoot (Putz 1984). Knowing the inter-support spanning
capacities of different species is important for predicting which vines are
likely to be stalled on their way to the canopy.
Most vines
that make it to the canopy do so with the help of a succession of taller supports. An exception are vines that climb up the stems of narrow-stemmed vines already
attached in the canopy (Pinard & Putz 1998). Vines hanging from the canopy may show no evidence of their stepwise ascent,
leading some observers to the mistaken conclusion that they “rode” to the canopy
on their current host trees (Photo 3). Although this might happen on occasion, the deleterious effects of
lianas on host trees, as well as the disadvantage to fast-growing lianas of
relying on slow-growing trees, diminishes the likely importance of this canopy
ascent strategy.
Once in the canopy, vines often grow between
tree crowns (Caballé 1977, 1998). These intercrown connections are of great
importance to animals that can’t fly or glide long distances (see below) and
also increase the likelihood of trees pulling down their neighbors when they
fall. Although vines growing between tree crowns are restricted by their
abilities to span intercrown distances, many grow on several canopy trees and
one individual in Panama, an Entada monostachya with a 51 centimeter diameter
stem, connected to the crowns of 49 canopy trees (Putz 1984). Only the
non-branching climbing palms (rattans) are completely restricted to the crown of
a single host tree. For them, the challenge is staying in their host’s crown as
their stem elongates (Photo 4; Putz 1990).
Vine Stem Anatomy and
Physiology
Although in cross section the stems of some
liana species are very tree-like, they differ by possessing large diameter vessels and
abundant soft tissues (parenchyma) in the xylem (Photo 5; Carlquist 1991).
Having large vessels is important to plants with
narrow stems because the flow rate through xylem vessels increases with the
fourth power of the radius; the xylem conducting capacities of even
narrow-stemmed lianas are therefore very large (Photo 6; Schenck 1892, Zimmermann 1983, Ewers et al. 1991). Consequently, per unit
cross-section area, vine stems can hydraulically support much larger total leaf
areas than trees. In fact, a liana 10 cm in diameter may have as much leaf area
(or leaf mass) as a tree five times as large (Putz 1983, Gerwing & Farias
2000). This difference in allometry helps explain how in tropical forests, where
lianas contribute only 5% to total above-ground biomass, liana leaves may
constitute 40% of the total forest leaf area (Hladik 1974, Schnitzer and Bongers
2002).
The abundance of soft tissues in vine stems
adds to their flexibility, helps them avoid mechanical damage, and speeds the
rate of recovery when damage does occur (Holbrook and Putz 1991, Fisher and
Ewers 1989). Bent and twisted stems of some vines react more like
multi-stranded cables than solid cylinders (Photo 7). This flexibility increases the likelihood of survival when
they fall with their host trees. Consequently, many of the lianas that
proliferate in treefall gaps are sprouts from lianas that survived the fall. Parenchyma tissues as well as bands and strands of phloem embedded in the xylem
of some species of lianas must have other physiological consequences, but these
have apparently not been studied.
Effects of Vines on Trees
and Forests
By displaying their leaves above those of the
trees that provide them mechanical support, vines are effective competitors for
light. Furthermore, because they invest little in thickening their stems and
branches, vines can use a large proportion of their resources to produce
additional leaves as well as for reproduction. Conversely, trees that are
heavily vine-laden grow more slowly and produce fewer seeds and fruits than vine-free trees
(Stevens 1987). Due to their generally deleterious effects on trees,
forest managers usually advocate the removal of vines, at least those growing
on future crop trees (e.g., Putz 1991).
The growth habit of vines also allows them to
be effective below-ground competitors for water and nutrients. In experimental
studies where vines and trees were allowed to compete in four situations (above-ground,
below-ground, both above- and below-ground, and not at all), Dillenberg et al. (1993) found strong vine effects on trees in
both domains. One mechanism for this impact was demonstrated in a seasonal
forest in Amazonian Bolivia by Diego Perez-Salicrup and Martin Barker (2000). They found that after vines growing on canopy trees were cut,
water stress decreased in the trees formerly infested with vines. A decrease could
even be detected one day after the cutting. The discovery that
vines are among the deepest-rooted plants in tropical forests (Jackson et al.
1995, Tyree & Ewers 1996) suggests that some vines may avoid competing with
trees while avoiding drought stress by tapping deeper stores of water. Other
experiments (Putz in prep) have revealed that vines generally colonize
nutrient-rich patches of soil much more quickly and with much less investment in
root biomass than trees. This versatility in root foraging can be explained as
another benefit of vine dependence on other plants for mechanical support; vines
have no need for the large diameter structural roots trees use to hold
themselves upright.
In addition to competing both above and
below-ground, lianas can cause mechanical damage to their host trees. Small
twigs and large stems of trees can be mechanically girdled by tendrils and twining stems,
respectively. Lianas can also proliferate so much that the branches of their
host trees break under their weight. Several light-demanding and dense wooded
liana species typically break their host trees, creating canopy gaps in which they proliferate (e.g. Acacia spp. in Central America
and Celtis spp. in South America). On the positive side, it has been
suggested that by growing between tree crowns, lianas help stabilize trees
(Smith 1973), but the evidence is that liana infested trees actually create
larger gaps when they fall. Slash-and-burn farmers are well aware of this
phenomenon and generally use heavily liana-laden trees as “king pins” when
clearing forests.
Tree species vary in their susceptibility to
liana infestations, and in their ability to shed lianas that colonize their
crowns (Putz 1984b). Because of limits in the diameter of supports that
lianas can use in their ascent to the canopy, fast growing trees tend to avoid
liana infestations, especially if they rapidly shed their lower branches.
Thick-stemmed palms are particularly immune to lianas, and lianas that do make
it into their coronas are shed along with the leaves to which they are attached. By creating large “crown shyness” gaps when they bash into their neighbors in
the wind, flexible stemmed trees can escape lianas attempting to grow from crown
to crown or shed those that do cross the gap, at least until they become firmly
attached. Smooth bark and bark that is shed in large flakes may also deter
lianas to some extent, but given the way that most lianas attach to their
supports, this mechanism would not seem to be particularly effective. Some tree
species also suffer reduced threats of vine colonization because the symbiotic
ants that they harbor and feed keep their host tree free of parasites, including
structural parasites such as lianas (Janzen 1969). Due to their vine avoiding
and vine shedding characteristics, sometimes including these ant symbionts,
pioneer trees are particularly common in severely vine infested areas where
slower growing trees are affectively suppressed (Schnitzer et al. 2000).
Herbaceous vines and lianas often play
important roles during forest succession after natural and anthropogenic
(i.e. human-induced) disturbances (Photo 8; Schnitzer
& Carson 2001). Most vines are light demanding (Hegarty & Caballé
1991) and thus benefit from disturbances, which they colonize with dispersed and
buried dormant seeds, formerly suppressed seedlings, and rampant vegetative
proliferation of vines that fell with the gap-making trees. Some lianas have
also been described as growing through the forest understory, essentially
“foraging” for canopy gaps and other areas of high light intensities (Peñalosa
1984). Liana proliferation on and near the edges of forests is also one of the
major causes of structural deterioration in fragmented forests (Laurance et al.
2001).
Vine Management
Where lianas are abundant, they represent a
severe nuisance for forest managers. Not only are crop trees suppressed and
damaged by the lianas they support, but forest operations are also rendered more
difficult and dangerous. Tree fellers are in particular danger when liana-laden
trees pull down their neighbors, often on top of the worker. For these reasons,
pre-felling liana cutting is a commonly prescribed silvicultural treatment for
reducing logging damage to the residual forest and for enhancing worker
safety (Putz 1991, Vidal et al. 1997, Gerwing 2001, Schnitzer et al. 2004).
Pre-felling liana cutting also diminishes
problems with post-logging liana infestations (Gerwing & Uhl 2002). This silviculturally beneficial effect is mostly due to the removal of the fallen stems that, where lianas have not been cut,
contribute the majority of stems that suppress regeneration in logging gaps (Alvira
et al. 2004). Although the effect of liana cutting on understory light
environments has not been well studied, it seems likely that the canopy opening
resulting from the fall of liana leaves increases the vigor of trees in the understory that are intended to form the next timber crop (Pérez-Salicrup 2001).
The Role of Vines: Summary
Although vines can be a major problem in
forests managed for timber, their beneficial roles in forest ecosystems should
not be overlooked. For example, lianas provide
important intercrown pathways for many canopy-dwelling animals and so are
important ecosystem engineers (Photo 9). Without these vine connections, moving from tree to
tree would entail descending to the ground where these animals are very susceptible to
predation (Emmons & Gentry 1983, Putz et
al. 2001). The abundant leaves, flowers, and fruits of lianas also represent
important food resources for animals, and contribute substantially to
biogeochemical cycles. Although many lianas have small and wind dispersed seeds,
some produce delicious fruits that are important to many forest animals. Many
canopy lianas also produce abundant flowers, making them an important component of
pollinator communities. A larger proportion of liana species than tree species
are pollinated by large bees and beetles. Because vines are favored by forest
disturbances and are more common as well as more diverse in warmer environments,
human-induced disturbances and global warming are likely to promote vine
abundance. Perhaps these environmental modifications are already responsible for the
reported increase in the growth rates and abundance of large diameter lianas in
tropical forests (Phillips et al. 2002), but more data are needed to evaluate
this trend and its proposed cause.
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Information
about this Review
This
review is also available in the following
languages:
Portuguese
Spanish
The author is: Dr. Francis E. Putz (PhD in Ecology)
The
proper citation is:
Putz FE
2012
Vine Ecology. ECOLOGY.INFO
24
If
you are aware of any important scientific publications about the ecology of
vines that were omitted from
this review, or have other suggestions for improving it, please contact the
author at his e-mail address:
fep
{at} botany.ufl.edu
The photo at the
top of the page shows grapes on a vine at Temecula, California, and
was taken by Chi Le (USA).
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