Drought resistance in tropical trees: (some of) what we know and (more
of) what we don’t know
Bettina Engelbrecht, Department of Biology, University of Utah
Tropical lowland forests occur in a wide range of rainfall regimes. Many tropical forests
have one or two pronounced dry seasons, and even in aseasonal tropical forests extended
periods of drought occur (e.g. Welsh & Newbery 1999, Burslem & Grubb 1996).
Diversity of tropical forests as well as species distribution patterns are strongly
influenced by the amount of annual rainfall and length of dry periods (e.g. Gentry 1988,
Swaine & Becker 1999, Bongers et al. 1999 ). There is also increasing evidence for the
importance of water availabitity for determining species microhabitat distributions (e.g.
review in Webb & Peart 2000, Swaine & Burslem, 2001).
During dry periods, plants can be exposed to considerable drought stress: wilting
has been observed, and water potentials down to –3.5MPa have been measured. Drought
has been associated with increased mortality and decreased growth in seedlings as well as
adult tropical trees (e.g. Veenendaal et al. 1995, Condit et al. 1995). Seedlings of 28
species of tropical woody plants showed a very large variation of the effects of drought
on both growth and survival among species (Engelbrecht 2001).
Differences in species’ drought resistance, their “ capacity … to withstand
periods of dryness” (Larcher 1980), together with the variation in water availability, may
therefore be a major factor influencing species’ population dynamics, and their
mircohabitat and large scale distribution in the tropics.
Mechanisms of drought resistance can be devided into two classes: traits that
delay dessication, and traits that provide tolerance to desiccation (see table).
Individually, most of these traits are well known and studied both in temperate and
tropical trees. However, it is quite difficult to evaluate the relative importance of the
individual traits for plant performance under drought conditions. Ecophysiologists are a
long way from achieving this objective. Knowledge of the relative importance of the
traits is essential, if we want to develop (relatively) easily measurable proxies for plant
drought resistance and if we want to establish meaningful functional groups in tropical
trees. We have to be able to link morphological and physiological traits with plant
performance, with population and community dynamics.
plant
TRAITS
WATER
availability
PLANT
PERFORMANCE
growth, survival,
propagation
SPECIES
ABUNDANCE
AND
DISTRIBUTION
Figure: Schematic representation on how plant traits, species performance and
community processes may be linked.
One approach is to rank relative drought resistance of a large number of species
and collect measures of the component traits. Multivariate statistical analysis may then
allow to analyze the relative importance of the individual traits. So far comparative,
quantitive assessments of plant drought resistance are almost entirely lacking. In our
study of drought resistance of seedlings of 28 species we are taking this approach
(Engelbrecht, 2001). So far, none of the more easily measurable plant traits (SLW,
cuticular conductance, plant height, seed size) correlated with the effect of drought on
growth or survival.
Table: Mechanisms of drought resistance (cf. Larcher 1980, Levitt 1980). Listed are
examples for traits contributing to drought resistance, how to assess them and a literature
example either on the method itself, of using the method.
TRAIT
traits that delay dessication :
deep roots
early stomatal closure
high WUE (carbon gain per water loss)
low cuticular conductance
high water storage
leaf shedding
MEASUREMENT
LITERATURE
rooting depth (digging), H and
O isotope ratios, for seedlings
plant height?
gas exchange: sensitivity of
stomates to water potential
gas exchange or better carbon
isotope ratios
gas exchange or leaf drying
curves
tissue water contents or whole
plant water flow rates
Jackson et al. 1995
Bonal & Guehl 2001
Farquhar et al. 1982, Huc et
al. 1994
Kerstiens 1996
Holbrook & Sinclair 1992
decidiousness, following leaf
area
traits that provide tolerance to desiccation :
low vulnerability to xylem embolism
vulnerability curves, possibly
wood density as a proxy
low turgor loss point
pressure-volume curves
osmotic adjustment
pressure-volume curves on
stressed and unstresssed plants
ability of cells to withstand low water
viability tests
potentials
Kolb et al. 1996, Hacke et al.
2001
Tyree & Hammel 1972
Tyree & Hammel 1972
Kyriapkopoulos and Richter
1991
In general, hot candidates for traits with some predictive power for plant drought
resistance are (1) vulnerability to xylem embolism, possibly with wood density as proxy
data (Tyree et al. 1994, Hacke et al. 2001), and (2) rooting depth (or it’s plasticity in
response to drought; Reader et al. 1992, Poorter & Hayashida-Oliver).
However, so far only Reader et al. (1992) could actually show a correlation of
‘plasticity of rooting depth’ to growth in drying soil for a large number of species in
northern England. Studies on the effect of drought on tropical species and rooting depth
in the tropics do not yet give a consistent picture, and our study of 28 seedlings showed
NO relation of drought resistance to rooting depth. Hacke et al. 2001 showed a relation
between wood density and vulnerability to xylem embolism, but (a) this relation may be
too weak to be of any significance in the range observed in tropical trees, and (b) the
relation between vulnerability and drought tolerance has not yet been empirically
established in the relevant range..
At the current state of knowledge, comparative bioessays for the effect of drought
(or other stresses) on species performance (growth and survival) may be a useful tool for
establishing plant functional groups. Stress resistance (instead of plant traits) could be
used for forming a working base for assessing e.g. effects of disturbance, deforestation of
global climate change.
References
This list is by no means complete!!! It is slightly scewed towards publications by participants of the workshop.
Bonal D, Guehl J-M. 2001. Contrasting patterns of leaf water potential and gas exchange responses to
drought in seedlings of tropical rainforest species. Funct Ecol 15: 490 – 496
Bongers F, Poorter L, Van Rompaey RSAR, Parren MPE 1999 Distribution of twelve moist forest canopy
tree species in Liberia and Cote d’Ivoire: response curves to a climatic gradient. J Veg Sci 10: 371 –
382
Burslem DFRP, PJ Grubb 1996 Responses to simulated drought and elevated nutrient supply among
shade-tolerant tree seedlings of lowland tropical forest in Singapore. Biotropica 28:636-648.
Condit R, Hubbell SP, Foster RB 1995 Mortality rates of 205 neotropical tree and shrub species and the
impact of severe drought. Ecological Monographs 65 (4): 419 – 439.
Engelbrecht BMJ. 2001. Drought resistance in seedlings of 28 tropical woody plant species.
EuroWorkshop: Functional Groupings of Tropical Trees.
Farquhar GD, O’Leary MH, Berry JA. 1982. On the relationship between carbon isotope discrimination
and the intercellular carbon dioxide concentration in leaves. Aust J Plant Physiol 9:121-137
Gentry AH 1988 Changes in plant community diversity and floristic composition on environmental and
geographical gradients. Annals of the Missouri Botanical Garden 75:1-34.
Hacke UG, Sperry JS, Pockman WT, Davis SD, McCulloh KA. 2001. Trends in wood density and structure
are linked to prevention of xylem implosion by negative pressure. Oecologia 126:457-461
Holbrook NM, Sinclair TR. 1992. Water balance in the arborescent palm, Sabal palmetto. I. Stem structure,
tissue water release properties and leaf epidermal conductance. Plant Cell Environ 15:393-399
Huc R, Ferhi A, Guehl JM. 1994. Pioneer and late stage tropical forest tree species (French Guyana)
growing under common conditions differ in leaf gas exchange regulation, carbon isotope
discrimination and leaf water potential. Oecologia 99: 297-305
Jackson PC, Cavelier J, Goldstein G, Meinzer FC, Holbrook NM. 1995. Partitioning of water resources
among plants of a lowland tropical forest. Oecologia 101:197-203
Kerstiens G. 1996. Diffusion of water vapour and gases across cuticles and through stomatal pores
presumed closed. In: Kerstiens G (ed) Plant Cuticles. BIOS Scientific Publisher. Ltd. , Oxford.pp 121134.
Kolb KJ, Sperry JA, Lamont BB. 1996. A method for measuring xylem hydraulic conductance and
embolism in entire root and shoot systems. J Exp Bot 47: 1805 – 1810
Kyriapkopoulos E, Richter H. 1991. Dessication tolerance and osmotic parameters in detached leaves of
Quercus ilex L. Acta…..
Larcher W. 1980 Physiological Plant Ecology. 2nd edition. Springer, Berlin.
Levitt J 1980 Responses of plants to environmental stresses. Volume II Water, radiation, salt and other
stresses. 2nd edition. Academic press. New York, USA
Swaine MD, Burslem DFRP. 2001. Functional species groups defined by regeneration requirements.
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Swaine MD, P Becker 1999 Woody life-form composition and association on rainfall and soil
fertility gradients in Ghana. Plant Ecology 145:167-173.
Tyree MT, Hammel HT. 1972. The measurement of the turgor pressure and the leaf water relations of
plants by pressure bomb technique. J Exp Bot 23: 267-282
Veenendaal EM, MD Swaine, VK Agyeman, D Blay, IK Abebrese, CE Mullins 1995 Differences in plant
and soil water relations in and around a forest gap in West Africa during the dry season may influence
seedling establishment and survival. Journal of Ecology 83:83-90.
Walsh RPD, DM Newbery 1999 The ecoclimatology of Danum, Sabah, in the context of the world’s
rainforest regions, with particular reference to dry periods and their impact. Philosophical
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Webb CO, Peart DR. 2000. Habitat associations of trees and seedlings in a Bornean rainforest. J Ecol 88:
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