Biotic Inventory:
Documenting Diversity at Katharine Ordway Natural History Study Area
Populus populus
Taxonomy
Kingdom Plantae
Phylum Magnoliophyta
Class Magnoliopsida
Sub-class Dilleniidae
Order Salicales
Family Salicaceae
Genus Populus
Section Populus
Common name for the plants belonging to the family Salicaceae: willow
Common name for the plants belonging to the genus Populus: poplar trees
Common name for the plants belonging to the section Populus: aspens
Diagnostic Characteristics
When identifying woody plant species, it is important to pay attention
to the leaf morphology, the presence of flowers or lack thereof, the
quality of the bark, and the height of the trunk. The leaves of the
described specimen consist of petioles, simple laminae and stipules.
The laminae are rounded and serrulated, with acute apex and secondary
veins arranged in a palmatopinnate fashion1. Such leaves are a distinct
feature of aspens. In addition, they are about 1 to 3 cm in diameter,
indicating that they are neoleaves of a young poplar tree. Morphology
of neoleaves is conserved among poplar trees, hence it is difficult to
distinguish to which one of the 29 described Populus species this
specimen belongs . The upper surface of the leaf is glabrous, and waxy.
In contrast, the lower surface of the leaf is hairy, especially in the
marginal area. The purpose of hairs could be to minimize extensive loss
of water. The lower surface of the leaf is of paler shade of green
compared to the upper one, probably due to greater abundance of
chlorophyll in the cells of the palisade mesophyll layer proximal to
upper epidermis. There are no visible flowers, since flowers in poplar
trees arise before leaves do . The identified specimen has greenish to
gray thin bark, also attributed to young poplar trees. In some aspens,
the bark also carries out photosynthesis. Its trunk is about 2.5 meters
tall, further implying that the specimen is juvenile, since the range
of trunk heights for adult poplar trees is from 12 to 50 meters.
Ecology
The trees of the genus Populus inhabit temperate zones of the northern
hemisphere. They can be found in agricultural as well as in urban
areas, in natural forests, and riparian zones1, . They are able to
survive temperature fluctuations between the seasons. Individuals
belonging to this genus are of immense ecological importance, as they
represent vegetational pioneers – the first plants to repopulate an
area that has been affected by fire or another harsh environmental
factor. Moreover, trees of the genus Populus host numerous insects
belonging to the order Lepidoptera, and develop symbiotic relationships
with fungi3.
Poplar trees grow very rapidly, and because of this convenient feature,
they are used for restoring the biodiversity of endangered riparian
zones. They play an important role in maintaining water and carbon
cycles in their eco-systems, since they develop an extensive root
system and carry out photosynthesis . Thus, they set the basis for
trophic interactions at multiple levels.
Aspens as well as other poplar trees are hosts to innumerable
herbivorous insects, some of which are monophagous (e.g. Bucculatrix
staintonella), while others are polyphagous (e.g. Lymantria dispar) . A
recent study carried out in Bulgaria uncovered 18 new lepidopteran
phytophages feeding on the poplar trees . This finding illustrates how
incredibly diverse the lepidopterans are, and how little we still know
about global biodiversity.
Poplar trees establish symbiotic relationships with soil fungi
(ectomycorrhizae). The fungus provides the tree with nitrogen,
phosphorus and water, while getting sugars and shelter in return. In
year 2008, scientists sequenced the genome of a fungus Laccaria
bicolor, which lives in close association with the poplars. They were
able to target some of the genes regulating rhizosphere colonization,
thus starting to uncover the underlying molecular mechanism of
symbiosis .
Not all fungi develop symbiotic relationships with poplar trees. The
poplar leaf rust Melamspora laricis is one of the most prominent
pathogens affecting the poplars, thus harming bioremediation. The
genome of this fungus has been partially sequenced, for the purposes of
revealing the genetic material guiding its pathogenicity3, .
Aspens, such as most other poplar trees, are able to hybridize
naturally thus yielding significantly genetically different offspring
and contributing to the genetic diversity of the population. Recent
droughts concurrent with hybridization events, yielded offspring with
four times greater survival rate compared to that of parents8. The
hybridization events ultimately lead to speciation of poplar trees.
Since many fungal and insect species depend on poplar trees, speciation
of poplar trees will lead to co-speciation of fungal and animal
species.
Life History
All 29 species of the genus Populus encompass dioecious individuals,
which can reproduce both sexually and asexually. Sexual reproduction
mechanism in these plants is identical to that of other angiosperms.
Asexual reproduction involves the formation and spread of underground
shoots (soboles). Aspen trees can live up to 150 years, and can reach
sexual maturity no sooner than the 10th or 15th year of their lives1.
Male and female aspens produce catkins, prior to producing leaves. Male
catkin comprises stamens that produce pollen. Female catkin comprises a
single-celled ovary positioned in a cup-shaped disk. Pollination occurs
thanks to the wind2.
Aspens mostly reproduce vegetatively. They extend their roots and
soboles substantially, especially after an environmental disturbance
such as fire. The ramets have been recorded growing as far as 40 meters
away from the parent aspen tree. Since the root system is so large, it
is able to absorb water and nutrients from the soil, thus ensuring
rapid growth of the ramets. As the ramets grow, they remain connected
with each other and with the parent tree via the root system. All the
interconnected trees are referred to as a clone, since they are all
essentially the same individual. Interestingly, aspen roots can survive
underground for a long time after parent tree had died. The researchers
came to this conclusion as they saw developing ramets in areas with no
adult aspen trees2.
Distribution
Aspens are widely distributed across the northern hemisphere, in
Europe, Asia and North America. Their natural range in North America
encompasses most of the territory of the United States of America, as
well as mountain ranges of Mexico. This wide geographical range
testifies about the adaptability of the aspen trees to environments
that differ in the extent of temperate character .
Voucher Information
Small, waxy leaves of the specimen.
The specimen was
found at the outskirts of a forest at the Ordway Natural History Study
Area. It resides close to shrubs and other lower woody plants.
References
Stettler, R.F., Bradshaw, H.D., Jr., Heilman, P.E., and
Hinckley, T.M. 1996. Biology of Populus and its implications for
management and conservation. NRC Press, Ontario, Canada. 539 p.
Information obtained from:
http://www.treesforlife.org.uk/tfl.aspen.html
[Accessed May 2010]
Information obtained from:
http://populuscenter.org/index.php?option=com_content&view=article&id=70&Itemid=91
[Accessed May 2010]
Information obtained from:
http://populuscenter.org/index.php?option=com_content&view=article&id=67&Itemid=68
[Accessed May 2010]
Havill, N.P., Raffa, K.F. 1999. Effects of elicitation treatment
and genotypic variation on induced resistance in Populus : impacts on
gypsy moth (Lepidoptera: Lymantriidae) development and feeding
behavior. Oecologia. Volume 120(2) 1999: 295-303.
Georgiev, G., Beshkov, S. 2001. New and little-known
lepidopteran (Lepidoptera) phytophages on the poplars (Populus spp.) in
Bulgaria. Anzeiger für Schädlingskunde. 73 (1): 1-4.
Martin, F. et al, 2008. The genome of Laccaria bicolor provides
insights into mycorrhizal symbiosis. Nature. 2008 Mar 6;452(7183):88-92.
Information obtained from:
http://genome.jgi-psf.org/Mellp1/Mellp1.home.html
[Accessed May 2010]
Information obtained from:
http://forestry.about.com/od/hardwoods/ss/aspen_3.htm
[Accessed May 2010]
Compiled by Tijana Martinov.
Biodiversity & Evolution (BIOL 270) Professr Sarah Boyer. Spring 2010.
Specimen collected at Macalester College’s Katharine Ordway Natural History Study Area on April 15, 2010.