How did trees evolve?

    It is believed that all trees evolved from early vascular plants such as Rhynia, a widespread plant about 420 million years ago.  These basic plants were supported by water pressure in the stem, and planted to the earth by rhizomes.  Rhizomes are basically underground extensions of the stem.  These very early precursors to the trees are noteworthy because they possessed a xylem and phloem, which is imperative to the evolution of trees and other vascular plants.  The xylem is very important because it makes the transportation of water and other nutrients from the root system to the rest of the organism possible.  The phloem transports nutrients created by photosynthesis to the rest of the organism.  For more on this, visit my page on nutrition.  As a result of the evolution of the xylem and phloem system, plants essentially became free to grow to significant heights.

Above: Cross-section of Rhynia, an early precursor to the trees.  Note the hollow xylem in the center of the stem and the water pressurized sacs throughout the cross-section.  The phloem can be considered the densly packed area surrounding the xylem.

Left:  one of the most important adaptations of conifer trees is the thick waxy cuticle that waterproof the leaves.

Leaves

One of the most notable adaptations of conifer trees are the presence of needle-like leaves.  These leaves are adapted to survive in harsher and colder conditions compared to broad leaves.  The needle leaf design is very similar to that of broad leaves, except everything is much more tightly packed, protecting the central vein of the leaf containing the vascular tissue.  The central vein is surrounded by a sheath for protection.  The photosynthetic cells are found in the ground tissue or mesophyll of the leaf.  The mesophyll can be located outside this sheath but below the epidermis.  these photosynthetic cells are surrounded by protective sclerenchyma and waxy cuticle, composing the dermal layer.  For more on the different tissues found in plants, visit my page on basic plant anatomy.  See the picture at the top of the page for a visual on leaves.

Shape

    One of the most easily recognizable features of conifer trees is the cone shape it exhibits.  This shape is an adaptation that allows for snow to easily glide off of the branches, so the branches do not accumulate too much weight.

Above:  Tamarack trees exhibit a cone shape, typical of Conifer trees.

Above:  Tamarack needles engaging in fall senescence.

   After the evolution of the xylem and phloem, many adaptations occurred over an extended period of time allowing the evolution of trees to progress from this basic vascular plant.  Some notable adaptations include the stem (which is becomes woody in trees), leaves, and roots.  The evolution of this three part anatomy sets up the basic framework for tree evolution.  For more on this, visit my page on basic plant anatomy.

    The next step that allowed for the evolution of trees was the evolution of the seed.  This adaptation allowed for the plants to reproduce in the absence of water, allowing plants to become fully terrestrial.  Shortly after the evolution of the seed came the gymnosperms.  The term ‘gymnosperm’ means ‘naked seed.’  This can be contrasted to the evolutionary more complex angiosperm mode of reproduction, involving seeds protected by fruits.  Larix laricina is an example of a gymnosperm tree. 

Specialized Conifer Adaptations

    Tamarack belongs to the phylum Coniferophyta.  These trees are known more commonly as conifer trees.  With 570 species, Conifer trees are by far the most numerous group of tree species that exist today.  Why have conifer trees become so successful?

Conifer trees are adapted for cold and harsh climates.

    Conifer trees live in cold climates.  This kind of cold weather can easily kill humans and other animals during prolonged exposure.  Conifer trees are specially adapted to protect themselves from freezing.  Tamarack and other conifer trees exhibit a protective behavior known as extracellular freezing.  In this process, some liquids are pushed through empty, exposed spaces on the tree and frozen on the outside of the cells.  This protects the interior cells from the cold.

    The needle-like leaves are an important adaptation to Conifer trees because they do not accumulate much snow, keeping the weight load mild.  Even though Tamarack trees lose their leaves in the winter, they are still subject to some snowfall.  The range of Larix laricina is immense, and in the northern part of the range, snowfall is inevitable. 

    The waxy cuticle on the epidermis is an extremely important adaptation because it protects the photosynthetic cells that are below in the mesophyll.  The waxy cuticle is the external part of the leaf, and makes the needles feel slick.  For more on leaf function, see my page on nutrition. 

Deciduous adaptation - Waste dumping

    As the leaves die, the chlorophyll and proteins from the leaves are transported from the leaves and recycled in the tree.  As this occurs, certain waste products of the tree like heavy metals, chlorine and silicone are transported to the dying leaves.  During this phase, the leaves of Tamarack shine golden yellow, the colors of carotenoids such as carotenes and xanthophylls and anthocyanins.  These pigments are actually always in the leaves, but they are usually concealed by the green chlorophyll pigment.  This waste dumping adaptation is important because it provides a convenient mechanism to get rid of wastes.  The leaves begin the dying and dropping process  known as senescence according to many stimuli. Some of these stimuli include decreasing day length, decreasing temperature, or decreasing soil moisture. 

    The Tamarack tree adapts the deciduous pattern of growing leaves because the energetic cost of producing new leaves is still less than trying to keep its leaves alive through the winter.  This is due to a decrease of moisture available and a decrease of daylight to allow for photosynthesis in the leaves.