Leaf adaptations

Leaves are the primary collectors of solar energy and the organ most directly affected by the environment. They also are the most responsive to environmental signals. Leaf properties are determined by light, nutrients, moisture, and the space-time parameters.

The leaves of trees have a number of adaptive features, including size, number, location, and chlorophyll content of chloroplasts; size, number, and structure of stomates (openings for gas exchange); thickness of epicuticular wax and cuticle; leaf stiffness and strength; and the size, number, and spacing of veins.

Trees of dry (xeric), moist (mesic), and wet (hydric) habitats have leaves that are specifically adapted structurally and functionally to these habitats. Dryness and cold induce some similar specializations, because cold conditions are often desiccating conditions as well. Tree leaves of mesic environments have a set of traits intermediate between xeric and hydric leaves.

Under xeromorphic conditions, the leaf has adopted features that decrease water loss. Leaf area that is exposed to the ambient air is reduced, although the ratio of internal surface to external surface area is high. The cells themselves are small, and the thickness of the wall is increased, as is the amount of fibrous tissue in the leaf, making the surface of the leaf rather hard. There are a larger number of veins. The epidermis is thick-walled and hairy, often with additional hypodermis and covered by a cuticle and epicuticular wax. Stomates are smaller, more closely spaced, sunken below the leaf surface, and covered with wax or hairs or both. Salt glands and water-storage cells are present in some species.

Tree leaves of supermoist environments, on the other hand, have fewer adaptations to minimize water loss. Large air spaces are present within the loosely packed mesophyll, and the cuticle is reduced, as are the number and frequency of veins. The stomates are larger but less closely spaced and either level with the leaf surface or elevated above it. The amount of fibrous tissue is reduced, and the hypodermis is absent. Water-secreting glands may be present. The walls of the epidermis are thinner.

Wood adaptations

In branches, reaction tissue forms where its inherent reaction force (pushing in the case of conifers and pulling in the case of hardwoods) will restore the intrinsic growth direction (equilibrium, or initial, position). This defines the locus of reaction tissue irrespective of the orientation of the structure with respect to gravity. Thus, reaction tissue is an adaptive morphogenetic phenomenon.

Many plant tissues show physiological and anatomical reactions due to physical displacement, but the response in wood is more permanent, more visible, and of greater economic importance, since reaction wood has in-built stresses that limit its use for most building projects, such as housing and furniture.

In the trunks of conifers, the reaction wood, called compression wood, forms on the lower side with respect to gravity and exerts a pushing force in the upward direction. In compression woods there is more growth on the lower side of the stem where the compression wood forms; this results in an oval cross section of the tree near the ground. This type of growth is called eccentric. In hardwood trunks the reaction wood is called tension wood and forms on the upper side of the lower trunk and exerts a contractive force that tends to pull the tree toward the upright position. In hardwoods there is generally less eccentricity associated with tension wood, but the annual rings may be wider. The names “tension wood” and “compression wood” are misleading, since they were assigned when the phenomena were thought to be due to such forces in the wood. Only later was it realized that the phenomenon was morphogenetic in nature and that tension or compression wood could form in wood that was in either tension or compression.

While reaction wood in the main stem occurs primarily in response to vertical displacement, reaction wood in branches acts against gravity to maintain the angle between the branch and the main axis. For example, the terminal shoots of pines exhibit negative geotropism throughout the growing season, and little or no compression wood is formed in the terminal shoots (although it is usually present in the laterals). In other species, such as the Canadian, or eastern, hemlock (Tsuga canadensis), the terminal shoots droop at the beginning of the season and gradually turn upward as the growing season progresses. During the drooping phase, the terminal (leader) is extremely flexible and sways freely in the wind. As the season progresses, the leader gradually increases in rigidity and, under the influence of compression wood formation, becomes erect to a vertical position. The rigidity is enhanced by the fact that compression wood is more highly lignified than regular wood. Concomitantly, the cellulose content is reduced.

In conifers a single cell type (the tracheid) is specialized for both conduction of sap and support. In compression wood the tracheid becomes quite round in cross section, forming intercellular spaces between neighbouring tracheids. Such spaces are not present in noncompression wood except in some species of junipers. The compression wood tracheids are so heavily lignified that the wood appears visibly reddish to the naked eye. The tracheids are thicker-walled, have spiral grooves along the length of the wall, and are shorter than noncompression wood tracheids.

In hardwoods the fibres are predominantly affected, although vessel diameter and frequency are generally reduced. The fibres of hardwoods develop a specialized layer in the cell wall—the so-called gelatinous layer—that is almost completely devoid of lignin, although in the other layers the fibre wall is lignified. The gelatinous layer is primarily composed of cellulose and hemicellulose. It is rubbery in texture and does not cut cleanly. Thus, tension wood fibres may be visible to the naked eye on a sawed board as a fuzzy surface. The lumber sawed from this wood will warp, cup, and exhibit much greater longitudinal shrinkage than nontension wood.