BOTANY VOLUME 4 - ECOLOGY - 2007

13. PLANTS IN THEIR HABITAT SPACE

13.4. Mechanical stresses

Resistance to self-weight, snow pressure, or epiphytes, coping with bending and “uplift” forces of wind and Water, tolerance to being buried by fallen litter, wind-blown sand, or slope erosion, as well as soil movements driven by gravity or ice formation — these are the criteria that determine whether specific species or life forms can survive in many biomes (Fig. 13.10).

Class="center">Fig. 13.10. Examples of mechanical loads on plants: A — ice (Eucalyptus pauciflora, 2050 m, Snowy Mountains, southeastern Australia); B — snow (timberline of birch forest in northern Sweden, 700 m); C — sand (dunes in eastern Australia); D — stony slope (Cerastium uniflorum in the Alps); E — fallen leaves (with beech seedlings); F — windthrow and stem breakage (Picea abies, after hurricane Lothar on December 26, 1999, in the Black Forest). Further examples include soil movements, trampling by grazing animals, the load of epiphytes and lianas (see Table 15.1), etc.

These issues are addressed by biomechanics. It evaluates the elasticity and strength of structures, investigates the causes of windthrow, and analyzes the mechanisms by which certain plants use others to reach the light “without unnecessary expenditure” (climbers, scramblers, and plants using others for support). Soil stabilization on slopes is largely determined by the mechanical properties of roots and rhizomes. Similarly, robust mechanical systems of underground Organs ensure that plants are not left rootless after grazing. Many plants can contract their roots and thus pull the sensitive growth point deep into the soil after germination (see Box 4.5, Fig. C). Of great ecological significance are the forces generated during the “release” of mechanical stress upon drying (“exploding” and seed ejection). Utilizing turgor pressure (20 bar or more), plants are capable of breaking apart massive structures and forcing their roots into soil crevices. Enormous mechanical work is performed by fine roots (protected at their tips by ROOT caps) as they penetrate very dense substrates. Leaf toughness is a key component of defense against herbivores, complemented by various “protective structures” (thorns, spines, cork formation). The ability to withstand mechanical stress, the generation of physical forces, and The Development of the aforementioned protective structures often ensure a species' successful persistence and determine the Stability of the entire ecosystem. Their ecological importance frequently outweighs the Organism's physiological capacity for adaptation.