The architecture of the forest is shaped by a phenomenon that is as distinctive as it is subtle. As early as the beginning of the last century, it was observed that certain tree species—such as oaks, willows, and poplars—develop true abscission zones at the base of their branches, allowing for their programmed shedding, almost like a self-imposed sacrifice for the benefit of the whole plant [4]. The most common explanation for this phenomenon of self-pruning has been framed in terms of carbon balance: when a branch receives less light than its compensation point, it becomes a net “cost” to the plant and is therefore shed [5].

However, more recent studies have shown that self-pruning may also be mediated by a progressive hydraulic “shut-down”: shaded lower branches receive less water, develop less efficient xylem, and are gradually isolated from the plant’s transport system [6]. In other words, less light means fewer auxins, reduced water flow, and diminished benefit. The outcome is not merely a cleaner trunk, but a strategic reallocation of resources toward the upper, better-illuminated portions of the crown.

Plants have no eyes, ears, or nervous system, yet when something touches them they respond with a precision that would put many animals to shame. Thigmotropism is a growth response oriented by contact: a plant structure perceives a mechanical stimulus and adjusts its development according to its direction. It is not a simple “movement,” but a reorganization of growth. The classic example is the tendril of a bean plant which, as soon as it encounters a support, stops growing symmetrically and begins to coil around the obstacle [7].

This response must be distinguished from the broader concept of thigmomorphogenesis, which refers to long-term structural changes induced by mechanical stimuli [8]. Thigmotropism, by contrast, is a matter of orientation: the plant determines where to grow—or where not to grow—based on what it touches.

The interesting—and often underestimated—point is that this is not a “defensive” response in the strict sense, but a strategy of spatial exploration. Thigmotropism allows climbing plants to colonize vertical niches, compete for light, and optimize mechanical support without investing in massive woody tissues. It is an elegant engineering solution: instead of building a tower, they use someone else’s.

Cover image: Beech tree seen under the canopy with a fish-eye lens. Arsiero (Vicenza, Italy). Photo: Author. 15.10.2024