The question of whether moss serves as a natural compass, indicating the north side of a tree, is an enduring piece of outdoor folklore. This idea suggests that the uneven growth of moss on a tree trunk can reliably point a person in a specific direction. To understand the true answer, it is necessary to examine the specific biological needs of the plant and the environmental factors that govern its growth patterns.
The Short Answer
The idea that moss grows only on the north side of trees is a misconception. Moss can grow on any side of a tree trunk, or even completely encircle it, provided the localized conditions are suitable. Moss is a non-vascular plant, meaning its growth is entirely dependent on environmental factors such as moisture and shade, not on any magnetic or directional sense. While a directional preference often exists, it is a consequence of microclimate rather than an inherent rule, and the direction of the densest growth can vary significantly based on location.
Fundamental Requirements for Moss Growth
Mosses, classified as bryophytes, are small, primitive plants that lack the vascular system found in trees and flowering plants. Unlike vascular plants, they do not possess true roots for drawing water from the soil. Instead, they have tiny structures called rhizoids primarily for anchoring themselves to a surface. This lack of a root system forces moss to rely on absorbing moisture directly through its surface from the air, rain, or fog.
The two factors that govern moss survival are consistent moisture retention and low light intensity, or shade. Excessive direct sunlight is detrimental because it causes rapid evaporation, quickly drying out the moss and preventing photosynthesis and reproduction. Reproduction requires a film of water for the male gametes to swim to the female gametes. Therefore, any surface that stays damp and shaded for the longest periods will be the preferred habitat for moss growth.
How Local Conditions Create Uneven Growth
The persistence of the north-side myth stems from the sun’s path in the Northern Hemisphere, where the sun primarily tracks across the southern sky. This trajectory means that the north-facing side of an object, like a tree trunk, receives the least amount of direct solar radiation. The reduced light exposure keeps the surface cooler and minimizes water evaporation, creating the ideal microclimate for moss growth.
This directional bias is easily overridden by other localized variables, which explains why the growth pattern is not universal. A dense forest canopy, for example, can filter the sunlight so effectively that all sides of a tree remain sufficiently shaded and moist, leading to uniform moss growth all around the trunk. Similarly, local topography, such as a steep hill or gorge, can create deep shade that causes moss to grow most heavily on a side other than north.
Prevailing wind patterns are another significant factor, as they can rapidly dry out one side of a tree, irrespective of sun exposure. In the Southern Hemisphere, the sun tracks through the northern sky, meaning the south side of a tree is the shadiest and most likely side to host the densest moss growth. These examples confirm that the specific microclimate of the tree, not its geographical orientation alone, dictates where the moss develops.
Reliability of Moss for Direction Finding
Relying on the distribution of moss for navigation is unreliable due to the numerous local factors that influence its growth. The growth is a reaction to the immediate, site-specific environment, not a consistent geographic indicator. A small change in terrain, the density of surrounding trees, or the presence of a nearby water source can cause the heaviest moss growth to shift from one side of the trunk to another.
Given that there are over 22,000 varieties of moss, some of which can tolerate direct sunlight, assuming a single, universal growth pattern is inaccurate. Using moss as the sole indicator for direction can lead to substantial errors in navigation. More reliable methods, such as using the sun’s position or a compass, are necessary for accurate direction finding.