Developing the Mosaic Theory from a Plant Perspective

As often happens in the scientific realm theories are developed according to the approaches utilized. This is true in ecology in which the focal entities are plants, animals, virus, bacteria, fungi, or man. By using organisms most of our investigations are devoted to discovering the relationships between the specified entity and the environment.

We have much empirical evidence about the relationship between plants and the environmental mosaic as well as between animals and their habitats.

The separation of knowledge often creates barriers when we try to explore the complexity of ecological systems.

When projected into a map, plant distribution appears as a mosaic. The patchy distribution of most plants, especially if we are dealing with trees, is considered as a static pattern if observed at a short time scale.

Plants intercept the geo-morphological and climatic heterogeneity at multiple scales. Plants are extremely sensitive to environmental conditions and adopt several mechanisms to avoid hostile conditions or interspecific stressors.

Plants form complex communities that are distributed mostly through coping with the heterogeneity of the environment, and as a consequence, plant communities are distributed patchily everywhere.

For many decades, the dominance of the climax theory alone has restricted the role of spatial heterogeneity in the dynamics of plants. Today we can better understand the dynamics of plant assemblages from algae to redwoods.

The idea of plants living in a static system is today abandoned by plant ecol-ogists and the dynamics are considered a rule in plant ecology. Dynamics allow plants to explore favorable habitats and to serve as involuntary engineers preparing the habitat for other species. That plant communities change species composition through time and that such communities can move around in a physical space are well documented.

The mosaic-cycle concept was developed in 1930 to explain the dynamism of forests and their successional stages toward a climatic stage, where climax status is not considered static but a transient phase (the final) of a long cycle. At the end of climax status, a complete rejuvenation occurs.

The dimensions of patches involved can vary greatly according to the soil heterogeneity and other climatic and edaphic conditions.

A common, yet wrong, belief is that a forest persists for a very long time if not disturbed. In reality the time lag of the involved processes is so long that we have no possibility to follow during a few human generations the entire cycle of a forest. Often forests are substituted by grassland or shrublands not due to disturbances, but due to reduction of soil fertility, or climatic changes. This is the case documented by Remmert (1989, 1991) in a forested area of Botswana.

Plants are extremely sensitive to geomorphological patterns that are quite fixed compared with other biological entities. For this reason, the pattern of plant distribution often appears static. With changing climate and/or the internal properties of the plant association, the mosaic expressed may change. This change has been called by various authors the "shifting mosaic steady state."

In reality, the changes that can occur inside a mosaic may be quite different and depend on the typology of vegetation considered. Grasslands and shrublands have quite different patterns than those of tree covers.

Grasslands and shrublands can modify the mosaic in a very short time and the agents involved may be soil arthropods, ground squirrels, moles, grazers, and diggers.

Trees (woodlands and forests) are influenced by at least three processes:

1. Development of vegetation along a successional gradient from the young stage to adult and senescent states. There is much evidence about this process common in every biome by which a succession of plants move in a direction of increase of respiration and decrease of productivity at ecosystem level (see Borman and Lickens 1979). The distribution of plant biomass changes along with the succession according to a cyclic fluctuation that can last a few months as well as many years (Fig. 3.13).

2. Gap dynamics. This process is linked to individual tree fall due to disease, senescence, attack by grazers, etc. The dynamic of gaps is very important in terms of regeneration in forests that do not have large disturbances, such as fires or hurricanes. The fall of an old tree has many effects on the soil and on the surroundings. For instance, the undergrowth close to the fallen tree suddenly receives light and is more exposed to desiccation. Several trees will incur damage to branches and competition for light and soil nutrients will be reduced. Around the gap an ecotonal area is created and this ecotone attracts new organisms like butterflies, birds, and new pioneering plants (Fig. 3.14).

3. Large-scale disturbance. The processes involved are fires, hurricanes, landslides, lava flows, earth-quakes, wind blows, climatic shock (severe frost, persisting drought). The changes that occur after these events depend on the typology of the forests, on the frequency of recurrence and on the season in which the event occurs.

In all cases plants can be substituted with other species, or a new generation of the same species might recover the open space (Fig. 3.15).

Fig. 3.13 Variation in biomass along an idealized patch. The increase of biomass is followed by a dramatic decrease for instance due to the death of a large tree. The decline occurs suddenly while the recovery takes longer (from Borman and Lickens 1979)

Time

Time

Fig. 3.14 The gap dynamic in a forest may produce a rejuvenation of the same species (A), or new species can enter into the forest allowing the empty gap to recover (B)

Reorganization

Aggradation Transition

Steady State o -E

Aggradation Transition

Steady State o -E

Fig. 3.15 Two models of patch development after a disturbance. Line A is an asymptotic model in which biomass accumulates until the Steady state. This model is well known and accepted by plant ecologists. Line B describes the Shifting-Mosaic Steady State proposed by Borman and Lickens (1979) in which after a phase of reorganization the aggradation phase grows quickly followed by a transition phase in which biomass decreases to enter into the final phase of the Steady State (from Borman and Lickens 1979)

Fig. 3.15 Two models of patch development after a disturbance. Line A is an asymptotic model in which biomass accumulates until the Steady state. This model is well known and accepted by plant ecologists. Line B describes the Shifting-Mosaic Steady State proposed by Borman and Lickens (1979) in which after a phase of reorganization the aggradation phase grows quickly followed by a transition phase in which biomass decreases to enter into the final phase of the Steady State (from Borman and Lickens 1979)

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