Vegetation: Encyclopedia II - Vegetation - Vegetation Processes

Vegetation - Vegetation Processes

Like all biological systems, plant communities are temporally and spatially dynamic; they change at all possible scales. Dynamism in vegetation is defined primarily as changes in either or both of species composition and vegetation structure.

Vegetation - Temporal Dynamics

Temporally, a large number of processes or events can cause change, but for sake of simplicity they can be categorized roughly as either abrupt or gradual. Abrupt changes are generally referred to as disturbances; these include things like fire, high winds, landslides, floods, avalanches and the like. Their causes are usually external (exogenous) to the community--they are natural processes occurring (mostly) independently of the natural processes of the community (such as germination, growth, death, etc.). Such events can change vegetation structure and species composition very quickly and for long time periods, and they can do so over large ares. Very few ecosystems are without some type of disturbance as a regular and recurring part of the long term system dynamic. Fire and wind disturbances are particularly common throughout many vegetation types worldwide. Fire is particularly potent because of its ability to destroy not only living plants, but also the spores and seeds representing the potential next generation, and because of fire's impact on faunal populations and soil characteristics.

Temporal change at a slower pace is ubiquitous; it comprises the field of ecological succession. Succession is the relatively gradual change in structure and composition that arises as the vegetation itself modifies various environmental variables, including light, water and nutrient levels over time. These modifications change the suite of species most adapted to grow, survive and reproduce in an area, causing floristic changes. These floristic changes contribute to structural changes that are already inherent in plant growth even in the absence of species changes (especially where plants have a large maximum size, i.e. trees), causing slow and usually predictable changes in the vegetation. Succession can be interrupted at any time by disturbance, setting the system either back to a previous state, or off on another trajectory altogether. Because of this, successional processes may or may not lead to some static, final state. Moreover, accurately predicting the characteristics of such a state, even if it does arise, is not always possible. In short, vegetative communities are subject to many and unpredictable variables that limit predictability.

Vegetation - Spatial Dynamics

As a general rule, the larger an area under consideration, the more likely the vegetation will be heterogeneous across it. Two main factors are at work. First, the temporal dynamics of disturbance and succession are increasingly unlikely to be in synchrony across any area as the size of that area increases. That is, different areas will be at different developmental stages due to different local histories, particularly their times since last major disturbance. This fact interacts with inherent environmental variability, which is also a function of area. Environmental variability constrains the suite of species that can occupy a given area, and the two factors together interact to create a mosaic of vegetation conditions across the landscape. Only in agricultural systems does vegetation ever approach perfect uniformity. In natural systems, there is always heterogeneity, although its scale and intensity will vary widely. A natural grassland may seem relatively homogenous when compared to the same area of partially burned forest, but highly diverse and heterogeneous when compared to the wheat field next to it.

Adapted from the Wikipedia article "Vegetation Processes", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki