Summary Report No. 38


Die Berücksichtigung natürlicher Störungen in der Vegetationsdynamik verschiedener Klimagebiete

K. Thonicke (February 1998)

The mortality of plants is also likely to affect the interactions between plants and their abiotic and biotic environment. Frequent causes of plant mortality are natural disturbances produced by different environmental factors, which change the vegetation in a certain area in different ways.

Natural disturbances are defined in different ways, according to their different theoretical foundations. In this report natural disturbances are understood as discrete events, determined by space, time and magnitude, which cause a change of state in part of an ecosystem in such a way, that plants as a function of the remaining resources repeatedly compete for the resources, and that modus and the rate of re-establishment is re-determined. Connected with this is the reorganisation of the inter- and intraspecific competition between plants at the site, which can result in a change in species composition. The resources mobilised by the disturbance favour the establishment of a new generation of plants and intensify the growth of the remaining plants on the affected patch, not least due to the reduced competitive stress. Therefore natural disturbances are regarded as natural elements of vegetation dynamics.

An approach is presented in this report showing how natural disturbances can be described in relation to other ecosystem compartments. After this, the natural disturbance regime is characterised by statistical variables, which describe the dimensions of disturbances in terms of space, time and magnitude. The reaction of the vegetation depends on the remaining amount of resources in the affected area, that together with the plant regeneration strategies and the characteristics of the disturbed patch determine the regeneration strategies the mode and rate of re-establishment.

The available literature was reviewed and natural disturbance regimes for each vegetation zone were produced. The vegetation zones are based on the ecosystemological classification of terrestrial vegetation of Walter and Breckle (1983).

The natural disturbance regime of evergreen tropical rain forest is characterised by small-scale natural disturbance with various causes. Bare patches are closed easily due to intensified growth of the juvenile plants in the former understorey. Destruction of the vegetation caused by infrequently occurring wind-throws, fires and shifting cultivation are usually compensated via cyclic successions, through which after several decades the postdisturbant primary forest is re-established.

In tropical savannahs and dry tropical forests droughts are frequent due to the high variability of precipitation. Grasses and trees are adapted to it in a different manner, because they have developed survival strategies as a result of annual dry season. Where there is sufficient soil moisture plant regeneration is principally vegetative after a period of drought. Fires reoccur after 2 to 4 years. Immediately after a fire, all nutrients previously fixed in the litter bcome bioavailable again. With decreasing annual mean precipitation intervals between fires become longer because of the decreasing plant productivity and consequent decreasing litter accumulation. Savannahs are exposed to intense grazing pressure due to the large number of large wild mammals and livestock, which migrate with the drying up of water sources and the exhaustion of the grazing area. Where the entire migration area is sufficiently large, the predisturbant vegetation structure re-establishes itself in the affected areas in the meantime.

The length of the fire return intervals in temperate grasslands and mediterranean biomes, which can vary between 3 and 4 years, but also between 15 and 100 years, are a function of litter accumulation, and also in part of the density of the understorey in the latter. Where conditions are dryer and water more scarce, primary production decreases, and thus the fire return intervals are longer, but the frequency of droughts higher. The plants resprout from lignotubers or from dormant buds, or regenerate from seeds. In nutrient-poor soils the mobilisation of nutrients fixed in plants is of great importance for the primary production of these ecosystems. The mediterranean biomes of the Mediterranean basin have been characterised by intense grazing for centuries, which can hardly be described in one regime due to many pronounced land use changes.

The vegetation dynamics of temperate deciduous forests are influenced by large-scale wind-throw caused by hurricanes and storms, and small-scale destruction due to lightning and winds of less intensity. The latter form patches of 30 to 1500 m² at intervals of 50 to 200 years, which are essential for the conservation of certain species whose juveniles require a minimum patch size to become successfully established.

In the Northern Hardwood Forest fire occurs at intervals of 20 to 200 years as a function of annual precipitation and soil structure. Large-scale wind-throws are caused by hurricanes. Where wind directions are constant, wave-generated Balsam Fir Forests develop, with regeneration cycles of 60 years due to the 60 yr. lifespan of the balsam fir Abies balsamea.

The equilibrium between deciduous and coniferous trees in the southern boreal of north America is probably maintained by the spruce budworm in connection with fires. The first colonizers following a fire are deciduous trees, which become more and more mixed with coniferous trees after 200 years of succession. The infestation of balsam fir Abies balsamea and white spruce Picea glauca by spruce budworm prevents deciduous trees being outcompeted by coniferous trees. In the boreal forest fires occur - depending on the moisture and amount of litter - at an average interval of 200 years. Fires are an important factor in the maintenance of the tree stability. Because of the reduction of the litter layer soil warming is guaranteed and therefore the expansion of the permafrost is limited. If this were not the case, a southward shift of the arctic tree line could be expected. Plants regenerate above all via wind dispersed seeds and lignotubers.

Fires can even occur in the arctic tundra, but much more rarely than in the boreal forest. Vegetation dynamics are rather influenced by mechanisms such as solifluction, that destroy vegetation cover.

The occurrence of fires depends on the occurence of dry periods and the amount of litter accumulation. As a result the return intervals decrease because less plant material being available to burn, or because with increasing moisture the ignition of flammable plant material becomes less probable. Weather conditions cause regular storms, which lead to large-scale windthrows. Insect outbreaks are determined by a variety of factors, especially by the population dynamics of the insect population itself. Large-scale natural disturbances are more rare than small-scale, but are of great intensity, whereas small-scale, but less intensive disturbances are more frequent.

Natural disturbances have been incorporated particularly in gap models by describing them with stochastic mortality functions in order to reproduce the behaviour of the vegetation and changes in vegetation caused by natural disturbances. But these functions fail to describe sufficiently all disturbance agents, which is why in the current modelling work, possibilities to create more deterministic models via essential effects and structure of ecosystems, also at larger scales, and to simplify the models to minimise the data needs are being discussed. Due to the variety of effects of various disturbance agents, it seems that opportunities to combine different modules representing certain agents into one general natural disturbance model are required.