Snow storm in the making, Kiev, Ukraine © Flickr/Pedro Moura Pinheiro
Storms bringing wind, snow, ice or hail or a combination of these factors have always impacted the health of forests and thus are a regular consideration in forest management plans. They can occur as catastrophic events affecting entire landscapes, the quality of wildlife habitats, and forest stand structure, which can lead to major disruptions in management goals. Alternatively they may occur as small scale disturbances that affect individual trees or groups of trees within a stand increasing the amount of dead wood and diversifying stand structure, which can have positive benefits for biological diversity.
Snow most commonly impacts trees by breaking stems but trees can also be bent or uprooted. The severity of snow damage is related to tree characteristics; factors controlling the stability of trees such as stem taper and crown characteristics are the most important. Conifers are particularly damaged by heavy snowfall, while broadleaved trees are generally more resistant to storms and snow in the late autumn and winter due to better root systems and lack of foliage.
Damaging winds vary from short-lived gusts, to strong prevailing winds, to powerful hurricanes, to brief but intense downdrafts from thunderstorms.
Approximately 2 000 hectares of forest were damaged in Estonia during the Eisma storm in August 2010 © O. Lavrentjeva
Storm damage can include initial mechanical damage from the storm, subsequent damage from other biotic or abiotic factors (i.e. insects, fire, sun, snow, ice, etc.), and loss of production. The impact of wind on forests is determined by a complex of many biotic and abiotic factors and is similar to those experienced during cyclones.
In a matter of minutes an ice storm can deposit a layer of ice heavy enough to bring down power and telephone lines and snap branches from trees. Impacts of individual storms are highly patchy and variable, and depend on the nature of the storm, its severity, frequency, timing and extent. Ice accumulation on trees can cause minor branch breakage; major branch loss, up to total crown loss; temporarily or permanently bending over of crowns; root damage (when soil is not frozen); breakage of trunks within or below the crown; and for some hardwoods, split trunks. Softwoods seem to suffer less damage than hardwoods. Recently-thinned stands can be highly vulnerable, as crowns have spread into newly-opened space but branch strength may not be fully developed. Trees damaged by ice storms or windthrow can be more susceptible to other disturbances such as insect pests or fire.
Large hailstones that can reach diameters of over 10 centimetres and can fall at speeds of over 150 km/h can also cause considerable damage to forests.
Dust storm in New South Wales, Australia, 2009 © Flickr/DabaYu
Dust storms and sandstorms are natural events that occur throughout the world, especially in dryland areas which occur in Central and South Asia, the Australian Outback, South American Patagonia, the North American Great Plains, Sahara and sub-Saharan Africa and the Mediterranean region.
Dust storms and sandstorms are a result of wind erosion and are driven by poor land management and degradation of the dryland vegetation cover. Strong winds and favourable surface atmospheric conditions (i.e. turbulence level, stability, soil moisture) can allow for large amounts of sand and dust to be lifted from bare, dry soils into the atmosphere. Every year one and a half tonnes of sand and dust are emitted from drylands into the atmosphere where it can be transported downwind affecting regions hundreds to thousands of kilometres away depending on meteorological conditions. Dust from the Gobi Desert, for example, is carried to the Pacific coasts of North America and dust from the Sahara Desert is carried to the Caribbean islands and the Amazon basin.
Dust can have numerous impacts on human and veterinary health, the environment, agriculture, marine ecosystems, fisheries, transport, visibility, aviation, and weather and climate at larger scales. Crops can be destroyed, trees can be damaged or blown down and greenhouses and other nursery structures can be broken.
In some cases, however, the deposition of dust can produce positive results. For example, mineral-rich Saharan dust transported across the Atlantic Ocean to the Amazon rain forest in South America provides iron and phosphorus to the nutrient-poor rainforest soils acting as fertilizer.
Measures to combat the occurrence and impacts of sand and dust storms include the use of windbreaks or shelterbelts to reduce the impact of wind speeds and decrease soil erosion.
Abiotic disturbances and their influence on forest health
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