TREES BY THE SEA: ADVANTAGES AND DISADVANTAGES OF OCEANIC CLIMATES

Abstract

Proximity to the ocean can have both positive and negative effects for tree survival. Across the world relict forests that were once trans-continental in their distribution depend for their continued survival on coastal refugia. The tree species that dominate the cloud-zone forests of Macaronesia, the coastal redwoods of California, the Atlantic forests of Brazil, and the Podocarp forests of New Zealand's South Island are all examples of palaeoendemic species that once had a much wider distribution and appear to owe their survival to the particular environmental conditions provided by coastal sites and oceanic islands. By contrast, in the islands of the North Atlantic, oceanic conditions appear to limit tree regeneration and make forests vulnerable to human disturbance. Paradoxically, winter warmth appears to be harmful to trees in northern cool and moist oceanic conditions. There may be many reasons why the warm winters of maritime environments can be detrimental for woody species. Insect attack, pathogenicity and metabolic dysfunction with loss of frost hardiness and over-wintering carbohydrate reserves are all possibilities. Where long, mild winters are combined with wet soil conditions, metabolic dysfunction brought about by prolonged periods of oxygen deprivation can deplete root meristems of carbohydrate reserves that are essential for avoiding postanoxic injury when the soil profile once again becomes aerated in spring. In areas where the temperature remains below zero for the greater part of the winter, this is not a danger. However, in oceanic habitats there is always the risk, even in the far north, of temperatures rising above zero. Model predictions and field observations confirm the potential dangers of warm winters by suggesting that any significant degree of winter warming will cause a retreat of Pinus sylvestris and other woody species from oceanic regions at northern latitudes.