A new study has identified the factors that cause deterioration of Mediterranean pine plantations, to develop a model of that can predict the risk of forest decline to help inform forests management strategies under a changing climate. It suggests that loss of needles is the most useful predictor of decline for the species studied.
Under the effects of climate change, the Mediterranean Basin is becoming warmer and drier, and increased mortality of pine plantations is expected. At particular risk are the populations growing on the edge of the range of the species. There are several potential causes of tree decline, including poor soil conditions, extreme drought and parasites, which can lead to reduced growth and defoliation or loss of leaves and needles. The study identified the main factors causing decline in replanted pine forests in a drought prone area of Southern Spain, which have experienced severe decline (i.e., extensive tree dieback) at the most southerly part of their natural range. It studied two types of pine plantation - Pinus sylvestris, or Scots pine, and Pinus nigra, the European black pine. Both these trees are native to Spain. Their natural occurence is limited to certain mountain areas with distinct ecological conditions. The pines were planted on sites in their range of natural distribution, but these sites are in the rear edge (southern limit of natural distribution in Europe). In some sites, Pinus sylvestris is out of the range.
Precipitation and temperature measurements from 1940 and 2009 were examined by the researchers to identify factors that lead to decline, as well as climate data from the Spanish weather monitoring system. The location, trunk size, age, and crown damage (percentage of defoliation) were measured and observed.
The results indicate that forest decline appears to be the result of a long, complex chain of events. The size of trees and increased competition between trees were found to predispose trees to decline, and drought events, such as those from 1994-1995, acted as triggers for decline. Further, more intense droughts, in 1999 and 2005, also contributed to damage. Spring temperature, precipitation and soil type are also important factors, as they have a strong influence on trees during the growing season. In the case of the replanted pine forests studied, decline occurred without any influence of pests and diseases.
Competition between trees for soil, nutrients and water affects the survival of the two species in different ways. Competition was more damaging to the introduced, non-native Scots pine which may be more vulnerable because it does not cope well when there is a lack of water. In comparison, the native black pine can survive better in conditions of low water. The impacts of competition can be managed by thinning densely planted and unmanaged areas of forest, the study recommends.
High levels of mortality were associated with high levels of crown damage. The study suggests that defoliation is a useful indicator to predict decline, especially as it is measured in forest inventories. Models using defoliation and growth can provide useful contributions to guidelines on forest management and the use of mitigation techniques, such as thinning treatments and sustainable harvest activities.
Source: Sánchez-Salguero, R., Navarro-Cerrillo, R.M., Swetnam, T.W. & Zavala, M.A. (2012) Is drought the main decline factor at the rear edge of Europe? The case of southern Iberian pine plantations. Forest Ecology and Management. 271:158-169. Doi: 10.1016/j.foreco.2012.01.040.