Changes in precipitation regimes and thermal patterns are important determinants driving possible alterations in plague and disease behaviour in forests and agrosystems. Extreme thermal events can reduce forest ecosystem production by as much as 30% in continental Europe (Ciais et al. 2005) and mild winters interrupted by periods of intense cold can alter ecosystem function, weakening the system and facilitating the spread of plagues within the affected populations. Nevertheless, gradual temperature changes may be accompanied by subtle but progressive changes in the plague and disease distribution area, which in turn may be facilitated by the increased mobility of people and goods at global scale (Anderson et al., 2004). The degree in which the decline of certain forest masses can be attributed to extreme climate events and the consequences of these events on the possible changes in plague and disease behaviour will be determined in the zones in which the phenomena have been observed to be important in recent years such as in the central and eastern Iberian Peninsula. Impacts attributable to extreme thermal events (cold/heat) in various forest ecosystems within relatively large geographical areas (thousands of hectares) and at different points in the drought cycle will be studied. The changes in plague behaviour for pines and Juniper will also be considered in the light of some of these extreme events.