Climate change and forest health
The oak jewel beetle, Agrilus pannonicus, has recently been associated with a European oak decline throughout its natural range and has increased in incidence in several countries including France, Germany, Hungary, Poland, the Netherlands, and the UK. Incidences of Agrilus species have increased worldwide (both in their countries of origin and by international movement) and their impacts are being linked to host tree stress potentially caused by climate change.
In the southeastern US, recent outbreaks of the southern pine beetle, Dendroctonus frontalis, in northern and high altitude ecosystems, where they were previously rare or absent, have been attributed to a warming trend in minimum winter air temperatures. In Central America, an unprecedented regionwide outbreak of pine bark beetles, mainly D. frontalis in association with other species, occurred after Hurricane Mitch. With climate change expected to increase the frequency and severity of extreme events such as hurricanes, the potential for future devastating outbreaks is quite high.
The mountain pine beetle, Dendroctonus ponderosae, is the most destructive pest of mature pines in North America, particularly lodgepole pine (Pinus contorta). In the western United States, outbreaks have been increasing in area after several years of drought and a major epidemic of this pest has also been ongoing in western Canada for several years. The problem has been exacerbated by successive years of mild winters, resulting in decreases of mortality of overwintering stages of the beetle. Their life cycle is generally completed in one year; warmer temperatures can result in two generations per year while cooler ones may result in one generation every two years. Drought conditions associated with warmer temperatures have also weakened the trees and increased their susceptibility to the beetles. Warmer temperatures have thus opened up previously climatically unsuitable mature pine stands to the pest.
Dendroctonus rufipennis is a North American pest of spruce that tends to attack weakened or windthrown trees; outbreaks are mostly linked to predisposing factors. As a result it can be expected that the impacts of climate change on trees and forests could enhance spruce beetle outbreaks. Research has shown that historic outbreak years in Utah and Colorado, USA were related to generally warm fall and winter temperatures and drought conditions. Similarly outbreaks in both Canada (Yukon Territory) and the US (Alaska) have been shown to be related to extremely high summer temperatures which influenced spruce beetle population size through a combination of increased overwinter survival, a halving of the maturation time from two years to one year, and regional drought-induced stress of mature host trees.
With short generation times and low developmental threshold temperatures, aphids can be expected to be strongly influenced by environmental and climatic changes. In general, it has been predicted that aphids will appear earlier in the spring, though the rate of advance will vary depending on location and species. The green spruce aphid, Elatobium abietinum, is believed likely to benefit from an increase in winter survival, leading to more intense and frequent defoliation of host spruce trees (Picea spp.). Studies show that warm weather caused spring migration of E. abietinum to start earlier, last longer and contain more aphids.
The spruce webspinning sawfly, Cephalcia arvensis, is monophagous on spruce (Picea) and endemic to the spruce range in Eurasia, where outbreaks have been seldom recorded. From 1985-1992 however there was a sudden outbreak of the sawfly in the Southern Alps during which populations developed an annual life cycle and grew exponentially, causing repeated defoliations resulting in extensive tree death. The outbreak corresponded to a period of high temperatures and low precipitation and severe water stress for the host trees. As a result, the insect exhibited lower mortality, faster development and higher feeding rates. In addition, the sudden increase in population density was not quickly followed by that of natural enemies, thereby facilitating unlimited population growth.
The European pine sawfly, Neodiprion sertifer, is an important pest species on pines in Europe, northern Asia, Japan and North America, where it was introduced, and is a serious defoliator of Scots pine (Pinus sylvestris) forests in northern Europe. It has been suggested that outbreaks of the sawfly on Scots pine in eastern and northern Finland are prevented by low winter temperatures which kill eggs, and it is predicted that outbreaks would become more common with winter warming.
Warmer temperatures have been linked to increasing populations of forest lepidopterans. A limiting factor, particularly for defoliators, is synchrony with bud burst in the spring while the key drivers are late winter temperatures for insect emergence and bud burst.
The spruce budworm, Choristoneura fumiferana, is a major defoliator of coniferous forests across North America. Climatic influences on life history traits are considered a major factor in restricting outbreaks and as a result, a changing climate is expected to impact the severity, frequency, and spatial distribution of spruce budworm outbreaks. Success in establishing feeding sites in the spring depends on initial egg weights and synchrony of development with that of buds of their hosts which is strongly influenced by climatic factors. This synchronization is critical in initiating outbreaks and thus determining the intensity of damage. In parts of its range, particularly at northern extremes, temperature can also influence the duration of outbreaks as collapses are often associated with the loss of suitable foliage often as a result of late spring frosts.
The autumnal moth, Epirrita autumnata, is a holarctic species that has been expanding its outbreak range in some areas. In the Nordic countries of Europe, Epirrita autumnata outbreak cycles are typically most prevalent in northernmost and continental birch forests but during the past 15-20 years it has expanded into the coldest, most continental areas previously protected by extreme winter temperatures. This pest overwinters in the egg stage and the level of egg survival is dictated by minimum winter temperatures. A rise in winter temperatures therefore will likely increase the area of forest susceptible to damage by the autumnal moth.
The gypsy moth, Lymantria dispar, is a significant defoliator of a wide range of broadleaf and even conifer trees. Two strains of gypsy moth exist - the Asian strain, of which the female is capable of flight; and the European strain, of which the female is flightless. The Asian strain is native to southern Europe, northern Africa, central and southern Asia, and Japan and has been introduced into Canada, the US and the UK as well as Germany and other European countries where it readily hybridizes with the European strain. The European strain is found in temperate forests throughout Western Europe and has been introduced into Canada and the US. There has been a noted increase in outbreaks in areas previously unaffected by this pest such as the Channel Islands and new areas in the UK. It is predicted that the gypsy moth will be able to extend its range in North America as a result of higher overwinter survival of egg stages because of milder winters and higher accumulation of day degrees for larval development; similar predictions have been made for Europe. A study has also noted an increased probability of gypsy moth establishment in New Zealand South Island.
The nun moth, Lymantria monacha, is a major pest of broadleaved and coniferous trees in Europe and Asia. In parts of Europe, the occurrence of outbreaks has increased, possibly as a result of the establishment of extensive pine plantations in poor quality areas or as a result of a changing climate. It has been predicted that nun moth will spread northwards in Europe because of higher accumulated day degrees and improved overwinter survival. Models have also predicted that climate warming would shift the northern boundary of distribution north by approximately 500-700 km and the southern edge of the range would retract northwards by 100-900 km.
Operophtera brumata is distributed throughout Europe, North Africa, Japan and Siberia and has also been introduced into Canada and the US. Climate change is impacting the spread of the winter moth. For example, in the Nordic countries of Europe, it has been noted that O. brumata had been climatically restricted to more southern and near-coastal locations in the regions but warmer temperatures has resulted in expansions in its outbreak area further northeast. Climate change has also affected the phenology of the winter moth. In the Netherlands over the past 25 years, early spring temperatures have increased while winter temperatures have not. As a result, a climate change induced asynchrony has occurred between winter moths and their host, pedunculate oak (Quercus robur), with eggs hatching before bud burst. Such a situation leaves no food for the larvae resulting in starvation and death.
The pine processionary caterpillar, Thaumetopoea pityocampa, is considered one of the most important pests of pine forests in the Mediterranean region. Climate change is having clear impacts on the distribution of this important forest pest. Latitudinal expansions northwards and altitudinal shifts upwards in France and Italy have been attributed to reduced frequency of late frosts as a result of a warming trend, which increases survival of overwintering larvae.
Native to central and southern Europe, Thaumetopoea processionea is a major defoliating pest of oak. Since the late 20th century it has been expanding its range northwards and is now firmly established in Belgium, Denmark, northern France, and the Netherlands and has been reported from southern Sweden and the UK. It is believed that the northward progression of the oak processionary moth is due to improved synchrony of egg hatch and reduction of late frosts as a result of warmer temperatures.
The larch bud moth, Zeiraphera diniana, is a European pest that has been defoliating large areas of larch forests in the Alps every 8 to 10 years for centuries. Increased temperatures associated with climate change have affected the relationship between egg hatch and host bud burst leading to asynchrony and thus reduced incidences of this moth in some areas. It has also been reported that abnormally high temperatures result in unusually high egg mortality.