Native and nonnative pest, pathogen, and plant species in the northeastern forests of North America all contain the capacity to alter and damage the ecological processes of the forest. This damage can lead to ecological and economic damage such as, among other things, causing tree mortality (Dukes et al. 2009). The northeastern forests of North America are being increasingly affected by invasive species in the form of fruit-bearing shrubs and vines that are altering young or physically disturbed forests. Essentially, the alien species create dense thickets that eliminate tree regeneration and reduce native understory shrub and herb diversity. These alien species are highly responsive to changes in the climate through changes in host distribution, population dynamics, nutrition, defense compounds, and evolving land use. Temperature increases in the northeastern US of 0.25 °C will increase the metabolism, reproductive rates, and survival of invasive species. In the case study, the possible responses of six key species were analyzed according to the effects of projected future climate change on the forests. For insect pests, climatic warming was found to accelerate insect consumption, development, and movement, which in turn can also influence fecundity, survival, generation time, and dispersal. The spread of pathogens in response to climate change is more difficult to predict because less is known about viral or bacterial sensitivity to climate in forest systems. However, likely affects of climate change on forest pathogens include increased growth and reproduction, altered dispersal, transmission rates, infection phenology, and changes in overwinter survival. And it is not just the temperature that affects the pathogens, precipitation, storm severity, nitrogen deposition, atmospheric ozone and CO2 concentration, and UV-B radiation all can affect forest pathogens. Plants also respond directly to changes in the climate. Direct effects to plants in the northeastern US forests as a result of climate change include: temperature, frost-free period length, and magnitude and duration of climate extremes. Indirect affects could also occur by altering ecosystem processes which could affect soil nutrients and moisture.—Patricio Ku
Dukes, J., Pontius, J., Orwig, D., Garnas, J., Rodger, V., Brazee, N., Cooke, B., Theoharides, K., Stange, E., Harrington, R., Ehrenfeld, J., Gurevitch, J., Leradu, M., Stinson, K., Wick, R., Ayres, M. 2009. Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: What can we predict?. Canadian Journal of Forest Research 39, 231–248
The species chosen to represent a sample of invasive aliens in northeastern US forests were selected because they represent a good sample of invasives and are not necessarily the most damaging species. The six case studies include two pests: Adelges tsuga (hemlock woolly adelgid) and the Malacosoma disstria (forest tent caterpillar); two common diseases: Armillaria (Armillaria root rot) and Cryptococcus fagisuga + Neonectria spp. (beech bark disease); and two invasive plant species: Frangula alnus Mill. (glossy buckthorn) and Celastrus orbiculatus (oriental bittersweet).
The result of almost all the case studies showed that climate change in almost certain to be a strong driver of evolutionary change in plant and pathogen populations. Although it is clear that invasive species are among the primary agents of biotic disturbance in northeastern US forests, it is unclear how exactly these forest dynamics will be played out. Uncertainties associated with internal ecosystem processes, climate projections, future human actions, and those arising from a lack of data on the invasive species themselves all make predicting the manifestations of these species more difficult. Nonetheless, the case studies did show that three of the six studied species (hemlock woolly adelgid, beech bark disease, and oriental bittersweet) are likely to become more widespread or abundant in northeastern US forests under projected climate change. None of the species that were studied were found to become less problematic as a result of climate change, but this does not mean that such species do not exist. The results were also difficult to interpret because of the relatively low statistical confidence level of the information on five of the six species studied. In the future it will be more useful and accurate to use whole-systems modeling of invasives to anticipate the range of possible responses of these complex systems as opposed to each individual species.