What's Killing Iowa's Trees? Diseases and Pests Putting Ecosystem Services at Risk

Over a short period of five years, my family lost seven of the ten old-growth white oaks in our surrounding yard. This does not take into account the numerous oak deaths that occurred during the same time period among our fifteen acres of northern Wisconsin old-growth forest. These massive, shade-providing trees I grew up under fell victim to oak wilt, a disease that causes rapid death and is easily spread by the transportation of firewood and lumber across states and counties. While I grieve the loss of my familial oaks because of childhood reminiscence, the widespread occurrence of tree death due to pests and disease affects the availability of numerous ecosystem services associated with forests around the Midwest and rest of the country.

             Before Euro-American settlement, the land that now makes up the state of Iowa was mostly prairie, with only roughly 18 percent of land covered by forests. This low percentage of forest cover has been reduced to approximately 7 percent today due to settlement and agriculture. Iowan land today is mostly cultivated for agricultural use, leaving riparian forests and wooded lots largely scattered around the state. The ecosystem services that these forest systems provide are vulnerable to change due to pests and diseases like Dutch Elm disease and the Emerald Ash Borer. In the 1960’s Iowa’s Elm population experienced a large decline due to the invasion of Dutch elm disease. Today, the Emerald Ash Borer presents a huge threat to the ash population in the state. Last month (10/15), the USDA mapped the effects of the pest in a map that declared the entire state of Iowa to be under quarantine, stating that wood is not to be transported across state lines or counties. The map can be seen below.

In their 2013 review article in Science, “The Consequence of Tree Pests and Diseases for Ecosystem Services,” Boyd et al discuss the introduction, consequences, and future management outlook for pests and diseases that induce mortality in global tree populations. The authors define pests and diseases in this context as herbivores or infections that “are perceived to reduce the value of trees to people,” whether that value is monetary or not. In their introduction to the concept of changes in ecosystem services, a figure is presented to display projected changes in forest structure in response to a disease that decreases abundance of native species (Figure 1). This figure shows an initial decline in a number of ecosystem services provided by the forest followed by eventual recovery of a select few as a result of compensatory growth of non-affected species. While this recovery may take place, the loss of biodiversity within the forest structure may have greater consequences, as presented by authors later in the paper.

Boyd et al. present examples of both pests and pathogens that currently account for large numbers of tree deaths globally. On the pest side, insects including the Asian longhorn beetle, Oak processionary moth, and Emerald ash borer are responsible for devastating loss of broad-leaved trees, oak species, and ash species respectively. These herbivorous creatures’ pathogenic counterparts come from a list of fungi, bacteria, viruses, and helminths that are the cause of commonly discussed diseases such as Pine wilt, Dutch elm disease, and chestnut blight. While both insect pests and disease-causing pathogens can result in widespread tree death, interaction between the two often exacerbates mortality within a forest community. Defoliation by an insect pest may leave a tree more vulnerable to infection by a pathogen that results in a deadly disease.

The geographic ranges of pest and pathogenic tree killers have increased with the evolution and development of world trade and human settlement. As discussed in the article, movement of trees beyond native ranges with the rise of international trade and agriculture brought with it the import of new pests and diseases to native tree species. Introduction of non-native insects and pathogens is detrimental to native forests that have not evolved to have mechanisms of resistance against such threats. Authors suggest that while not all introduced non-native species are harmful to native flora, genetic reassortment and hybridization can result in acquired virulence factors that become harmful to native trees.

Boyd et al. discuss evidence in support of climate warming’s detrimental influence on tree susceptibility to pest and disease. In addition to increased mortality, climate change introduces another level of importance for sustained areas of arboreal forests. Along with monetary-driven services such as pulp, timber, and food sources, trees act as a major carbon sinks. Authors suggest that large arboreal forests, when exposed to extreme pest-induced defoliation, can become carbon sources that contribute to the greater global climate change.

            In their suggestions for management and future disease control methods, the authors emphasize the need for risk-focused approaches to consider the larger set of ecosystem services being provided by arboreal species in question. Suggested approaches to management procedures are broken into three steps presented as the identification of 1) agent (whether fungal, insect, etc.) 2) pathway of transport (such as water or man made pathways.) 3) category of at-risk ecosystem services. While many tree diseases and pests affect local forest communities, the transport of timber and firewood can exacerbate the spread of such endemics.

Boyd, I. L., P. H. Freer-Smith, C. A. Gilligan, and H. C. J. Godfray. "The Consequence of Tree Pests and Diseases for Ecosystem Services." Science 342.6160 (2013): 1235773. Web.

Iowa Department of Natural Resources "Emerald Ash Borer."  Web. 15 Nov. 2015. <http://www.iowadnr.gov/Conservation/Forestry/Forest-Health/Emerald-Ash-Borer>.