Forest Health Status in North America

The forests of North America provide a variety of benefits including water, recreation, wildlife habitat, timber, and other forest products. However, they continue to face many biotic and abiotic stressors including fires, native and invasive pests, fragmentation, and air pollution. Forest health specialists have been monitoring the health of forests for many years. This paper highlights some of the most damaging forest stressors affecting North American forests in recent years and provides some projections of future risks.

Although the total forest area in the U.S. has been relatively stable, some portions of the country have been increasingly fragmented. Fragmentation of forests may lead to changes in ecological processes and reduce biological diversity. Analyses of forestland maps derived from satellite imagery indicate that large portions of the forestland are fragmented with about 44% being within 90 m of the forest edge [2].

FOREST PESTS
Forest insects and pathogens are biotic disturbance agents that can be either beneficial or detrimental to forests. While they play critical roles in forest ecosystems, they can be devastating when populations reach high levels. Outbreaks can lead to damaging levels of defoliation or mortality under suitable climatic and site conditions.

Canada
Historic trends in forest pests for Canada are presented in Fig. 4

U.S.
Historic trends in forest pest activity in the U.S. are presented in Fig

Gypsy Moth Southern Pine Beetle Mountain Pine Beetle
Western Spruce Budworm Spruce Budworm In addition to the gypsy moth, several other invasive forest pests are threatening forests in the U.S. The hemlock wooly adelgid (Adelges tsugae), a native of Asia, continues to spread in eastern hemlock forests. Since its introduction in 1924, it has spread to hemlock forests from southeastern Maine to northeastern Georgia and west to eastern Tennessee. Biological control agents have been released in an attempt to control populations. The emerald ash borer (Agrilus planipennis), also a native of Asia, was first reported killing ash trees in the Detroit and Windsor areas of Michigan in 2002. Since then, infestations have been found throughout lower Michigan and neighboring areas in Ontario (Canada), northwest Ohio, and northern Indiana. Infestations were also recently found in the Chicago area. The European woodwasp (Sirex noctilio) has recently been found infesting pine trees in New York State and Ontario. Introductions of this insect into other countries have resulted in significant mortality levels in pine plantations. Monterey, lodgepole, ponderosa, jack, and most species of southern pines (especially loblolly) are known to be susceptible. The susceptibility of other North American conifers is not known. A new disease called "sudden oak death" (caused by Phytophthora ramorum) is killing thousands of tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), and California black oak (Q. kelloggii) in coastal areas of California. An isolated infestation, discovered in Oregon, is being treated with goal of eradication. National surveys of oak forests have not found infestations outside California and Oregon.

Mexico
Historic trends in forest pest activity in Mexico are presented in Fig. 6

AIR POLLUTION
Air pollutants, including sulfur, nitrogen, and tropospheric ozone, can have significant cumulative effects on forests. Canada and the U.S. have cooperated in monitoring air pollutant deposition, concentrations, and effects under the 1991 Canada -U.S. Air Quality Agreement [5]. Spatial distribution of wet sulfate and wet nitrate deposition is presented in Figs. 7 and 8, respectively. Canada has also recently calculated critical loads exceedances of acidifying compounds for forest soils in eastern provinces (Fig. 9)[6]. For the U.S., cumulative distribution functions and frequency distributions were used to estimate the percent forest by region of the country exposed to specific levels of air pollution [7]. In the North and South, approximately 50% of the forests were exposed to sulfate deposition of more than 15 kg/ha/year for 1994-2000 (Fig. 10) compared to the Pacific Coast and Rocky Mountain regions, where approximately 50% of the forests received less than 2 kg/ha/year. Nitrate deposition was highest in the North where approximately 50% of the forests received an annual average input of more than 13 kg/ha/year. The North and South regions experienced the highest ammonium deposition rates. Ozone concentrations were relatively high across much of the South with only 10% of the forests exposed to ozone index concentrations of less than 6 ppm-h/year. Although most of the Pacific Coast region forests were exposed to relatively low ozone index, 10% of the forested area experienced exposure between 41.2 and 117.8 ppm-h/year.

FOREST HEALTH INDICATORS
In the U.S., the Forest Health Monitoring (http://fhm.fs.fed.us) and the Forest Inventory and Analysis (http://fia.fs.fed.us) programs monitor a suite of forest health indicators to determine the effects of air pollution and other stressors.

Crown Conditions
Crown dieback and foliar transparency measurements were used to calculate a crown index [8]. Overall, less than 15% of the basal area was associated with unhealthy crowns. Ecoregion sections having greater than 10% average basal area associated with unhealthy crowns were mostly located in the Interior West.

Tree Mortality
Tree mortality on plots has been assessed using two indices: MRATIO, ratio of annual mortality volume to annual gross growth volume; and DD/LD, the ratio of the average dead tree diameter to the average live tree diameter [8]. The highest rates of mortality occurred in Idaho and western Washington.

Soil Condition
Forest soils are critical components of forest ecosystems. The soil condition indicator collects information on physical and chemical properties of soil on measured plots [9]. The mean soil pH value for measured plots is 4.8, with acidic tendency of soil pH most clear east of the Mississippi River. The southeastern U.S. tended to have the greater proportion of forest soils with low effective cation exchange capacity levels. Total

Ozone Biomonitoring
The effects of ozone on forest ecosystems are monitored by assessing damage to ozone-sensitive species on ozone biomonitoring sites located in forests throughout the U.S. The severity of foliar injury is assessed according to an injury score: 0-4.9 for no or minute injury, 5-15 for light to moderate injury, 15-25 for moderate to severe injury, and greater than 25 for severe injury. Spatial interpolations of plot injury scores for the period from 1999-2002 show that the highest foliar injury occurred in the mid-Atlantic and the southeast, with significant injury recorded in southern California.

Lichens
The FHM and FIA programs also monitor effects of air pollution on lichen communities. Biotic indices have been developed based on lichen community data along air pollution and climate gradients. For example, spatial interpolation of lichen index scores across Washington and Oregon show a decrease of air pollution-sensitive lichens near major metropolitan areas [10].

FUTURE CHALLENGES
In the future, we hope to enhance timely detection, analysis, and reporting of adverse changes in forest health to facilitate effective management responses. To increase our understanding of the adverse changes