Urban Forest Health

Model results project that species currently near their northern range limits in the region may become more abundant and more widespread under a range of climate futures. Results from forest impact models suggest that species such as bitternut hickory, black oak, bur oak, and white oak may have increases in both suitable habitat and biomass, and some deciduous forest types have the potential for productivity increases across the assessment area.

Across northern latitudes, warmer temperatures are expected to be more favorable to individuals near the northern extent of their species’ range and less favorable to those near the southern extent. Results from climate impact models project declines in suitable habitat and landscape-level biomass for northern and high elevation species such as black spruce, balsam fir, red spruce, and paper birch. Forest ecosystems dominated by boreal species, such as spruce-fir or paper birch, are consistently rated as the most vulnerable across numerous regional vulnerability assessments.

Changes in climate may allow some nonnative plant species, insect pests, and pathogens to expand their ranges farther north as the climate warms and the growing season increases. The abundance and distribution of some nonnative plant species may be able to increase directly in response to a warmer climate and also indirectly through increased invasion of stressed or disturbed forests. Similarly, forest pests and pathogens are generally able to respond rapidly to changes in climate and also disproportionately damage-stressed ecosystems.

Given that warmer temperatures and seasonal changes in precipitation are expected across the region, it is reasonable to expect that soil moisture regimes will also shift. Longer growing seasons and warmer temperatures would generally be expected to result in greater evapotranspiration losses and lower soil-water availability later in the growing season, thereby increasing moisture stress on forests. Further, increases in extreme rain events suggest that greater amounts of precipitation may occur during fewer precipitation events, resulting in longer periods between rainfall.

All global climate models agree that sea level will rise. Sea levels have increased over the past century, and this trend is expected to continue. Additional warming is expected to increase global sea levels by up to 1m (3 ft) by the end of the century. In the Mid-Atlantic, sea-level rise is significantly greater than observed global sea-level rise, due to sinking of the land surface as it adjust to the melting of former ice sheets and the withdrawals of natural resources from underground.

Some national and global studies suggest that conditions favorable for wildfire will increase, but few studies have specifically looked at wildfire risk in the Mid-Atlantic region. The duration of the fire season in the Mid-Atlantic region is closely linked with increases in average temperature during the summer (Liu et al. 2010). If drought or prolonged dry periods increase in this region as expected, fire risk will increase in both forests and local communities.

Heavy precipitation events have increased substantially in number and severity in the across the Northeast over the last century, and many models agree that this trend will continue over the next century. Under the higher scenario (RCP8.5) the number of extreme events is projected to increase by two to three times the historical average in every region by the end of the 21st century, with the largest increases in the Northeast. Under the lower scenario (RCP4.5), these events are projected to increase by 50%–100%.

Seasonal differences in temperatures across the Mid-Atlantic and Northeast have decreased in recent years as winters have warmed three times faster than summers. By the middle of this century, winters are projected to be milder still, with fewer cold extremes, particularly across inland and northern portions of the Northeast. Warmer temperatures are expected to cause more winter precipitation to be delivered as rain. Snowfall, snow depth, and snow pack are all expected to be reduced.

Warmer temperatures can lead to decreased soil moisture even without an associated decrease in precipitation, resulting in a temporary inability for a tree to meet water demand.

Evidence at both global and local scales indicates that growing seasons have been getting longer, and this trend is projected to become even more pronounced over the next century. Warmer temperatures will result in fewer days with minimum temperatures below 32°F and a shorter freeze-free season. Winter or early-spring warmth has caused plants to start growing and emerge from winter dormancy earlier in the spring.