This study documents the scale and intensity of drying over the last half century in the Kenai Lowlands of south-central Alaska. Using historical aerial photos and field sampling of wetlands, including muskegs, kettle ponds, and closed and open basin lakes, we present data on drying and successional changes in woody vegetation between 1950 and 1996. The results of this study suggest that the Kenai Peninsula is becoming both woodier in its vegetation and drier. A regional analysis of 1113 random points indicated increased forest cover and decreased open and wet areas in both burned and unburned areas between 1950 and 1996. A census of water bodies in three subregions indicates that almost two-thirds of water bodies visited show some level of decrease in spatial area. Over 80% of field sites visited have experienced some level of drying, where vegetation transects indicate substantial invasion into former lake beds by facultative upland plants. These results are consistent with a regional change in climate that is both warming and drying as documented in Kenai and Anchorage weather records.

We document accelerating invasion of woody vegetation into wetlands on the western Kenai Peninsula lowlands. Historical aerial photography for 11 wetland sites showed that herbaceous area shrank 6.2%/decade from 1951 to 1968, and 11.1%/decade from 1968 to 1996. Corresponding rates for converting herbaceous area to shrubland were 11.5% and 13.7%/decade, respectively, and, for converting nonforest to forest, were 7.8% and 8.3%/decade, respectively. Black spruce ( Picea mariana (Mill.) BSP) forests on three wetland perimeters established since the Little Ice Age concluded in the 1850s. Dwarf birch shrubs at three wetland sites showed median apparent tree-ring age of 13 years, indicating recent shrub colonization at these sites. Peat cores at 24 wetland sites (basal peat ages 1840 – 18 740 calibrated years before present) indicated that these peatlands originated as wet Sphagnum –sedge fens with very little woody vegetation. Local meteorological records show a 55% decline in available water since 1968, of which one-third is due to higher summer temperatures and increased evapotranspiration and two-thirds is due to lower annual precipitation. These results suggest that wet Sphagnum–sedge fens initiating since the end of the Wisconsin glaciation began to dry in the 1850s and that this drying has greatly accelerated since the 1970s.

We used tree ring data (AD 1601–2007) to examine the occurrence of and climatic influences on spruce beetle (Dendroctonus rufipennis) outbreaks in south‐central and southwest Alaska and found evidence of regional‐scale outbreaks dating from the mid‐1700s, related to climate variability at multiple temporal scales. Over interannual time scales (∼1–3 years), El Niño years, combined with severe late‐summer drought, appeared to contribute significantly to spruce beetle outbreaks in the study area. Over multidecadal time scales (up to ∼40 years), cool‐phase Pacific Decadal Oscillation (PDO) conditions tended to precede beetle outbreaks, regardless of the phase of El Niño‐Southern Oscillation (ENSO). All sites showed low‐severity disturbances attributed to spruce beetle damage, most notably during the 1810s. During other major periods of disturbance (i.e., 1870s, 1910s, 1970s), the effects of spruce beetle outbreaks were of moderate or higher severity. The highly synchronized timing of spruce beetle outbreaks at interannual to multidecadal scales, and particularly the association between cool‐phase PDO conditions and beetle disturbance, suggests that climate (i.e., temperature, precipitation) is a primary driver of outbreaks in the study area. Our disturbance chronologies (mid‐1700s to present) suggest that recent irruptions (1990s to present) in south‐central and southwest Alaska are within the historical geographic range, but that outbreaks since the 1990s show greater spatiotemporal synchrony (i.e., more sites record high‐severity infestations) than at any other time in the past ∼250 years.

Increasing temperatures have resulted in reduced growth and increased tree mortality across large areas of western North American forests. We use tree‐ring isotope chronologies (δ¹³C and δ¹⁸O) from live and dead trees from four locations in south‐central Alaska, USA, to test whether white spruce trees killed by recent spruce beetle (Dendroctonus rufipennis Kirby) outbreaks showed evidence of drought stress prior to death. Trees that were killed were more sensitive to spring/summer temperature and/or precipitation than trees that survived. At two of our sites, we found greater correlations between the δ¹³C and δ¹⁸O chronologies and spring/summer temperatures in dead trees than in live trees, suggesting that trees that are more sensitive to temperature‐induced drought stress are more likely to be killed. At one site, the difference between δ¹³C in live and dead trees was related to winter/spring precipitation, with dead trees showing stronger correlations between δ¹³C and precipitation, again suggesting increased water stress in dead trees. At all sites where δ¹⁸O was measured, δ¹⁸O chronologies showed the greatest difference in climate response between live and dead groups, with δ¹⁸O in live trees correlating more strongly with late winter precipitation than dead trees. Our results indicate that sites where trees are already sensitive to warm or dry early growing‐season conditions experienced the most beetle‐kill, which has important implications for forecasting future mortality events in Alaska.