Remote sensing studies indicate that slow and subtle degradation of forest canopies, or forest canopy declines, has increased in extent during recent decades, possibly due to hotter and drier droughts. However, it is difficult to collect consistent, high-quality time series of forest canopy decline occurrence observations needed for examining multi-year drought contributions to forest canopy decline at an annual scale. In this research, we leveraged a tool for visual interpretation of annual Landsat satellite imagery (TimeSync) and a hierarchical Bayesian time series modeling approach (stochastic antecedent modeling, SAM) in five forest type groups located in the western United States to assess (1) what seasonal and interannual patterns in vapor pressure deficit (VPD) and precipitation preceded forest canopy decline events, (2) how drought effects on forest canopy decline events differed by forest type group, and (3) whether or not drought effects on forest canopy decline events were uniform within forest type groups. We examined observations of forest canopy decline over three decades (1985–2013) at 126 plots where we collected annual TimeSync observations. Stochastic antecedent modeling indicated that January–March VPD and July–September precipitation anomalies for the current year and 1–3 yr in the past contributed to defining drought conditions in relation to forest canopy decline dynamics. The probability of forest canopy decline decreased with summer precipitation for all forest type groups and increased with winter VPD for the warmest and the coldest forest type groups. However, the magnitude and direction of forest canopy decline sensitivity to drought varied substantially within forest type groups. The ubiquitous, but not uniform, effects of drought on forest canopy decline dynamics implied that local biotic (e.g., forest structure and composition, tree genetics) and abiotic (e.g., topography and soils) factors act to mediate effects of drought on forest change. The integration of the TimeSync satellite image interpretation tool with SAM provides a promising approach to link ecological understanding of tree drought responses to forest and landscape responses at regional and continental scales.
Diameter growth is seasonal in Douglasfir, the evergreen tree found in much of western Washington, Oregon, and northern California. Initiation and cessation of diameter growth are both triggered by environmental cues. The tree responds to these cues to improve its chances of growing under favorable conditions. As environmental conditions change, however, land managers want to know how warmer summers and falls may affect diameter growth in Douglas-fir.
Forest Service scientists Connie Harrington, Brad St. Clair, and Kevin Ford conducted a study to examine the environmental cues that drive diametergrowth cessation in Douglas-fir under the current climate. They also modeled seasonal cessation into the future, assuming increasing emissions of greenhouse gasses. In the warmer parts of Oregon and northern California, day length will be the limiting factor, preventing an extended fall growing season. In the cooler parts of Oregon and Washington, growth may extend nearly 4 weeks into fall by 2100 because low temperatures are currently the limiting factor.
Data gathered during record-high temperatures in 2015 revealed that Douglasfir appears to stop growing when high temperatures coincide with long daylight hours. Research continues on this summer growth-cessation pathway, which currently may be occurring across 2 percent of the coast Douglas-fir range, but by 2100 may affect more than 30 percent of the area.
The direct effects of climate change on alpine treeline ecotones – the transition zones between subalpine forest and non-forested alpine vegetation – have been studied extensively, but climate-induced changes in disturbance regimes have received less attention. To determine if recent increases in area burned extend to these higher-elevation landscapes, we analysed wildfires from 1984–2012 in eight mountainous ecoregions of the Pacific Northwest and Northern Rocky Mountains. We considered two components of the alpine treeline ecotone: subalpine parkland, which extends upward from subalpine forest and includes a fine-scale mosaic of forest and non-forested vegetation; and non-forested alpine vegetation. We expected these vegetation types to burn proportionally less than the entire ecoregion, reflecting higher fuel moisture and longer historical fire rotations. In four of eight ecoregions, the proportion of area burned in subalpine parkland (3%–8%) was greater than the proportion of area burned in the entire ecoregion (2%–7%). In contrast, in all but one ecoregion, a small proportion (≤4%) of the alpine vegetation burned. Area burned regionally was a significant predictor of area burned in subalpine parkland and alpine, suggesting that similar climatic drivers operate at higher and lower elevations or that fire spreads from neighbouring vegetation into the alpine treeline ecotone.