Old-growth forests offer safe haven to bird species struggling with climate change
Researchers from the College of Forestry at Oregon State University report that the more sensitive a bird species is to warm temperatures during its breeding season, the more likely it is to benefit from being near an old-growth forest (also known as a primary forest). The findings are based on observations of 13 bird species that have been tracked annually in the U.S. Geological Survey’s Breeding Bird Survey (BBS), among the most comprehensive bodies of data of its kind in North America.
The team, led by Matthew Betts, a professor in the College of Forestry, pooled together data pertaining to forest structure and climate patterns across northwestern North America with figures on bird populations. Then, they used satellite imagery to determine the area of primary forest within roughly 450 yards of each bird survey route (forming roughly 25-mile-long by 450-yard-wide corridors).
Over the last 30 years, the team reports, only two species tracked in the BBS exhibited negative effects which were, unequivocally associated with rising temperatures — Wilson’s warbler (Cardellina pusilla) and the hermit warbler (Setophaga occidentalis). While they are struggling to cope overall, population counts performed in the field show that both species are stable, or even thriving, in areas with a high proportion of primary forests.
“Managers hoping to combat the effects of climate change on species’ populations may now have an additional tool – maintaining and restoring old-growth forest,” said Betts.
Wilson’s warbler winters in Mexico and breeds during the late spring and early summer along the West Coast and across northern North America, from Alaska to New England and the Canadian Maritimes. Although the species occurs in early-stage as well as mature forests, it is declining at a rate of about 2 percent per year in the Pacific Northwest. Hermit warblers also winter in Mexico but breed exclusively along the West Coast as far north as Washington. Its populations are relatively stable but declining in landscapes with low amounts of old-growth forest.
More work will be needed in order to identify the specific features of mature forests that buffer the effects of warming temperatures on birds. One possibility is that the large trees themselves function as “heat sinks” during warm periods and thus moderate temperatures, which would align with previous research.