Forest Drought and Loss

Fig 4: Abundance of spotted owl pairs vs percent old forests.  (Figure 4, Bart et.al.1992)

Spotted Owls like Old Growth Forests:

 Since there is compelling evidence  that the Northern Spotted Owl were found in old growth forests 40 times greater than that of areas lacking old growth (Bart et al. 1992), the conservation of these forests will be key for conserving these owls that prefer this habitat. Figure 4 supports the increase of  spotted owl pairs as increases in the  percent of older forests occurs in the Pacific Northwest. The recent and current trends of the increased loss of old growth forests due to logging was the primary issue looked at in previous studies and debates. However, investigating the current warming trends will be interesting and relevant.

What Do Old Growth Forests Supply?

Research by Gutierrez suggests a couple of hypothesis as to why the spotted owl prefers these old growth forests. The main advantages suggested by Gutierrez is that spotted owls are better able to thermoregulate as these forest provide a more stable climate and protection from inclement weather. These forests provide good protection (in comparison to a fragmented thin forest) for these owls as the heavy cover prevents predators from invading. The prey abundance theory suggest old growth forests will provide an abundant and accessible source of prey not available in other habitats, which reduces the owl's need to travel to other fragmented forests for prey(Gutierrez. 1985). Since the increased energy needed to exploit habitat patches in a fragmented forest may negatively affect reproductive output (Gutierrez. 1985).



Warming and Forest Decline:

Virtually all future climate scenarios predict increases in wildfire in western North America, especially east of the Cascades, due to higher summer temperatures and earlier spring snow melt.  Fire frequency and intensity have already increased in the past 50 years, and most notably the past 15 years in the shrub steppe and forested regions of the West. The area burned by fire regionally is projected to double by the 2040s and triple by the 2080s. The probability that more than two million acres will burn in a given year is projected to increase from 5% (observed) to 33% by the 2080s. USFS and CIG researchers have linked these trends to climate changes. Drought and hotter temperatures have also led to an increase in outbreaks of insects, such as the mountain pine beetle, increasing the risk of fire. (Littell et al., 2009)

      “Tree death rates have more than doubled in old-growth forests across the western United States” during the study period, says Phillip van Mantgem. Much of these forests studied by Mantgem were 450 year old forests to 1000 year old forests which overlap with the ranges of the Northern Spotted Owl. As Figure 5 below (Mantgem et al.), shows from the 1960s to 2000's in the Pacific Northwest in particular, there seems to be a drastic change in mortality rate, from a 0.25 %/yr to almost 1.5%/yr change; showing almost a 6 fold increase in the 40 year period.

Fig. 5: Trends of Tree mortality rate vs year, A) regions, (B) elevational class, (C) stem
diameter class, (D) genus, and (E) historical fire return interval class. (Mantgem et al. 2009)

Increased mortality in old growth PNW forests, through regional warming (.3 to .5 deg C./ decade) lead to increased water deficits, contributing to tree mortality rates. To make situations worst, as climate change is inducing stress upon these trees, the chances of pine beetle infestation increases. Therefore, the risks of loosing forests from climate change on top of the problems of deforestation from logging ,that was such a big controversy, doesn't help the case  of the already decline numbers of the spotted owl.