Western hemlock growth response to increasing intensity of Swiss needle cast on Douglas-fir: changes in the dynamics of mixed-species stands

Abstract

Western hemlock (Tsuga heterophylla (Raf.) Sarg.) is a major commercial tree species in western Oregon and Washington and is often associated with coast Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var menziesii) and other species in coniferous forests of the Coast Ranges and Cascade Mountains. Growth of Douglas-fir in many coastal forests has been negatively affected by Swiss needle cast (SNC), a foliar disease caused by the ascomycete Phaeocryptopus gaeumannii (T. Rohde) Petr. and characterized by premature foliage loss on severely infected Douglas-fir trees. The effect of SNC on stand dynamics in mixed Douglas-fir-western hemlock stands was tested by constructing a diameter increment model for western hemlock that quantified its growth response to varying SNC severity in Douglas-fir. Diameter increment of western hemlock in any given growth period increased with increasing initial SNC severity as measured by Douglas-fir foliage retention (FR), here defined as the number of annual needle cohorts held by the tree. Furthermore, a decline in Douglas-fir FR during the growth period was associated with an additional increase in diameter increment of western hemlock trees. Assuming no change in FR over the growth period, western hemlock trees in stands with severely impacted Douglas-fir (initial FR ≤1.5 years) averaged 79 per cent greater diameter growth than that in relatively healthy stands (initial Douglas-fir FR ≥3.5 years). The implied annual diameter growth response of a western hemlock with initial diameter at breast height of 10, 20, 30 or 40 cm was 0.29, 0.52, 0.65 and 0.68 cm year 21 , respectively. Compensatory growth by western hemlock in mixed-species stands alters stand dynamics by allowing this species to surpass the growth of Douglas-fir experiencing severe SNC.

Persistence of the Swiss Needle Cast Outbreak in Oregon Coastal Douglas-Fir and New Insights from Research and Monitoring

“…However, in the two most heavily affected catchments, with ~50–90% of the area mapped by ADS, there was no effect. They hypothesized that this was due to vegetation compensation—that is, growth of non-Douglas-fir conifers, hardwoods, and shrubs increased because of the negative impact of SNC on Douglas-fir ( Zhao et al 2014a ).…”

“…Precommercial and commercial thinning can be used in conjunction with species selection to improve the stand ( Zhao et al 2014b ). Variation in disease tolerance within individuals is observed in the field ( Zhao et al 2014a ); therefore, thinning from below can remove Douglas-fir with sparse crowns and small sizes indicative of low disease tolerance. Western hemlock is a codominant species in the near-coastal region of Oregon that naturally seeds into Douglas-fir plantations.…”

Environmental variables associated withNothophaeocryptopus gaeumanniipopulation structure and Swiss needle cast severity in Western Oregon and Washington

“…Nakash., U. Braun & Crous. However, the loss of vigor associated with SNC may diminish the ability of Douglas-fir to compete with other conifer species (e.g., Tsuga heterophylla), thereby altering the ecology and successional trajectory of Douglas-fir in mixed natural stands (Lee et al, 2017;Stone, Coop, & Manter, 2008;Zhao, Maguire, Mainwaring, & Kanaskie, 2014). Impaired carbon assimilation and reduced photosynthetic capacity due to foliage loss result in height and diameter growth reductions of 20%-50% relative to healthy trees (Maguire, Kanaskie, Voelker, Johnson, & Johnson, 2002;Manter et al, 2003).…”

“…The physiological stress caused by this disease rarely leads to mortality, as even the most severely diseased trees retain the current year's foliage (Hansen et al, 2000;Maguire et al, 2002). However, the loss of vigor associated with SNC may diminish the ability of Douglas-fir to compete with other conifer species (e.g., Tsuga heterophylla), thereby altering the ecology and successional trajectory of Douglas-fir in mixed natural stands (Lee et al, 2017;Stone, Coop, & Manter, 2008;Zhao, Maguire, Mainwaring, & Kanaskie, 2014).…”