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

Abstract

Westernhemlock(Tsugaheterophylla(Raf.)Sarg.)isamajorcommercialtreespeciesinwesternOregonandWashingtonandisoftenassociatedwithcoastDouglas-fir(Pseudotsugamenziesii(Mirb.)Francovarmenziesii)andother species in coniferous forests of the Coast Ranges and Cascade Mountains. Growth of Douglas-fir in many coastal forestshasbeennegativelyaffectedbySwissneedlecast (SNC),afoliardiseasecausedbytheascomycetePhaeocryptopusgaeumannii(T.Rohde)Petr.andcharacterizedbyprematurefoliagelossonseverelyinfectedDouglas-fir trees.TheeffectofSNConstanddynamicsinmixedDouglas-fir-westernhemlockstandswastestedbyconstructingadiameterincrementmodelforwesternhemlockthatquantifieditsgrowthresponsetovaryingSNCseverityin 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 growthperiod,westernhemlocktreesinstandswithseverelyimpactedDouglas-fir(initialFR ≤1.5years)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 inmixed-speciesstandsaltersstanddynamicsbyallowingthisspeciestosurpassthegrowthofDouglas-firexperiencing severe SNC.