Impact of eastern dwarf mistletoe (Arceuthobium pusillum) on host white spruce (Picea glauca) development, growth and performance across multiple scales

Abstract

Infection by eastern dwarf mistletoe (Arceuthobium pusillum) modifies needle and branch morphology and hastens white spruce (Picea glauca) mortality. We examined potential causal mechanisms and assessed the impacts of infection-induced alterations to host development and performance across scales ranging from needle hormone contents to bole expansion. Needles on infected branches (IBs) possessed higher total cytokinin (CK) and lower abscisic acid contents than needles on uninfected branches (UBs). IBs exhibited greater xylem growth than same-aged UBs, which is consistent with the promotive effect of CKs on vascular differentiation and organ sink strength. Elevated CK content may also explain the dense secondary and tertiary branching observed at the site of infection, i.e. the formation of 'witches' brooms' with significantly lower light capture efficiencies. Observed hormone perturbations were consistent with higher rates of transpiration, lower water use efficiencies (WUEs) and more negative needle carbon isotope ratios observed for IBs. Observed reductions in needle size allowed IBs to compensate for reduced hydraulic conductivity. Severe infections resulted in dramatically decreased diameter growth of the bole. It seems likely that the modifications to host hormone contents by eastern dwarf mistletoe infection led white spruce trees to dedicate a disproportionate fraction of their photoassimilate and other resources to self-shaded branches with low WUE. This would have decreased the potential for fixed carbon accumulation, generating a decline in the whole-tree resource pool. As mistletoe infections grew in size and the number of IBs increased, this burden was manifested as increasingly greater reductions in bole growth.

Growth of hartweg’s pine (<em>Pinus hartwegii</em>) parasitized by two dwarf mistletoe species (<em>Arceuthobium</em> spp.)

“…This scenario where two dwarf mistletoes are coexisting is rare (Hawksworth and Wiens, 1996;Queijeiro-Bolaños et al, 2014); thus, it raises the question about how resources are used and shared among the two species. In addition, although these parasites may not be arresting pine growth in a noticeable way, we cannot state that these have no effect on forest industry activities, such as poor wood quality and lower roundwood production (Andrade and Cibrián, 1980;Madrigal et al, 2007;Logan et al, 2013).…”

Impacts of eastern dwarf mistletoe on the stem hydraulics of red spruce and white spruce, two host species with different drought tolerances and responses to infection

“…Infections are known to negatively impact wood quality and reduce harvestable stand volumes through reductions in tree growth and increases in tree mortality (Hawksworth and Johnson 1989;Singh and Carew 1989;Hawksworth and Weins 1996;Hadfield et al 2000 Geils andBaker and Knowles 2004). Despite the well-documented negative impacts of dwarf mistletoe infections on their hosts (e.g., Singh and Carew 1989;Hawksworth and Weins 1996;Geils and Hawksworth 2002;Hadfield et al 2000;Shaw et al 2008), relatively less is known about the physiological mechanisms of dwarf mistletoe-induced host decline (Hull and Leonard 1964a, b;Clark and Bonga 1970;Fisher 1983;Wanner and Tinnin 1986;Hawksworth and Weins 1996;Sala et al 2001;Logan et al 2002Logan et al , 2013Meinzer et al 2004;Reblin et al 2006;Xia et al 2012;Marias et al 2014).…”

“…This may allow dwarf mistletoes to act as a stronger sink to host mineral resources dissolved in the xylem stream (Hawksworth and Weins 1996) but presumably places the host under even greater water stress which may ultimately reduce host survival during periods of low-moisture availability (Page 1981;Sangüesa-Barreda et al 2012). While the endophytic system allows the mistletoe to access host resources, it may also interfere with water transport within infected branches, potentially reducing water availability to distal host tissues (Meinzer et al 2004;Logan et al 2013). For example, western hemlock (Tsuga heterophylla (Raf.)…”

“…Jones) experience a * 52 % reduction in water transport capacities (i.e., reduced sapwood area specific conductivities) (Meinzer et al 2004). Similarly, in white spruce (Picea glauca (Moench (Voss)) branches parasitized by the eastern dwarf mistletoe (A. pusillum Peck), sapwood area specific conductivities are reduced by 32 % (Logan et al 2013). Presumably these flow reductions result from the development of the parasitic oscula that protrude into host tracheids and are used to appropriate water and xylem contents for the dwarf mistletoes.…”

“…Presumably these flow reductions result from the development of the parasitic oscula that protrude into host tracheids and are used to appropriate water and xylem contents for the dwarf mistletoes. While reductions in stem conductivities may increase the potential for water stress within parasitized branches, they may be offset by reductions in the size of needles on infected branches (Tinnin and Knutson 1980;Logan et al 2002;Reblin et al 2006;Chhikara and Ross-Friedman 2008;Littley et al 2008;Xia et al 2012) and an overall reduction in amount of total host leaf area on parasitized branches (Meinzer et al 2004;Logan et al 2013). For example, needles on eastern dwarf mistletoe-infected branches in white spruce are 40-50 % smaller than needles from uninfected branches or trees (Logan et al 2002;Reblin et al 2006).…”