Dwarf mistletoe affects whole-tree water relations of Douglas fir and western larch primarily through changes in leaf to sapwood ratios

Abstract

Dwarf mistletoes induce abnormal growth patterns and extreme changes in the biomass allocation of their hosts as well as directly parasitizing them for resources. Because biomass allocation can affect the resource use and efficiency of conifers, we studied the influences of dwarf mistletoe infection on above-ground biomass allocation of Douglas fir and western larch, and the consequences of such changes on whole-tree water use and water relations. Sap flow, tree water potentials, leaf:sapwood area ratios (A :A), leaf carbon isotope ratios, and nitrogen content were measured on Douglas fir and western larch trees with various degrees of mistletoe infection during the summer of 1996 in western Montana. Heavy dwarf mistletoe infection on Douglas fir and western larch was related to significant increases in A :A. Correspondingly, water transport dynamics were altered in infected trees, but responses were different for the two species. Higher A :A ratios in heavily infected Douglas firs were offset by increases in sapwood area-based sap flux densities (Q ) such that leaf area-based sap flux densities (Q) and predawn leaf water potentials at the end of the summer did not change significantly with mistletoe infection. Small (but statistically insignificant) decreases of Q for heavily infected Douglas firs were enough to offset increases in leaf area such that whole-tree water use was similar for uninfected and heavily infected trees. Increased A:A ratios of heavily infected western larch were not offset by increases of Q. Consequently, Q was reduced, which corresponded with significant decreases of water potential at the end of the summer. Furthermore, mistletoe-infection-related changes in A:A as a function of tree size resulted in greater whole-tree water use for large infected larches than for large uninfected trees. Such changes may result in further depletion of limited soil water resources in mature infected stands late in the growing season. Foliage from infected trees of both species had lower water use efficiencies than non-infected trees. Our results demonstrate substantial changes of whole-tree processes related to mistletoe infection, and stress the importance of integrating whole-tree physiological and structural processes to fully understand the mechanisms by which pathogens suppress forest productivity.

Effects of Dwarf Mistletoe on Climate Response of Mature Ponderosa Pine Trees

“…Sala et al (2001) found that foliage in infected Douglas-fir branches have lower water-use efficiency, but infected trees do not have higher water use overall. Their results indicate that infected trees compensate for lower water-use efficiency by increasing sap flow during times of moderate water availability.…”

“…Additional evidence indicates that infection increases needle surface area while decreasing volume (Tinnin and Knutson 1980;Wanner and Tinnin 1986). Such shifts in the distribution of biomass in the host crown could lower water-use efficiency, resulting in lower tolerance to drought stress (Sala et al 2001). Sala et al (2001) also reported that foliage in infected branches has lower water-use efficiency when compared to uninfected branches in Douglas-fir (Pseudotsuga menziesii Mirb.)…”

“…Such shifts in the distribution of biomass in the host crown could lower water-use efficiency, resulting in lower tolerance to drought stress (Sala et al 2001). Sala et al (2001) also reported that foliage in infected branches has lower water-use efficiency when compared to uninfected branches in Douglas-fir (Pseudotsuga menziesii Mirb.) and western larch (Larix occidentalis Nutt.…”

Transpiration and stomatal conductance of mistletoe (Loranthus europaeus) and its host plant, downy oak (Quercus pubescens)

“…4b). This may be partially ascribed to the higher osmotic potential of the mistletoe foliage (Sala et al 2001) and to the higher water capacitance of the "succulent-like" Loranthus leaves (Glatzel 1983). In the second hypothesis (transpiration from the succulentlike leaves without the propagation of pressure to the xylem), rehydration of the leaf tissues would have to occur at the expense of evening transpiration (i.e.…”

A holoparasitic plant severely reduces the vegetative and reproductive performance of its host plant in the Caatinga, a Brazilian seasonally dry forest

“…In addition to photosynthate reallocation, holoparasitic plants may stimulate the secretion of hormones by the host plant, directly inoculating the host structures with their own hormones or mRNA (Knutson 1979;Ihl et al 1984;Westwood et al 2010). In this sense, holoparasitic plants may negatively impact many metabolic and physiological aspects of their host plants, such as stomata opening, photosynthesis rate, transpiration, hormone secretion or water-use efficiency (Goldstein et al 1989;Fernandes et al 1998;Sala et al 2001;Runyon et al 2008;Prider et al 2009). …”