Implications of mistletoe infection from tree to ecosystem scale
Mistletoe can be perceived as a friend or a foe, depending on the ecological focus: animal ecologists tend to regard mistletoes as a keystone species and as ecosystem engineer, in light of the wildlife habitat, biodiversity and nutrient cycling it promotes. Tree physiologists and forest managers however tend to be concerned about decreasing stand health and productivity losses when mistletoes overburden their hosts and cause reduced growth rates and a degradation of the canopy.
Mistletoe presence has profound effects on tree physiology, soil nutrient cycling as well as stand health and stand dynamics. Potential modifications of mistletoe presence on the energy budget and on forest vulnerability to climate change will further feed back into stand dynamics and disturbance patterns. If you’re interested in ecosystem implications of mistletoe infection, then check out my synthesis in Environmental Research Letters where I particularly focus on mistletoe as friend and foe in light of tree mortality (Griebel et al., 2017).
Mistletoe and host mortality
Numerous recent studies highlighted an alarming trend of increasing tree mortality across terrestrial ecosystems when infected hosts are subjected to prolonged drought stress. In southeastern Australia, mistletoe distributions are increasing, and particularly young infected eucalypts were identified to be vulnerable to premature death. While the mechanistic effects of mistletoe infection on host physiology are reasonably well understood, quantifying the effects of mistletoe infection on stand productivity and ecosystem structure remains challenging. Moreover, the physiological processes leading to the death of infected hosts remain difficult to decipher, as the lavish water use of mistletoes increases xylem embolisms while simultaneously reducing carbon assimilation of their hosts when they attempt to preserve water.
Structural changes in infected stands
To monitor how mistletoe infection alters aboveground biomass distribution and to quantify mistletoe and host population dynamics in a severely infected stand, I’ve coupled annual inventories of mistletoe infection rates with measurements of host tree stem growth, canopy turnover and stand structure (Griebel et al., 2021). I’ve found that trees with >60% mistletoe foliage almost completely stopped to grow, which is providing a first threshold for productivity losses due to mistletoe infection in eucalypts. Canopy volume declined at the peak of the infection, and mistletoe leaves contributed >40% to stand litter fall during drought. However, mistletoes are also vulnerable to heat and drought stress, as the population nearly crashed at the end of the 3-year drought. Host tree recovery however was surprisingly rapid, with substantial increases in basal area and thickening of canopy volume a few months after drought.
Physiological processes in infected trees
To quantify the increased risk of hydraulic dysfunction due to water stress caused by mistletoe infection, I’ve monitored transpiration of infected and uninfected branches from two mature eucalypt species (E. moluccana and E. fibrosa) as well as stem and leaf water potentials of infected and uninfected trees at the Cumberland Plain woodland in south-eastern Australia. I’ve further used hydraulic vulnerability curves to estimate percent loss in conductivity for each species at the peak of the drought.
As expected, infected branches used significantly more water due to less efficient restriction of afternoon water loss compared to uninfected branches. Remarkably, infected branches used up to 4-fold more water on hot days compared to uninfected branches on typical summer days, which highlights that the lavish water use of mistletoes adds tremendous water stress when droughts are compounded by heatwaves. Ultimately, the lavish water use of mistletoes resulted in significantly decreased stem and leaf water potentials, and an increase of up to 11% loss in conductivity. This supports that the excessive water use of mistletoes increased xylem embolisms during the drought, and confirms greater hydraulic dysfunction of infected trees which places them at higher risk of hydraulic failure.
Read more about ‘Mistletoe, friend and foe: Synthesizing ecosystem implications of mistletoe infection‘ here: Griebel et al. (2017) Environmental Research Letters, 12 115012. https://doi.org/10.1088/1748-9326/aa8fff.
Read more about ‘Recovery from severe mistletoe infection after heat and drought-induced mistletoe death‘ here: Griebel et al. (2021) Ecosystems. https://doi.org/10.1007/s10021-021-00635-7.
Read more about ‘Tapping into the physiological responses to mistletoe infection during heat and drought stress‘ here: Griebel et al. (2021) Tree Physiology. https://doi.org/10.1093/treephys/tpab113.