School of the Environment, Washington State University, United States
Background/Question/Methods In the last 30 years, bark beetle outbreaks have killed hundreds of millions of conifers globally. Drought is frequently implicated in the triggering of these landscape-scale events, yet the exact physiological mechanisms that leave trees vulnerable to bark beetles remain unresolved. A leading hypothesis is that drought-induced stomatal closure puts trees into a state of carbon limitation, limiting defense capabilities. In the absence of photosynthesis, trees may deplete their stored carbohydrates (we refer to starch specifically). At some critical threshold, carbon limitation may become so severe that the production of secondary metabolites becomes impossible.
We test the hypothesis that starch availability limits secondary metabolite (SM) production in carbon limited trees. Using the biosynthetic pathways for a variety of SMs we determined the glucose cost to produce one milligram of each compound. Next, we developed a theoretical maximum defense production model. This model accounts for simultaneous allocation to respiration and SM, assuming growth has stopped. We parameterized our model for mature P. edulis, using data from a large field experiment in New Mexico. Here, we present results comparing observed secondary metabolite production against a theoretical maximum, and evaluate when, if ever, carbon limitation inhibits induced SM production.
Results/Conclusions The constitutive SM levels at our field site ranged from 10 mg g-1 DW at the maximum starch levels observed (80 mg g-1 DW) to less than 2 mg g-1 DW as starch approached it’s minimum. Yet, according to our model, trees had enough starch to produce 3x the SM concentrations we observed when starch was near its maximum. As local carbon limitation set in and trees approached a minimum starch concentration in the xylem and phloem, SM production became stoichiometrically infeasible beyond constitutive levels (~1.9 mg g-1 DW).
We provide strong evidence that carbon limitation inhibits SM production in trees. During drought, photosynthesis becomes limited as water availability declines. The longer plants spend in this source limited state, the more likely they are to reduce carbohydrate stores. This can fuel bark beetle outbreaks if enough trees across a landscape are unable to mount sufficient defenses. With historic droughts occurring across Western North America, this may become a more likely scenario. Still, questions remain surrounding the resilience of different conifer species to drought-induced carbon limitation, whether non-local carbohydrate stores can be mobilized to support SM production, and if a shift toward cheaper SM species might occur as carbon availability declines.