Try it with fire and lime: phytochemical responses to prescribed fire, soil amendments, and simulated herbivory

Abstract

Phytogenic defensive compounds mediate important multitrophic interactions in terrestrial ecosystems, yet we have limited understanding of how the methods used to restore and maintain degraded ecosystems influence subsequent ecosystem chemical ecology. To elucidate the chemical ecology of applied ecological restoration management techniques and phytochemical mechanisms of biotic interactions, I carried out an observational and an experimental study in a restored grassland. In my observational study, I investigated the effects of prescribed fire regime (quantity of historical burns and time since burn) on plant defensive chemistry. In my experimental study, I tested the effects of simulated herbivory, prescribed burning, and fast-acting soil lime on plant defensive chemistry at three phenological time steps. I tested foliar tissues across two growing seasons in two perennial forbs: (1) Castilleja levisecta (observational and experimental study), a hemiparasite Pacific Northwest native that produces the defensive iridoid glycosides aucubin, catalpol, macafadienoside, and, putatively, methyl shanzhiside; and (2) Plantago lanceolata (experimental study only), a European exotic that produces the defensive iridoid glycosides aucubin and catalpol. In my observational study, quantity of historical burns was a significant factor for C. levisecta iridoid glycoside concentrations. Total iridoid glycosidee concentrations were negatively related to quantity of historical burns. Time since most recent burn was not a significant factor for total iridoid glycosides. In my experimental study, unique patterns emerged in response to simulated herbivory for one constituent iridoid glycoside in each of my two plant species. A prescribed autumn burn had no effect on C. levisecta iridoid glycoside concentrations, but had a lessening effect on foliar iridoid glycoside concentrations in P. lanceolata. Phenology significantly modified some of these patterns. Fast-acting soil lime had no significant short-term effect on iridoid glycoside concentrations in either species. Understanding how these landscape scale land management techniques interact with phenology and biotic factors to affect plant defensive chemistry is crucial to developing restoration and management plans rooted in sound chemical ecology theory. My research confirms that environmental factors, phenology, and land management techniques have the potential to create important chemical legacies that should be planned and monitored alongside other response variables in long-term terrestrial restoration projects.