Mortality of resprouting chaparral shrubs after a fire and during a record drought: physiological mechanisms and demographic consequences

Abstract

We examined postfire regeneration of chaparral shrubs during an intense drought. This study focused on the demography and physiology of shrub species that resprout from a basal lignotuber following fire. We found significant levels of resprout mortality when intense drought occurred in the year following fire during the period of shrub recovery. Three of the seven sampled resprouting species had the greatest or near greatest levels of mortality ever recorded when compared to previous studies. Most shrub mortality occurred during the drought after individuals had resprouted (i.e. individuals survived fire, resprouted and then subsequently died). Physiological measurements of species with high mortality suggested that resprout stems were highly embolized and xylem hydraulic conductivities were close to zero during the peak of the drought. In addition, lignotubers of two of the three species experiencing high mortality were depleted of starch. Population densities of most shrub species declined after the drought compared with their prefire levels, with the exception of one drought tolerant obligate seeding species. Resprouting shrub species may deplete their carbohydrate reserves during the resprouting process, making them particularly vulnerable to drought because of the need to transpire water to acquire the CO2 that is used to supply energy to a large respiring root system. Drought appears to interact with fire by altering postfire shrub recovery and altering species abundances and composition of chaparral communities.

Decade-Long Plant Community Responses to Shrubland Fuel Hazard Reduction

“…Therefore, seasonal differences in surface soil moistures were minimized. Other physiological processes also correlate with seasonal drought, including reduced water potential in mature shrubs and reduced carbohydrate storage in lignotubers, which may leave plants more susceptible to disturbance by reducing resources needed for resprouting (Pratt et al 2013).…”

“…Today, frequent accidental ignitions increasingly convert chaparral from native shrubland to non-native annual grassland and drastically reduce species diversity, especially in southern California (Haidinger and Keeley 1993). Chaparral conversion to grassland is especially likely under global-changetype drought (Syphard et al 2007, Pratt et al 2013. Non-native plant invasion can create a positive feedback cycle: once non-native plants establish, they can increase fire frequency, which favors more non-native plants (Brooks et al 2004, Brennan and.…”

Fire Severity and Regeneration Strategy Influence Shrub Patch Size and Structure Following Disturbance

“…In part, this likely reflects large canopy openings as well as destruction of below-ground plant tissues from which resprouts could emerge, in part because resprouting vigor may be greatest following moderate-severity fire [55]. In high-severity patches, shrub regeneration frequently relies on seed rain from unburned or less-severely-burned areas because of soil seedbank mortality [35,56,57] and destruction of rhizome and root crown tissue [2,56]. Some post-fire seeding species may have a competitive advantage over resprouting species following intense wildfire because of their long-lived, heat-resistant seeds [58] and the tendency of seeds to be fire stimulated [59].…”

“…In low and moderate burned areas, post-fire resprouting appears to be a more effective strategy than seeding, based on proportional abundances of resprouters compared to seeders (Figure 4). This is likely a result of the competitive advantage obtained from undamaged pre-fire biomass [2,5,8] over germination.…”

“…Post-disturbance vegetation response and recovery reflects uncertainties in community composition, structure, and recovery trajectories [1][2][3]. Wildland fire is an especially complicated disturbance agent because spatial and temporal variability in fire severity can compound one another to produce highly variable outcomes [3,4].…”