Post‐fire forest regeneration shows limited climate tracking and potential for drought‐induced type conversion

Abstract

Disturbance such as wildfire may create opportunities for plant communities to reorganize in response to climate change. The interaction between climate change and disturbance may be particularly important in forests, where many of the foundational plant species (trees) are long‐lived and where poor initial tree establishment can result in conversion to shrub‐ or graminoid‐dominated systems. The response of post‐disturbance vegetation establishment to post‐disturbance weather conditions, particularly to extreme weather, could therefore provide useful information about how forest communities will respond to climate change. We examined the effect of post‐fire weather conditions on post‐fire tree, shrub, and graminoid recruitment in fire‐adapted forests in northern California, USA, by surveying regenerating vegetation in severely burned areas 4–5 yr after 14 different wildfires that burned between 2004 and 2012. This time period (2004–2016) encompassed a wide range of post‐fire weather conditions, including a period of extreme drought. For the most common tree species, we observed little evidence of disturbance‐mediated community reorganization or range shifts but instead either (1) low sensitivity of recruitment to post‐fire weather or (2) weak but widespread decreases in recruitment under unusually dry post‐fire conditions, depending on the species. The occurrence of a single strong drought year following fire was more important than a series of moderately dry years in explaining tree recruitment declines. Overall, however, post‐fire tree recruitment patterns were explained more strongly by long‐term climate and topography and local adult tree species abundance than by post‐fire weather conditions. This observation suggests that surviving adult trees can contribute to a “biological inertia” that restricts the extent to which tree community composition will track changes in climate through post‐disturbance recruitment. In contrast to our observations in trees, we observed substantial increases in shrub and graminoid establishment under post‐fire drought, suggesting that shifts in dominance between functional groups may become more likely in a future with more frequent and intense drought.

Divergent responses to climate change and disturbance drive recruitment patterns underlying latitudinal shifts of tree species

“…Using physiologically based simulations in the northeastern United States, Liang et al () reported that disturbances may expedite species' recruitment into new sites, but they had little effect on the velocity of simulated range boundary shifts. Following fires in California that occurred since 2004, Young et al () found little evidence of disturbance‐mediated vegetation community reorganization on range shifts, from either low sensitivity of species to post‐disturbance weather or widespread recruitment declines precipitated by unusually dry conditions. Interactions between climate change and disturbances warrant further investigation as management practices will need to be adapted to a changing climate, particularly to facilitate migration.…”

From canopy to seed: Loss of snow drives directional changes in forest composition

“…Drought-induced canopy mortality is well-established (Allen et al, 2015;Anderegg et al, 2013), and recent studies have quantified the impacts of prolonged drought (Redmond, Weisberg, Cobb, & Clifford, 2018) and postfire drought conditions (Stevens-Rumann et al, 2017;Young et al, 2019) on regeneration. However, relatively little is known about the consequences of reduced snowpack and early season frost damage on canopy mortality and regeneration response, although these phenomena may become more common as the climate warms (Woldendorp, Hill, Doran, & Ball, 2008)-leading to root mortality and nutrient loss (Decker, Wang, Waite, & Scherbatskoy, 2003), needle and bud injury (Man, Kayahara, Dang, & Rice, 2009), canopy mortality (Buma, 2018), and seedling mortality (Camarero & Gutiérrez, 2004).…”

Rethinking fire‐adapted species in an altered fire regime

“…, Young et al. ). Therefore, it is likely that regional shrub influences not only are stronger than measured in this study, but also may become increasingly important under future climate change.…”

“…Because shrub‐free areas are rare in stand‐replacing fire patches of the Sierra Nevada (Young et al. ), we quantified juvenile conifer growth across a gradient of shrub competition rather than comparing high‐shrub areas to shrub‐free areas. To ensure that environmental conditions were similar across sites, we limited sites to fire footprints that met the following requirements: between the North and South forks of the American River; >400 ha in size; 5–50 yr old; within the Tahoe or Eldorado National Forests; not planted or herbicided following fire; and containing identifiable shrub fields surrounded by mixed‐conifer forest according to satellite imagery.…”

“…, Young et al. ), but are missing the critical demographic transition between establishment and sexual maturity.…”