Demographic processes underpinning post-fire resilience in California closed-cone pine forests: the importance of fire interval, stand structure, and climate

Abstract

The resilience of serotinous obligate-seeding plants to fire may be compromised if increasing fire frequency curtails time available for canopy seed bank accumulation (i.e., immaturity risk), but how various drivers affect seed availability at the time of fire is poorly understood. Using field data from California closed-cone pine (Pinus attenuata and P. muricata) stands, we assess two critical demographic processes during the inter-fire period-reproductive capacity and mortality.At tree-and stand-levels, we test how these processes are affected by stand age and are mediated 2 by biotic and abiotic factors. We found that stand age was the key driver of reproductive capacity; older stands had a greater proportion of reproductively mature individuals and greater closed cone density. Stand density mediated the effect of age; greater stand density resulted in greater closed cone density and a lower proportion of reproductively mature individuals, but reproductive capacity in low-and high-density stands converged over time. Increased moisture stress reduced the stand-level proportion reproductively mature but had no effect on closed cone density. Mortality was strongly associated with density-dependent thinning and increased in stands with high moisture stress. Reproductive capacity began to increase sharply 10 years after fire and by 20 years immaturity risk is low. However, prior to 20 years, low-density stands with high moisture stress may be more susceptible to immaturity risk. Understanding these relationships is critical to predicting serotinous population persistence under changing climate and disturbance conditions.

Climate change and altered fire regimes: impacts on plant populations, species, and ecosystems in both hemispheres

“…Some changes will arise from climate-driven alterations to different dimensions of fire regimes, such as increases in severity that are governed by the combination of fire behavior and fire-adaptive (or sensitive) plant traits (Franco et al 2022). Other changes will emerge when shifting fire regimes interact with key demographic and community dynamics pre-fire (Agne et al 2022;Gosper et al 2022;Souto-Veiga et al 2022) and post-fire (Arroyo-Vargas et al 2022;Dawe et al 2022;Brehaut and Brown 2022); some occurring over evolutionary time scales (Lamont 2022;Ladd et al 2022). Finally, some changes will emerge as surprising outcomes from disturbances interacting in a linked (Lindenmayer et al 2022) or compound (Bendall et al 2022) manner.…”

“…Incredibly valuable insights are born of such rigorous cone measurements across large populations of established trees, though they present a snapshot in time of reproductive capacity if a disturbance were to occur, leaving the temporal development of this critical stage less well understood. In this issue, (Agne et al 2022) present data on cone abundance across a chronosequence of past stand-replacing fire across most of the geographic distribution for knobcone pine (Pinus attenuata) and bishop pine (Pinus muricata) on the west coast of the USA. These two serotinous species are adapted to a stand-replacing fire regime and demonstrate tremendous resilience to fire (Harvey and Holzman 2014;Reilly et al 2019), but given their short fire-return-intervals relative to other serotinous conifer forests in North America, are an ideal setting to test how immaturity risk varies with fire interval (Keeley et al 1999).…”

“…These two serotinous species are adapted to a stand-replacing fire regime and demonstrate tremendous resilience to fire (Harvey and Holzman 2014;Reilly et al 2019), but given their short fire-return-intervals relative to other serotinous conifer forests in North America, are an ideal setting to test how immaturity risk varies with fire interval (Keeley et al 1999). Taking a chronosequence approach allows for identifying the period of time where immaturity risk is greatest (very high < 10 years since fire and moderate 10-20 years since fire in this case), and how drought-related mortality in inter-fire periods can lengthen the period of immaturity risk (Agne et al 2022).…”

“…Species that store mature seeds in various structures can routinely build the available seedbank from near zero immediately following fire to levels sufficient to support resilience to fire after some time (Agne et al 2022). However, temporal variability (specifically reduction) in seed production during inter-fire periods (Redmond et al 2016;LaMontagne et al 2021) via masting, and untimely cone opening and subsequent seed release/leakage, can affect seedbanks during this critical window of time.…”

“…Understanding the mechanisms that deplete the seedbank, once formed, are an important additional dimension of exploration. When post-fire populations are sparse, plants can produce more cones/seeds per individual, potentially buffering otherwise slower seedbank accumulation assumed from the number of mature plants alone (Turner et al 2007;Agne et al 2022). However, the limits to these compensatory responses (e.g., a single tree is fixed in one location and can only produce so many cones) may not fully make up for lower numbers of cones/ seeds that would be present with a larger population of mature individuals.…”