Regeneration strategies and forest resilience to changing fire regimes: Insights from a Goldilocks model

Abstract

Disturbances are ubiquitous in ecological systems, and species have evolved a range of strategies to resist or rebound following disturbance. Understanding how the presence and complementarity of regeneration traits will affect community responses to disturbance is increasingly urgent as disturbance regimes shift beyond their historical ranges of variability. We define “disturbance niche” as a species' fitness across a range of disturbance sizes and frequencies that can reflect the fundamental or realized niche, that is, whether the species occurs alone or with other species. We developed a model of intermediate complexity (i.e., a Goldilocks model) to infer the disturbance niche. We parameterized the model for subalpine forests in Yellowstone National Park (USA) adapted to infrequent stand‐replacing fires and included the three major tree‐regeneration strategies: (1) obligate seeders that rely on ex situ seeding into burned areas (non‐serotinous lodgepole pine, Pinus contorta var. latifola), (2) obligate seeders that depend on in situ seedbanks (serotinous lodgepole pine, Pinus contorta var. latifola), and (3) species that can resprout from surviving roots following fire (quaking aspen, Populus tremuloides). Our results showed which regeneration strategies increase or decrease in prevalence as fire rotation declines. Non‐serotinous pines were extirpated when fire rotation was below 50 years in a monoculture and 100 years in a mixed forest; serotinous pines were extirpated when fire rotation was below 20 years; and aspen was extirpated when fire rotation fell below 6 years. The fundamental and realized disturbance niches pinpointed the key mechanisms limiting regeneration for each strategy, namely, increasing fire size for non‐serotinous pine (ex situ seeders), decreasing fire frequency for serotinous pine (in situ seeders), and interspecific competition for aspen (resprouters). In a mixed forest, the three regeneration strategies were complementary and each dominated at different combinations of fire size and frequency. Consequently, diversity of regeneration strategies enhanced forest resilience to declining fire rotations. Despite its simplicity, our Goldilocks model produced realistic dynamics and could be readily adapted to other disturbance‐prone ecosystems to explore the generality of these results. The disturbance niche is a key concept for anticipating community resilience to changing disturbance regimes.

Evaluating immaturity risk in young stands of the serotinous knobcone pine (Pinus attenuata)

“…We can merely say that our estimate represents a lower limit on the current risk of this species. Interestingly, Ramiadantsoa et al (2023) developed a simulation of serotinous species persistence in a stand-replacing fire regime and found that monocultures were extirpated at a fire rotation of around 20 years, not much less than the minimum cycle estimate we examined here. Nonetheless, a comparison of the likelihood of persistence across sites and species clearly depends on the time to reach reproductive maturity, which in turn depends on site productivity.…”

“…Furthermore, a fire history study in mixed-conifer forests upslope to one of our Whiskeytown study sites had a much shorter fire cycle of 8 years (Fry & Stephens, 2006), which may indicate a higher probability of more frequent fires in the adjacent knobcone pine ecosystems in this region. It is important to point out our sample size was relatively small and does not accurately represent the full variability of knobcone pine across its entire range, but it fits the findings of Ramiadantsoa et al (2023), who report that the risk of extirpation in serotinous conifers drastically decreases between a 20-and 50-year fire cycle. Finally, given that the fire cycle is predicted to shorten in the western United States, inspection of Figure 8 means that our optimism about the resilience of this species must be guarded.…”