High‐severity fire drives persistent floristic homogenization in human‐altered forests

Abstract

Ecological disturbance regimes across the globe are being altered via direct and indirect human influences. Biodiversity loss at multiple scales can be a direct outcome of these shifts. Fire, especially in dry forests, is an ecological disturbance that is experiencing dramatic changes due to climate change, fire suppression, increased human population in fire-prone areas, and alterations to vegetation composition and structure. Dry western conifer forests that historically experienced frequent, low-severity fires are now increasingly burning at high severity. Relatively little work has been done looking at the effects of this novel disturbance type on affected plant communities, and little is known about how these impacts change over time. To fill in these knowledge gaps, we examined a fire that burned in a yellow pine and mixed conifer forest in the central Sierra Nevada in California, USA. We sampled at five time steps across the nine years following the fire (1, 3, 5, 8, and 9 years postfire). We found a generally unimodal relationship between fire severity and plant alpha and gamma diversity, but found that areas that burned at high severity supported progressively lower plant diversity as time since fire increased. Similarly, beta diversity decreased drastically through time for the high-severity areas, while remaining more static in the other severity classes. The combination of these findings indicates that significant floristic homogenization can result from high-severity fire in this ecosystem type. We also saw consistently lower diversity in unburned areas in comparison to area burned at low and moderate severity, underlining that both lack of fire and high-severity fire can have negative impacts on postfire plant diversity. Unburned areas that experienced forest thinning after the first sample year saw an increase in plant diversity over time, suggesting that some-but not all-of the effects of fire on plant diversity can be approximated through forest management.

Peeking under the canopy: anomalously short fire‐return intervals alter subalpine forest understory plant communities

“…Many drivers of decreased community dissimilarity (i.e. homogenization) have previously been identified, including historical land use (Velland et al ., 2007), non‐native species introductions (Qian & Ricklefs, 2006), silvicultural treatments (Macdonald & Fenniak, 2007; Jaeger et al ., 2022), and fire severity (Weeks et al ., 2023). Repeat short‐interval fires (i.e.…”

“…Such effects may be further amplified in already warm, dry landscape positions (Hoecker et al ., 2020) or where tree cover remains sparse (Andrade et al ., 2021); reduced tree recruitment following fire in lower montane forests (Donato et al ., 2016; Davis et al ., 2019; Young et al ., 2019) suggests plant communities in arid areas may be particularly vulnerable to change. What's more, natural and prescribed fire may homogenize previously distinct plant communities through homogenization of abiotic conditions (Mattingly et al ., 2015; Stevens et al ., 2015; Richter et al ., 2019; Weeks et al ., 2023), but whether such changes occur following anomalously short‐interval fire is not known. As changing climate and fire regimes trigger postfire conversion of forest to sparse or nonforest (Coop et al ., 2020), elucidating effects on the whole plant community is needed to understand the future of these burned landscapes.…”

A roadmap for pyrodiversity science

“…Such shrublands can persist for decades in areas that were formerly forest by outcompeting less established tree seedlings (Weeks et al, 2023). Further, the seasonality of fire events may influence the survival and availability of propagules for immediate colonisation.…”

“…For example, in high‐severity burn patches in conifer forests in California, USA, tree propagule availability can determine whether forest stands regenerate or whether the landscape is converted to a shrubland dominated by species that exhibit fire‐cued seed generation (Welch et al., 2016). Such shrublands can persist for decades in areas that were formerly forest by outcompeting less established tree seedlings (Weeks et al., 2023). Further, the seasonality of fire events may influence the survival and availability of propagules for immediate colonisation.…”

Response of pollinator taxa to fire is consistent with historic fire regimes in the Sierra Nevada and mediated through floral richness†

“…For example, fire severity has significantly increased in many fire‐prone regions of the world that historically burned at low or moderate severity (Flannigan et al., 2013; Jones et al., 2022; Stephens et al., 2014). The impacts of high‐severity fire on these systems include lower recruitment and establishment of native plant species (Collins et al., 2017; Etchells et al., 2020), shifts in vegetation communities (Parks et al., 2019; Scheffer et al., 2001), and loss of plant diversity (Miller et al., 2018; Weeks et al., 2023) and ecosystem services (Adams, 2013; Benavides‐Solorio & MacDonald, 2001; Hurteau & Brooks, 2011). These changes may further alter disturbance regimes through positive feedback (Pausas & Keeley, 2019).…”