Legacies of stress and altered disturbance regimes threaten shrubland resilience

Abstract

This article comments on:

Zomer MA, Moreira B, Pausas JG. 2025. Pre-disturbance plant condition drives intraspecific resprouting variability in Mediterranean shrubs. Journal of Experimental Botany 76, https://doi.org/10.1093/jxb/eraf246.

This article comments on:

Zomer MA, Moreira B, Pausas JG. 2025. Pre-disturbance plant condition drives intraspecific resprouting variability in Mediterranean shrubs. Journal of Experimental Botany 76, 5559–5572. https://doi.org/10.1093/jxb/eraf246

Following disturbance, many species recover by resprouting shoots, particularly within fire-prone Mediterranean-type climate regions. Global changes in drought, temperature, and fire create multiple disturbances that affect resprout success and are difficult to disentangle. Zomer et al. (2025) examined the effects of water availability, plant condition, fire history, site characteristics, and repeated crown removal on two Mediterranean shrub species. Variability in resprouting success occurred mostly among individuals within populations, and was predominantly driven by pre-disturbance condition and history, disturbance frequency, and water availability. Reduced resprout persistence and vigour threaten ecosystem function and biodiversity in a changing world.

Within Mediterranean-type ecosystems, shrub species rely on two primary strategies for recovery following crown removal by wildfire, namely resprouting and seeding. For seeding species, new individuals are recruited from a dormant seed bank. For resprouting species, plants recover through growth from buds that are protected from fire, often from specialized structures at their root crown. Each of these strategies is associated with risks at multiple points along the pre- to post-disturbance timeline (Table 1). A long-standing question in Mediterranean-type environments considers the trade-offs between resprouting and seeding life history strategies (Pausas and Keeley, 2014).

Table 1.

Seeding and resprouting species experience different risks during post-disturbance recovery

Life History	Pre-disturbance key trait	Fire	Post-disturbance (short term)	Post-disturbance (long term)
Seeding	Seed bank	Seed survival	Recruitment risk	Immaturity risk/Senescence risk
	Risks: [Pre-dispersal] Limited seed inputs due to reduced flowering or fruiting, increased ovule abortion, old age (senescence risk), reduced seed dispersal, fruit or seed herbivory, pathogens, dieback, and mortality of adult plants [Post-dispersal] Reduction of seed bank due to long fire interval, granivores, pathogens, seed senescence	Risks: Factors that alter fire severity and intensity Altered timing of fire	Risks: Factors that reduce seed germination Factors that reduce seedling survival and success: drought, competition, herbivory, disturbance	Risks: Factors that delay reproductive maturity and replenishment of the seed bank Long time between fires and old plant age leading to decline of vigour, fecundity, and mortality
Resprouting	Plant condition	Plant survival	Resprouting risk	Recovery risk
	Risks: Factors that reduce plant water status Factors that reduce stores of nutrients, carbohydrates, buds	Risks: Factors that alter disturbance severity and intensity Factors that alter fire intensity, size, and timing	Risks: Factors that reduce resprout survival and success	Risks: Factors that delay replenishment of plant access to and stores of water, nutrients, carbohydrates, or buds

Risk factors	Examples
Site characteristics	Soil, slope, nutrients
Climate	Aridity, seasonality, rainfall reliability
Environmental strains and extreme events	Drought, temperature, wind
Time since last disturbance	Plant age, plant size, plant maturity
Biotic factors	Pathogens, herbivory, invasive species

The ability to persist at a site is linked to key traits that span the pre-disturbance to post-disturbance continuum, and which differ for the two life history types of seeding and resprouting. Risk factors impact these key traits and may reduce species abundance following disturbance. The key traits and risk factors included here are focused specifically on fire as the primary disturbance, and these key traits and risk factors may vary for other types of disturbances.

For both strategies, the seedling and juvenile stages are fraught; however, in the context of short-term demography, seeding species have the most at stake. Drought, herbivory, disturbance, and other stresses all may increase mortality of seedlings, thereby reducing populations to below pre-fire levels. At the extreme, obligate seeding species may be locally extirpated if fire re-occurs prior to reproductive maturity and the re-establishment of a seed bank (juvenile or immaturity risk). Climate change is expected to increase juvenile risk as described by the interval squeeze concept, which predicts that warming and drying climates lead to a longer juvenile interval (protracted juvenile risk), which, coupled with shortened fire return intervals, reduces the likelihood of population persistence (Enright et al., 2015). Additionally, obligate seeding species may also be particularly sensitive to drought-associated mortality as adults, probably associated with shallow rooting (Gazol et al., 2017; Jacobsen and Pratt, 2018). When recruitment fails, closed-canopy shrublands lose diversity and canopy cover, resulting in an altered community structure that is more open and often dominated by annual herbs. These degraded open-canopy shrublands are referred to as ‘type-converted’ (Pratt, 2022), and display a continuum of canopy cover from mostly closed to entirely open (Jacobsen and Pratt, 2018).

Resprouting is an ancestral trait in most lineages and may commonly be a ‘safer’ strategy than seeding in many environments (Pausas et al., 2016). Safety is promoted by an established root system that can survive multiple disturbances, thus retaining their position within the landscape, and supporting rapid recovery and reaching reproductive maturity. In partially type-converted shrublands in which diversity and canopy cover are reduced, it is often the more ‘resilient’ resprouting species that remain as low-density shrubs within a community that is structurally similar to a savannah (Jacobsen and Pratt, 2018; Pratt, 2022). Yet, post-fire resprouting success can be highly variable across resprouting species (Keeley, 2006; Moreira et al., 2012; Marais et al., 2014; Cowan et al., 2023). Altered disturbance regimes and increased stress may reduce the resilience of even the most apparently invulnerable resprouting members of these shrubland communities (Aguirre et al., 2024). The most vulnerable resprouting strategy is likely to be facultative seeding that comprises both resprouting and seeding after fire (Marais et al., 2014).

Intraspecific variability in resprouting success

Despite often being treated as a binary trait at the species level, resprouting success can show significant intraspecific variability. Numerous factors affect intraspecific resprouting: life history traits (facultative seeders versus obligate resprouters), anatomy and bud bank traits, individual health/carbohydrate stores, landscape factors, aridity, time since last fire, and disturbance intensity and timing (Table 1; Keeley, 2006; Moreira et al., 2012). The main causes of resprouting success remain poorly understood and may vary across species. Hence, more research is needed to address this important topic.

One challenge in understanding predictors of resprouting lies in how resprout success is assessed. This often requires detailed monitoring and tracking of individual plants from pre-fire populations through the resprouting process, which has been rare (see Malanson and Trabaud, 1988, Moreira et al., 2012; Marais et al., 2014; Pratt et al., 2014 for examples). Resprouting shrubs may suffer fire-induced mortality or they may successfully resprout but then subsequently die (Fig. 1). For plants surviving fire, young resprouting plants may experience mortality due to drought (Pratt et al., 2014), herbivory (Ramirez et al., 2012), pathogens (Jacobsen et al., 2012; Aguirre et al., 2024), and repeated disturbance (Enright et al., 2015; Zomer et al., 2025). These factors are predicted to be exacerbated by climate change (Enright et al., 2015; Pausas et al., 2016).

Fig. 1.

Resprouting plants differ in their success in resprouting post-fire as well as in the vigour of resprouting shoots. Some plants may (A) not survive the fire and will not resprout (plant 1), or will successfully resprout and subsequently die (plant 2), while yet others will resprout but with differing degrees of vigour, as evidenced by different crown heights and volumes (B–D). The images are the California Mediterranean-type climate region shrub, Adenostoma fasciculatum Hook. & Arn. (Rosaceae), during the first winter post-fire. (Photo credit: A.L. Jacobsen.)

Zomer et al. (2025) examined the complex patterns of variable resprout vigour of two facultative seeding species to investigate the main factors associated with resprout success. Plants may vary in their pre-fire condition, crown replacement, and growth post-fire (Fig. 1). Populations may differ in resprouting ability across their range along environmental gradients, thus being dependent upon local conditions (Ojeda, 1998). Zomer et al. (2025) found that variability in post-disturbance vigour was associated with pre-disturbance plant condition (plant biomass), and linked to plant age, past disturbances, and stress. Site effects were indirect and mediated by plant condition, which the authors were able to tease apart by using structural equation modelling. These are exciting results, and they sharpen the focus for future studies on where to look for main causes of resprout success.

Over the longer term, plant recovery and resource accumulation are important in re-establishing the ability to resprout following future disturbances (Clark et al., 2013; Enright et al., 2015). Within a site, plant age and size appear linked to resource availability for some species, and therefore larger plants have a higher likelihood of resprouting success (Moreira et al., 2012; Marais et al., 2014; Pratt et al., 2014; Zomer et al., 2025).

Short intervals between fires represent a critical threat

Species are adapted to specific disturbance regimes. Changes to fire regime, including alterations to the frequency, season, size, and intensity of fires, result in reduced capacity of natural systems to recover. This applies to both seeding and resprouting species (Table 1). For seeding species, a short period of time in between consecutive fires (i.e. a short fire return interval) leads to heightened immaturity risk that may result in the local extirpation of seeding species from areas with repeated fires (Jacobsen et al., 2004).

For resprouting species, short intervals between fires significantly reduce resprouting success and vigour due to depleted carbohydrate reserves and buds. Zomer et al. (2025) found significant declines in resprout success with repeated crown removal. This is an important result highlighting that resprouters are vulnerable to short disturbance return intervals. For their study, which examined facultative seeders, a short fire return would also limit their seedbank, leading to an overall population contraction due to repeat fires. The results of Zomer et al. (2025) are likely to be conservative estimates as they used clipping to remove crowns. It is plausible that removal of biomass by fire and the additional damage to the bud bank and shallow roots would lead to a greater reduction in resprout success in the case of repeated wildland fire (Malanson and Trabaud, 1988).

When disturbances/stresses occur jointly, such as when drought corresponds with fire, recovery may be further diminished. Young plants, lacking extensive root systems, were especially sensitive to water availability during resprouting (Zomer et al., 2025). Intermediate to larger shrubs may better tolerate drought both with and without fire (Venturas et al., 2016). Changes to risk factors affecting different stages of development have the potential to shift population demographics in ways that alter community composition (Pausas and Keeley, 2014).

Understanding resprouting differences across populations is a challenge for numerous reasons such as the ubiquitous entanglement of environmental differences across populations and phenotypic plasticity. Additionally, site history is an added factor to consider. Natural field experiments coupled with common garden studies is a powerful approach to elucidate a comprehensive and mechanistic understanding of the vulnerabilities of and risks to shrubland communities in a modern and changing world, and to sort out genetically based traits from phenotypic plasticity responses (Table 1). The work of Zomer et al. (2025) is noteworthy in combining field studies with a common garden approach to separate phenotypic plasticity across different field sites from genetically based adaptive responses. Their field and common garden results were complementary, and the absence of a significant population effect suggests that resprouting responses were not driven by local adaptation.

Uncertain future for shrublands

Following fire, shrublands are at risk from numerous factors that may impact the recovery of stands. With compounded disturbances, such as fire and drought, resprout failure and reduced resprout vigour contribute to declines in resprouting shrub diversity, stand density, and stand recovery. These changes combine with shrubland declines that occur due to separate stresses during fire-free intervals (Jacobsen and Pratt, 2018). Overall, these factors convert Mediterranean-type shrublands into alternative states such as grasslands (Pratt, 2022). These losses are already occurring and are extensive in many Mediterranean-type climate regions (Syphard et al., 2019).

While the mechanisms driving decline in seeding and resprouting plants may differ, both are susceptible to changing disturbance regimes and face increasing risks. In a drier and disturbance-prone future, Mediterranean-type climate region shrublands face unpredictable challenges driven by complex interactions between drought, fire regime change, declining plant vigour, introduced species, disease, and other stresses. The effects of these changes are uncertain, but no strategy may be ‘safe’ for shrubs in an era of unprecedented change. In a future in which compound disturbances are expected to increase, studies such as those by Zomer et al. (2025), which seek to understand and disentangle the interactions between multiple disturbances and plants experiencing legacies of drought and stress, are increasingly necessary.