Drought‐induced woody plant mortality in an encroached semi‐arid savanna depends on topoedaphic factors and land management

Wonkka, Carissa L.; Taylor, Charles A.; Zou, Chris B.; Twidwell, Jeremiah J.; Rogers, William E.

doi:10.1111/avsc.12044

Cited by 40 publications

(43 citation statements)

References 40 publications

(47 reference statements)

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“…Further, warmer temperatures in the absence of significant drought have been shown to increase tree growth rates, particularly in deciduous species (Way and Oren 2010). Overall, the direct mortality consequences of warming could be relatively small and limited when considered globally, as earth system feedbacks (e.g., greater atmospheric moisture and precipitation) combined with multiple treelevel compensatory processes can buffer future Xiong et al 2011, Kharuk et al 2013, Zhang et al 2014b, Zhou et al 2013, 2014bAustralasia, Semple et al 2010; Europe,Čater 2015; North America, Vogelmann et al 2009, Zegler et al 2012, Baguskas et al 2014, Hart et al 2014, Kane et al 2014, Twidwell et al 2014, Gu et al 2015South America, Brienen et al 2015. v www.esajournals.org tree mortality relative to hotter droughts (e.g., Klein et al 2014a).…”

Section: Temperaturementioning

confidence: 99%

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

2015

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Abstract. Patterns, mechanisms, projections, and consequences of tree mortality and associated broadscale forest die-off due to drought accompanied by warmer temperatures-''hotter drought'', an emerging characteristic of the Anthropocene-are the focus of rapidly expanding literature. Despite recent observational, experimental, and modeling studies suggesting increased vulnerability of trees to hotter drought and associated pests and pathogens, substantial debate remains among research, management and policy-making communities regarding future tree mortality risks. We summarize key mortalityrelevant findings, differentiating between those implying lesser versus greater levels of vulnerability. Evidence suggesting lesser vulnerability includes forest benefits of elevated [CO 2 ] and increased water-use efficiency; observed and modeled increases in forest growth and canopy greening; widespread increases in woody-plant biomass, density, and extent; compensatory physiological, morphological, and genetic mechanisms; dampening ecological feedbacks; and potential mitigation by forest management. In contrast, recent studies document more rapid mortality under hotter drought due to negative tree physiological responses and accelerated biotic attacks. Additional evidence suggesting greater vulnerability includes rising background mortality rates; projected increases in drought frequency, intensity, and duration; limitations of vegetation models such as inadequately represented mortality processes; warming feedbacks from die-off; and wildfire synergies. Grouping these findings we identify ten contrasting perspectives that shape the vulnerability debate but have not been discussed collectively. We also present a set of global vulnerability drivers that are known with high confidence: (1) droughts eventually occur everywhere; (2) warming produces hotter droughts; (3) atmospheric moisture demand increases nonlinearly with temperature during drought; (4) mortality can occur faster in hotter drought, consistent with fundamental physiology; (5) shorter droughts occur more frequently than longer droughts and can become lethal under warming, increasing the frequency of lethal drought nonlinearly; and (6) mortality happens rapidly relative to growth intervals needed for forest recovery. These high-confidence drivers, in concert with research supporting greater vulnerability perspectives, support an overall viewpoint of greater forest vulnerability globally. We surmise that mortality vulnerability is being discounted in part due to difficulties in predicting threshold responses to extreme climate events. Given the profound ecological and societal implications of underestimating global vulnerability to hotter drought, we highlight urgent challenges for research, management, and policy-making communities.

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Section: Temperaturementioning

confidence: 99%

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

2015

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“…In the future, if climate becomes warmer and drier or if the frequency, magnitude, and duration of drought increase, present-day grasslands in some areas may become desert shrubland. In contrast, woodlands and forests could also shift to savanna or grassland (e.g., Allen et al 2010;Anadón et al 2014a) and increases in woody cover realized in recent decades may be reduced by a higher frequency of "hot droughts" (Bowers 2005;Breshears et al 2005;Twidwell et al 2014). Climatechange simulations under elevated atmospheric CO 2 predict pronounced shifts toward tree-dominated biomes (Scheiter and Higgins 2009).…”

Section: Climatementioning

confidence: 99%

“…Interannual and spatial variability in rainfall is high in the semiarid zone. Given the potential longevity of woody plants, exceptionally good recruitment years can set the stage for seed production and opportunities for recruitment decades into the future, whereas only exceptionally strong drought years can significantly reduce mature tree cover (Fensham and Holman 1999;Twidwell et al 2014). Models incorporating these dynamics predict that decadal or longer deviations from mean tree density may result (Fig.…”

Section: Climatementioning

confidence: 99%

“…However, if there are fissures or gaps in the impermeable layers that allow resources and woody plant roots to pass through, woody plants may thrive. Aboveground patterns in distribution, size, and mortality rates of woody plants that accompany drought may reflect variation in these edaphic heterogeneities (Bestelmeyer et al 2011;Rossatto et al 2014;Twidwell et al 2014).…”

Section: Topography and Soilsmentioning

confidence: 99%

“…However, in regions where annual precipitation is highly variable-a characteristic of many water-limited environments-it is challenging to pinpoint an absolute carrying capacity for woody plants, as mortality and recruitment in any given year are tied to that year's or the recent series of years' precipitation, not the long-term average. Precipitation deficits will decrease recruitment and increase adult mortality (Bowers 2005;Twidwell et al 2014), but density-dependent mortality may also occur during more benign conditions (Meyer et al 2008;Dwyer et al 2010). Precipitation-induced fluctuations in recruitment and mortality rates (Fig.…”

Section: Woody Plant Carrying Capacitymentioning

confidence: 99%

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Woody Plant Encroachment: Causes and Consequences

Archer

Andersen

Predick

et al. 2017

Springer Series on Environmental Management

372

379

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Woody vegetation in grasslands and savannas has increased worldwide over the past 100-200 years. This phenomenon of "woody plant encroachment" (WPE) has been documented to occur at different times but at comparable rates in rangelands of the Americas, Australia, and southern Africa. The objectives of this chapter are to review (1) the process of WPE and its causes, (2) consequences for ecosystem function and the provision of services, and (3) the effectiveness of management interventions aimed at reducing woody cover. Explanations for WPE require consideration of multiple interacting drivers and constraints and their variation through time at a given site. Mean annual precipitation sets an upper limit to woody plant cover, but local patterns of disturbance (fire, browsing) and soil properties (texture, depth) prevent the realization of this potential. In the absence of these constraints, seasonality, interannual variation, and intensity of precipitation events determine the rate and extent of woody plant expansion. Although probably not a triggering factor, rising atmospheric CO 2 levels may have favored C 3 woody plant growth. WPE coincided with the global intensification of livestock grazing that by reducing fine fuels, hence fire frequency and intensity, facilitated WPE. From a conservation perspective, WPE threatens the maintenance of grassland and savanna

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Ungulates mitigate the effects of drought and shrub encroachment on the fire hazard of Mediterranean oak woodlands

Lecomte,

Bugalho,

Catry

et al. 2024

Ecological Applications

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Climate change is increasing the frequency of droughts and the risk of severe wildfires, which can interact with shrub encroachment and browsing by wild ungulates. Wild ungulate populations are expanding due, among other factors, to favorable habitat changes resulting from land abandonment or land‐use changes. Understanding how ungulate browsing interacts with drought to affect woody plant mortality, plant flammability, and fire hazard is especially relevant in the context of climate change and increasing frequency of wildfires. The aim of this study is to explore the combined effects of cumulative drought, shrub encroachment, and ungulate browsing on the fire hazard of Mediterranean oak woodlands in Portugal. In a long‐term (18 years) ungulate fencing exclusion experiment that simulated land abandonment and management neglect, we investigated the population dynamics of the native shrub Cistus ladanifer, which naturally dominates the understory of woodlands and is browsed by ungulates, comparing areas with (no fencing) and without (fencing) wild ungulate browsing. We also modeled fire behavior in browsed and unbrowsed plots considering drought and nondrought scenarios. Specifically, we estimated C. ladanifer population density, biomass, and fuel load characteristics, which were used to model fire behavior in drought and nondrought scenarios. Overall, drought increased the proportion of dead C. ladanifer shrub individuals, which was higher in the browsed plots. Drought decreased the ratio of live to dead shrub plant material, increased total fuel loading, shrub stand flammability, and the modeled fire parameters, that is, rate of surface fire spread, fireline intensity, and flame length. However, total fuel load and fire hazard were lower in browsed than unbrowsed plots, both in drought and nondrought scenarios. Browsing also decreased the population density of living shrubs, halting shrub encroachment. Our study provides long‐term experimental evidence showing the role of wild ungulates in mitigating drought effects on fire hazard in shrub‐encroached Mediterranean oak woodlands. Our results also emphasize that the long‐term effects of land abandonment can interact with climate change drivers, affecting wildfire hazard. This is particularly relevant given the increasing incidence of land abandonment.

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Drought‐induced woody plant mortality in an encroached semi‐arid savanna depends on topoedaphic factors and land management

Abstract: "Droughtinduced woody plant mortality in an encroached semi-arid savanna depends on topoedaphic factors and land management" (2014).

Cited by 40 publications

References 40 publications

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

Woody Plant Encroachment: Causes and Consequences

Ungulates mitigate the effects of drought and shrub encroachment on the fire hazard of Mediterranean oak woodlands

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