water Review Wildfires in Grasslands and Shrublands: A Review of Impacts on Vegetation, Soil, Hydrology, and Geomorphology Ilan Stavi Dead Sea and Arava Science Center, Yotvata 88820, Israel; [email protected] Received: 2 April 2019; Accepted: 16 May 2019; Published: 20 May 2019 Abstract: Wildfires are prevalent in grasslands and shrublands. The objective of this study is to provide land managers with a general overview, by assessing the main impacts of wildfire, including those on plant communities (e.g., secondary succession and species invasion), soil characteristics (e.g., water repellency (hydrophobicity), aggregation and structure stability, and contents of organic carbon and nutrients), and surface processes (e.g., ash deposition, ground surface clogging, water runoff, soil erosion, hillslope debris flow, and dry ravel). Additionally, the study discusses the effects of livestock grazing on the functioning of post-fire grasslands and shrublands. Although mesic regions are mentioned, this review focuses on drylands. The comparatively low-to-moderate fuel loads that characterize grasslands and shrublands generate wildfires of relatively moderate intensity, resulting in moderate burn severity. Yet, it seems that because of decreased soil aggregate stability following burning, the hoof action of livestock that access burnt lands shortly after the fire increases the shearing and detachment of mineral material from the ground surface; this increases soil erodibility, with the possible risk of accelerated land degradation. The review ends with an assessment of general implications for environmental sustainability and health, and provides recommendations on wildfire control in rangelands, and on restoration of burnt lands. Keywords: climatic change; drought episodes; ecosystem services; herbaceous and woody vegetation community; hillslope processes; invasive species; organic matter; prescribed burning; shrub encroachment; water and wind erosion 1. Introduction Grasslands and shrublands are important for the conservation of floral and faunal biodiversity. Also, they provide a wide range of additional ecosystem services, including carbon sequestration, flood control, soil erosion mitigation, and pasture for livestock [1]. Grasslands and steppes are 1 defined as ecosystems with a tree density of up to 10 trees ha− . Shrublands have been characterized as ecosystems with shrub or sub-shrub densities of at least 30% cover, plus tree densities of up to 1 10 trees ha− [2]. Therefore, the shrubland definition includes garigue, phrygana, chaparral, brush, maquis, and scrub [3]. These lands are vulnerable to increasing anthropogenic pressures, including infrastructures, rural or urban construction, agriculture, and forestry. These disturbances can degrade ecosystems, specifically, through the fragmentation of spatial connectivity, which is needed for the healthy existence of complex systems [4]. One of the major factors that affect grasslands and shrublands across the world is wildfires [5], Figure1. Although some studies consider wildfires as a disturbance, this article stresses that fires are an important ecological factor that fill crucial roles and functions in grasslands and rangelands [6]. Water 2019, 11, 1042; doi:10.3390/w11051042 www.mdpi.com/journal/water Water 2019, 11, 1042 2 of 20 Water 2019, 11, x FOR PEER REVIEW 2 of 19 Figure 1. The world map, indicating sites of key studies (for more details, follow the numbers in the list of references). Among the main factors that determine the behavior of fires,fires, the most important is the fuel load quantity and and quality. quality. In In this this regard, regard, greater greater quantiti quantitieses of fuel of fuel elevate elevate the thefire's fire’s burning burning potential, potential, and andlonglong dry dryperiods periods enhance enhance flammability flammability and and increase increase probability probability of of wildfires wildfires in in these these ecosystems. ecosystems. Compared toto forests,forests, thethe availabilityavailability ofof fuel fuel in in grasslands grasslands and and shrublands shrublands is is low; low; however, however, this this fuel fuel is veryis very dry. dry. Therefore, Therefore, fire fire intensity intensity in in these these lands lands is is relatively relatively low low [ 7[7],], but but fires fires spreadspread fast.fast. Quantities 1 of available fuel in grasslandsgrasslands andand shrublandsshrublands rangerange betweenbetween 2–102–10 MgMg haha−−1,, all all of which is burntburnt during a typical wildfire.wildfire. In In comparison, the the quantity of available fuel in woodlands and forests 1 1 ranges between 10–5010–50 MgMg haha−−1 andand 200–1500 200–1500 Mg Mg ha −1, ,respectively, respectively, of of which which only only 5–95% 5–95% and and 5–25%, 5–25%, respectively, isis burntburnt duringduring aa typicaltypical wildfire.wildfire. Prevailing meteorological conditions during a firefire dictate firefire behavior,behavior, in in particular, particular, wind wind velocity velocity and and direction, direction, and relativeand relative air humidity. air humidity. In this regard,In this 1 1 fourregard, ranges four ofranges wind of velocity wind velocity have been have defined: been defined: (1) weak: (1) weak:< 10 km < 10 h− km; (2) h−1 moderate:; (2) moderate:10–20 10–20 km hkm− ; 1 1 (3)h−1; strong:(3) strong:20–30 20–30 km km h− h;−1 and; and (4) (4) extreme: extreme: >> 3030 h h−−1. Four. Four ranges ranges of relative of relative air humidity air humidity have have also alsobeen beendefined: defined: (a) very (a) low: very < low:10%; <(b)10%; low: (b)10–20%; low: (c) 10–20%; moderate: (c) moderate:20–30%; and 20–30%; (d) high: and > 30%. (d) high: The >likelihood30%. The of likelihood wildfiresof increases wildfires as increases the wind as velocity the wind increases velocity and increases relative and air relative humidity air decreases humidity decreases[8]. [8]. Among the meteorological conditions, the air temperature has the smallest direct effect effect on the probability of wildfireswildfires [[8].8]. Yet, the heat released during wildfireswildfires might form a positive feedback by drying the near-surface vegetation at the fire fire front,front, thereby expanding the burnt area and increasing the burn severity [7]. [7]. In In additi addition,on, topography topography is is another another importan importantt determinant determinant of of fire fire development. development. Overall, the steeper the hillslope incline, the faster the firefire front cancan advanceadvance uphill.uphill. Three ranges of hillslope incline have have been been defined: defined: (i) (i) gentle: gentle: < 10%;< 10%; (ii) (ii)medium: medium: 10–30%; 10–30%; and and(iii) (iii)steep: steep: > 30%.> 30%Dry. Drychannels channels can behave can behave as fire ascorridors fire corridors (even in (even the direction in the direction opposite oppositeto the wind), to the whereas wind), watershed whereas watershedlines tend to lines stop tend theto advancement stop the advancement of the fire offront. the fireThe front. combination The combination of large quantities of large quantities of dry fuel of dryand fuel'suitable' and ‘suitable’ meteorological meteorological and topographic and topographic conditions conditions constitutes constitutes a 'promising a ‘promising recipe' for recipe’ wildfires for wildfires[8]. [8]. Fires areare frequentlyfrequently caused caused by by humans. humans. Although Although some some fires fires may may be be ignited ignited accidently, accidently, others others are aare deliberate a deliberate management management practice practice (prescribed (prescribed fires), fires), in which in which the land the managers land managers can achieve can achieve specific goals,specific for goals, example, for example, sustaining sustaining plant diversity plant diversity by maintaining by maintaining a certain frequency a certain frequency of fires [7]. of Some fires fires [7]. areSome started fires are by arsonists.started by Forarsonists. example, For inexample, Israel, about in Israel, one-third about ofone-third wildfires of betweenwildfires2012 between and 20152012 fitand this 2015 definition fit this definition [9]. Such ignitions,[9]. Such ignitions, mostly of mostly forests, of have forests, been have named been pyro-terrorism, named pyro-terrorism, and have occurredand have inoccurred other countries in other as countries well, including as well, Greece including [10] andGreece Spain. [10] Theand threat Spain. of The pyro-terrorism threat of pyro- has alsoterrorism been reportedhas also been for Estonia reported [11 for] and Esto thenia United [11] and States the [United12]. States [12]. Water 2019, 11, 1042 3 of 20 The following terms are important for discussing wildfires. Fire intensity represents the energy that 2 is released during the different stages of fire. Fire intensity is measured in energy units (e.g., kW m− ) released over a certain time interval along the fire front. Fire severity quantifies the above- and below-ground vegetal material that is consumed by fire. This term is helpful in describing burnt lands, where knowledge gaps exist regarding their fire intensity [7,13]. Further, the greatest proportion of heat released in fire is transferred to the air, whereas only a small share of it is transferred to the ground. Therefore, fire intensity is not considered a good indicator for changes in soil properties. The heat which reaches the ground surface is transferred into the soil profile through a combination of different processes, including radiation, conduction,