Evaluating How Fire Affects Forest Biodiversity in the Sierra Nevada Mountains

Transcription

1 Evaluating How Fire Affects Forest Biodiversity in the Sierra Nevada Mountains By: Michael Shun ENVS 190A December 15, 2016

2 Table of Contents 1. Abstract 3 2. Introduction 3 3. Fire Intensity 6 4. Fire Frequency 9 5. Non-native Species LANDIS Conclusion Figures Tables References 26 2

3 Abstract Fire is a natural occurrence in Mediterranean-type ecosystems. The Sierra Nevada Mountains in California consist primarily of chaparral shrublands and mixed-conifer forested habitats that have adapted to fires of varying frequencies and intensities. Fire management practices have altered the chaparral habitat over the last century however, so understanding how fire frequency and intensity affect the region s biodiversity is necessary for use in conservation and restoration practices. Infrequent high intensity crown fires now dominate the chaparral habitat due to an altered fire regime which changes forest composition and structure so prescribed fires are being implemented for use in forest management practices to help reduce the intensity of these fires. Non-native species are an increased concern due to the altered chaparral habitat and changes in local fire regimes using prescribed fire. Frequent, low intensity fires reduce the chance for non-native species to die as well as creating more space available for non-native species to colonize the burn area. Higher intensity fires create a better opportunity for fire adapted species to colonize and reduce the risk for non-native species to survive post fire. Computer models such as LANDIS (LANscape DIsturbance and Succession) have been utilized to better understand how fire frequency and intensity may affect specific habitats for conservation and restoration methods without using prescribed fire in natural habitats. In conclusion, more research needs to be conducted to better understand the extent that fire frequency and intensity play in Sierra Nevada chaparral biodiversity but current research indicates that frequent low intensity fires mixed with infrequent high intensity fires provide the best opportunity to restore this region to a natural fire regime. Improving native biodiversity of chaparral habitats in the Sierra Nevada Mountains will improve and preserve chaparral habitat services or functions for future use. Introduction Mediterranean-type ecosystems are prominent throughout much of the California Sierra Nevada Mountain range. Mixed-conifer forests and shrublands, also known as chaparral in California, dominate this ecosystem (Rundel 1998, Figure 1). Fire plays a major role in forest structure and composition of chaparral plant biodiversity in the Sierra Nevada Mountains (Webster and Halpern 2010, Collins et al. 2007). Sierra Nevada chaparral shrublands and mixed-conifer forests contain many unique plant species that are adapted to fire in which they exhibit fire-adapted seeds or cones that require fire for seed germination (Keeley 2000, Keeley and Fotheringham 1998, Keeley et al. 2003). Some 3

4 annual plant species in the Sierra Nevada Mountains are also adapted to recolonize or regenerate during specific fire interactions such as fire intensity, fire frequency, or fuels consumed by fire (Keeley and Brennan 2012). Understanding how fire affects local species is important since certain burn methods or practices may be detrimental to chaparral plant community s long term (Keeley 2000). Historically, frequent occurrences of low or moderate intensity fire created a more spread out forest structure which allowed for more understory sunlight and reduced leaf litter (Collins et al. 2007). For almost a century, however, fire suppression methods have been used to control natural and human induced fires throughout much of the Sierra Nevada Mountains which have impacted chaparral shrublands and mixed-conifer forests in the region (Keeley et al. 2005b). Fire suppression methods are believed to have changed the natural fire intensity, frequency, and nutrients available of this habitat (Skinner and Chang 1996, Keeley 2002, Keeley and Brennan 2012). This regime has altered forest structure and composition of chaparral habitats and it is believed that long periods of fire suppression deteriorate seed banks as well as increase the intensity of fire (Keeley et al. 2005b, Collins et al. 2007). Land management agencies have recognized the importance of fire in chaparral environments so controlled burns have been put into place to counteract the increased amount of fuel created by fire suppression methods (Odion and Hanson 2006). These burn practices, also known as prescribed fires, have been implemented to try and restore the integrity of chaparral and mixed-conifer forest habitats as well as reducing fire intensity and the potential for human related impacts (Stephenson 1999, Collins et al. 2007). Fire 4

5 however, shapes the structure and composition of Sierra Nevada chaparral habitat but fuel reduction methods have created a promising method to increasing plant biodiversity (Aerts and Honnay 2011, Stevens-Rumann and Morgan 2016). Current fire management practices focus primarily on understory and ground fuel reduction using prescribed fire in forests habitats; however, few studies have been conducted to understand how human induced fires affect chaparral plant communities (Collins et al. 2007). The integrity of forested ecosystems is valuable to maintain due to the many services that Sierra Nevada chaparral habitat provides to the local habitat as well as people living around the region (Noss 1999). Ecosystem services are categorized as any resource or benefit that the ecosystem provides and ecosystem function is controlled by biodiversity patterns in mixed-conifer and chaparral shrublands (Keeley et al. 2006). Some of the services that chaparral habitats provide are the decomposition of waste, water purification, and producing fresh oxygen for the surrounding ecosystems. Fire helps to reduce tree biomass as well as decreasing dead debris on the ground therefore understanding how fire frequency and intensity affects forest biodiversity is necessary to maintain and preserve these ecosystem functions and services (Aerts and Honnay 2011, Stevens-Rumann and Morgan 2016). Unmanaged forests provide insight as to how natural fire regimes work in Sierra Nevada chaparral so fire management practices can be altered to increase plant biodiversity in managed forests instead of strictly focusing on fuel reduction techniques. These forests have not been altered from fire suppression methods or fuel reduction techniques over the past century which allows scientists an insight of how a natural fire 5

6 regime works. Comparing the differences in forest biodiversity through fire intensity, fire frequency, and the presence of non-native species is necessary to start understanding natural forest structure and composition in chaparral habitats. Long term successional changes caused by fire alter ecosystem structure and species composition so understanding and restoring natural fire regimes in the Sierra Nevada region may increase overall plant biodiversity (Aerts and Honnay 2011). Restoring a natural fire regime in the region may preserve these ecosystem services for future generations so evaluating how fire intensity and fire frequency affect biodiversity in this region is paramount to restoring, conserving, and preserving chaparral shrublands and mixed-conifer forested habitats for future generations. Fire Intensity Fire intensity, which is determined by a number of forest biodiversity factors including forest stand age, woody debris composition, or plant density of the ecosystem, is shown to affect forest biodiversity in Sierra Nevada chaparral shrublands and mixedconifer forest ecosystems (Keeley et al. 2008). Many native plant species in Sierra Nevada chaparral shrublands and mixed-conifer forests are adapted to intense fires, some of which burn all above ground biomass every years (Keeley and Brennen 2012, Stevens- Rumann and Morgan 2016). Older forest systems contain more biomass and more woody debris which increase amount of fuel available increasing the risk for high intensity fire (Keeley and Fotheringham 2003). Understanding how fire intensity affects Sierra Nevada 6

7 chaparral biodiversity may help to provide insight for creating forest management practices that do not disrupt natural structure or composition of these ecosystems. Chaparral shrublands in the Sierra Nevada Mountains may benefit from a high intensity fire regime instead of current fuel reduction methods utilizing prescribed fire since this region naturally experiences crown fires that burn at high intensity (Keeley et al. 2008). Current fire management practices however, focus on reducing fire intensity to maintain more control over naturally occurring fires in California but forest gaps are created after intense fires which result in increased light and nutrients available for recolonizing or germinating plants (Keeley et al. 2003, Huisinga et al. 2005). High intensity fires many hold a potential for ecosystem restoration practices by decreasing the richness, cover, and abundance of non-native species as well as increase richness and cover of native species in years following high intensity fire. Lower intensity fires may be less beneficial since non-native species will have a better chance of regeneration (Keeley et al & Keeley et al. 2005b). A crown fire in 2002, the McNally Fire, presented an opportunity to study fire intensity in a chaparral habitat that had never experienced fire in recorded history. Ancient forest stands, or forests that have survived for over 150 years without fire, and mature stands, which have experienced fire within the last 60 years, were chosen to observe successional changes in forest structure and biodiversity measures. Ancient stands experienced higher intensity fires than mature stands but plant cover and species richness in ancient stands was greater than that of mature stands (Table 1). There was no evidence suggesting that chaparral habitat was altered from long periods of fire suppression which 7

8 shows the capacity for chaparral shrublands to rebound after a century without fire (Keeley et al. 2005b). Mixed-conifer forests in the Sierra Nevada Mountains naturally experience low to moderately intense fires but fire suppression methods have been used in much of the Sequoia and Kings Canyon National Parks region and many other parts of the Sierra Nevada for over a century (Stephenson 1999, Figure 2). These methods have changed the mixedconifer forest fires from moderately intense surface fires to highly intense crown fires which affect plant species diversity in this region (Keeley et al. 2003, Keeley and Brennan 2012). To better understand how fire suppression regimes affect Sierra Nevada forests Odion and Hanson (2006) looked at the three largest fires that have occurred since 1999 based on burned area and focused on fire rotation intervals by assessing fires that took place between 1950 and Moderate levels of fire were most common with less than a 15% occurrence of high intensity fires and the risk for high intensity fires were not great in any of the three Sierra Nevada fires researched (Odion and Hanson 2006, Table 2). Restoration techniques using induced fires could be useful with restoring Sierra Nevada chaparral habitats by reducing understory and ground cover though controlled burning using prescribed fire which reduces the chance for high intensity crown fires and closer mimics natural fire regimes in mixed-conifer forests (Odion and Hanson 2006, Webster and Halpern 2010). 8

9 Fire Frequency Fire suppression methods used to reduce the frequency of naturally occurring fires have altered chaparral composition and structure in Sierra Nevada Mountain habitats but fuel reduction techniques are being implemented to counter their long term effects (Noss 1999, Keeley et al. 2005b, Wayman and North 2007). These methods are being utilized to mitigate or restore changes to the ecosystem and the services that it provides; however questions arise as to how prescribed fires in chaparral habitats may affect local species richness, cover, and overall biodiversity in shrublands and mixed-conifer forests (Huisinga et al. 2005). Fire is essential for chaparral habitats to increase plant richness and cover by reducing woody debris, leaf litter on the canopy floor, and canopy cover. Intense crown fires are common in chaparral shrublands so fire frequency has been the primary focus for alterations in chaparral biodiversity (Keeley et al. 2003). Frequent fires help to reduce fire intensity and increase chances for many fire adapted seeds to germinate by helping to create forest gaps which allow more light to penetrate the understory allowing for increased levels of seed germination (Wayman and North 2007, Webster and Halpern 2010, Stevens-Rumann and Morgan 2016). A better understanding how fire frequency affects Sierra Nevada plant biodiversity is necessary to help create fire management practices that may allow for a limited use of fire suppression and prescribed fires without altering natural chaparral habitat structure or composition. Prescribed fire was used in the Teakettle Experimental Forest which consists of annuals, shrubs, and trees to better understand how fire frequency affects annual ground cover (Figure 3). Richness and cover increased in post fire burn treatment sites due to a 9

10 thinning of leaf litter and canopy cover suggesting that frequent prescribed fires may enhance mixed-conifer forest understory biodiversity. In burned treatment plots, shrub cover decreased but annual herbaceous cover increased shortly after the disturbance which was attributed to shrubs and trees taking longer to rebound post fire. Increased herb cover and richness in the treatment sites was attributed to reduced ground litter, understory light allowance, and soil moisture and showing the greatest increase in plant composition occurring within the first year of the burn treatment with smaller increases each year following the prescribed fires (Wayman and North 2007). Implementing the use of frequent low intensity fires may help fire management practices to adopt methods to maintain mixed-conifer forest biodiversity for groundcover plant species due to reduced shrub cover and canopy cover. After a 1993 Rim fire in Arizona, prescribed fire was used in a mixed-conifer forest to better understand how fire frequency affects forest biodiversity. Annual groundcover and shrub cover increased shortly after prescribed fire but tree species did not have increased canopy cover almost a decade post fire (Huisinga et al. 2005). Fire decreased species richness immediately after the fire but in the following years after the decrease, there was a steady increase in plant richness as well as increased resilience and lower levels of non-native species (Keeley et al. 2008, Stevens-Rumann and Morgan 2016). To better understand changes in plant richness, abundance, and fire intensity, Webster and Halpern (2010) observed successional changes that took place in mixed-conifer forests during multiple burns over a 20 year period. Forest composition changed for control plots, first-burn plots, and second-burn plots showing that burned plots contained twice as many species as control plots within the first 10 years of being burned. After 20 years, the burned 10

11 plots contained 4-5 times more shrubs than the control sites and canopy cover increased in the burned sites but was not evident until almost 20 years after the burn. (Webster and Halpern 2010) This information helps to provide insight that prescribed burning can be beneficial to chaparral habitat and how frequently prescribed burning can be utilized to increase biodiversity measures in chaparral habitats. Non-native Species Non-native plant species are an increased concern in Sierra Nevada ecosystems (Keeley et al. 2003). Sierra Nevada chaparral ecosystems have experienced various fire management practices for the last century including fire suppression and prescribed burns that have altered fire intensity, frequency, as well as nutrients available which increase the chances for non-native species to colonize this region (Keeley and Brennan 2012). Fire frequency and intensity are two components used to better help understand how fire may affect both native and non-native plant biodiversity in the Sierra Nevada Mountains. Increased frequency of fire helps increase all plant species richness and diversity but frequent low intensity fires also increase the risk for non-native species to colonize burn locations (Griffis et al. 2001, Keeley et al. 2003). Prescribed fire reduces intensity of crown fires and mixed-conifer forest fires which increases the ability for non-native species to colonize burn areas from competing native shrubs and grasses due to lack competition or predators which alters habitat structure and composition (Keeley 2000, Keeley et al. 2008). In the Southern Sierra Nevada foothills, Keeley et al. (2003) looked at unburned and burned plots to better understand how burn locations are affected by non-native species. 11

12 Non-native plants were nonexistent in unburned locations but richness and cover for nonnative species increased with increased fire frequency (Table 3). Total species richness was over 2.5 times higher and shrub cover was five times as high in third year burns compared to first year burns although much of the increase in richness and cover was due to nonnative species colonizing burned primarily in locations with low fire intensity (Keeley et al. 2003, Table 4). Diversity and richness of non-native plant species in Sierra Nevada chaparral habitats increased in the years following the fire as well indicating that fire frequency is essential for non-native grasses to colonize in chaparral regions (Keeley et al. 2003). The McNally Crown Fire in 2002 presented an opportunity to better understand how non-native species influences chaparral burn locations post fire. Non-native plant species abundance and richness increased post fire but native species abundance and richness was initially reduced suggesting that fire adapted plant species need decades to regenerate and recolonize completely. Increased fire frequency and reduced intensity from prescribed fires however, have made it easier for non-native plants to colonize the chaparral shrublands in the Sierra Nevada Mountains (Keeley et al. 2012). Increased fire frequency reduces the ability for native woody plants to regenerate post fire and reduced fire intensity increases chances for non-native plant annual seed banks to survive and recolonize increasing non-native plant richness and abundance in Sierra Nevada chaparral habitats. However, these changes in non-native plant species were determined to not be an immediate threat to Sierra Nevada chaparral habitat during the McNally fire (Keeley et al. 2005b). More research needs to be conducted to better understand if increased richness 12

13 and abundance of non-native species would be a long term threat to forest structure and composition throughout the Sierra Nevada region (Collins et al. 2007). Sierra Nevada crown fires were also observed to better understand how fire intensity affects non-native plant species richness and cover in mixed-conifer forested chaparral habitats (Keeley et al. 2005b, Keeley et al. 2008). In higher severity fires, some plant species died out which gave fire adapted seeds a chance to germinate increasing plant richness, abundance and plant composition for native species (Webster and Halpern 2010). Plant species in low severity fires had a reduced mortality rate so plant diversity and richness was maintained but plant cover was reduced which further increased the chances for seed banks of non-native species to survive and recolonize. Low to moderately intense fires increased the chance for non-native species to colonize burned locations and many species observed consisted of non-native annual grasses or forbs which burn easily during intense fires. Much of the decrease in plant cover regardless of fire intensity was due to fire sensitive non-native annual plant species indicating that frequent low intensity fires favors non-native species by allowing their seeds more area to germinate post fire (Keeley et al. 2008). Higher intensity fires increase the chances for non-native seed banks and recolonizers to die which increases the chances for native fire adapted plant species to slowly recolonize the habitat. A better understanding of how native and non-native plants react to fuel reduction regimes in chaparral habitats is needed to be certain that increased frequency of human induced or natural fires is beneficial in restoring or conserving Sierra Nevada chaparral (Keeley 2000). Human induced fires did not negatively impact native plant species richness 13

14 or diversity but richness and abundance of non-native species increased but the increase was minor. Changes in forest or shrubland composition do not jeopardize the integrity of chaparral habitats within the first decade, but long term effects of increased non-native species are widely unknown for this region. Fire intensity and frequency in chaparral habitats both affect native and non-native biodiversity and it is paramount for fire management practices to better understand how controlled and natural fires affect Sierra Nevada chaparral shrublands and mixed-conifer forests to mitigate the invasion of nonnative plant species by integrating a forest management system that benefits native plant species over non-native plant species. Landscape Disturbance and Succession (LANDIS) Computer models such as LANDIS (LANscape DIsturbance and Succession), were created to help understand how fire and other disturbances may affect natural environments. LANDIS was designed to simulate successional patterns in a specific location and is used to better understand how fire frequency affects succession in habitats after experiencing a large-scale disturbance by using a system of small grids to make up a larger landscape (Franklin et al. 2001, Gustafson et al. 2000, Mladenoff and He 1999). The LANDIS simulation program was created to take management methods, site conditions, and specific species into account while running simulations after inserting a fire disturbance into the computer model and producing multiple pathways of succession used for interpretation. This model is designed to create and interpret fire frequency models dependent upon the 14

15 fire intervals and providing many possible outcomes for use in forest management practices. The LANDIS model depicted a couple of interesting results however (Franklin et al. 2001). Variability in fire frequency was large which was attributed to accidental simulated fires being an infrequent occurrence within the LANDIS simulation. Furthermore, the program is set to use time interval units of 10 years creating a design flaw in which individual species or fires cannot be recorded or accounted for (Franklin et al. 2001, Mladenoff et al. 1996). This flaw causes an issue that residual stand volume or tree density cannot be accounted for since individual plant species are not recognized by the LANDIS model (Gustafson et al. 2000). Determining tree density and stand volume of plant species is essential in understanding forest structure and composition in the simulated environment so fire intensity can be accounted for in Sierra Nevada chaparral habitats as well as fire frequency. Currently, LANDIS only accounts for obligate resprouters, a type of plant species that does not die post fire. This design flaw in LANDIS may not properly account for the recolonization of some plants that are fire adapted for seed germination. In naturally occurring fires, fire tolerant species show slow recolonization rates immediately post fire and during the successional years following fire so successional simulations are not entirely accurate (Franklin et al. 2001). The simulation also did not account for the available space created by fire where obligate resprouters were located before fire however; the simulation did provide similar observed results to the expected results known from field studies. Further work needs to be done to improve the LANDIS 15

16 simulations to include obligate seeders within the simulations to better understand how chaparral habitats may be affected by fire frequency. LANDIS is not completely effective in understanding successional shifts post fire currently but it can be used as a piece of the puzzle to help understand natural patterns in fire (Franklin, et al. 2001). The LANDIS computer model may one day help to provide an understanding of how fire intensity and frequency affect similar ecosystems with different forest structure and composition (Mladenoff and He 1999). Various management practices could be implemented based on the simulations once LANDIS is programmed to closer mimic natural fire regimes in Sierra Nevada Mountain chaparral habitats. The LANDIS program and other models have the potential to help scientists better understand how fire frequency and intensity affect specific habitats and may once day provide insight to creating fire management practices that will benefit native biodiversity. Conclusion Chaparral shrublands and mixed-conifer forested ecosystems in the Sierra Nevada Mountains experience high intensity and frequent fires so both are important factors in determining plant biodiversity (Keeley et al. 2003). Understanding how fire frequency and intensity affect native and non-native biodiversity is paramount for conservation or restoration efforts as well as maintaining ecosystem benefits and services (Aerts and Honnay 2011). Interactions between fire and chaparral habitats consist of processes that involve soil composition, seed banks, and the organ regeneration of some plant species 16

17 (Borchert and Odion 1995). More research needs to be done to better understand how varying intensities and frequency of fires affect mixed-conifer forest and shrubland biodiversity in order to restore or preserve the environment to natural conditions (Keeley et al. 2006, Stevens-Rumann and Morgan 2016). Fire suppression methods have altered the forest structure of Sierra Nevada chaparral and forested ecosystems by reducing understory plant richness, abundance, and plant cover which has occurred due to an increased canopy cover, woody debris present, and leaf litter on the ground as well as from a reduced fire frequency but an increase in the intensity (Rundel 1998 and Keeley 2002, Collins et al. 2007, Aerts and Honnay 2011). Many non-woody plant species in the Sierra Nevada chaparral and mixed-conifer forests achieve their highest levels of richness and abundance post fire (Keeley 2000). There has not been enough research done to positively identify whether fire suppression techniques are the primary driving force behind more intense fires in the Sierra Nevada forests however (Odion and Hanson 2006). More research needs to be conducted to accurately understand to what extent fire suppression techniques have and will affect Sierra Nevada chaparral shrublands and mixed-conifer forests in the Sierra Nevada Mountains. Fire intensity and frequency do affect forest structure and composition but to what extent is widely unknown since each individual forest contains different species so a general understanding of general habitat such as chaparral or mixed-conifer forests does not suffice. Burning practices focus on reducing human hazards and fuel reduction which are created from a general understanding of similarities between similar forest systems and not differences between them but mitigating alterations to forest structure and 17

18 composition needs be a primary focus (Keeley 2002). Frequent and severe fires both allow for a greater increase in annual plant richness and abundance by reintroducing nutrients and increasing sunlight available in the burn location suggesting chaparral habitats may benefit from occasional high intensity fires or frequent low intensity fires by reducing the likelihood for non-native plant seed banks to survive (Webster and Halpern 2010). Fire practices and regimes need to be altered based on specific conditions in a location but it would be difficult for forest management practices to mimic natural fire occurrences. Focusing research on species populations or demographics would provide a better understanding as to how forests are altered when forest management protocol differs from natural fire regimes (Noss 1999). Adaptive management can be used to adjust fire regimes in Sierra Nevada Mountain forested and shrubland habitats as to help maintain and conserve Sierra Nevada habitats by increasing forest biodiversity and richness of native species which would allow for an overall increase in ecosystem services and function (Noss 1999). Creating fire management goals focusing on restoring Sierra Nevada chaparral ecosystems back to natural fire regimes is important for use in conservation biology as well as maintaining the region for future use, however, an emphasis needs to be put on individual mixed-conifer forests or shrublands however since forest structure and composition varies for each location. In conclusion, more research is needed to better understand how forest biodiversity differs upon location is needed to improve burning regimes for specific habitats within the Sierra Nevada Mountains (Keeley 2002). 18

19 Figure 1: Vegetation Types of the Sierra Nevada Mountains (Wayman and North 2007) 19

20 Figure 2: Kings Canyon and Sequoia National Parks. These locations have experienced almost a century of fire exclusion. They are located in the southern portion of the Sierra Nevada Mountains. (Keeley et al. 2003) 20

21 Figure 3: Location of Teakettle Experimental Forest. Prescribed fire was used on each of the plot sites to better understand how fire frequency affects understory plants. (Wayman and North 2007) 21

22 Table 1: Species richness of Ancient and Mature Stands after the McNally Fire in Data indicates burned sites in the first year post fire. (Keeley et al. 2005b) 22

23 Table 2: Area (ha), fire intervals (y), and the percentages of fire severity after the McNally Fire in (Odion and Hanson 2006) 23

24 Table 3: Understory plant cover and richness for native and non-native species in a mixed-conifer forest post fires that occurred in 1996 and (Keeley et al. 2003) 24

25 Table 4: Chaparral biodiversity patterns for non-native plant species during first and third year burns. Data was gathered following fires in 1996 and 1998 in a Sierra Nevada mixed-conifer forest. (Keeley et al. 2003) 25

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