THE VALUE OF RIPARIAN TO BUFFER EFFECTS OF CLIMATE CHANGE IN CALIFORNIA’S CENTRAL VALLEY

Abbygayle Britton

Dr. Foran | ENVS 190 Senior Thesis April 17, 2019 THE VALUE OF RIPARIAN 1

Abstract

The ecosystem services provided by riparian habitats are a potential alternative to mitigate the impacts of climate change on the Central Valley of California (CVC). The rise in regional temperature increasingly alters the hydrological regime which degrades aquatic ecosystems, contributes to water scarcity, and imposes stress on the flora and fauna throughout the CVC.

Though riparian habitats historically characterized much of the CVC, its current potential in onset of climate change is not as widely acknowledged. A literature review supports the capacity for riparian habitats to provide biological refugia through thermal cover, enhanced habitat quality and role as a corridor for migration. Further research determined that riparian habitats can likely influence aquifer recharge and effectively store water resources. As the effects of climate change become more severe, it will be essential to incorporate the role of riparian habitats. THE VALUE OF RIPARIAN HABITATS 2

Table of Contents

Abstract...... 1

Introduction ...... 3 Reduced snowfall and snowpack ...... 5 Modified flow regime...... 6 Increase in extreme climatic events ...... 8

Functions of Riparian Habitat as a Buffer to the Effects of Climate Change...... 10 Biological refugia...... 10 Heterogeneity...... 10 Thermal cover and Microclimates ...... 12 Enhance habitat quality...... 13 Infiltration and Aquifer recharge ...... 14 Corridor for migration...... 15

Risks to riparian habitat ...... 16 Water divergence...... 16 Land Use Competition ...... 17

Potential for Restoration...... 17 Course Restoration...... 17 Easements ...... 19 Restoration of natural floodplain...... 20 Protection through policies...... 20

Conclusion...... 21

References ...... 22 THE VALUE OF RIPARIAN HABITATS 3

Introduction

The California Central Valley is a region that is highly vulnerable to climate change because it has limited water resources, climate sensitive agriculture, and severe habit degradation

(Scanlon et.al, 2012; Lee et. al, 2011; Kucera and Barrett, 1995). Each of these conditions will be magnified by climate change. However, research has suggested that riparian habitat, once a major feature of the Valley, has the potential to lessen the negative impacts of climate change

(Capon et. al 2013; Seavy et. al, 2009).

Riparian habitats play an integral role in maintaining the stability of the California

Central Valley (Capon et. al, 2013; Seavy et. al, 2009; Naimen et. al, 2000). Valley oaks

(Quercus lobata) and Fremont cottonwoods (Populus fremontii), amongst other tree species, provide optimal habitat for migrating water fowl and native avifauna (Gilmer et. al, 1982;

Katibah, 1984). The diverse flora is also home to multiple listed species, like the nearly extirpated Least Bell’s Vireo (Vireo belli pusillus) and Valley elderberry longhorn beetle

(Desmocerus californicus dimorphus), endangered California tiger salamander (Ambystoma californiense), and threatened giant garter snake (Thamnophis gigas) (Gardali et. al, 2006; fws.gov). Along with habitat, riparian areas provide other ecosystem services like protection, groundwater recharge, and nutrient cycling (Brauman et. al, 2007; Chaimson, 1984).

However, a drastic change of land use from floodplains to agriculture and urbanization has led to a considerable decline in riparian habitat (Nelson et. al, 2003; Reid et. al, 2016; Katibah, 1984).

The loss of riparian habitat inhibits the ecological processes of the Central Valley and makes the region vulnerable to environmental disturbance.

This study aims to assess the viability of riparian habitats to serve as an effective and practical resource in lessoning the impacts of climate change in the Central Valley of California THE VALUE OF RIPARIAN HABITATS 4

(CVC). Through literature review this paper will: 1) Identify the regional impacts of climate change on CVC; 2) Determine ecosystem services of riparian habitat that will function as a buffer against climate change; 3) Identify potential risks to riparian habitat; 4) Provide a conceptual framework of necessary elements to riparian habitat restoration.

The Regional Impacts of Climate Change

A major consequence of climate change is the rise in temperature that will range from

1.1°C to 6.4°C (Solomon et. al, 2007). One of the expectations for the Central Valley in response to this temperature rise is a severely altered hydrological cycle. The winter and spring temperatures of the surrounding mountain ranges that provide water to the region continue to show warming trends (Cayan et. al, 2008; Mote et. al, 2005). The warmer temperatures reduce snow accumulation, which alternatively falls as rain, and the spring snow melt is happening 10-

30 days earlier in the season (Cayan et. al, 2001; Hayhoe et. al, 2004; Mote et. al, 2005; Stewart et. al, 2005). These factors have several implications downstream in the Central Valley.

The Central Valley (CVC) derives most of its’ water from the Sierra Nevada, which highly influences the water regimes throughout the landscape (ca.water.usgs.gov). The CVC is split into the Sacramento Valley and San Joaquin Valley, which depend on surface and groundwater, respectively, to support water resource needs (Katibah, 1984). The Sacramento and

San Joaquin merge in the Delta, where they continue to flow to the San Francisco Bay and into the ocean (ca.water.usgs.gov). THE VALUE OF RIPARIAN HABITATS 5

Reduced snowfall and snowpack

The reduction in snowpack and the addition of more winter rainfall can increase instream temperatures enough to exceed optimal temperatures for many aquatic species. In the best-case climate change scenario, an air temperature rise of 2°C can lead to a 57% habitat reduction for cold water aquatic species (Null et. al, 2013). Higher levels of increased air temperature of 4°C results in 91% habitat reduction, while a 6°C increases is nearly 99.3% (Null et. al, 2013). If the increase of instream temperature goes beyond the optimal range of native aquatic species, it will impact metabolic processes, increase susceptibility to disease, and overall lower fitness.

Furthermore, the lifecycles of many native aquatic species are tied to the seasonal patterns of instream temperatures, including several Salmonid species (Poff et. al, 2002; Null et. al, 2013).

Summer temperatures already reach the upper temperature range for many Salmonid species, like steelhead trout (Oncorhynchus mykiss) and Chinook salmon (O. tshawytscha) (Katz et. al,

2013). The successful migration of most salmonid species depends on minimizing the amount of time they are exposed to relatively warm temperatures. However, stream temperatures of

21°C already extend well into September, which coincides with California’s largest salmon run, the Fall run of Chinook salmon (Null et. al, 2013). The temperature strain imposed on the various Salmonid species continues to result in unsuitable habitat that will contribute to the decline in productivity of aquatic ecosystems.

The most severe increases in stream temperature have been found in lower elevation watersheds of the Central Valley, like the Yuba, Tuolumne, Tule and Kaweah watersheds (Katz et. al, 2013; Null et. al, 2013). Many salmonid species utilize lower elevation watersheds for migration, breeding and spawning ground, or primary habitat. The resulting change in stream temperature poses the risk of extinction to native steelhead, salmon and trout species (Moyle et. THE VALUE OF RIPARIAN HABITATS 6 al, 2017). Currently, at least 47% of the present salmonid species are already listed as endangered, vulnerable, or near threatened (Katz et. al, 2013). Additionally, under the current climate change scenario, 45% of California’s native salmonids will likely be extinct within the next 50 years (Moyle et. al, 2017). The native steelhead, salmon, and trout in the CVC cannot sustain their populations with the continued warming of , so limiting stream temperature within their optimal range will be critical to their future survival.

Modified flow regime

The trend of reduced snow pack and earlier spring melt have altered the Central Valley’s summer flow regime (Cayan et. al, 2008; Mote et. al, 2005; Stewart et. al 2005). Although there is more winter rain, the reservoirs are not designed to maintain supplemental winter rain and off season water storage. During winter and early spring, reservoirs are kept at a percentage of their capacity to accommodate seasonal flooding (Knowles et. al, 2006). However, this is under the assumption that the snowpack will act as temporary water storage. To avoid flood risk or infrastructure failure, freshwater is released and essentially lost to the ocean where it cannot be utilized (Knowles et. al, 2006; Stewart et. al, 2005). The lack of storage in snowpack and inability to delay winter freshwater releases results in an altered flow regime that impacts the aquatic ecosystems of the Central Valley.

A major consequence of a modified flow regime is habitat degradation (Nilsson and

Svedmark, 2002; Poff et. al, 1997). The quality of aquatic systems depends on efficient flow to regulate the temperature, salinity, oxygen and pollutant concentration (Mount et. al, 2012).

Similarly, flow plays an integral role in sediment transfer, which supports increased productivity in the benthic community and provides spawning habitat for fish in washed out gravel beds THE VALUE OF RIPARIAN HABITATS 7

(Nilsson and Svedmark, 2002; Poff et. al, 1997). The aquatic ecosystems of the CVC are increasingly becoming unsuitable for many indigenous species, like the juvenile Chinook salmon and delta smelt, because of a lack of spawning habitat (Mount et. al, 2012). The intensity and frequency of flow is also important in producing different stages of succession that supports habitat heterogeneity (Nilsson and Svedmark, 2002). For example, high flow events erode and scour the channel which produces habitat for some birds, like the spotted sandpiper, who utilize the exposed gravel bars for nest sites (Venture, 2004). Without a sufficient flow regime, aquatic ecosystems in the CVC will continue to degrade and risk the survival of the various species that depend on this habitat.

A modified flow regime has further consequences for aquatic species whose lifecycles are in sync with the normal timing of floodplain inundation and flood pulses (Morey, 1998;

Sommer et. al, 2001). Amphibian development relies on the inundation period of ephemeral pools outlasting the time it takes to complete their juvenile stages (Morey, 1998). Some species, like the western spadefoot toad, require about 5 weeks of inundation after breeding. However, the endangered California tiger salamander can require up to 3 months. The premature drying associated with lower summer flows can potentially inhibit the full development of these amphibians (Morey, 1998). Equally important is the co-occurrence of fish spawning and breeding to flood pulses (Moyle et. al, 2017). In lower elevation floodplains, like the Yolo

Bypass, the timing of winter and spring flood pulses coincide with spawning and breeding of many native fish like splittail, steelhead, sturgeon, and Chinook salmon (Moyle et. al, 2017).

Access to these floodplains provide a greater supply of nutrient that supports bigger fish and enhances the probability of future survival (Moyle et. al, 2017; Opperman et. al, 2010).

Consequently, an altered flood pulse make native fish populations susceptible to decline THE VALUE OF RIPARIAN HABITATS 8

(Opperman et. al, 2010). Many aquatic species of the central valley have adapted to a natural flow regime and a significant change between this relationship can impact their ability to have sustainable populations.

Increase in extreme climatic events

Both and droughts are predicted to increase in severity and frequency with the onset of climate change (Hayhoe et. al, 2004; Stewart et. al 2005). The heightened occurrence of floods and droughts will have direct consequences on CVC aquatic ecosystems, like habitat alteration (Morey, 1998; Hanak and Lund, 2012).

Generally, aquatic systems benefit from floods through ecosystem processes like increased nutrient cycling and habitat regeneration (Nilsson and Svedmark, 2002; Hanak and

Lund, 2012). Nutrient cycling is promoted both longitudinally, as flow encourages nutrient movement downstream, and laterally as nutrients are deposited past the high-water marks of the river channel onto the floodplain (Nilsson and Svedmark, 2002). Additionally, major flood events have high flow variability which contributes to a more complex aquatic ecosystem by influencing upland riparian habitat. The different magnitude of floods will influence riparian habitat differently depending on if it is a low, intermediate, or high flood which includes modification of plant species, plant communities, and large geomorphic features, respectively

(Nilsson and Svedmark, 2002; Hanak and Lund, 2012). Reliable flooding produces a mix of successional stages that help maintain optimal understory vegetation, which is essential for species that prefer to nest in 5-10-year-old trees (Duffy and Kahara, 2011; Franzreb, 1989). The flora and fauna within the CVC are conditioned to utilize the effects of flooding to their benefit THE VALUE OF RIPARIAN HABITATS 9 to some degree, although it is unclear whether future flooding will overwhelm these aquatic ecosystems (Brauman et. al, 2007).

The increase in duration and frequency of droughts will cause a considerable decline in suitable habitat for aquatic species. The lack of water associated with drought will limit or inhibit the seasonal inundation period for amphibians that is necessary for breeding and larvae stages

(Morey, 1998). Amphibians, like the California tiger salamander, have already declined to unstable populations and will likely have a difficult time recovering from these circumstances

(Fisher and Shaffer, 1996). Similarly, droughts exacerbate the effects already occurring from a modified flow regime and increase in stream temperature, which limits suitable habitat for native fish throughout the CVC (Hanak et. al, 2015). These conditions lower the fitness of native fish, while invasive species tend to thrive under the same circumstances. The competition from invasive fish populations makes native fish populations, like steelhead and delta smelt, susceptible to decline (Hanak et. al, 2015). As drought conditions worsen in the future, aquatic species will continue to endure compounding environmental stress that can potentially lead to their extinction (Hanak et. al, 2015).

The water shortages associated with drought have the potential to create severe dry conditions that reduce suitable habitat (Duffy and Kahara, 2011). The migrating waterfowl that depend on managed wetlands for food and habitat throughout winter and spring are subjected to conditions of overcrowding and poor water quality that increase their likelihood for disease

(Duffy and Kahara, 2011; Hanak et. al, 2015). Droughts reduce the amount of surface water available for human use so the increased irrigation demands are targeted towards groundwater

(Hanak et. al, 2015). The CVC is a critical part of the Pacific Flyaway where over 5 million water birds depend on this habitat. The impact of drought has already cut nearly 25% of water THE VALUE OF RIPARIAN HABITATS 10 delivery to these managed wetlands and the amount is expected to increase over time (Hanak et. al, 2015). The lack of water degrades available habitat that influences lower numbers of nests, eggs, and fledging’s amongst waterfowl populations (Franzreb, 1989; Gilmer et. al, 1982).

Furthermore, the smaller area of habitat makes individuals more susceptible to infection and disease. Waterfowl populations that utilize the CVC will find it increasingly difficult to sustain themselves amidst the impacts of predicted drought.

Functions of Riparian Habitat as a Buffer to the Effects of Climate Change

The effects of climate change on aquatic ecosystems contributes to a significant loss of biodiversity within the Central Valley (Capon et. al 2013; Seavy et. al, 2009). Globally,

California has one of the highest levels of biodiversity and risks substantial loss if habitats are no longer suitable (Kucera and Barnett, 1995). On the other hand, if there is suitable habitat, stationary species or species with a restricted range have limited opportunity to migrate faster than the onset of climate change. As biodiversity declines, the Central Valley becomes less resilient and loses its ability to withstand future impacts. However, riparian habitat has the capacity to help mitigate some of these negative effects. The ecological processes maintained through riparian habitat provide biological refugia, enhances habitat quality of neighboring ecosystems, and enables migration.

Biological refugia

Heterogeneity

The high heterogeneity of riparian habitats is a key characteristic that enables these zones to serve as biological refugia with the onset of climate change (Alpert et. al, 1999; Oakley et. al, THE VALUE OF RIPARIAN HABITATS 11

1985; Sork et. al, 2010). On a regional scale, riparian habitats share a similar consistency, however individual river systems throughout the CVC show variability that contributes to heterogeneity amongst riparian zones (Naimen et. al, 2000). Though not always entirely obvious, these differences can be seen within the genetic diversity between riparian habitats and the range of evolutionary processes that occur (Naimen et. al, 2000). Some of the major factors that influence riparian habitat heterogeneity are variations in topography and its role as a transition zone (Naimen et. al, 2000; Oakley et. al, 1985).

Since riparian zones are found in relation to watersheds and aquatic ecosystems, the changing dynamics over the course of a stream influences the composition of vegetation

(Naimen et. al, 2000; Oakley et. al, 1985). The differences in topography throughout the span of a river’s course are suitable to different plant types. This encourages high species density in close proximities (Oakley et. al, 1985; Alpert et. al, 1999). Also, subtle climatic variations in temperature and precipitation influence local genetic selection, which in conjunction with high species density increases the likelihood that local populations in CVC will contain individuals that can tolerate the effects of climate change (Sork et. al, 2010).

As the intermediate between upland and aquatic ecosystems, the role of riparian habit as a transition zone contributes to its heterogeneity (Oakley et. al, 1985; Capon et. al, 2013). Since riparian habitats are ecotones they contain a mixture of plant species from neighboring habitats, while also including some riparian habitat specialists that are disturbance adapted (Oakley et. al,

1985; Lyon and Gross, 2005). As a transition zone, riparian areas withstand high levels of disturbance that influence the composition of the area (Oakley et. al, 1985). High disturbance events, like floods, scour, erode, and transfer sediment, all contribute to reshaping of the channel and succession (Steiger et. al, 2005). These events add to the edge effect, where the variety of THE VALUE OF RIPARIAN HABITATS 12 plant structures provide many niches that support high diversity (Oakley et. al, 1985; Capon et. al, 2013). Some species, like the spotted sand piper, utilize the exposed sand bar for nesting sites, while others, like herons or egrets, prefer to roost in old growth trees (Venture, 2004). Riparian habitats consistently have more species diversity abundance compared to upland habitats

(Oakley et. al, 1985). Even though some species, like migrating birds are not there for the entirety of their life, 70% of CVC vertebrate species will utilize riparian habitat at some critical point in their lifecycle (Naiman et. al, 2000).

The increase in severity and frequency of extreme climatic events, like floods and drought, makes heterogeneity an essential factor in species ability to thrive (Steiger et. al, 2005;

Capon et. al, 2013). The nuances embedded within a riparian habitat support a variety of niches that provide more opportunity for species to thrive. Furthermore, since riparian habitats have the capacity to support a relatively large number of species, when individuals or populations are lost, as an effect of climate change, there will be a greater chance that riparian communities will still be able to thrive and maintain diversity (Sork et. al, 2010).

Thermal cover and Microclimates

The unprecedented rate at which climate change occurs has made it difficult for many species to acclimate successfully to the new conditions. Increased temperature and lack of water availability are major hurdles to the survival of individuals because the quality and amount of suitable habitat is greatly reduced. Fortunately, the streams in relation to riparian habitats influence air temperature, anywhere from 10 to 60m laterally, depending on the vegetation

(Naiman et. al, 2000; Rykken et. al, 2007). These microclimates are notably cooler, which provides suitable habitat for temperature sensitive species (Naiman et. al, 2000; Rykken et. al, THE VALUE OF RIPARIAN HABITATS 13

2007; Dugdale et. al, 2018). When riparian habitat is clear cut, it can result in an increase of over

10°C in surface air temperature, with lower relative humidity, which can easily exceed the optimal temperature range of native species (Rykken et. al, 2007). Likewise, riparian habitat influences local stream temperature through canopy cover and groundwater release (Capon et. al,

2013). The canopy cover shades the water and can provide habitat several degrees cooler, depending on the composition of the vegetation (Dugdale et. al, 2018). On a bigger scale, the groundwater of riparian habitats drains into the river to create pockets of cooler water relative to the average stream temperature. These pockets have been a critical microhabitat utilized by salmon to help them successfully migrate through warmer rivers (Seavy et. al, 2009). Since reduced snowpack, modified flow regime, and drought contribute to the predicted rise in temperatures throughout CVC habitats, riparian areas will have an integral role in providing protection from temperature extremes (Oakley et. al, 1985).

Enhance habitat quality

Riparian ecosystems play a substantial role in maintaining the quality of aquatic habitats, which will become increasingly important as the negative effects of drought and a modified flow regime heighten (Mount et. al, 2012, Naimen et. al, 2000). Some ecological processes of riparian zones, like energy flow, enhance biological activity, while other processes benefit surrounding habitats by removing harmful constituents (Naimen et. al, 2000, Seavy et. al, 2009). For example, the complex root and vegetation matrix traps sediment, filters pollutants, and reduces erosion that would otherwise impact aquatic habitats (Capon et. al, 2013; Duffy and Kahara,

2011; Naimen et. al, 2000). Furthermore, biological activity is augmented through the supply of large woody debris (LWD) and occurrence of hyporheic zones (Steiger et. al, 2005; Naiman et. THE VALUE OF RIPARIAN HABITATS 14 al, 2000; Seavy et. al, 2009). Usually, LWD is introduced to the aquatic ecosystem through disturbance events, which is a food source for aquatic invertebrates (Seavy et. al, 2009). The hyporheic zone, where groundwater and surface water mix supports greater amounts of decomposition and nutrient retention, as well (Steiger et. al, 2005; Naiman et. al, 2000). Since aquatic habitat degradation throughout the CVC will continue to degrade with the effects of climate change becoming more apparent, there is great potential to use the capacity of riparian habitats to lessen environmental stressors at a larger scale.

Infiltration and Aquifer recharge

The increase in winter rains and earlier spring melt make it difficult for man-made structures to efficiently store this water for future use and instead the water is lost to the ocean or becomes a flood risk (Knowles et. al, 2006; Stewart et. al, 2005; Hayhoe et. al, 2004). However, in the CVC riparian habitats have historically reinforced groundwater storage, which preserves fresh water resources and limits flood risk downstream (Brauman et. al, 2007; Steiger et. al,

2005). When there is a floodplain, riparian networks allow a longer retention time which supports greater rates of infiltration (Duffy and Kahara, 2011; Brauman et. al, 2007; Steiger et. al, 2005). In 1985, there was an accidental levee breach along the Cosumnes river that began the regrowth of riparian forests (Swenson et. al, 2003). Although the riparian habitat was relatively young, an established floodplain created a substantially longer residence time for flood waters that influenced flood management and aquifer recharge (Swenson et. al, 2003).

The influence of riparian habitat on aquifer recharge is especially important as groundwater depletion becomes more severe with the increase in drought and unreliable water availability (Hayhoe et. al, 2004; Stewart et. al, 2005; Famiglietti et. al, 2011; Scanlon et. al, THE VALUE OF RIPARIAN HABITATS 15

2012). The limited availability of surface water puts additional stress on groundwater resources, particularly in the summer when agriculture requires most of CVC fresh water resources

(Famiglietti et. al, 2011). Throughout CVC, aquifer collapse is a threat as land subsidence continues to occur with the over pumping of groundwater (Luhdorff et. al, 2014). However, by utilizing ecological processes of riparian habitats, the additional winter rains and spring melt that can be stored in aquifers. Though this will likely not solve all of CVC water needs, it’s important to consider what natural ecological processes can increase water storage.

Corridor for migration

The change in temperature and precipitation patterns associated with climate change has influenced a shift in suitable habitat where the ability to migrate is critical to species’ survival.

Unfortunately, since CVC land has largely transitioned to agriculture and urbanization, available habitat is severely fragmented and migration to suitable habitat is difficult (Hunter et. al, 1999).

However, in the past riparian zones have shown that they play an essential role as corridors when movement is critical to survival (Grivet et. al, 2008). For example, riparian corridors in CVC connected Sierra Nevada foothills to the coastal ranges in the west and facilitated movement of endemic oaks (Grivet et. al, 2008). Ensuring there are opportunities for species’ movement will increase the chances of maintaining genetic diversity when they are faced with the environmental stress of climate change.

Many species are trying to move towards cooler areas in response to climate change, however the topographic diversity of California means cooler areas are not necessarily upward and poleward (Sork et. al, 2010; Beier, 2012). Although predicted species movement is limited, through a strong network of riparian habitat creates potential routes that support movement and THE VALUE OF RIPARIAN HABITATS 16 dispersal (Beier, 2012; Oakley et. al, 1985). Available corridors will be especially important for species, like the threatened elderberry longhorn beetle, who has limited dispersal and relies on spatial redundancy (Collinge et. al, 2001). Since riparian zones have high heterogeneity and connects ecological communities, they support a variety of animals that are searching for new territory (Oakley et. al, 1985; Seavy et. al, 2009). With the impacts of climate changing limiting suitable habitat, riparian corridors play an essential role in maintaining biodiversity amongst

CVC communities (Grivet et. al, 2008; Oakley et. al, 1985; Seavy et. al, 2009).

Risks to riparian habitat

Water divergence

As fresh water becomes an increasingly limited resource in the CVC with the onset of climate change, there is a heightened risk to the quality and resiliency of riparian zones (Scanlon et. al, 2012; Capon et. al, 2013; Nilsson and Svedmark, 2002). California’s growing population and agricultural demand relies on accessible water year around (Capon et. al, 2013). However, the reduction in snow pack, modified flow regime, and increase in severity and frequency of droughts have reduced the availability of surface water (Cayan et. al, 2008; Mote et. al, 2005;

Hayhoe et. al, 2004). Thus, the source of freshwater shifts to groundwater reserves. Over- irrigation of groundwater, especially during droughts, poses additional stress on riparian habitats that also depend on these reserves (Scanlon et. al, 2012; Famiglietti et. al, 2011). Lack of groundwater supply particularly affects mature trees that rely on access to the water table as a mechanism to withstand drought conditions (Nilsson and Svedmark, 2002). If groundwater continues to be diverted at the current rate, then riparian habitat will become more vulnerable to climatic stressors. THE VALUE OF RIPARIAN HABITATS 17

Land Use Competition

In the CVC, riparian habitats are threatened by land use competition from urban sprawl and agriculture. Within the next decade, California’s population is predicted to increase between

19-30% which means there will be a need for more infrastructure (Duffy and Kahara, 2011).

Since land is often valued in terms of economic benefit and not ecological function, remaining riparian areas risks being developed or converted to agriculture (National Research Council,

2002; Hunter et. al, 1999). Many areas in CVC have already experienced a shift towards grasslands in response to livestock and human influence (Hayhoe et. al, 2004; Oakley et. al,

1985). The pressure to reduce remaining riparian habitat makes them vulnerable to physical alteration and will reduce ecological services these areas provide (Oakley et. al, 1985). If riparian zones continue to decline they cannot be effective buffers against climate change. Fragmented habitat will not aid species, like the threatened elderberry longhorn beetle, who benefit from the connectivity of riparian areas (Hunter et. al, 1999). Even current areas may be too small to be a functional mechanism against lessoning the impacts of climate change (Hunter et. al, 1999).

Potential for Restoration

Course Restoration

Since riparian habitats are characterized by high species density and diversity, it is challenging to determine effective restoration plans that will encompass the ideal conditions for every species involved (Beier, 2012; Choi et. al, 2008). The optimal option for riparian habitat restoration would be “coarse” restoration, in that it is not species specific but instead focuses on entire riparian communities (Beier, 2012). Instead, mitigation should focus on restoring habitat THE VALUE OF RIPARIAN HABITATS 18 to its most natural state that is best fitted towards future change. This requires an understanding of what underlying gradients are influencing the dynamic of specific riparian areas, from abiotic factors like runoff concentration to soil conditions (Lyon and Gross, 2005).

The complications associated with restoration is the idea of what natural state a habitat should be restored to. For example, in the past the CVC could support huge populations of bird species. However, complete restoration of the CVC to mirror past composition of riparian habitat is not possible, but also may not be completely necessary to achieve restoration goals (Choi et. al, 2008). The goal for restoration is not to restore the past, but instead to utilize the understanding of the ecosystem services riparian habitats provide and restore its capacity to sustain these systems on its own (Choi et. al, 2008). Part of this means promoting genetic diversity through more than just local material (Harris et. al, 2006). Planting species that are from all along a river gradient, as opposed to just local flora, can increase the likelihood of the vegetation community successfully adjusting to the regional effects of climate change (Harris et. al, 2006). This focuses on planting a wide range of species influenced by predicted future conditions, which means in one area planting both flood and drought tolerant plants (Seavy et. al,

2009; Oakley et. al, 1985). The other element is that restoration in riparian habitats should not necessarily favor old growth (Duffy and Kahara, 2011). There is major role for old growth, as habitat and regulation of ecosystem processes, however it is important to keep in mind that ongoing succession is critical to the sustainability of riparian habitats. The likelihood of successful restoration depends on what adaptation the various species have towards impacts of climate change, which cannot necessarily be determined from past conditions of the CVC. THE VALUE OF RIPARIAN HABITATS 19

Easements

Since about 94 % of the CVC is privately owned, effective collaboration with landowners is vital to the success of riparian restoration (Duffy and Kahara, 2011). One mechanism is through land easements that limit land use to varying degrees based on the terms of the easement

(Hunter et. al, 1999). However, the private land use in CVC is primarily dedicated to agriculture, which represents a $30 billion-dollar industry for California (Hayhoe et. al, 2004). The trade-off to obtain private land will become increasingly expensive as that land is valued more for agriculture, or even urbanization. Also, the cost of easements that are tailored to benefit riparian habitats could outweigh available state funds allocated towards restoration (Hunter et. al, 1999).

The easements that can be afforded potentially lack enough area to provide efficient ecosystem services from riparian habitat, or in other words it may not be fiscally worth it (Hunter et. al,

1999).

Nonetheless, the stakeholders of the Cosumnes River Preserve (CRP) have about 2,400 acres under restrictive easements, which they utilized parts of to support the growth of riparian ecosystems (Swenson et. al, 2003). With the help of established easements, local farmers agreed to a “non-structural flood management project” in 1997 that allowed intentional levee breaches that spanned about 5.5 miles (Swenson et. al, 2003). The new access to its floodplain created sand splay that resulted in the growth of willows and cottonwoods, which were the beginning of future established riparian habitat (Swenson et. al, 2003). Successful establishment of riparian habitats in CRP provide a potential framework of utilizing easements for restoration.

The amount of private lands and reserves that lack effective riparian zones have the potential to connect patches of habitat that will supplement restoration efforts (Hunter et. al,

1999). The Wetlands Restoration Program (WRP), is an opportunity for landowners to promote THE VALUE OF RIPARIAN HABITATS 20 protection and restoration of wetlands. However, WRP about 0.3% of its focus is on riparian restoration efforts, which is around 2311 ha (Duffy and Kahara, 2011). Though land use competition makes riparian habitat restoration more difficult, there is an abundance of private land that represents the opportunity for future efforts.

Restoration of natural floodplain

A controversial, yet crucial, element to successful restoration is establishing more acreage for natural floodplains throughout the CVC. A natural disturbance regime and natural processes essentially provides the framework for riparian habitats to repair themselves (Naimen et. al, 2000, Hunter et. al, 1999). Like the emergence of the Accidental Forest in the CRP, a floodplain increases sediment deposition that jumpstarts the growth of riparian habitat and these habitats become self-sustaining (Florsheim and Mount, 2002; Naimen et. al, 2000; Hunter et. al,

1999). However, the solution of flooding the CVC for riparian restoration meets opposition with other goals for land use.

Protection through policies

Future restoration and establishment of sustainable riparian habitats risk being undermined by present demands for water. Since the rights to water in CVC is highly competitive and often controversial, the necessary water rights to the environment is easily overlooked. There are many benefits to riparian habitats that will play a bigger role as the effects of climate change become more severe. However, without enough water delivered, the fundamental processes of riparian habitat are inhibited (Naimen et. al, 2000). To ensure enough THE VALUE OF RIPARIAN HABITATS 21 water is supplied to riparian habitats, the environment must be conceived as an essential consumer of water and allowed similar rights (Naimen et. al, 2000).

Currently, there are no federal programs that specifically manage against harmful ecological activities towards riparian habitats (Hunter et. al, 1999; National Research Council,

2002). The National Policy Act (NEPA) requires a certain level of examination of potential impacts to any environment, but it is not a regulatory law that represents the importance of riparian habitat. Other federal regulations involve the Wetland regulation section 404 under the

Clean Water Act and flood plain regulation, however neither of these have a primary focus on riparian habitat and many riparian areas end up not meeting the necessary qualifications to be represented through these regulations (National Research Council, 2002). The most powerful regulation that benefits riparian areas is the Endangered species act because many species like steelhead, Coho salmon, riparian brush rabbit, and Least bell’s vireo are closely linked riparian areas (National Research Council, 2002). The lack of a formal protection status undercuts the value of riparian habitats and diminishes it potential to play an active role towards mitigating the effects of climate change.

Conclusion

Though riparian habitats have the capacity to lessen the consequences of climate change in the CVC, future research is necessary to effectively utilize these areas. The driving question is how much riparian habitat is enough to mitigate the impacts of climate change? Part of the benefit of riparian ecosystems is that they can support many species within a relatively small area, but there is not a comprehensive understanding of the amount of land that is required to have a functional riparian network in the CVC (Hunter et. al, 1999; Hilty and Merenlender, THE VALUE OF RIPARIAN HABITATS 22

2004). With land use competition and limited funds, it will make a considerable difference if restoration only requires a percentage of historical range of riparian habitat. However, the dynamics of riparian habitat are heavily influenced by geomorphic processes that are misunderstood or entirely unknown when a restoration plan is made (Kondolf, 1998).

Furthermore, restoration budgets often do not include post restoration funds, which makes it difficult to build a comprehensive understanding of what is necessary for riparian habitats to thrive in the CVC (Kondolf, 1998). Although, riparian habitats are an optimal alternative to mitigate the effects of climate change, lack of knowledge regarding the dynamics of these ecosystems limit this ability.

As regional temperatures continue to rise, the impacts of climate change will become more severe and ecosystem disturbance throughout the CVC will be more frequent. The CVC hydrological cycle is drastically changing in response to a decrease in snowfall and snowpack, modified flow and increase in extreme climatic events. The environmental drawbacks involve shift in location and amount of suitable habitat, which has led to notable declines in many species in CVC. Overall, the ability of aquatic ecosystems in the CVC to sustain ecosystem services and provide optimal habitat is diminishing with the onset of climate change. However, the high heterogeneity and ability to adjust to disturbance makes riparian ecosystems an effective mechanism to address the impacts of climate change in the Central Valley of California, which aims to balance current human needs with long term environmental and human requirements. THE VALUE OF RIPARIAN HABITATS 23

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