CHAPTER 11: NATIVE ADAPTATIONS TO FLOW IN

MEGAN KELSO

INTRODUCTION The immense power and seasonality of the as it cut though the harsh landscape of Grand Canyon forged a native riparian plant community composed of species with special adaptations suited to historical flow patterns. The creation of Glen Canyon Dam just up- stream of Grand Canyon dramatically altered the timing and size of these flows (see Burley, this volume), which shifted the composition and distribution of the riparian (river corridor) plant community.

Riparian are distributed on the banks of the Colorado in elevation zones according to their tolerances for disturbance, inundation, and desiccation (see Figures 1 and 2). Closest to the river are herbaceous marsh plants that can tolerate inundation, but not desiccation. Slightly higher on the river bank is the low riparian zone, dominated by fast-growing woody plants that can tolerate moderate disturbance and grow best in moist soils. These trees also grow in the middle riparian zone intermixed with bunchgrasses and perennial . The high riparian zone is the most removed from the river, so it experiences the least flood disturbance but the most desiccation stress. It is inhabited by a mix of drought tolerant riparian plants and desert plants (Kennedy and Ralston 2011).

Since the construction of Glen Canyon Dam, there has been a dramatic increase in vegetation in the low zones. Marsh plants, once virtually absent along the main stem due to violent scouring floods, have proliferated (Webb et al. 2002). The low riparian and middle riparian zones are much more densely vegetated. Poplars and willows now crowd the shoreline because there is little extreme seasonal violence to deter them (Kennedy and Ralston 2011). High riparian zone plants that relied on periodic extreme high flows for reproduction are not recruiting as successfully in the high riparian zone, while desert plants that do not rely on moisture from the river may expand down the riverbank to take advantage of the drier conditions that other plants cannot tolerate (Anderson and Ruffner 1987).

An interesting sub-plot through this story of changing vegetation is the differential impact of the dam on native versus invasive plant species, particularly invasive Tamarisk (Tamarix ramosissima). While both native and invasive plant abundances have increased since the construction of Glen Canyon Dam, invasive Tamarisk has increased proportionally more due to its tolerance of a broader range of salinities and moisture levels (see Eskra, this volume).

Figure 1. Pre-dam native plant zones (modified from Anderson and Ruffner 1987).

Figure 2. Post-dam native plant zones (modified from Anderson and Ruffner 1987).

2 ADAPTATIONS OF NATIVE RIPARIAN PLANTS TO PRE-DAM CONDITIONS

Floods and high flows From the perspective of a riparian plant, one of the most important components of the pre-dam flow regime was the recurrence interval of huge scouring floods. These floods could uproot whole plants, break off limbs, and reshape river banks and sandbars (Schmit and Schmidt 2011). Enormous floods like this (some >100,000 cfs) occurred roughly every 6 years in the Grand Canyon before Glen Canyon Dam (Kennedy and Ralston 2011). These raging floods created such high levels of disturbance that plant cover in the Grand Canyon before the construction of Glen Canyon Dam was dramatically sparser (see Cookingham, this volume).

Riparian plants have a range of strategies to survive in such a highly disturbed system. Woody trees in the low riparian zone have adapted to tolerate extreme disturbance. For example, many willow species (Salix spp.) have flexible branches that bend with the flow to reduce resistance. Freemont cottonwood (Populus fremontii) has brittle branches that break off easily, reducing strain on the and trunk during intense flows (Karrenberg et al. 2002). Plants in the high riparian zone, such as catclaw acacia (Acacia greggii) and western honey mesquite (Prosopis glandulosa), avoid the brunt of the flood disturbance by growing higher up on the river banks. To survive so far from the water they must tolerate extremely dry conditions, for which they have long tap roots (Anderson and Ruffner 1987). The huge scouring floods were large enough to wet the banks of the high riparian zone, providing ideal recruitment (successful seedling establishment) conditions for high riparian zone plants such as catclaw acacia and western honey mesquite.

Just as important as surviving a scouring flood is having a strategy for rapidly colonizing the fresh, bare substrate left behind. Some herbaceous plants in the low zone, such as common reed (Phragmites australis), send out runners to colonize bare areas (Kennedy and Ralston 2011). Even if only a few stems remain in the ground during a flood, the plant can recover quickly. Many low riparian zone trees can re-sprout from branch fragments that break off during a flood and float downstream to a bare patch of sediment (Karrenberg et al. 2002).

Even in years without huge scouring floods, the Colorado River still experienced large peak flows in spring and early summer due to snowmelt higher in the watershed (Hamill 2009). These peak flows, while not as dramatic as the enormous scouring floods, were still extremely powerful (roughly 50,000 cfs) and would thrash plants, wet the riverbanks, and deposit fresh sediment. Many native trees in the low riparian zone are adapted to disperse their seeds at the tail end of these seasonal floods so that their seedlings can take advantage of the freshly- deposited, nutrient-rich sediment and moist riverbanks (Karrenberg et al. 2002, Lytle and Poff 2004). For example, Fremont poplar (Populus fremontii) disperses its seeds March-April and willows seed March-June (Karrenberg et al. 2002, Glenn and Nagler 2005). Under moist conditions with abundant sunlight, willow and poplar seedlings can grow quickly, even outcompeting highly competitive Tamarisk (Karrenberg et al. 2002, Glenn and Nagler 2005). In addition to providing ideal recruitment habitat for native riparian tree seedlings, these seasonal floods also improved conditions for mature native trees by providing moisture in the desiccating heat of summer and flushing salts from riverbank soils. Many native plants that grow in the low riparian zone of the Colorado River do poorly in high salinity or extremely dry conditions (Stromberg et al. 2007).

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Plants in the marsh zone and low riparian zone are adapted to survive extended periods of inundation that would drown most plants. Flooding kills plants by depriving their roots of oxygen, which they need to break down sugars into useable energy (a process called aerobic respiration). Cattail (Typha latifolia) is a marsh plant that survives flooding by transporting oxygen to the soil from the atmosphere through spongy tissue in its stem called aerenchyma. Willows have a similar adaptation: they have aboveground roots that grow from the base of the trunk (called adventitious roots) that transport oxygen from the atmosphere down into the soil to support aerobic respiration (Lytle and Poff 2004).

It is intuitive that large floods play an important role in plant survival and reproduction in the Grand Canyon. Perhaps less intuitive is the importance of the rate of flood recession. But if you imagine that your offspring are trying to grow roots fast enough to keep pace with a rapidly receding water table after a flood, it becomes clear why rate of flood recession is critical to recruitment of many species. Cottonwood seedlings are well-adapted to historic rates of flood recession on the Colorado River. Their roots can keep pace with a water table declining by 2-4 cm/day (Karrenberg et al. 2002, Glenn and Nagler 2005). Sometimes floods would recede faster than that, stranding and killing seedlings, but favorable conditions occurred occasionally.

Low flows We have discussed the seasonally high flows, but not their counterpart: the very low flows from late summer through winter. During this period, plants that are not right next to the river must find another source of water or do with very little. Many of the plants in the low and mid-riparian zones, including seep willow (Baccharis salicifolia), willows (Salix), and poplar (Populus) are phreatophytes, meaning they send roots down to the water table. However, they can only extend their roots moderate distances, limiting how high on the riverbank they can grow. Plants in the high riparian zone, such as western honey mesquite (Prosopis glandulosa) and catclaw acacia (Acacia greggii) can extend their roots much deeper in search of water, and therefore tend to be more drought tolerant (see Anderson and Ruffner 1987). As a result of these adaptations, mesquite and catclaw acacia can grow higher on the riverbank, occupying alluvial terraces and talus slopes. Additionally they are long-lived species, able to persist in the canyon even if suitable recruitment conditions (high flows that moisten the high river bank) only occur every 10-30 years (Anderson and Ruffner 1987).

Sediment So far we’ve exclusively discussed the size and timing of flows, but don’t be fooled – it’s not all about water. The amount of sediment in the river is also very important to plants because it affects the availability of substrate and likelihood of being buried in sediment during a large flood. Before the construction of Glen Canyon Dam, the Colorado River carried as much as 60 million tons of sediment past Lee’s Ferry every year (Schmit and Schmidt 2011). Willow (Salix spp.) and poplar (Populus) seedlings are adapted to survive burial in sediments from late- season floods. Tamarisk seedlings are not as well adapted to burial, so conditions that favor high sediment deposition give native tree seedlings an advantage over invasive tamarisk (Glenn and Nagler 2005).

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* Desert plants are do not require river inundation to survive or recruit. However, they may be able to expand their zone towards the river to take advantage of drier areas that become un-useable by plants that require inundation.

Table 1. Native riparian plants and their adaptations to flow, by zone.

5 EFFECTS OF GLEN CANYON DAM ON NATIVE RIPARIAN PLANTS The completion of Glen Canyon Dam in 1963 dramatically changed the flow regime and sediment load of the Colorado River through Grand Canyon (see Burley, this volume). The most immediately striking result of the dam on the riparian plant community is an overall increase in vegetation. Another dramatic change is the increase in abundance of invasive Tamarisk relative to native plants (see Eskra, this volume). There are also subtler changes, like the movement of plant communities along the elevation gradient of the riverbanks in response to changes in the disturbance regime and water provisioning.

The most dramatic change caused by the dam was the elimination of huge scouring floods (which historically occurred roughly every 6-10 years) and annual spring-summer peak flows. These much milder conditions led to an increase in vegetation along the river. Marshes filled in areas that were once open-water. Poplars, willows, and invasive tamarisk crowded the low riparian zone, overgrowing once-sandy beaches (Kennedy and Ralston 2011). Additionally, high riparian zone plants such as catclaw acacia (Acacia greggii) and western honey mesquite (Prosopis glandulosa), have expanded down into lower zones on the riverbank from which they were once excluded by high levels of disturbance (Anderson and Ruffner 1987).

The loss of annual spring-summer floods deprived the riverbanks of moisture and freshly deposited sediment, and caused the soils in the high zone to accumulate salts (Stromberg et al. 2007, Kennedy and Ralston 2011). The seedlings of many native tree species require moist soil to survive. For these species, which include western honey mesquite (Prosopis glandulosa), seep willow (Baccharis salicifolia), poplar (Populus fremontii) and willows (Salix spp.), the elimination of spring-summer floods compressed the zone of successful recruitment to a narrow strip close to the river because seedlings could not survive the drier, saltier conditions in the un-wetted higher riverbank zones (Stromberg et al. 2007, Hamill 2009). This drying of high riverbank zones gives an advantage to plants that can tolerate drier, saltier substrate, such as arrowweed ( sericea) and invasive tamarisk (T. ramosissima) (Vandersande et al. 2001, Glenn and Nagler 2005).

While spring and early summer peak flows were dramatically reduced, base flows were increased, causing overall homogenization of flows on a seasonal timescale. This relatively constant flow throughout the year provides a stable water source, further increasing vegetation density close to the water (Stromberg et al. 2007).

The dam dramatically decreased seasonal variability in flow but simultaneously increased intra-daily variability. In the first 20 years of dam operation, the Colorado saw daily swings in flow up to 20,000 cfs for power generation purposes (see Burley, this volume). Since 1996, the dam has been operated according to a Mean Low Fluctuating Flow (MLFF) regime, which limits rapid fluctuations in flow (Hamill 2009). MLFF regulations have provided a relatively constant water source to riparian plants, which is thought to have contributed to an increase in herbaceous wetland species and woody riparian vegetation (both native and non-native) near the shoreline (Hamill 2009).

Glen Canyon dam dramatically reduces the sediment load of the Colorado River. This has led to an increase in net erosion rates from the canyon, reducing substrate availability for riparian plants (see Gibson, this volume). Net erosion of fine sediments leaves behind coarser

6 sediments, which hold water and nutrients less well, disfavoring mesic (moisture-loving) plants (Stromberg et al. 2007).

EFFECTS OF HIGH FLOW EXPERIMENTS ON NATIVE RIPARIAN PLANTS To mitigate some of the negative effects of Glen Canyon Dam on the Colorado River ecosystem, the Department of the Interior has conducted three High Flow Experiments (HFEs) in March 1996, November 2004, and March 2008 (see Burley, this volume). The three HFEs to date have been smaller in magnitude and earlier or later than historic spring-summer peak flows. They have restored some, but not all of the function of historic flows with respect to riparian vegetation.

Because the HFEs are smaller in magnitude than historic scouring floods (32,000-45,000 cfs as compared to historic flows of >100,000 cfs) they have not removed much woody vegetation close to the shoreline (e.g. willows, poplars, tamarisk). HFEs did successfully clear some herbaceous wetland plants from the shoreline, but this reduction in plant cover only lasted a year (Kennedy and Ralston 2011).

The timing of HFEs has been earlier or later than that of historic spring-summer floods. Reproduction of many native riparian trees tightly corresponds to historic peak flow timing, so the HFEs have not benefitted recruitment of native trees as much as natural flows. Interestingly, tamarisk (Tamarix ramosissima) has a longer reproductive window than many native trees, so it is likely to benefit more from HFEs than the native trees (see Eskra, this volume).

REFERENCES

Anderson, L.S. and G.A. Ruffner. 1987. Effects of Post-Glen Canyon Dam Flow Regime on the Old High Water Line Plant Community along the Colorado River in Grand Canyon (Report number GCES/22/87). National Park Service, Grand Canyon National Park. Grand Canyon, .

Glenn, E. and P. Nagler. 2005. Comparative ecophysiology of Tamarix ramosissima and native trees in western U.S. riparian zones. Journal of Arid Environments 61:419-446.

Hamill, J. 2009. Status and trends of resources below Glen Canyon Dam update - 2009. USGS Fact Sheet 2009-3033.

Karrenberg, S., P. Edwards and J. Kollmann. 2002. The life history of Salicaceae living in the active zone of floodplains. Freshwater Biology 47:733-748.

Kennedy, T. and B. Ralston. 2011. Biological responses to high-flow experiments at Glen Canyon Dam. Pages 93-125 in T. Melis, editor. Effects of three high-flow experiments on the Colorado River ecosystem downstream from Glen Canyon Dam, Arizona. U.S. Geological Survey, Circular 1366.

Lytle, D. and L. Poff. 2004. Adaptation to natural flow regimes. TRENDS in Ecology and Evolution 19(2):94-100.

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Schmit, L. and J. Schmidt. 2011. Introduction and overview. Pages 1-15 in T. Melis, editor. Effects of three high-flow experiments on the Colorado River ecosystem downstream from Glen Canyon Dam, Arizona. U.S. Geological Survey, Circular 1366.

Vandersande, M., E. Glenn and J. Walworth. 2001. Tolerance of five riparian plants from the lower Colorado River to salinity drought an inundation. Journal of Arid Environments 49:147-159.

Webb, R., T. Melis and R. Valdez. 2002. Observations of environmental change in Grand Canyon, Arizona. U.S. Geological Survey Water-Resources Investigations Report 02-4080.

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