3 Hydrology and Landscape Connectivity of Vernal Pools

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Hydrology and 3 Landscape Connectivity of Vernal Pools

Scott G. Leibowitz and Robert T. Brooks

CONTENTS

Deﬁnitions...... 32 Vernal Pool Hydrology...... 33 Hydrologic Budget ...... 33 Precipitation ...... 33 Groundwater...... 33 Surface Water ...... 35 Evapotranspiration...... 36 Basin and Catchment Morphology...... 37 Hydrologic Dynamics...... 38 Hydrologic Connectivity ...... 39 Population Dynamics and Landscape Connectivity...... 42 Metapopulation Theory ...... 42 Connectivity and Dispersal...... 44 WetlandÐTerrestrial Connectivity ...... 46 Conservation Implications ...... 47 Hydrologic Impacts ...... 47 Timber Harvesting...... 47 Land Development ...... 48 Climate Change...... 49 Loss of Landscape Connectivity ...... 49 Summary...... 50 Acknowledgments...... 50 References...... 51

Hydrology is fundamental to wetland establishment and maintenance of wetland processes (Cole et al. 2002). Hydrology has been shown to affect, if not control,

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many aspects of wetland ecology, including litter decomposition and the accumula- tion of organic matter and sediment (Bärlocher et al. 1978), the composition and productivity of pool fauna (Paton and Couch 2002), and amphibian diversity (Burne and Griffin 2005). Vernal pools are a type of wetland that normally experiences drawdowns and dry periods. Tiner (2003) notes that although alternating wet and dry periods occur in most wetlands, vernal pools experience extreme fluctuations in moisture conditions. This hydrologic variability, both within- and between-year, is a primary factor influencing species composition and productivity (Paton and Couch 2002). Vernal pool organisms have special life history strategies for completing their life cycle under fluctuating wet and dry conditions (Chapter 6, Colburn et al., Colburn 2004). Despite these strategies, a pool species can go locally extinct if hydrologic conditions are sufficiently harsh. In spite of this, the biotic diversity of vernal pools can be maintained over time if local extinctions are offset by recolonizations from surrounding sites. This can occur through passive or active dispersal of organisms between pools, either over the ground or through the air. Recolonization may also occur, to a more limited extent, through surface-water connections. Landscape connectivity between vernal pools can be a major factor that influences recolonization of unoccupied pools. This chapter examines the hydrology and landscape connectivity of vernal pools of glaciated northeastern North America. We begin by defining vernal pools and addressing their relationship to isolated wetlands. We then review the hydrology of northeastern pools, and population dynamics and landscape connectivity. The chapter concludes with a discussion of conservation implications.

DEFINITIONS Northeastern vernal pools are temporary to semipermanent bodies of water occurring in shallow depressions that typically fill during the spring or fall and may dry during the summer or in drought years. These systems are bounded on the drier end by ephemeral pools that do not normally contain standing water, and on the wetter end by semipermanent ponds that only rarely dry (see Preface, Calhoun and deMaynadier). This book focuses on vernal pools associated with forests in the glaciated northeast of North America. Distinguishing features of these pools include fluctuating hydrologic conditions, presence of seasonally standing water, and occurrence within the glaciated forest biome (see Figure 1, Preface). Vernal pools often occur as depressional wetlands surrounded by uplands. These pools conform to Tiner’s (2003: 495) definition of a geographically isolated wetland: “wetlands that are completely surrounded by upland at the local scale.” However, vernal pools are often embedded within larger wetlands, and they can also occur in floodplains (Preface, Calhoun and deMaynadier, and Chapter 2, Rheinhardt and Hollands). For example, Colburn (2004) reports that 7% of 48 vernal pools in the glaciated northeast were found in flood plain settings. Although they are often included as a type of isolated wetland (Tiner 2003), northeastern vernal pools do not have geographic isolation as a distinguishing feature.

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VERNAL POOL HYDROLOGY The hydrology of vernal pools is determined by patterns in precipitation and temperature, connections to ground- and surface-water resources, losses from evapotranspiration, and the physical and biotic features (including plant community composition and structure) of pool basins and catchments. Factors related to weather are external to the pools and vary with time; physical site characteristics are intrinsic to each pool, are essentially ﬁxed, and vary spatially among pools. The hydrology of vernal pools is characterized by both hydroregime, which is the temporal pattern of inundation, drying, and water-level change, and hydroperiod, or the duration of inundation (a component of hydroregime). The physical attributes of pools determine the general length of pool hydroperiod, whereas annual patterns in precipitation and temperature-driven evapotranspiration determine the year-to- year variation in hydroregime and hydroperiod. In the following, we discuss the hydrologic budget of vernal pools, some of the physical characteristics of pools and their basins that affect pool hydrology, and their hydrologic dynamics and connectivity. As there are few published studies on the hydrology of northeastern vernal pools, we also draw on information from other ephemeral, ponded wetland systems.

HYDROLOGIC BUDGET The hydrology of vernal pools can be characterized by a simpliﬁed climate–water balance equation where the change in the amount of water in a pool is equal to the sum of inputs from precipitation, groundwater, and surface water, minus loss from evapotranspiration (Lide et al. 1995; Figure 3.1); note that groundwater or surface water inputs can be negative, representing net losses. Studies have shown that wetlands in different hydrogeologic and climatic settings vary considerably in the inﬂuence of these factors on the change in storage (Winter et al. 2001).

Precipitation Precipitation is, almost by deﬁnition, a major source of water input to many types of wetlands such as vernal pools. Precipitation can enter a vernal pool directly at the pool surface or indirectly as surface runoff from the adjacent catchment during a rainfall event. In a 10-year study of geographically isolated pools in central Massachusetts, Brooks (2004) showed that amounts of weekly precipitation accounted for more than half the variation in the change in weekly pool depths. The importance of precipitation as a major water source has also been reported for some California vernal pools (Zedler 1987; Pyke 2004), Carolina bays (Lide et al. 1995), cypress pond–pine ﬂatwood ecosystems (Mansell et al. 2000), Mississippi forest pools (Bonner et al. 1997), and prairie potholes (Hayashi et al. 1998).

Groundwater GroundwaterÐsurface water interactions occur in nearly all freshwater systems, including wetlands and vernal pools (Winter and LaBaugh 2003). The interactions

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34 Science and Conservation of Vernal Pools in Northeastern North America

Precipitation Transpiration

Evaporation SW runoff SW spillage Water table

GW discharge GW recharge Local flow system

Intermediate flow system

Regional flow system Confining layer

FIGURE 3.1 Schematic diagram illustrating elements of a vernal pool water budget. Inputs can include precipitation, ground water discharge, and surface-water runoff. Ground water can originate from local, intermediate, or regional ﬂow systems. Outputs can include evaporation, transpiration, ground water discharge, and intermittent surface-water spillage. Vernal pools in riverine settings can also have stream input and output (not shown). (Adapted from Sando, S.K. (1996). South Dakota wetland resources. In Fretwell, J.D., Williams, J.S., and Redman, P.J. [compilers] National Water Summary on Wetland Resources. U.S. Geological Survey, Reston, VA. Water-Supply Paper 2425, pp. 351Ð356.)

are affected by the positions of the pools relative to groundwater level or flow and the geologic, climatic, and edaphic (soil-related) settings of the pools (Chapter 3, Rheinhardt and Hollands). Groundwater flow can occur at scales ranging from regional to local (Winter and LaBaugh 2003; Figure 3.1). On a regional scale, topographically high locations function as recharge areas and topographic lows are discharge areas. Local groundwater flow, relative elevations, and site-specific hydrologic processes also need to be considered and may have greater influence (Rains et al. 2006). Changes in direction of groundwater flow, from recharge (outflow of pool water to groundwater) to discharge (inflow of groundwater to the pool), are determined by the relative elevations of a pool and local groundwater. These directional changes are mostly climate driven, as long-term weather patterns control groundwater levels. The hydraulic conductivity (rate of water movement through the soil) of pool basins and catchments depends on soil permeability and controls the exchange of pool water and groundwater (Winter and LaBaugh 2003). These authors suggest that a hydraulic conductivity of 0.3 m dÐ1 (1.0 ft dÐ1) separates permeable from nonper- meable soils. Vernal pools often occur because local soils have relatively low hydraulic conductivity or because topographic depressions have filled with impervious

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sediments. Gay (1998) found the till soils adjacent to vernal pools in central Mas- sachusetts had hydraulic conductivities between 1.5 × 10Ð3 and 6.4 × 10Ð6 m dÐ1 (4.9 × 10Ð3 and 2.1 × 10Ð5 ft dÐ1), clearly indicating they occurred on low-permeability materials. Pools can also occur on highly permeable soils where the water table is high. On the outer Cape Cod, vernal pools occur on fine to coarse grain glacial outwash sediments with horizontal hydraulic conductivities of 100 m dÐ1 (328 ft dÐ1) (Sobczak et al. 2003). Water levels in these pools match regional groundwater levels during periods of high groundwater levels. During periods of low groundwater levels, pool levels are perched above the ground water with exchange restricted by relatively impervious peat mats. At these times, pool water levels are largely affected by frequency and intensity of precipitation events. Where it has been investigated, groundwater exchange appears to occur mostly at the pool margins, resulting in short groundwater flow paths (Phillips and Shedlock 1993). Loss of pool water to adjacent catchment groundwater seems to be driven mainly by transpiration from plants along the margins of the pools (Hayashi et al. 1998). In prairie potholes, lateral infiltration to shallow groundwater, driven by transpiration from adjacent catchment vegetation, was estimated to be up to 70% of total pool water loss in the summer months (Parsons et al. 2004). Pool water specific conductance measures that are in excess of precipitation conductance values indicate the contribution of groundwater, which has a greater mineral concentration than precipitation or surface runoff. Palik et al. (2001) suggested that conductance measurements above 100 µS cmÐ1 indicate groundwater contributions for seasonal pools in Minnesota forests. For 65 vernal pools in Rhode Island, pool water conductance measurements ranged between 19 and 376 µS cmÐ1, with a mean of 64 µS cmÐ1 (Skidds and Golet 2005).

Surface Water Surface water can enter a pool through runoff or surface water inlets and exit through surface water outlets. There is no published quantification of surface water inputs to, or outflow from, geographically isolated vernal pools, and groundwater is assumed to be a more significant water source (Brooks 2005). However, surface runoff from the adjacent catchment should play a significant role during snow melt or rainfall events when soils are saturated (Chapter 2, Rheinhardt and Hollands, Colburn 2004). Small depressional wetlands that have low storage capacity, such as some vernal pools, are vulnerable to filling at these times, which can cause surface- water overflow or spillage. Whether spillage will enter neighboring water bodies depends on the volume of outflow, the distance to the neighboring surface water, the permeability of the soils over which the flow travels, and the elevational differ- ence between the water bodies (Leibowitz and Vining 2003; Winter and LaBaugh 2003). Low-volume surface outflows will generally infiltrate into the ground before reaching neighboring waters. However, intermittent connections between isolated wetlands and other waters have been reported (Leibowitz 2003; Leibowitz and Vining 2003). In addition, vernal pools that occur within riverine settings are not strictly isolated and can be connected by surface waters from ephemeral or intermittent streams or be inundated by flooding of larger streams and rivers. Thus, surface-water

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input from other vernal pools or other aquatic systems may occur during episodic events in these settings.

Evapotranspiration Evapotranspiration includes evaporation of pool surface water and transpiration from pool and adjacent catchment vegetation, especially forest trees. Evapotranspiration is the principal pathway for water loss from geographically isolated pools (Brooks 2004, 2005). Many pools experience a drop in water after trees leaf out in the spring. Evapotranspiration has also been reported as the major source of water loss for other isolated, ponded wetlands including Carolina bays (Lide et al. 1995), cypress ponds (Mansell et al. 2000), prairie potholes (Hayashi et al. 1998), and California vernal pools (Pyke 2004). Evapotranspiration is principally a temperature-driven process. Annual temperature patterns are less variable in the Northeast than annual precipitation patterns. Brooks (2004) showed that weekly water level change was significantly related to potential evapotranspiration (PEt) but that the effect of precipitation was 2Ð5 times greater. PEt peaks in the late spring and summer months when forest trees are in full foliage. PEt losses typically exceed precipitation inputs from mid-June through mid-September (Figure 3.2). This period of water deficit coincides with the period of maximum vernal pool drying. Pools dried earlier in those years with larger cumulative water deficits, especially when early spring groundwater resources were below long-term means and late winter snowpack was reduced or absent.

Ppt and PEt (cm) Pool depth (cm) 16 60 14 50 12 40 10 8 30 6 20 4 10 2 0 0 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Month PptPEt 503 1710

FIGURE 3.2 Thirty-year average (1971Ð2000) precipitation (Ppt) and potential evapotranspiration (PEt) for the Quabbin Reservoir watershed, central Massachusetts, and eight-year average monthly surface-water depths for two vernal pools (503, 1710) on the Quabbin.

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BASIN AND CATCHMENT MORPHOLOGY The effects of precipitation inputs and evapotranspiration losses on vernal pool hydrology are dependent on the pool’s basin and catchment characteristics. Basin volume also determines storage capacity. In addition, the soil characteristics of the basin and catchment control permeability, which affects groundwater dynamics, groundwater storage, and surface runoff. Thus, basin and catchment characteristics are fundamental factors in the hydrologic budget of a vernal pool. Surprisingly little is known about the morphology of vernal pool basins and catchments, or their relationships to pool hydrology. What information is available has been reviewed by Brooks and Hayashi (2002) and Brooks (2005). Surface area is the most commonly reported morphological attribute of pool basins, probably as it is most easily determined (Brooks 2005). Limited empirical data show that 66% of 430 Massachusetts pools (Brooks et al. 1998) and about 75% of 304 Maine pools (Calhoun et al. 2003) are less than 500 m2 (0.12 ac) in maximum surface area. Longer hydroperiods generally occur in pools that exceed 1000 m2 (0.25 ac) (Brooks and Hayashi 2002). Pool size, either surface area or volume, affects the duration of standing water. Smaller or shallower pools have a larger perimeter- to-area (or volume) ratio, and higher rates of water loss (relative to maximum pool volume). This is caused by either transpiration from forest trees along the perimeter or groundwater leakage (Millar 1973). For larger pools, water loss relative to available volume is so low that these pools may be permanent. Measures of basin volume, area, and depth are positively correlated (Brooks and Hayashi 2002). Larger pools typically have greater maximum depths, but this relationship is also dependent upon the basin profile. For pools of equal surface area, concave basins will have greater depths. Lastly, maximum pool volume is a mathematical function of maximum pool surface area, depth, and basin profile (Brooks and Hayashi 2002). We are unaware of any published studies on the extent and characteristics of vernal pool catchments and their effects on pool hydrology. However, an unpublished thesis by Gay (1998) concluded that differences in the hydrologic systems of two pairs of vernal pools were attributable to differences in bedrock beneath pool catchments, catchment morphologies, and transmissivities of catchment soils. The pair of pools with the longer hydroperiods occurred in catchments over bedrocks com- posed of gabbro, which weathers more easily than the gneiss underlying the shorter hydroperiod pools. The longer hydroperiod pools were located in bowl-shaped catchments, whereas the shorter hydroperiod pools were located on relatively flat terrain. Finally, hydraulic conductivities were one to two orders of magnitude lower in the shorter hydroperiod pools. Gay (1998) felt that these characteristics collec- tively resulted in greater surface-water/groundwater connections in the catchments of the longer hydroperiod pools. The shorter hydroperiod pools, with weaker groundwater connections, were more direct expressions of precipitation. The area that actually contributes surface water or groundwater to a pool may be different than the catchment area defined strictly by topography. Two studies that attempted to quantify the spatial extent of catchments of cypress ponds (Mansell et al. 2000) and karst ponds (O’Driscoll and Parizek 2003) determined that the area

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of potential surface-water and groundwater contributions to the pools was much smaller (i.e., an order of magnitude) than the catchment areas defined by surficial topography. Conversely, groundwater that originates outside a catchment may enter a pool if intermediate or regional groundwater flow discharges to the pool (Winter and LaBaugh 2003; Figure 3.1).

HYDROLOGIC DYNAMICS

Hydroregime and hydroperiod vary among vernal pools and, over time, within any single pool. These important characteristics affect pool structure and habitat availability, which strongly influence the composition and reproductive success of pool biota. For example, vernal pool-breeding species are adapted to fluctuating water levels, whereas annual drying precludes organisms that require permanent standing water. Effects of hydrologic dynamics on pool biota are discussed by Colburn et al. (Chapter 6) and Semlitsch and Skelly (Chapter 7). Despite its importance, there has been little research on the hydrologic variability of vernal pools. The hydroperiods of ephemeral forest pools occur over a temporal gradient, from highly ephemeral rainwater pools to semipermanent water regimes (Colburn 2004). Historically, there has been very limited information on the occurrence and distribution of pools by hydroperiod. On a year-to-year basis, Brooks (2004) found that pool hydroperiods were shorter and pools dried earlier in years with less rainfall and larger cumulative water deficits. Hydroperiod can be correlated with pool depth and volume, but it is only moderately to poorly correlated with pool area (Brooks and Hayashi 2002; Skidds and Golet 2005). The hydroregime of vernal pools is best described by a repeating, annual pattern of inundation and drying. In central Massachusetts, this pattern is best captured by a hydrologic year that starts in October and ends the following September (Brooks 2004). Geographically isolated pools in this area are typically dry or at minimum depths by the first of October, fill partially with late fall rains after leaf fall, fill to capacity with spring rains and snow melt, and then dry through late spring and summer after full forest canopy development (Figure 3.2). This hydroregime pattern can differ dramatically for any single pool in any one year, e.g., a pool can be dry or full to overflowing in almost any month (Table 3.1). This pattern can also differ for pools in other locations and in other hydrogeomorphic settings. Colburn (2004) has proposed a hydrologic classification for vernal pools that considers the timing of annual drying and filling. Short-cycle, spring-filling pools are usually dry or mostly dry during the winter. They then fill and reach their maximum depth and volume in the spring due to snow melt and spring rains. These pools quickly shrink in size as inputs decline and evapotranspiration increases, drying by late June or early July. Short-cycle, fall-filling pools behave similarly, except they fill in late fall or early to mid winter. Short-cycle, spring-filling pools are typically flooded for three to four months per year, compared with seven to nine months for short-cycle, fall-filling pools. Long-cycle pools, in contrast, usually dry in mid to late summer or early fall. These pools are also divided into spring or fall filling. Long-cycle, spring-filling pools typically have water for five to eight months,

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TABLE 3.1 Thirteen-Year (1993–2005) Average Monthly Mean, Standard Deviation, Minimum, and Maximum Water Depths for Vernal Pool PR243, Quabbin Watershed, Central Massachusetts

Average Number of Monthly Standard Weekly Month Mean Deviation Minimum Maximum Observations

October 4.8 (1.9) 12.0 (4.7) 0 (0) 51 (20) 48 November 14.3 (5.6) 18.6 (7.3) 0 (0) 51 (20) 43 December 30.4 (12.0) 21.3 (8.4) 0 (0) 51 (20) 31 January 34.0 (13.4) 21.6 (8.5) 0 (0) 51 (20) 5a February 22.5 (8.9) 31.8 (12.5) 0 (0) 45 (18) 2a March 39.2 (15.4) 13.7 (5.4) 16 (6) 51 (20) 9a April 46.2 (18.2) 6.7 (2.6) 26 (10) 51 (20) 32 May 40.6 (16.0) 11.2 (4.4) 3 (1) 51 (20) 52 June 23.9 (9.4) 17.2 (6.8) 0 (0) 51 (20) 50 July 7.0 (2.8) 14.8 (5.8) 0 (0) 50 (20) 50 August 5.2 (2.0) 13.1 (5.2) 0 (0) 50 (20) 49 September 1.8 (0.7) 6.6 (2.6) 0 (0) 33 (13) 49

Note: Depths in cm (inches). Maximum depth before surface overﬂow is 51 cm (20 in.) for this pool.

a Pool is frozen during the winter season; accessibility is limited and few visits were made to the pool.

Source: Brooks (unpublished data).

compared to nine to eleven months for long-cycle, fall-filling pools. Finally, semipermanent vernal pools contain water throughout the year, generally remaining flooded for several years at a time. Water levels in these pools also reach a maximum during the spring and then decrease during the summer. These five classes represent a continuum of increasing hydroperiod (Colburn 2004); however, hydroperiod also varies between years within classes. Given the lack of field data on hydrologic variability, a classification system such as Colburn’s may be useful for categorizing pools according to their hydrologic variability and for inferring possible effects on pool organisms.

HYDROLOGIC CONNECTIVITY A vernal pool can have hydrologic connections to other pools and water bodies through groundwater flow. In addition to local groundwater flows, this may also include connections between distant pools through regional flow systems (Winter and LaBaugh 2003; Figure 3.1). It is also possible that vernal pools can have permanent surface- water connections to other bodies of water, as pools can occur in larger wetlands that have perennial inlets (Garret Hollands, personal communication). Permanent

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FIGURE 3.3 Examples of intermittent surface-water connections between vernal pools (dotted lines denote intermittent connections). (a) Geographically isolated pools connected through a swale zone by snow melt and spring runoff. (b) Depressional pools occurring within a larger, forested wetland merged during high water events. (c) Riverine pools within the floodplain of a permanent stream connected during floodplain inundation. (d) Riverine pools occurring as a headwater complex are connected when the intermittent or ephemeral stream is flowing.

surface-water connections would be expected to occur in a relatively small number of pools. However, a larger number of northeastern vernal pools may be connected by surface water intermittently. Such intermittent connections have been reported for California vernal pools (Zedler 1987), prairie potholes (Leibowitz and Vining 2003), and other wetland types (e.g., Snodgrass et al. 1996; Babbitt and Tanner 2000). Intermittent surface-water connections could occur between vernal pools during wetter conditions in various ways. For example: geographically isolated vernal pools could be connected to each other through swale zones during snow melt and spring runoff (Figure 3.3a); individual pools within a larger forested wetland can merge if conditions are wet enough to cause standing water in the larger wetland (Figure 3.3b); riverine vernal pools occurring in floodplains of permanent streams can connect to each other and to the stream during floodplain inundation (Figure 3.3c); and riverine vernal pools that occur as a headwater complex connect to each other during times of intermittent or ephemeral stream flow (Figure 3.3d). Intermittent surface-water connections should occur at a hierarchy of scales (Figure 3.4). There is also a temporal hierarchy of intermittent connections, though

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FIGURE 3.4 Spatial hierarchy of hydrologic interactions (dotted lines denote intermittent surface-water connections). (a) Micropools connect annually as water fills the entire pool. (b) Pools within a geographically isolated wetland connect during flood events (annual or less frequent) when the larger wetland fills. (c) Geographically isolated wetlands containing pools can connect during extreme flood events when the wetland spills over into an adjacent isolated wetland. Some hydrologic interactions may not fit in this hierarchy, e.g., connections between riverine pools or between geographically isolated pools.

this need not match the spatial pattern. For example, micropools, riverine pools in a 1-year floodplain, and vernal pools connected through swale zones will usually have annual intermittent connections, though the micropool connections should have the longest duration. At the other extreme, geographically isolated pools in areas with large relief and riverine pools in 100-year floodplains will rarely have intermittent surface-water connections. Thus, vernal pools occur within an isolation- connectivity continuum over time and space (Leibowitz and Vining 2003). Groundwater connections can serve as an important mechanism for transport- ing soluble materials between vernal pools and other waters. Surface-water connections can transport not only soluble compounds but also insoluble materials such as sediment, organisms, and reproductive propagules. While there have been some studies of groundwater interactions between vernal pools, surface-water interactions have received almost no attention. Thus, we are not able to say at this time how widespread or significant these connections are. Characterizing the frequency distribution of these surface-water connections (Leibowitz and Vining 2003) could promote a better understanding of the ecological functioning of these systems.

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POPULATION DYNAMICS AND LANDSCAPE CONNECTIVITY Vernal pools support a diversity of aquatic organisms. These organisms must survive in a stressful environment that experiences regular drying. Survival is made possible by a number of life-history strategies that allow organisms to successfully grow and reproduce in these systems (Colburn 2004). Eggs of pool-breeding organisms must hatch and develop while water is present, and populations must be able to withstand or avoid drought. Yet the timing and duration of water availability is highly variable, both spatially (among pools) and temporally (among years). Thus, in spite of adaptive life-history strategies, populations of a particular species may go locally extinct in pools that experience particularly harsh conditions. Classical ecological theory dealt with complete extirpation of a species over its entire range. More recently, metapopulation theory has been proposed to explain the dynamics of local extinctions and recolonizations over a group of individual sites (Hanski 1999). In the absence of ﬁeld studies on these dynamics, metapopulation theory can provide an explanation of how a species might persist in the face of local extinctions, and the factors that could inﬂuence their persistence (Chapter 8, Gibbs and Reed).

METAPOPULATION THEORY Levins (1970) introduced the term metapopulation to refer to a population of local populations. His theory examines the long-term viability of a species in a landscape where various factors affect its ability to persist. Metapopulation dynamics are increasingly recognized as playing an important role in the long-term sustainability of certain wetland species (Gibbs 1993; Lehtinen et al. 1999). Metapopulation dynamics are a function of extinctions of local populations within distinct sites, due to random environmental and demographic variation, and recolonization from neighboring sites (Hanski 1999). Levins (1970) was able to demonstrate that the metapopulation could be maintained if the rate of recolonizations is greater than the rate of extinctions. According to this perspective, it is the metapopulation that can persist over time; local populations wink on and off in response to local extinction and recolonization events. If recolonizations do not offset local extinctions, the proportion of occupied sites will decrease over time, and the metapopulation will eventually go extinct. A species’ extinction rate is related to local population dynamics, community structure, and genetic change (Figure 3.5). For vernal pools, extreme hydrologic variability would be expected to have a major inﬂuence on the extinction rate. This could include direct effects on the population, such as increased mortality from desiccation and reduced reproduction because of delayed ﬂooding, as well as indirect effects through lowered habitat quality. The recolonization rate of a species depends on the rate of emigration from occupied sites, which is a function of local population dynamics (for example, movement due to overcrowding) and behavior, e.g., emigration in response to poor conditions (Hanksi 1999). The recolonization rate also depends on the probability

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Hydrology and Landscape Connectivity of Vernal Pools 43 colo- vins, R. xtinction and re xtinction rate. A species’ behavior affects the distance affects behavior A species’ xtinction rate. . American Mathematical Society, Providence, RI. pp. 75Ð107. Providence, American Mathematical Society, . ) is a function of the environment’s spatial pattern. (Adapted from Le ) is a function of the environment’s D Some Mathematical Questions in Biology ), whereas the distribution of distances between sites ( ), whereas the distribution d Factors inﬂuencing metapopulation dynamics. The equilibrium number of occupied sites is a balance between the e inﬂuencing metapopulation dynamics. Factors With permission.) With nization rates. Metapopulations can survive only if the recolonization rate is greater or equal to the e nization rates. Metapopulations can survive it can disperse ( (1970). Extinction. In Gerstenhaber, M. (Ed.) (1970). Extinction. In Gerstenhaber, FIGURE 3.5 FIGURE

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that these emigrants will successfully recolonize an empty site (Figure 3.5). This is affected by the emigrants’ ability to arrive at an unoccupied site and, once there, to become successfully established. In the original Levins (1970) model, extinction is random with an average rate that is constant across all sites. This would cause local populations to wink on and off independently of each other. However, local extinctions in vernal pools are likely to be clustered in time during harsh hydrologic conditions. In addition, some pools will have greater hydrologic variability than others and therefore experience higher rates of extinction. For example, pools that are shallow and totally dependent on precipitation would experience more frequent than average dry conditions. This would represent marginal habitat for a species requiring longer and more predictable wet periods, and so local extinctions would be relatively common. In contrast, deeper pools that receive groundwater would be less responsive to variations in precipitation, and would have a lower than average extinction rate for that species. These pools would remain wet during mild droughts, when other pools were dry. At such times the wet sites could function as source pools (Hanksi 1999), producing emigrants that could recolonize pools with marginal habitat (sinks) that experienced more frequent extinctions. However, pools may shift from being sources to sinks and vice versa, due to year-to-year variability within and between pools. This suggests that long-term maintenance of metapopulations may require a variety of pools having different hydrologic regimes to sustain local populations and serve as source areas under different conditions (Elizabeth Colburn, Harvard University, personal communication).

CONNECTIVITY AND DISPERSAL We noted in the previous section that recolonization is dependent on the emigrants’ ability to arrive at an unoccupied site. The probability that an emigrant will successfully get to an unoccupied site depends on the dispersal distance of the species (a function of behavior) and the distribution of distances between sites, which is a function of the spatial pattern of the environment (Figure 3.5). For a given species with a ﬁxed dispersal distance, the level of connectivity is higher in landscape settings with smaller distances between pools (Figure 3.6). Distances between sites are a function of pool density, which has been reported in the literature to range between 0Ð13.5 pools kmÐ2 (0Ð35 pools miÐ2) over the glaciated northeastern United States (Chapter 3 in Colburn 2004). This means that regions with higher pool densities should have greater recolonization rates and, consequently, sustain more species. Similarly, connectivity in a given landscape is greater for species with larger dispersal distances. For example, wood frogs are highly mobile, with ﬁrst-time breeders able to disperse 1.1Ð1.2 km (0.68Ð0.75 mi) (Colburn 2004). Thus, these frogs should be able to readily move between pools. Colburn (2004), deMaynadier and Houlahan (Chapter 13), and Semlitsch and Skelly (Chapter 7) include dispersal distances for some vernal pool organisms. There are many different mechanisms by which species disperse between vernal pools. Dispersal can be active or passive, can happen during different life stages, and can occur over land or through the air. For example, adult frogs and salamanders

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(a)(b) (c)

7000 6000 5000 4000 3000 2000 Connections (#) 1000 0 605 4 3 2 1 Pool Density (# km–1) (d)

FIGURE 3.6 Effect of reduced pool density on landscape connectivity. Pools were randomly generated in a 100 km2 (38.6 mi2) landscape and then randomly removed to simulate the effects of pool conversion. Initial pool density is based on a glacial collapse valley in Cape Cod, MA, and connections are for a species having a ﬁxed dispersal distance of 1.15 km (0.7 mi), e.g., wood frog (Colburn 2004). (a) Density of 5.8 pools kmÐ2 (15.0 pools miÐ2). (b) Density of 3 pools kmÐ2 (7.8 pools miÐ2). (c) Density of 1 pool kmÐ2 (2.6 pools miÐ2). (d) Plot showing number of connections vs. pool density.

actively travel between pools over land, whereas birds and certain insects disperse through flight. For many other species, reproductive propagules or resting stages are passively dispersed between sites. As an example, plant seeds can be transported by wind, and fingernail clams and fairy shrimp can be carried on the feet or feathers of birds (Chapter 6, Colburn et al.). Although it has not received much attention, surface-water connections between wetlands — both permanent and intermittent — may also serve as corridors for movement (Leibowitz 2003). Dispersal of seeds by water (hydrochory) has been observed in southeastern swamp forests (Schneider and Sharitz 1988). Small-fruited spike-rush (Eleocharis microcarpa Torrey var. filiculmis), which can be abundant in vernal pools south of New England, may disperse through hydrochory (Hickler 2004). Thus, it is possible that certain animals, plant parts, and reproductive propagules disperse between northeastern vernal pools through surface-water connections. As few vernal pool animals have adaptations to disperse through flowing water (Elizabeth Colburn, personal communication), surface water is probably not a major dispersal mechanism. However, surface water could be important for particular

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taxa (e.g., crayfish) and for certain types of pools, such as those occurring in floodplains. Given these different modes of dispersal, interpreting distances between pools as straight line distances — as we implicitly did above — can be problematic. For example, an organism might require forest cover, moist habitat, or a hedgerow for movement. In this case, the effective distance between sites would need to reflect the length of the actual travel path — which would take into account the presence of corridors and barriers — rather than straight-line distance (Wiens 1997). The use of simple, straight-line distances can also be inappropriate for aquatic organisms and reproductive propagules that are carried between sites through permanent or intermittent surface-water connections. Another complication in this case is that downstream flow — either through swale zones (Figure 3.3a) or intermittent or ephemeral streams (Figure 3.3d) — adds directionality; e.g., passively transported organisms and propagules cannot travel upstream and actively transported organisms can move greater distances and expend less energy in the downstream direction. This would not be the case with bi-directional connections that occur when water surfaces merge (Figure 3.3b) or floodplains fill (Figure 3.3c). The temporal pattern of intermittent surface-water connections, both frequency and duration, must also be considered. Timing could be the more critical factor for organisms that disperse in this manner. For example, two riverine pools located 1 km (0.6 mi) apart within a 1-year floodplain would have greater hydrologic connectivity than two geographically isolated pools that are 0.1 km (0.06 mi) apart but, due to high relief, only merge during 10-year storm events.

WETLAND–TERRESTRIAL CONNECTIVITY The Levins metapopulation model conceptualizes the landscape as consisting of sites, or “pockets of suitable habitat” (Levins 1970: 80), embedded within surrounding unsuitable habitat. Up until this point, we have interpreted “site” as an individual vernal pool. This would be the case for obligate wetland species that live their entire lives within the confines of the pool basin. Such permanent residents include fingernail clams, nematodes, flatworms, fairy shrimp, spreadwing damselflies, and clam shrimp (Chapter 6, Colburn et al.). These organisms have different life-history strategies that allow them to feed and breed during wet conditions and survive dry periods. Permanent residents would also include plants adapted to vernal pool hydrology. Many other organisms, however, are migrants that use the pool for only a portion of their life. This includes nonbreeding migrants (e.g., some species of predaceous diving beetles, turtles, snakes, birds, and mammals) that feed but do not breed in vernal pools (Chapter 6, Colburn et al.; and Chapter 9, Mitchell et al.). Nonbreeding migrants can include both wetland obligate and facultative wetland (not requiring wetlands to survive) species. In contrast, migratory breeders use the vernal pool basins to breed (this may take place during wet or dry conditions, depending on the species) but then leave as the pool dries out. These species are pool-dependent, they breed most successfully in seasonal pools. Migratory breeders include mole

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salamanders (Ambystoma spp.), wood frogs, and certain dragonflies and caddisflies (Chapter 6, Colburn et al.; and Chapter 7, Semlitsch and Skelly). Migrants that are wetland obligates require vernal pools to complete their life cycle. However, they are only obligates during a particular life stage. These “stage- specific” wetland obligates also require core terrestrial habitat (Chapter 7, Semlitsch and Skelly). Gibbons (2003) argued eloquently that the strong focus on jurisdictional delineation and presence of hydric conditions for defining wetlands has caused wetland scientists to overlook how critical terrestrial habitat is for many wetland species. Thus, the term “site” needs to take on a different meaning for these migratory species; it does not represent a single, homogeneous habitat type, but rather a mixture of core habitats, including vernal pools, that fulfill the particular species’ life history requirements. The arrangement of these habitats is also important; e.g., amphibian migratory breeders require vernal pools and terrestrial forests to be located within their maximum adult migratory travel distance (Regosin et al. 2005; Baldwin et al. 2006). This means that the concept of landscape connectivity must be expanded to include not only movement between pools but also movement between vernal pools and other required habitat (Chapter 7, Semlitsch and Skelly; Chapter 8, Gibbs and Reed; and Chapter 12, Windmiller and Calhoun).

CONSERVATION IMPLICATIONS

HYDROLOGIC IMPACTS Vernal pool organisms have numerous life history strategies for surviving, including different life stages for wet and dry conditions. These stages are tightly linked to the timing of flooding and drawdown. Although flooding and drawdown exhibit significant temporal variability from year-to-year, any impacts that have a long-term effect on the frequency, duration, magnitude, or variability of flooding will inevitably alter community composition. We discuss several potential impacts below. These range in scale from impacts to individual pools to large-scale effects throughout the entire glaciated northeast.

Timber Harvesting The hydrologic effects of timber harvesting on forest pools are determined by the impacts of the harvesting on the sensitive components of the hydrologic budget, namely runoff, evapotranspiration, and soil permeability. The effects of timber harvesting and associated reductions in evapotranspiration on isolated ponds have been studied for various types of forest in the southeastern United States. This research has generally revealed elevated groundwater levels and an increase in runoff to ponds, resulting in longer hydroperiods (Sun et al. 2000). However, these effects are very ephemeral and become insigniﬁcant by the tenth year following harvesting, due to rapid regeneration and growth of other forest vegetation (Sun et al. 2000). Although relatively short-lived, these effects could still be signiﬁcant for species requiring short hydroperiods if source populations are unavailable for recolonization.

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In contrast to the effects observed by Sun et al. (2000), no relationship was found in Minnesota between time since harvesting and forest pool hydroperiod (Palik et al. 2001). However, these researchers observed that the youngest stand in their study had been harvested seven years prior, so that hydrology may have already returned to preharvest conditions. Batzer et al. (2000) also were unable to find an effect of timber harvesting on pond hydrology, concluding that natural annual variation in hydroperiods overwhelmed any timber harvesting effect. Although informative, studies in other areas may not reflect the impact of timber harvesting on northeastern vernal pools. Based on a conceptual understanding of pool hydrology, any impacts that do occur should be greater in dry climates with low potential evapotranspiration than in wetter areas, and greater for geographically isolated pools vs. pools associated with other waters. Impacts should also be directly related to the intensity of the management. Recommended modifications of timber management practices for protecting vernal pools and their biota are discussed by deMaynadier and Houlahan (Chapter 13).

Land Development Urbanization and development for associated land uses is a significant threat to vernal pools in many areas of the glaciated Northeast (McKinney 2002). Urbaniza- tion can cause hydrologic impacts through cumulative watershed modifications that alter hydrology (Richter and Azous 1995). Windmiller and Calhoun (Chapter 12) discuss best management practices for minimizing the impact of development on vernal pools. Wetland hydrology in urban areas is usually highly altered in amounts, sources, and flow rates. Urbanization can increase the area of impervious surfaces (e.g., buildings, roads, parking lots) and alter natural flow pathways (e.g., construction of storm drainage systems). These changes can be expected to alter the hydrology of urban wetlands so that they will have more frequent, rapid, and large (“flashy”) changes in water level and have a lower frequency of flooded and/or saturated conditions (Ehrenfeld et al. 2003). In a study of 21 wetlands in northeastern New Jersey, including four depressional wetlands, flashiness was significantly different among disturbance classes; sites with high levels of disturbance had larger fluctuations (Ehrenfeld et al. 2003). However, Rubbo and Kiesecker (2005) found that the modified hydrologic regimes of urbanized wetlands in central Pennsylvania tended to result in longer hydroperiods and a tendency towards permanence of standing water. Other land use changes can also impact vernal pool hydrology. Permanent clearing of forests for field crops or pastures could alter evapotranspiration rates and cause surface-water temperatures to rise (due to reduced shading). Farm machinery could also alter soil permeability. The impact of this change in land use can be regional in scope: according to Colburn (2004), up to 85% of most of southern and central New England and much of the Canadian Maritime Provinces were converted from forest to fields and pastures by the mid-1800s. However, much of this land subsequently reverted to forest cover.

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Climate Change The dominant effect of weather patterns on pool hydrology means that these systems will be affected by climate change (Sun et al. 2002). Furthermore, vernal pools and their biota are likely to be affected by climate change early on, due to their relative hydrologic isolation and location at the land/water interface (Root and Schneider 2002). Climate change predictions of more episodic precipitation and increased evapotranspiration would cause pools to dry earlier in the year and remain dry longer (Brooks 2004). In addition, climate change could increase the frequency of extreme rainfall events and cause more frequent and longer interevent droughts. This could increase the frequency of the drying and reﬁlling cycle, compared with the slow, extended drying that now occurs (Brooks 2004). Increasing magnitude and variability of temperature could alter quantities and timing of snow melt. This could poten- tially affect many species, especially early spring migrants that deposit eggs around the time of snow melt. Finally, any of these hydrologic changes could affect stream ﬂow, thereby impacting riverine pools.

LOSS OF LANDSCAPE CONNECTIVITY According to metapopulation theory (Levins 1970), the distribution of pool species across a region represents a dynamic equilibrium between factors that cause local extinctions and those that affect recolonization. Any impacts that increase local extinctions or reduce recolonization will therefore decrease species distributions. One such impact is the conversion of pools to different land uses, which can cause the local extinctions of all the species dependent on those pools. Conversion can also indirectly affect populations in surrounding pools by lowering recolonization rates. This might occur because the converted pools are no longer able to serve as sources of emigrants. Conversion can also reduce recolonization because the cumulative loss of pools increases the average distance between remaining sites (Figure 3.6). Reduced landscape connectivity can be caused by other impacts besides direct conversion. Impacts that decrease pool ﬂooding, such as climate change, would lower landscape connectivity by making intermittent surface-water connections less frequent or shorter in duration. In addition, roads in upland areas can act as barriers to dispersal by increasing mortality or changing behavior. Pool-dependent species can also be affected by impacts that alter the spatial relationship between pools and other core habitat areas. For example, deforestation can cause the direct loss of a semiaquatic pool species that also requires forested habitat if the distance between the pool and remaining forest exceeds its maximum migratory travel distance. Vernal pool conservation has historically focused on protecting pool habitat. Yet because pool species depend on landscape connectivity for long-term persistence, conservation programs must also preserve connectivity. In addition, conservation efforts should include the other core habitat types needed by some pool organisms. A landscape perspective that considers connectivity and supplemental habitat is necessary to conserve the rich biota associated with these seasonally ﬂooded pools. 3675_C003.fm Page 50 Tuesday, June 12, 2007 7:39 AM

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SUMMARY Vernal pools are shaped by hydrologic processes which influence many aspects of pool function. The hydrologic budget of a pool can be summarized by a water- balance equation that relates changes in the amount of water in the pool to precipitation, ground- and surface-water flows, and evapotranspiration. For many vernal pools, precipitation and evapotranspiration are the major determinants of the water cycle. However, groundwater can also be significant in specific settings and at particular times. Surface-water flows can be important for certain vernal pools, e.g., those in riverine settings. Basin and catchment characteristics influence the relative role of surface water and groundwater. A limited number of vernal pools may have permanent surface-water connections to other waters. Intermittent surface-water connections may also occur during episodic events, either annually or less frequently. It is suggested that these intermittent surface-water connections result in a spatial hierarchy of hydrologic interactions. There is also a temporal hierarchy of intermittent connections, though this need not match the spatial pattern. Thus, vernal pools occur within an isolation-connectivity continuum over time and space. Theory suggests that the persistence of a species across vernal pools represents a balance between factors that cause local extinctions and those that allow for unoccupied areas to be recolonized. Landscape connectivity makes it possible for species to disperse between vernal pools and recolonize pools that are unoccupied due to local extinctions. Connectivity is greater for species with larger dispersal distances and in landscape settings with greater pool densities. In addition to connections between pools, migratory species also require landscape connectivity between pools and other core habitat areas, such as forests. Any impacts to vernal pools that have a long-term effect on the frequency, duration, magnitude, or variability of flooding will inevitably alter community composition. These impacts, which range in scale from changes to individual pools to large-scale effects throughout the entire glaciated Northeast, include timber harvesting, land development, and climate change. A landscape perspective that considers connectivity and supplemental habitat is necessary to conserve the rich biota associated with these seasonally flooded pools.

ACKNOWLEDGMENTS Special thanks to E. Colburn, R. McKinney, P. Zedler, and an anonymous reviewer for their thoughtful comments on this manuscript. We also received useful sugges- tions from A. Calhoun, G. Hollands, and R. Rheinhardt. Thanks to D. White for helpful discussions on metapopulations and dispersal. The information in this document has been funded by the U.S. Environmental Protection Agency (EPA) and the U.S. Forest Service. This document has been subjected to EPA’s peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. 3675_C003.fm Page 51 Tuesday, June 12, 2007 7:39 AM

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