A Long‐Term Comparison of Hydrology and Plant Community Composition in Constructed Versus Naturally Occurring Vernal Pools

Successful restoration of ephemeral wetlands worldwide is particularly challenging, given the often‐precise relationship between hydrological features and plant community dynamics. Using a long‐term experiment in vernal pool restoration, we compare hydrological and vegetative characteristics of constructed pools with those of adjacent, naturally occurring reference pools. Although constructed and reference pools were similar in maximum water depth and duration of inundation at the beginning of our experiment in 2000, constructed pools were shallower and inundated for shorter periods by 2009. Native vernal pool species were able to establish populations in many constructed pools, and seeding sped their establishment. Comparing seeded plots in constructed pools with unseeded plots in reference pools, we found no significant difference in the cover of seeded species, native species, or exotic species in most years. In recent years, however, native species have declined in both constructed and reference pools. Finally, the cover of native vernal pool species was positively and non‐linearly associated with both water depth and seeding treatment. We conclude that the establishment of appropriate hydrological conditions was necessary, but not sufficient to promote successful performance of vernal pool species in constructed pools. Constructed pools with hydrologic conditions similar to those of reference pools were more likely to support populations of native vernal pool plant species, but only seeded pools were similar to reference pools in abundance of native cover. Most importantly, hydrological conditions in experimental pools have worsened since their construction, which may hamper persistence of native species in this restoration effort.     

Hydroperiods of created and natural vernal pools in central Ohio: A comparison of depth and duration of inundation

Water levels were recorded weekly from six natural vernal pools and 10 created vernal pools at two forested wetland complexes in central Ohio. Vernal pool median water depth and duration of inundation were significantly greater at the created vernal pools than at the natural vernal pools (α = 0.05, P < 0.05). The average period of inundation for created pools was 309 ± 32 days, compared with 250 ± 16 days for natural pools. The created pools produced a range of inundation times, from 163 to 365 days in length, with three pools permanently inundated.     

Hydrology,Physiochemistry, and Amphibians in Natural and Created Vernal Pool Wetlands

This study compared the hydrology, physiochemistry, and amphibian biomass between a complex of created vernal pools and a complex of natural vernal pools in 2007 in central Ohio, United States. Hydrologic connectivity of surface water and groundwater differed between the natural and the created pool complexes. Surface inundation duration for created pools exceeded that of natural pools, although spring water depths were similar. Dissolved oxygen (p= 0.05) and hourly temperature (p= 0.00) were 1.2% and 1.1% higher, respectively, in the created pools, and conductivity was 1.5% higher (p= 0.00) in the natural pools. Amphibian dip net results found no significant difference in biomass between natural and created pools or family (hylid, ranid, and ambystomatid) biomass in both pool types. Amphibian families were evenly represented by both capture methods in the created wetlands; however, the distribution of families was not even in natural pools and the proportion of ranids was four times greater for samples obtained by funnel traps than dip netting. Eleven years after construction, the created vernal pools did not mimic natural pools in surface inundation and groundwater–surface water exchange, dissolved oxygen, and water temperature. The created pools are perched wetlands and are never likely to mimic reference pool hydrology. Dissolved oxygen and temperature differences are likely due to the separation of surface water and groundwater in the created pools. However, the created pools exhibited a higher taxa diversity than the natural pools due to a more even distribution of organisms between the three families.     

Changes in vernal pool edaphic settings through mitigation at the project and landscape scale

Vernal pool mitigation is a highly controversial process that has been frequently criticized for its inability to adequately replicate the ecosystem functions of the original intact wetlands. We analyzed past mitigation practices in two rapidly growing counties in California's Great Central Valley to determine if mitigation procedures are re-arranging the vernal pool landscape by substituting more common or less ecologically significant pool types (as defined by soil type and geomorphology) for rarer or ecologically richer pool types. Results indicate that most development projects impacting vernal pools conduct at least a portion of their mitigation requirements at a site with similar edaphic settings. However, when examined at a landscape-scale across all development projects, the more common edaphic settings such as Northern Hardpan and Low Terrace pools are increasing while more rare types such as Northern Claypan and Volcanic Mudflow pools are decreasing. Results also show that Drainageway pools, a less-specialized pool type with generally lower species richness, are becoming more common through mitigation. These results are confirmed by an analysis of landscape diversity, which showed that overall landscape diversity was lower at mitigation sites than at project sites. Despite these results, the ecological significance of vernal pool mitigation practices remains unclear for several reasons. The lack of maps showing exact locations of vernal pools at project sites make it difficult to precisely determine vernal pool acreage and distribution among edaphic settings. Additionally, more research is needed to determine precise relationships between edaphic settings and species distributions and the effects of mitigation area management practices on species distribution and persistence.     

The effects of an unpredictable precipitation regime on vernal pool hydrology

1. Vernal pools are small precipitation‐fed temporary wetlands once common in California. They are known for their numerous narrowly endemic plant and animal species, many of which are endangered. These pools experience the typical wet season/dry season regime of Mediterranean climates. Their hydrological characteristics are determined by a complex interaction between the highly variable climate and topographic relief. 2. Hypotheses regarding the effects on ponding of total precipitation, storm intensity and pattern were examined using long‐term weather records combined with two decades of data on the length and depth of inundation in 10 individual pools. Similarly, data on pool landscape position and microtopography allowed examination of the interactions between topography and rainfall amount and pattern. 3. The total amount of precipitation and length of inundation were strongly correlated. Landscape position affected ponding duration, with collector pools holding water longer than headwater pools. Basin microtopography interacted with climatic variability to determine the nature and extent of within‐basin microhabitats sufficiently different in hydrological and/or soil conditions to support or exclude individual species. The effect on hydroperiod of precipitation concentrated in a few months rather than spread more evenly over the season depended on total precipitation. 4. Changes in climate, the mound‐and‐depression landscape or pool microtopography could have profound impacts on the hydrology of individual pools as well as the array of hydrological conditions in the system. Given the individualistic responses of the numerous endemic species supported by vernal pools, any of these environmental changes could diminish their sustainability and increase the risk of species extinction. Conservation, restoration and management decisions should take these factors into account.     

The effects of pH change and NO(-) (3) pulse on microbial community structure and function: a vernal pool microcosm study

Forest vernal pools experience strong environmental fluctuations, such as changes in water chemistry, which are often correlated with changes in microbial community structure. However, very little is known about the extent to which these community changes influence ecosystem processes in vernal pools. This study utilized experimental vernal pool microcosms to simulate persistent pH alteration and a pulse input of nitrate (NO3 -), which are common perturbations to temperate vernal pool ecosystems. pH was manipulated at the onset and microbial respiration was monitored throughout the study (122?days). On day 29, NO3 - was added and denitrification rate was measured and bacterial, fungal, and denitrifier communities were profiled on day 30 and day 31. Microbial respiration and both bacterial and fungal community structure were altered by the pH treatment, demonstrating both structural and functional microbial responses. The NO3 - pulse increased denitrification rate without associated changes in community structure, suggesting that microbial communities responded functionally without structural shifts. The functioning of natural vernal pools, which experience both persistent and short-term environmental change, may thus depend on the type and duration of the change or disturbance.     

Loss of biodiversity and hydrologic function in seasonal wetlands persists over 10 years of livestock grazing removal

Ecological restoration provides a means to increase biodiversity in ecosystems degraded by natural and human‐induced changes. In some systems, disturbances such as grazing can be key factors in the successful restoration of biodiversity and ecological function, but few studies have addressed this experimentally, especially over long time periods and at landscape scales. In this study, we excluded livestock grazing from plots within a grassland landscape containing vernal pools in the Central Valley of California for 10 years and compared vernal pool hydrology and plant community composition with areas grazed under an historic regime. In all 10 years, the relative cover of native plant species remained between 5 and 20% higher in the grazed versus ungrazed plots. This effect was particularly prominent on the pool edges, though evidence of invasion into the pool basins was evident later in the study. Native species richness was lower in the ungrazed plots with 10–20% fewer native species found in ungrazed versus grazed plots in all years except the first year of treatment. Ungrazed pools held water for a shorter period of time than pools grazed under an historic regime. By the ninth year of the study, ungrazed pools took up to 2 weeks longer to fill and dried down 1–2 weeks sooner at the end of the rainy season compared to grazed pools. The results of this study confirm that livestock grazing plays a key role in maintaining biodiversity and ecosystem function in vernal pools.     

Livestock grazing affects vernal pool specialists more than habitat generalists in montane vernal pools

Questions

Do livestock grazing and seasonal precipitation structure species composition in montane vernal pools? Which grazing and precipitation variables best predict cover of vernal pool specialists and species with broader habitat requirements? Is vernal pool species diversity correlated with livestock exclosure, and at what spatial scales?

Location

Montane vernal pools, northeast California, USA.

Methods

Vegetation was sampled in 20 vernal pools, including pools where livestock had been excluded for up to 20 years We compared plant species composition, functional group composition and species diversity among sites that varied in grazing history and seasonal precipitation using CCA and LMM.

Results

Although vernal pool specialists were dominant in montane vernal pools, over a third of plant cover was comprised of species that occur over a broad range of wetland or upland environments. The species composition of vernal pool plant communities was influenced by both livestock grazing and precipitation patterns, however the relative effects of these environmental variables differed by functional group. Livestock exclosures favoured perennial vernal pool specialists over annual vernal pool specialists. In contrast, the cover of habitat generalists was more strongly influenced by seasonal precipitation than livestock grazing. At small spatial scales, species richness and diversity decreased as the number of years a pool had been fenced increased, but this relationship was not significant at a larger spatial scale.

Conclusions

Both livestock grazing and seasonal precipitation structure the montane vernal pool plant community. We found that livestock grazing promotes the cover of annual vernal pool specialists, but at the expense of perennial vernal pool specialists. Wetter vernal pools, however, support higher cover of wetland generalist species regardless of whether pools are grazed.     

Inundation effects on small-scale plant distributions in San Diego, California vernal pools

California vernal pools were used to develop and test hypotheses related to species response patterns and distributions along environmental moisture and elevation gradients. Data from three rainfall years were examined for patterns of plant distribution in relation to length and frequency of inundation and relative elevation. Field distributions on elevation and inundation duration gradients were characteristic of each species, with species distributions overlapping along the gradient. Plants with limited inundation tolerance (`Non-Pool' species) responded to wet years by shifting their distributions toward the dry end of the moisture gradient and in the dry year toward the wetter end. These species have most of their distribution outside of pools. Species with limited tolerance to the gradient extremes had their peak frequency in intermediate positions (`Edge' species). `Pool' species had substantial inundation tolerance. Several were near the limits of inundation tolerance in the deepest study pools, but others withstood longer inundation. `Pool' species had static distributions on the elevation gradient, growing in whatever conditions prevailed in a particular year. A number of annual `Pool' species had their highest frequency in the intermediate, moderately wet year. The dry year had a strong negative effect on the frequency of a few `Pool' annuals, but established perennials persisted regardless. In general, results of the regression analyses can be understood given the other information available on pool hydrology and vegetation. They do not produce any obvious hypotheses to test. For a given species, variables have different predictive value, depending on the year.     

The germination process in vernal pools: sensitivity to environmental conditions and effects on community structure

Variation in timing and amount of rainfall in California has been noted to cause strong year-to-year differences in the composition of vernal pool communities. We explored the effects of possible variation of the “rainy season” in a seedbank germination study conducted with three factors: monthly timing of first soil moistening, length of moist period before inundation, and length of inundation. Monthly timing was the most important factor in determining the number of plants and number of species that germinated in the pots. Many species, mostly generalist wetland and pool-endemic species, showed sharp decreases in number in later trials. Five species did not germinate in the final trial, and two of these were abundant only in the first trial. Inundation was required for the germination of two species. Treatments with a combination of inundated and moist conditions resulted in higher numbers of species and numbers of plants compared to treatments with only moistness or only inundation. Our results indicate that vernal pool species have mechanisms that keep them from emerging under unfavorable conditions. The sensitivity of vernal pool species to out-of-season germination suggests that they could be negatively affected by climatic change, with generalist species like non-native Lythrum hyssopifolium having a potential advantage. Received: 15 July 1996 / Accepted: 14 July 1997     

Quantifying the Hydroregime of a Temporary Pool Habitat: A Modelling Approach for Ephemeral Rock Pools in SE Botswana

Ecological and evolutionary processes in temporary rock pools operate within constraints imposed by their hydrologic regimes. These shallow pools flood when seasonal rains accumulate on impermeable substrates. Despite the ecological importance of hydrologic conditions for these ecosystems, we typically lack tools and empirical data required to understand the implications of hydrologic variability and climate change for biotic populations and communities in these habitats. In this study, we developed a hydrologic model to simulate rock pool hydrologic regimes based on rainfall, evapotranspiration, and basin geometry. The model was used to investigate long-term patterns of seasonal and inter-annual variation in hydroregime. In addition, hydrologic conditions associated with potential climate change scenarios were simulated and evaluated with respect to the biological requirements of the anostracan Branchipodopsis wolfi. The model’s output for daily inundation matched with field observations with an overall accuracy of 85% and correctly estimated complete hydroperiods with an overall accuracy of 70%. Simulations indicate large variation in individual hydroperiods (76–115%) as well as in the number of hydroperiods per year (19–23%). Furthermore, this study suggests that climate change may significantly alter the rock pool hydroregime. These findings confirm the hydrologic sensitivity of these ephemeral habitats to precipitation patterns, and their potential sensitivity to future climate change. Modelling indicates that the suitability of average inundation conditions for B. wolfi deteriorates significantly under future climate predictions. High levels of spatial and temporal variation in hydrologic conditions are dominant features of these habitats and an essential consideration for understanding population and community-level ecological processes.     

Invasive plant feedbacks promote alternative states in California vernal pools

Invasive species have the potential to create positive feedbacks and push an ecosystem into an alternative state through a variety of mechanisms. Unless the drivers behind these feedbacks are understood, restoring a system to a more desirable state may not be possible. We used a long‐term vernal pool restoration project based out of Travis Airforce Base, Fairfield, CA, U.S.A. to examine natural pools dominated by either invasive or native plant communities, and restored pools predominately composed of invasive plants. We determined that plant community structure is drastically altered towards invasive grasses with the addition of a single centimeter of litter. In the absence of this litter layer, community structure was driven by a non‐native forb rather than native species. We also found that native plant‐dominated vernal pools have a longer inundation duration and are deeper compared to invasive‐dominated pools, regardless of construction status. These results suggest that once invasive grasses establish through lower inundation depths, their litter deposition can initiate a positive feedback to maintain an invasive alternative state. However, even after litter removal, non‐native forbs can replace the grasses causing a second alternative state still separate from the most desirable native dominated state. This study directly demonstrates that invasive species, and their positive feedbacks, may limit the success of ecological restoration. To effectively restore a system all constraints must be identified and removed before successful restoration can occur.     

Projected Shifts in Coffea arabica Suitability among Major Global Producing Regions Due to Climate Change

Regional studies have shown that climate change will affect climatic suitability for Arabica coffee (Coffea arabica) within current regions of production. Increases in temperature and changes in precipitation patterns will decrease yield, reduce quality and increase pest and disease pressure. This is the first global study on the impact of climate change on suitability to grow Arabica coffee. We modeled the global distribution of Arabica coffee under changes in climatic suitability by 2050s as projected by 21 global circulation models. The results suggest decreased areas suitable for Arabica coffee in Mesoamerica at lower altitudes. In South America close to the equator higher elevations could benefit, but higher latitudes lose suitability. Coffee regions in Ethiopia and Kenya are projected to become more suitable but those in India and Vietnam to become less suitable. Globally, we predict decreases in climatic suitability at lower altitudes and high latitudes, which may shift production among the major regions that produce Arabica coffee.     

Cattle waste reduces plant diversity in vernal pool mesocosms

In California, much of the remaining vernal pool habitat is used for cattle grazing. Some studies suggest that grazing helps promote native plant diversity on grasslands, but the impact of grazing on plants that reside in pool basins is largely unknown. We investigated how one aspect of cattle grazing, the deposition of waste, affects these plant species by adding dung and urine to mesocosms lined with vernal pool soil. As a result of dung input, orthophosphate, conductivity, and turbidity increased in our mesocosms while dissolved oxygen decreased. Such changes in water quality are consistent with a shift toward a eutrophic state. Algal biomass and percent-cover also increased in dung-treated mesocosms. When the mesocosms dried, vascular plant species richness and percent-cover in dung-treated mesocosms were reduced by up to 54% and 87%, respectively. We attribute this to light attenuation by algal mats that flourished in the nutrient-enriched water. We also found that dung input caused significant, but weak, shifts in the composition of the vascular plant community. We conclude that cattle grazing may be detrimental to plant communities in vernal pools via increased nutrient loading, which promotes algal growth. Any beneficial effects of grazing may thus be limited to the surrounding grassland. Studies that examine the regional-scale impacts of grazing on vernal pool grasslands should separately consider the impacts to local-scale (i.e., within-pool) plant diversity, as most of the threatened and endangered plant species of California vernal pools reside primarily in pool basins.     

The role of demography,intra‐species variation,and species distribution models in species' projections under climate change

Organisms are projected to shift their distribution ranges under climate change. The typical way to assess range shifts is by species distribution models (SDMs), which predict species’ responses to climate based solely on projected climatic suitability. However, life history traits can impact species’ responses to shifting habitat suitability. Additionally, it remains unclear if differences in vital rates across populations within a species can offset or exacerbate the effects of predicted changes in climatic suitability on population viability. In order to obtain a fuller understanding of the response of one species to projected climatic changes, we coupled demographic processes with predicted changes in suitable habitat for the monocarpic thistle Carlina vulgaris across northern Europe. We first developed a life history model with species‐specific average fecundity and survival rates and linked it to a SDM that predicted changes in habitat suitability through time with changes in climatic variables. We then varied the demographic parameters based upon observed vital rates of local populations from a translocation experiment. Despite the fact that the SDM alone predicted C. vulgaris to be a climate ‘winner’ overall, coupling the model with changes in demography and small‐scale habitat suitability resulted in a matrix of stable, declining, and increasing patches. For populations predicted to experience declines or increases in abundance due to changes in habitat suitability, altered fecundity and survival rates can reverse projected population trends.     

Multiscale resistant kernel surfaces derived from inferred gene flow: An application with vernal pool breeding salamanders

The importance of assessing spatial data at multiple scales when modelling species–environment relationships has been highlighted by several empirical studies. However, no landscape genetics studies have optimized landscape resistance surfaces by evaluating relevant spatial predictors at multiple spatial scales. Here, we model multiscale/layer landscape resistance surfaces to estimate resistance to inferred gene flow for two vernal pool breeding salamander species, spotted (Ambystoma maculatum) and marbled (A. opacum) salamanders. Multiscale resistance surface models outperformed spatial layers modelled at their original spatial scale. A resistance surface with forest land cover at a 500‐m Gaussian kernel bandwidth and normalized vegetation index at a 100‐m Gaussian kernel bandwidth was the top optimized resistance surface for A. maculatum, while a resistance surface with traffic rate and topographic curvature, both at a 500‐m Gaussian kernel bandwidth, was the top optimized resistance surface for A. opacum. Species‐specific resistant kernels were fit at all vernal pools in our study area with the optimized multiscale/layer resistance surface controlling kernel spread. Vernal pools were then evaluated and scored based on surrounding upland habitat (local score) and connectivity with other vernal pools on the landscape, with resistant kernels driving vernal pool connectivity scores. As expected, vernal pools that scored highest were in areas within forested habitats and with high vernal pool densities and low species‐specific landscape resistance. Our findings highlight the success of using a novel analytical approach in a multiscale framework with applications beyond vernal pool amphibian conservation.     

Comparing amphibian habitat quality and functional success among natural,restored, and created vernal pools

The fact that several vernal pool restoration and creation attempts in eastern Pennsylvania and New Jersey have been paired with conservation of natural pools in the same area provided a valuable research opportunity to compare amphibian habitat quality between project sites and natural reference pools. To measure desired outcomes, we used successful reproduction and metamorphosis of two vernal pool indicator species, the wood frog and spotted salamander. Although many previous studies indicate that restored and created pools rarely replace function lost in the destruction of natural pools, success of vernal pool indicator species was not necessarily related to pool type in this study. Results indicate a strong correlation between reproductive success for both species and vernal pool size (i.e. mean depth and volume), regardless of pool type. Although overall survival rates of wood frog larvae were significantly higher in natural pools with hydroperiods between 12 and 35 weeks, wood frogs were also successful in one restored and one created vernal pool. Salamander survival rates were highest in two natural and two created pools, which had in common both greater volumes and higher proportions of forest land cover in the surrounding 1,000 m. The documented success of vernal pool indicator species in two well‐established created pools demonstrates that pool creation can sometimes restore communities and ecological functions lost, especially when nearby natural pools are degraded or destroyed.     

Conservation of rare species with island-like distributions: A case study of Lasthenia conjugens (Asteraceae) using population genetic structure and the distribution of rare markers

Californian vernal pools, a patchy, island-like habitat, are endangered as a result of habitat destruction. Conservation of the remaining vernal pool habitat is essential for the persistence of several endangered species. We present the first study examining DNA-level genetic diversity within and among populations of a vernal pool plant species. We investigated genetic variation across eight populations of the US federally endangered vernal pool endemic Lasthenia conjugens (Asteraceae) using intersimple sequence repeat (ISSR) markers. Genetic diversity within the species was high (Nei's gene diversity estimate was 0.37), with moderate differentiation among populations (Bayesian F _ST analog of 0.124). Using an amova analysis, we found that the majority of the genetic variation (84%) was distributed within populations. There is a significant relationship between geographical distance and pairwise genetic differentiation as measured by the Bayesian estimate θ^B. The alternative hypotheses of historic geological processes within the Central Valley and contemporary gene flow are discussed as explanations of the data. Because of the vulnerability of the populations, we calculated a probability of loss for rare alleles (fragments) in the populations. Calculations show that sampling only one of the eight populations for ex-situ conservation or restoration will capture approximately 54% of the sampled rare fragments. We believe that one of the sampled populations has become extinct since it was sampled. When removing this population from the above-mentioned calculations, sampling one population will capture only 41.3% of the sampled rare fragments. We recommend sampling strategies for future conservation and restoration efforts of L. conjugens.     

Biogeochemical Hotspots in Forested Landscapes: The Role of Vernal Pools in Denitrification and Organic Matter Processing

Quantifying spatial and temporal heterogeneity in ecosystem processes presents a challenge for conserving ecosystem function across landscapes. In particular, many ecosystems contain small features that play larger roles in ecosystem processes than their size would indicate; thus, they may represent “hotspots” of activity relative to their surroundings. Biogeochemical hotspots are characterized as small features within a landscape that show comparatively high chemical reaction rates. In northeastern forests in North America, vernal pools are abundant, small features that typically fill in spring with snow melt and precipitation and dry by the end of summer. Ephemeral flooding alters soil moisture and the depth of the soil’s oxic/anoxic boundary, which may affect biogeochemical processes. We studied the effects of vernal pools on leaf-litter decomposition rates, soil enzyme activity, and denitrification in vernal pools to assess whether they function as biogeochemical hotspots. Our results indicate that seasonal inundation enhanced leaf-litter decomposition, denitrification, and enzyme activity in vernal pools relative to adjacent forest sites. Leaves in seasonally flooded areas decomposed faster than leaves in terra firme forest sites. Flooding also influenced the C, N, and P stoichiometry of decomposing leaf litter and explained the variance in microbial extracellular enzyme activity for phosphatase, β-d-glucosidase, and β-N-acetylglucosaminidase. Additionally, denitrification rates were enhanced by seasonal flooding across all of the study pools. Collectively, these data suggest that vernal pool ecosystems may function as hotspots of leaf-litter decomposition and denitrification and play a significant role in decomposition and nutrient dynamics relative to their size.