Beth A. Lawrence

Formation of tussocks by sedges: effects of hydroperiod and nutrients

Tussock formation is a global phenomenon that enhances microtopography and increases biodiversity by adding structure to ecological communities, but little is known about tussock development in relation to environmental factors. To further efforts to restore wetland microtopography and associated functions, we investigated Carex stricta tussock size in relation to elevation (a proxy for water depth) at a range of sites in southern Wisconsin, USA, and tested the effect of five hydroperiods and N+P addition (15 g N/m2 + 0.37 g P/m2) on tussock formation during a three-year mesocosm experiment. Wet meadows dominated by C. stricta averaged 4.9 tussocks/m2, with a mean volume of 1160 cm3 and height of 15 cm. Within sites, taller tussocks occurred at lower elevations, suggesting a structural adaptation to anoxic conditions. In our mesocosm experiment, C. stricta accelerated tussock formation when inundated, and it increased overall productivity with N + P addition. Within two growing seasons, continuous inundation (+18 cm) in the mesocosms led to tussocks that were nearly as tall as in our field survey (mean height in mesocosms, 10 +/- 1.3 cm; maximum, 17 cm). Plants grown with constant low water (-18 cm) only formed short mounds (mean height = 2 +/- 0.4 cm). After three growing seasons, the volume of the largest tussocks (3274 +/- 376 cm3, grown with +18 cm water depth and N + P addition) was 12 times that of the smallest (275 +/- 38 cm3, grown with -18 cm water depth and no N + P). Though tussock composition varied among hydroperiods, tussocks were predominantly organic (74-94% of dry mass) and composed of leaf bases (46-59%), fine roots (10-31%), and duff (5-13%). Only the plants subjected to high water levels produced the vertically oriented rhizomes and ascending shoot bases that were prevalent in field-collected tussocks. Under continuous or periodic inundation, tussocks achieved similar heights and accumulated similar levels of organic matter (range: 163-394 g C/m2), and we conclude that these hydroperiods can accelerate tussock formation. Thus, C. stricta has high utility for restoring wetland microtopography and associated functions, including carbon accumulation.

DIRECT AND INDIRECT EFFECTS OF HOST PLANTS: IMPLICATIONS FOR REINTRODUCTION OF AN ENDANGERED HEMIPARASITIC PLANT (CASTILLEJA LEVISECTA)

ABSTRACT Rare, parasitic plants pose an interesting challenge to restoration practitioners. In addition to the problems associated with small population size, rare parasites may also be limited by their host requirements. We examined how the performance of a rare Pacific Northwest hemiparasite, Castilleja levisecta, was affected by the availability of different host combinations in the greenhouse and in the field. Castilleja levisecta individuals were grown with two individuals of the grass Festuca roemeri, two individuals of the aster Eriophyllum lanatum, one individual of each of these species (a “mixed” treatment), or without any host. We did not find support for the complimentary diet hypothesis, which predicts that parasites grown with multiple host species perform better than individuals grown alone or with a single host. In the greenhouse, C. levisecta individuals grown in the mixed treatment had greater stem growth than those planted with F. roemeri, but did not differ from E. lanatum or no-host treatments. In the field, vole activity had indirect effects on C. levisecta survival mediated through host species: vole tunneling and C. levisecta mortality were strongly associated with host treatments including E. lanatum. Vole tunneling and C. levisecta mortality were strongly associated with host treatments including E. lanatum. Field survival of no-host and F. roemeri treatments were significantly higher than those grown with E. lanatum. Our results emphasize the importance of basing conservation decisions on experimental research conducted under conditions similar to those of the intended application, as greenhouse results were a poor predictor of field performance. For restoration of endangered hemiparasitic plants, we recommend planting with hosts that are not attractive to herbivores.

Herbicide management of invasive cattail (Typha × glauca) increases porewater nutrient concentrations

Invasive wetland plants are the primary targets of wetland management to promote native communities and wildlife habitat, but little is known about how commonly implemented restoration techniques influence nutrient cycling. We tested how experimental mowing, herbicide application, and biomass harvest (i.e., removal of aboveground biomass) treatments of Typha-invaded mesocosms altered porewater nutrient (NO3−, NH4+, PO4−3) concentration and supply rate, vegetation response, and light penetration to the soil surface. We found that while herbicide application eliminated the target species, it also reduced native plant density and biomass, as well as increased porewater nutrient concentration (PO4−3, NO3−) and supply rates (N, P, K) up to a year after treatments were implemented. Because herbicide application promotes nutrient enrichment, it may increase the likelihood of reinvasion by problematic wetland invaders, as well as cause eutrophication and deleterious algal blooms in adjacent aquatic systems. Our data suggest that biomass harvest should be considered by managers aiming to reduce Typha abundance without eradicating native diversity, avoid nutrient leaching, as well as possibly utilizing biomass for bioenergy.