Christa P. H. Mulder

Putting plant resistance traits on the map: a test of the idea that plants are better defended at lower latitudes

• It has long been believed that plant species from the tropics have higher levels of traits associated with resistance to herbivores than do species from higher latitudes. A meta-analysis recently showed that the published literature does not support this theory. However, the idea has never been tested using data gathered with consistent methods from a wide range of latitudes. • We quantified the relationship between latitude and a broad range of chemical and physical traits across 301 species from 75 sites world-wide. • Six putative resistance traits, including tannins, the concentration of lipids (an indicator of oils, waxes and resins), and leaf toughness were greater in high-latitude species. Six traits, including cyanide production and the presence of spines, were unrelated to latitude. Only ash content (an indicator of inorganic substances such as calcium oxalates and phytoliths) and the properties of species with delayed greening were higher in the tropics. • Our results do not support the hypothesis that tropical plants have higher levels of resistance traits than do plants from higher latitudes. If anything, plants have higher resistance toward the poles. The greater resistance traits of high-latitude species might be explained by the greater cost of losing a given amount of leaf tissue in low-productivity environments.

EFFECTS OF HERBIVORY ON ARROWGRASS: INTERACTIONS BETWEEN GEESE, NEIGHBORING PLANTS, AND ABIOTIC FACTORS

Herbivores may affect plants by removing biomass, altering competitive interactions, and altering the abiotic environment. Changes in the size and quality of forage species and in species composition as a result of herbivory, in turn, affect future herbivory. We investigated the direct and indirect effects of herbivory by Brant Geese (Branta bernicla nigricans) on Triglochin palustris (arrowgrass) in a subarctic salt marsh in southwestern Alaska. In the first experiment, we compared arrowgrass in exclosed plots, unexclosed plots with feces removed, and control plots. In the second experiment, we used a full-factorial design to examine the effects of clipping arrowgrass, clipping neighboring plants, depositing goose feces, and their interactions on arrowgrass size and biomass allocation. In the third experiment, we placed hand-reared goslings on premanipulated plots from the second experiment to examine the effects of arrowgrass size, density, and species composition on the probability of an individual arrowgrass plant being grazed. For unclipped plants, feces deposition resulted in reduced bulb mass, reduced percentage of biomass in bulb and roots, and increased percentage of biomass in leaves, whereas feces deposition had no effect on clipped plants. Clipping neighbors resulted in increased arrowgrass root and stolon mass only for unclipped plants. Feces deposition resulted in increased vegetative reproduction when neighbors were clipped, but had no effect on vegetative reproduction when neighbors were not clipped. Plants in exclosed plots were larger, had greater allocation to leaves, had higher concentrations of C and N, and were more likely to flower than were plants in unexclosed plots. These results indicate an increase in competition for light with neighbor plants under feces deposition, which may be ameliorated by biomass removal of neighbors. Our results predict that an increase in grazing pressure is not necessarily detrimental to arrowgrass, provided that it is accompanied by increases in consumption of neighbor plants. The number of arrowgrass plants completely removed was not related to arrowgrass density, but the number of plants partially grazed increased with arrowgrass density. The probability that an individual arrowgrass plant would be grazed was negatively related to biomass or percent cover of several other species (Potentilla egedei, Chrysanthemum arcticum, Carex spp., Salix spp.). These results suggest that some neighbor species may provide a measure of protection (associational refuge) from herbivory, and that an increase in grazing intensity may have a strong negative effect on arrowgrass populations by reducing this protection and through an increased likelihood of consumption of the arrowgrass remaining after neighbors are removed. We conclude that the way in which we view neighboring plants (as competitors or potential protectors) affects our predictions regarding the effects of changing herbivore populations. Feedbacks from the plant community to herbivores following grazing should be included in studies that aim to extrapolate to the population level the effects of herbivory on individuals.

Direct and indirect effects of rats: does rat eradication restore ecosystem functioning of New Zealand seabird islands?

Introduced rats (Rattus spp.) can affect island vegetation structure and ecosystem functioning, both directly and indirectly (through the reduction of seabird populations). The extent to which structure and function of islands where rats have been eradicated will converge on uninvaded islands remains unclear. We compared three groups of islands in New Zealand: islands never invaded by rats, islands with rats, and islands on which rats have been controlled. Differences between island groups in soil and leaf chemistry and leaf production were largely explained by burrow densities. Community structure of woody seedlings differed by rat history and burrow density. Plots on islands with high seabird densities had the most non-native plant species. Since most impacts of rats were mediated through seabird density, the removal of rats without seabird recolonization is unlikely to result in a reversal of these processes. Even if seabirds return, a novel plant community may emerge.

Improved water retention links high species richness with increased productivity in arctic tundra moss communities.

A positive relationship between plant species richness and ecosystem functioning has been found in a number of experimental studies. Positive species interactions at high species numbers have been suggested as a cause, but mechanisms driving positive interactions have not often been tested. In this experiment we asked three questions: (1) What is the relationship between species richness and productivity in experimentally constructed moss communities? (2) Is this relationship affected by plant density? and (3) Can changes in moisture absorption and retention explain observed relationships? To answer these questions we exposed arctic tundra moss communities of different species richness levels (1–11 species) and two different densities in the greenhouse to two levels of drought (short and long). Biomass (by the community and individual species), height and community moisture absorption and retention were measured as response variables. High species diversity increased productivity (more so in low-density plots than in high-density plots), but only when plots were watered regularly. Plot moisture retention was improved at high species richness as well, and plant height and variation in height was increased compared to plants in monoculture. Under high-density and short-drought conditions 10 out of 12 species grew better in mixture than in monoculture, but under the long drought treatment only six species did. A positive feedback loop between biomass and improved humidity under high diversity was supported by path analysis. We conclude that in this community the relationship between species richness and productivity depends on moisture availability and density, with improved water absorption and retention likely to be the mechanism for increased plant growth when drought periods are short. Furthermore, since this is the opposite of what has been found for temperate moss communities, conclusions from one system cannot automatically be extrapolated to other systems.

Growth responses of Carex ramenskii to defoliation, salinity, and nitrogen availability: Implications for geese-ecosystem dynamics in western Alaska

Abstract:The Yukon-Kuskokwim River Delta in western Alaska is the principal nesting area for several species of geese, including Pacific black brant. Grazing by geese on Carex ramenskii, one of the...

EFFECTS OF ENVIRONMENTAL MANIPULATIONS ON TRIGLOCHIN PALUSTRIS : IMPLICATIONS FOR THE ROLE OF GOOSE HERBIVORY IN CONTROLLING ITS DISTRIBUTION

1 Arrowgrass (Triglochin palustris) is a preferred forage species of geese in the YukonKuskokwim Delta (south-western Alaska) where it is found primarily on slough levees in coastal areas. Geese may affect nutrient availability, interspecific light competition, and salinity. These variables were manipulated in order to identify interactive effects of interspecific competition and abiotic factors on arrowgrass size, biomass allocation and distribution, which are likely to be significant in relation to the effects of herbivory on arrowgrass abundance and distribution. 2 Arrowgrass individuals were transplanted from two slough levee communities to the same two communities and to the adjacent slough margin and wet Carex meadow communities. Geese were excluded and nutrient availability, light competition and salinity levels were manipulated. 3 When light levels were not manipulated, fertilization had a negative effect on plant biomass and allocation to bulbs. Under decreased competition for light, plant biomass of fertilized plants was not significantly different from that of control plants. Fertilization appears to have a negative effect on arrowgrass as a result of increased competition for light. 4 Plants in the slough margin habitat were smallest, had the lowest allocation to leaves and stolons, and the lowest N concentrations and total N mass. Results from the fertilization treatment suggest plants in this community are limited primarily by physical factors. 5 Plants in the Carex wet meadow had higher allocation to leaves than in other communities under unfertilized conditions, but decreased allocation to leaves under fertilization. Plants in this community appear lightand nutrient-limited under unfertilized conditions, and primarily light-limited under fertilization. 6 Our results suggest that the presence of geese may control arrowgrass distribution because (a) faeces deposition has a negative effect on arrowgrass, (b) this negative effect is ameliorated by consumption of neighbours, and (c) the combination of high light competition and highly selective foraging for arrowgrass limit expansion of arrowgrass into the Carex meadow community. These explanations can now be tested.

Invasive rats alter woody seedling composition on seabird-dominated islands in New Zealand

Invasive rats (Rattus rattus, R. norvegicus, R. exulans) have large impacts on island habitats through both direct and indirect effects on plants. Rats affect vegetation by extirpating burrowing seabirds through consumption of eggs, chicks, and adults. These seabirds serve as ecosystem engineers, affecting plant communities by burying and trampling seeds and seedlings, and by altering microclimate. Rats also directly affect plant communities by consuming seeds and seedlings. We studied the direct and indirect impacts of rats on the seedlings of woody plants on 21 islands in northern New Zealand. We compared seedling densities and richness on islands which differed in status with respect to rats: nine islands where rats never invaded, seven islands where rats were present at the time of our study, and five islands where rats were either eradicated or where populations were likely to be small as a result of repeated eradications and re-invasions. In addition, we compared plots from a subset of the 21 islands with different burrow densities to examine the effects of burrowing seabirds on plants while controlling for other factors that differ between islands. We categorized plant communities by species composition and seedling density in a cluster analysis. We found that burrow densities explained more variation in seedling communities than rat status. In areas with high seabird burrow density seedling densities were low, especially for the smallest seedlings. Species richness and diversity of seedlings, but not seedling density, were most influenced by changes in microclimate induced by seabirds. Islands where rats had been eradicated or that had low rat populations had the lowest diversity and richness of seedlings (and adults), but the highest seedling density. Seedling communities on these islands were dominated by Pseudopanax lessonii and Coprosma macrocarpa. This indicates lasting effects of rats that may prevent islands from returning to pre-invasion states.

Effects of invasive rats and burrowing seabirds on seeds and seedlings on New Zealand islands.

Rats (Rattus rattus, Rattus norvegicus, Rattus exulans) are important invaders on islands. They alter vegetation indirectly by preying on burrowing seabirds. These seabirds affect vegetation through nutrient inputs from sea to land and physical disturbance through trampling and burrowing. Rats also directly affect vegetation though consumption of seeds and seedlings. Seedling communities on northern New Zealand islands differ in composition and densities among islands which have never been invaded by rats, are currently invaded by rats, or from which rats have been eradicated. We conducted experimental investigations to determine the mechanisms driving these patterns. When the physical disturbance of seabirds was removed, in soils collected from islands and inside exclosures, seedling densities increased with seabird burrow density. For example, seedling densities inside exclosures were 10 times greater than those outside. Thus the negative effects of seabirds on seedlings, by trampling and uprooting, overwhelm the potentially beneficial effects of high levels of seed germination, seedling emergence, and possibly seed production, which result from seed burial and nutrient additions. Potential seedling density was reduced on an island where rats were present, germination of seeds from soils of this island was approximately half that found on other islands, but on this island seedling density inside exclosures was 7 times the density outside. Although the total negative effects of seabirds and rats on seedling densities are similar (reduced seedling density), the differences in mechanisms and life stages affected result in very different filters on the plant community.

Species removal and experimental warming in a subarctic tundra plant community

Neighbor interactions are likely to play an important role in subarctic plant communities. We conducted experiments in Interior Alaska in which we crossed species removal with greenhouse warming manipulations. We examined changes in community biomass, and in plant survival and growth of individual species in response to experimental warming and to: (1) removal of whole species versus an equivalent amount of biomass across many species, and (2) removal of subdominant (locally common) versus minor (locally uncommon) plants. Community biomass indicated compensation in growth after removal of minor species and after biomass removal without elimination of entire species, but under-compensation after removal of subdominants. Growth and survival of individual species showed facilitation between some species. Warming increased growth of dominant vascular plants, but at the same time reduced survival, and these impacts were greater for larger, more mesic species than for the smaller species associated with drier habitats. Growth of mosses was reduced by the warming. Removal effects did not differ between warming and ambient conditions. The results indicate that common species are able to reduce resources for others (competitive effect) and increase their growth after neighbor removal, whereas locally uncommon species are not able to respond rapidly to increased resources made available by neighbor removal. Therefore, the impact of the presence of common species on locally uncommon species was facilitative overall, but not vice versa. The balance between disturbances such as changes in temperature and species losses from the community will likely be crucial in determining shifts in subsequent community composition.

Herbivores and pathogens on Alnus viridis subsp. fruticosa in Interior Alaska: effects of leaf, tree, and neighbour characteristics on damage levels

Parasite damage strongly affects dynamics of boreal forests. Damage levels may be affected by climate change, either directly or indirectly through changes in properties of host trees. We examined how herbivore and pathogen damage in Alnus viridis subsp. fruticosa (Rupr.) Nym. depend on leaf morphology and chemistry, tree size, and tree neighborhood. Damage and tree properties were measured in 2003 and 2004 on eight trees at each of 20 sites in Interior Alaska. Damage varied significantly among sites and among trees within sites, but Cartesian distances between sites were not correlated with similarity in damage levels. Compared with middle leaves, terminal leaves experienced less damage from phloem-feeding insects and pathogens, whereas leaf-roller damage was largely confined to terminal leaves. Summer drought in 2004 strongly reduced damage from phloem-feeding insects, while damage from chewing insects increased. Overall, herbivore damage was best explained by leaf morphology and chemistry, and pathogen...