Spatial and temporal variation in herbivore resources at northern latitudes

Abstract

Herbivores may affect plants directly through foraging, trampling and fertilization or indirectly through altered competitive relationships between plant species. In addition, herbivores may cause indirect interactions between species that do not compete for the same resources, but are object to grazing from a shared herbivore. A reintroduction of a large herbivore, the Svalbard reindeer (Rangifer tarandus platyrhynchus), was found to generate a strong trophic top-down effect on the vegetation, including a reduction in cover of mosses, vascular plants and lichens. This supports the “exploitation ecosystem hypothesis”. This caused a reduction in reindeer carrying capacity K and sustained reduction in herbivore densities. Plant groups differed in the rate of recovery after the reindeer population peak, and lichens did not show any sign of recovery. Consequently, the irruption caused a shift in the composition of the vegetation.

In addition to top-down effects of herbivores on plants, plant-herbivore interactions may work through variation in foraging resources caused by variation in environmental conditions. We investigated which climatic variables were able to explain local growth of two long-lived evergreen plant species Cassiope tetragona and juniper (Juniper communis), sampled over a large geographic area at Svalbard and Finnmark (Norway), respectively. Both species responded positively to high summer temperatures. C. tetragona growth was also negatively related to the summer Arctic Oscillation (AO) index and number of overcast days in July. Juniper growth was positively associated with a high winter AO, September temperature and precipitation. Juniper growth was positively correlated with a rapid phenological development and high net primary production (measures derived from satellite images), growing conditions that are generally recognised to decrease nutritional value of plants as forage for ungulates. Juniper with high average growth responded more to high a winter AO/NAO, summer temperature and derived spring NDVI than plants with low average growth. We found that juniper was able to capture important environmental factors and can thus be used as a proxy for foraging conditions.

Because temporal variation in climate affects plants at both temporal and spatial scales, it may cause synchronised population growth rates among plants and subsequently ungulate populations over large distances. We analysed whether growth of C. tetragona and juniper was correlated among localities and to what extent this correlation decreased with distance was performed. In addition, we analysed which climatic variables could be capable of generating synchronised growth. The spatial synchrony was high over large areas for C. tetragona. However, which climatic variable caused this synchrony was not possible to identify. The spatial correlation in juniper growth was on the other hand low and even increased slightly with increasing distance.

Body growth in ungulates is influenced by both demographic and environmental variability. The effect of demographic stochasticity is density dependent. Autumn body mass (measured as carcass weight) of semi-domesticated reindeer calves (R. t. tarandus) in Finnmark, northern Norway, was negatively affected by high population density. Furthermore, there was a negative relationship between calf body mass and unfavourable winter conditions (mainly through heavy snow accumulation), whereas body mass was positively associated with slow plant development during summer. Accounting for the within-herd variation in population density did not affect the relationship between environment variables and body mass to any extent. There was a negative relationship between calf body mass and juniper growth, indicating that juniper can capture environmental variation of importance for reindeer growth. Accounting for both density and juniper growth resulted in several changes in how environmental variables affected reindeer body mass, but still the general impression of negative effects of snow rich winters persisted.

Global climate is warming rapidly. One result of the change in global climate is the frequency of periods with temperature above 0°C during winter will increase in arctic regions. If above zero temperatures are combined with precipitation, this can cause formation of ground-ice or crusty, hard snow. This will directly affect forage availability for herbivores, but little is known about how it may affect plant species of importance for herbivores. We experimentally constructed ground-ice during early winter on high arctic heath vegetation to simulate the effect of an icing event on plant performance. Growth response of a small willow (Salix polaris) and a rush (Luzula confusa) was measured the following summer. Both species showed some response to icing. S. polaris that had been subject to experimental icing, showed increased growth later in season compared to the control groups. L. confusa showed a weak negative response throughout the summer to the experimental icing event.