Estimating the distribution of forage mass for ungulates from vegetation plots in Bavarian Forest National Park

Herbaceous ground vegetation is an important pool of biomass and nutrients, which is also used as the major forage source for wild ungulates. Up to now no standard methods exist to estimate herbaceous biomass on a landscape level for temperate forests, which are characterised by deciduous trees with closed canopies. Quantity and quality of the herbaceous forage accessible to herbivores can be estimated from estimated cover in vegetation plot data and information on biomass and element concentrations in plant species. Vegetation was sampled stratified by community types and forest developmental phases in Bavarian Forst National Park, Germany. We adopted the PhytoCalc model to estimate biomass and bioelement stocks from vegetation plot data and adjusted species assignments and absolute levels of biomass to the conditions in the national park. We categorised attractiveness of plant species as forage for red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Multiple controls of total biomass and of plant groups (graminoids, ferns, herbs, Vaccinium, Rubus) were studied by stepwise regression against stand and environmental predictors. Herbaceous mass had a highly skewed distribution in the park, with 75% of plots having less than 231 g*m-2 of biomass or 24 g*m-2 of raw protein. Contributions of plant groups were site-dependent and variable, but decreased in the order Vaccinium-graminoids-Rubus-herbs-ferns. Biomass appeared to be controlled by deciduous tree cover, by total cover of canopy and coarse woody debris and by site quality, with nutrient-poor, high elevation sites having higher herb biomass. As a consequence, montane beech forests offered less forage mass than coniferous communities of high elevations and mires. Stand disturbances by bark beetles and the corresponding forest developmental phases had no systematic effects on total biomass. Dominant grass and Vaccinium species are of intermediate attraction to foraging deer and reached higher mass in coniferous forests of poorer, colder and waterlogged sites, where higher summer activity of the intermediate feeder Cervus elaphus were predicted. It is hypothesised that forest sites suboptimal for tree growth raise the park's carrying capacity for deer. Highly attractive, nitrophytic plant species were usually scarce (75% of plots with < 3 g*m-2) and occurred in clumps after disturbances in mortal and juvenile phases of stand development, where up to Manuskript eingereicht am 6. Dezember 2013, angenommen am 24. März 2014 Koordinierender Editor: Norbert Hölzel 53 666 g*m-2 of attractive herbaceous plant mass were modelled. High soil fertility, as found in basiphytic beech forests, additionally favours these attractive plants. A relative preference of the selective browser Capreolus capreolus for these transient stages was predicted. The distribution of herbaceous forage mass and quality is subject to complex spatio-temporal patterns, the detection of which requires detailed vegetation data. The results suggest that it is possible to model the distribution of herbaceous vegetation for analyses at the stand and homerange scale. However, for a more comprehensive analysis of habitat choice the proposed method cannot deliver the required extent and accuracy.