J. den Ouden

Effects of introduction and exclusion of large herbivores on small rodent communities

In this study we analysed the effects of large herbivores on smallrodent communities in different habitats using large herbivore exclosures. Westudied the effects of three year grazing introduction by red deer(Cervus elaphus L.) in previously ungrazed pine and oakwoodland and the exclusion of grazing by red deer, roe deer(Capreoluscapreolus L.) and mouflon (Ovis ammon musiminL.) in formerly, heavily grazed pine woodland and heathland. At eight exclosuresites within each habitat type, small rodents were captured with live trapsusing trapping grids. At each trapping grid, seed plots of beechnuts(Fagus sylvatica L.) and acorns (Quercusrobur L.) were placed to measure seed predation by rodents.Exclusion of grazing by large herbivores in formerly, heavily grazedhabitats had a significant effect on small rodent communities. Insideexclosureshigher densities of mainly wood mice (Apodemus sylvaticusL.) and field voles (Microtus agrestis L.) were captured.Introduction of grazing by red deer appeared to have no significant negativeeffects on small rodent communities. The seed predation intensity of beechnutsand acorns by small rodents was significantly higher in ungrazed situations,particularly in habitats that were excluded from grazing. The differencesbetween grazing introduction and exclusion effects on small rodent communitiescan be explained by differences in vegetation structure development. Therecovery of heavily browsed understory vegetation after large herbivore grazingexclusion proceeded faster than the understory degradation due to grazingintroduction. Small rodents depend on structural rich vegetations mainly forshelter. We conclude that large herbivores can have significant effects onvegetation dynamics not only via direct plant consumption but also throughindirect effects by reducing the habitat quality of small rodent habitats.

Wood structural differences between northern and southern beech provenances growing at a moderate site

Planting provenances originating from southern to northern locations has been discussed as a strategy to speed up species migration and mitigate negative effects of climate change on forest stability and productivity. Especially for drought-susceptible species such as European beech (Fagus sylvatica L.), the introduction of drought-tolerant provenances from the south could be an option. Yet, beech has been found to respond plastically to environmental conditions, suggesting that the climate on the plantation site might be more important for tree growth than the genetic predisposition of potentially drought-adapted provenances. In this study, we compared the radial growth, wood-anatomical traits and leaf phenology of four beech provenances originating from southern (Bulgaria, France) and northern locations (Sweden, the Netherlands) and planted in a provenance trial in the Netherlands. The distribution of precipitation largely differs between the sites of origin. The northern provenances experience a maximum and the southern provenances experience a minimum of rainfall in summer. We compared tree productivity and the anatomy of the water-conducting system for the period from 2000 to 2010, including the drought year 2003. In addition, tree mortality and the timing of leaf unfolding in spring were analysed for the years 2001, 2007 and 2012. Comparison of these traits in the four beech provenances indicates the influence of genetic predisposition and local environmental factors on the performance of these provenances under moderate site conditions. Variation in radial growth was controlled by environment, although the growth level slightly differed due to genetic background. The Bulgarian provenance had an efficient water-conducting system which was moreover unaffected by the drought in 2003, pointing to a high ability of this provenance to cope well with dry conditions. In addition, the Bulgarian provenance showed up as most productive in terms of height and radial growth. Altogether, we conclude that the similarity in ring-width variation among provenances points to environmental control of this trait, whereas the differences encountered in wood-anatomical traits between the well-performing Bulgarian provenance and the other three provenances, as well as the consistent differences in flushing pattern over 3 years under various environmental conditions, support the hypothesis of genetic control of these features.

Fuel load, humus consumption and humus moisture dynamics in Central European Scots pine stands

Samples of Scots pine (Pinus sylvestris L.) humus were burned under different moisture and fuel load scenarios to model humus consumption. For moisture levels below 120% on a dry mass basis, a parabolic increase of humus remaining with increasing moisture content was observed while, for higher moisture levels up to 300%, humus was reduced by a constant 10–15% on a dry mass basis. Both fuel load and humus moisture had a highly significant influence on humus consumption. Humus gross calorific value of Scots pine (19 509 KJ kg–1) is lower than that of other pine species. We found a desorption time-lag for humus moisture of 85 h in this study. Field data show a steady accumulation of humus in Central European Scots pine stands (up to 45 t ha–1 in 120-year-old stands). Amounts of litter remain constant over the different stand ages (~15 t ha–1). This study provides important information to predict humus consumption in Scots pine stands. The results can be used to build a fire severity and post-fire succession model for Scots pine stands in Central Europe.

Dendrogeomorphology - a new tool to study drift-sand dynamics

dendrogeomorphological approach is presented, using wood characteristics of native oak (Quercus robur L.) to infer dynamics of aeolian sediment transport in drift-sand areas. Wood samples, taken from oaks in two drift-sand areas, were analysed to study changes in tree-ring pattern and wood anatomy as a consequence of burying or exposure from drift sand. In all cases, the wood of the sampled oaks showed sudden changes in anatomy and tree-ring width due to burial by drift sand or subsequent exposure after erosion of the new soil surface. After aerial stems became covered by drift sand, the wood lost its characteristic ring-porous features, and tree rings became strongly reduced in width with less distinct ring boundaries. Buried stems that became exposed after erosion showed an abrupt increase in ring width and turned distinctly ring porous again. Roots that were exposed also adopted clear ring-porous features, increased in ring width and anatomically resembled aerial stem wood. Using tree-ring analysis, it is possible to precisely date sand deposition and erosion events by detecting the concurrent changes in anatomy of woody structures. This study indicates the high potential of dendrogeomorphology as a tool to study drift-sand dynamics with a high temporal, i.e. annual, resolution for a period going back as long as the maximum age of the trees present (in this study at least 250 years). Since the signals of past deposition and erosion events are conserved in the wood, this is the only method that can be used to reconstruct drift-sand dynamics when the actual landforms are no longer present.

Successional trends and biomass of mosses on windthrow mounds in the temperate rainforests of Southeast Alaska

We investigated successional trends on windthrow mounds in two old-growth Tsuga heterophylla-Picea sitchensis forests in northern southeast Alaska to determine the influence of windthrow disturbance on the maintenance of plant diversity. We were particularly interested in assessing the value of mosses in detecting long-term effects of disturbance in temperate rainforests. Mosses established a dense carpet on windthrow mounds within the first few decades after the disturbance. No consistent changes were noted in total moss and vascular plant cover, moss biomass, or species diversity between young mounds (±50 yrs), intermediate mounds (±150 yrs) or old mounds (> 200 yrs), or between mounds and the undisturbed forest floor, despite consistent differences in soils development. Classification and ordination of the vegetation data did not show a consistent relationship between soil surface age or soil depth and overall species composition on the two sites. Young mounds were the most compositionally distinctive, primarily due to moss species. Pogonatum alpinum var. sylvaticum, P. contortum and Polytrichum formosum were generally confined to young mounds with unstable substrata, while Dicranum majus and Sphagnum girgensohnii were associated with old soil surfaces and deep organic soils. Vascular plant species with affinities for riparian or deep shade habitats (Tiarella trifoliata, Coptis asplenifolia and Dryopteris expansa) showed a general preference for the forest floor. Gymnocarpium dryopteris was the only vascular plant with a significant association with young mounds. Mosses comprised approximately 25% of understory plant biomass and as much as 50% of understory productivity. In cool temperate forests, the inclusion of mosses in vegetation analysis may provide valuable insights into the nature of vegetation patterns over subtle environmental gradients. The distinctiveness of the temperate rainforest type and the unique ecological effects of windthrow disturbance in this type are also suggested by this study.