Search

Tree architectures reflect the main abiotic and biotic selection pressures determining tree growth and survival. Studies have shown that trees growing in herbivore‐dominated ecosystems, such as savannas, develop denser, more divaricate 'cage'‐like architectures in response to chronic browsing pressure (also known as 'brown world' architectures). In contrast, trees growing under resource‐limited conditions develop traits that allow them to better compete for abiotic resources (referred to as 'green world' architectures); for example, light limitation can induce trees to form more pole‐like structures that allow them to grow faster in height. In temperate forests, abiotic drivers have traditionally been assumed to be the main factors affecting plant architecture. However, gap dynamics typically characterize old‐growth temperate forests, where disturbances such as storms create gaps with temporary high‐light availability, but also attracting intense mammalian browsing. Under such conditions, one would expect tree saplings to exhibit high architectural plasticity enabling them to develop a 'cage' in response to high herbivore pressure, but when herbivore pressure declines switch to a more pole‐like structure that allows them to grow quickly in height and compete for light.We tested the ability of five dominant European tree species to induce a 'cage' architecture (hereafter referred to as 'cage plasticity') under contrasting light and herbivory regimes in a full‐factorial experiment in Białowieża Forest. We planted 720 young tree saplings in six replicated blocks in which we manipulated ungulate browsing (exclosure vs. control) and light levels (gap vs. closed forest).Two out of the five species (hornbeam Carpinus betulus and lime Tilia cordata) were able to develop a cage when exposed to ungulate herbivores but only under high‐light conditions. These two species had the highest survival rates in the herbivory treatment over a 10‐year period.Synthesis. Cage plasticity in temperate forest tree species may reflect an overlooked adaptation to mammal herbivory originating in a past when large mammals exerted strong selection pressures in European ecosystems. Our findings challenge the view of closed‐canopy forest communities being solely driven by abiotic drivers and call for a re‐evaluation of the importance of large herbivores in shaping the functional traits of temperate tree species. [ABSTRACT FROM AUTHOR]

1. We tested the interactions between biotic and abiotic factors in structuring temperate forest communities by comparing tree recruitment after 7 years inside 30 pairs of exclosure (excluding ungulates: red deer, roe deer, bison, moose, wild boar) and control plots (7 × 7 m each) in one of the most natural forest systems in Europe, the Białowieża Primeval Forest (eastern Poland). The strictly protected part of that forest hosts the complete native variety in trees, ungulates and their carnivores and excludes human intervention. 2. We analysed whether the exclosure effect interacted with abiotic factors, relevant for tree recruitment (canopy cover, ground vegetation cover, soil fertility and soil wetness) at different stages of tree regeneration (seedlings, saplings ≤ 50 and > 50 cm). 3. Contrary to our expectations, a single factor dominated at each stage of tree regeneration. Herbaceous vegetation cover was the main factor determining the number of seedlings with an optimum at 38% of cover. Soil fertility determined the density of saplings ≤ 50 cm, with on average three times higher density on eutrophic than on oligotrophic soils. Herbivory was the main factor determining recruitment rate of trees into > 50 cm size classes only. 4. The density of saplings that grew into the > 50 cm size class was more than three times higher in the exclosures than in the control plots during 7 years. In the absence of ungulates, on average 3.1 species recruited into the > 50 cm size class compared to 1.7 in control plots. Tree species occurred in more equal proportions inside exclosures, whereas species composition was pushed towards strong dominance of a preferred forage species, Carpinus betulus, in the presence of ungulates. This suggests that preference of species by ungulates can coincide with tolerance to browsing. 5. Synthesis. The study showed that abiotic conditions dominated the early stages and ungulate impact the later stages of tree regeneration, indicating the context-dependence of herbivore top-down effects. Heterogeneity in abiotic and biotic conditions may, therefore, have an important influence on the strength of top-down effects and the role that herbivores play in natural ecosystems. [ABSTRACT FROM AUTHOR]

1. Fires are nowadays small, yet frequent, in temperate Central European conifer forests, but little is known about the fire history in this region. This is likely due to the lack of intact forests that contain old trees and dead wood from which fire history may be reconstructed. An exception is the Białowieża Primeval Forest (BPF) in Poland for which we were able to reconstruct the fire history in detail. 2. From 886 tree ring samples collected in a 13-ha conifer-dominated area, we reconstructed fire events and tree establishment back to the mid 1600s. From 1653 to the late 1700s fires were very frequent, with mean point (single tree) fire intervals of 18 years and mean stand scale fire intervals of 6 years. After 1781, the intervals between the fires increased dramatically, and since 1874 no major fire was recorded. 3. Tree establishment underwent substantial changes, closely tracking shifts in fire frequency. When fires were frequent, Pinus sylvestris establishment occurred only sporadically. Later, less frequent fires promoted massive establishment of both P. sylvestris and Picea abies. At present, only P. abies and a few deciduous trees regenerate. 4. Synthesis. We present the first high-resolution fire history in the Central European temperate lowland forest area. The discovery of old P. sylvestris trees and stumps with fire scars in many conifer-dominated parts of BPF show that fire was a major component in the past dynamics of this forest. We also show that historically, fires were recurring at very close intervals, supporting an open, Pinus-dominated forest. These result contrasts with the written history of BPF, which focus on a few, large fires from the past. Human influence on the fire regime was probably substantial, although the disentangling of climatic and human impacts needs further studies. We propose that fire should be increasingly taken into consideration in models of disturbance, vegetation development and forest openness in the whole Central European lowland forest region. [ABSTRACT FROM AUTHOR]