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B2) Mechanisms of Vegetation Change

Underlying mechanisms of vegetation change and diversity in retention forestry

Michael Scherer-Lorenzen
Doctoral researcher: Sara Klingenfuß (since 2019)

University of Freiburg, Faculty of Biology, Institute of Biology II, Department of Geobotany

Background

Applying retention measures to enhance structural elements in rather homogeneous, even-aged stands alters microclimate and resource availability across spatial and temporal scales, to which plants respond sensitively (1). Introducing structural diversity by creating gaps, or by leaving habitat trees, will affect light quality and quantity, wind speed, air humidity, soil temperature and moisture, litter input, and nutrient availability on the forest floor.

In addition, these abiotic conditions also change across several temporal scales (daily fluctuations, seasonal changes, year-to-year variation). In particular, the spatio-temporal variability of light intensity at the forest floor is usually increased in more structurally complex forest stands (2). Thus, the complex interplay of these changes results in altered resource (light, nutrients, water) availability for plants and hence competitive advantages of certain species over others. Therefore, understorey plant diversity in terms of species composition, species richness, and functional diversity has been shown to react sensitively to any retention measures (3). Less known are the influence of the landscape context, however, as well as the relative contributions of the different resource axes and their variability for understorey diversity. 

 

  

Research questions and hypotheses

Here, we aim to mechanistically understand the effects of retention measures for understorey vegetation, with a focus on habitat trees and dead wood, according to the overall design of ConFoBi, and their context within a landscape (i.e. forest connectivity) by disentangling various effects of abiotic changes on plant performance. We hypothesize that increasing forest structural diversity will increase the spatio-temporal heterogeneity of resource availability, allowing for species-coexistence and hence higher species and functional diversity of the understorey. Additionally, we will determine the relative importance of light vs. soil resource heterogeneity for understorey vegetation diversity, hypothesizing a dominant effect of light. We will thereby be able to understand the effects of stand structure on the light regime, nutrient availability, and forest floor community structure and diversity.

 

Approach, methods and linkages

In the first three years (cohort 1), the analyses of PhD student Jan Helbach focused on the overall question of how heterogeneity in forest structure influences resource heterogeneity and the availability of light for understorey vegetation, and hence species diversity and composition. This analysis relied on two steps:

  • Effects of forest structure and forest connectivity on understorey diversity
  • Scale-dependence of resource heterogeneity and its effects on species composition

We determined the composition and spatial distribution of understorey higher plants and ground-dwelling mosses within each of the 135 study plots at different spatial scales (plot: presence-absence data of higher plants for the entire hectare; sub-plot: species abundances of higher plants on six permanently marked 5 x 5-m quadrats; transect: species abundances of higher plants and mosses on a 9.5-m N-S transect with 12 0.4 x 0.4-m quadrats). Light conditions were measured in all plots at the same spatial scales as for the vegetation analyses (plot: average of six hemispheric photos taken at the sub-plots; sub-plot: one hemispheric photo per sub-plot; transect: PAR measurements for 2-4 weeks on each transect quadrat, i.e. 12 in total). Soil properties were also determined in all plots at the same spatial scales as for the vegetation analyses (plot: average taken from sub-plot samples; sub-plot: three mixed soil samples to a depth of 15 cm; transect: one sample per transect quadrat, i.e. 12 in total). Soil samples were analysed for total C, N, and P; NH4+; NO3-; and cations (K, Fe, Mg, Ca, and others). Measures for canopy structure included openness and leaf area index (based on hemispherical photos), and canopy roughness (from remote sensing products, with A1).

 

 

Findings

The work of the first PhD student, Jan Helbach, showed that understorey species richness ranges from 2 to 71 species on the 1 ha plots. Overall, 323 species were found on all plots. Further analysis revealed that heterogeneity of light and soil C:N ratio increases with increasing stand structural complexity. Additionally, increasing light heterogeneity leads to increased understory plant species richness, supporting the heterogeneity-diversity hypothesis. Results imply that forest management could diversify light levels in forests by following an irregular tree harvesting strategy to enhance understory diversity.

B2’s second PhD student Sara Klingenfuß adopts a trait-based approach, shifting the focus from the taxonomically-based species composition to the characteristics and functional traits of the understorey species present. She thus quantifies the role of forest structure and resource heterogeneity on trait distributions and the functional diversity of the understorey. The changes in resource heterogeneity/availability along the gradient of forest structure will thus be related to these functional measures of understorey plants. Changes in growing conditions can lead to diversification in plant strategies and thus increase variability in trait expression. Limiting similarity as well as environmental filtering are known mechanisms that cause interspecific trait variability to change. However, plants are able to adapt and trait plasticity within species might interplay with the effect of the environment and allospecific competition for resources. Observing the effect of these mechanisms on finer – scales including trait measurements and intraspecific variability in the forest understory has not received much attention in the past, and will be tackled by the second phase of B2.
In order to investigate said mechanisms on an inter- and intraspecific scale, Sara Klingenfuß  performed detailed vegetation surveys and quantified functional trait distribution of all higher plants on a subset of all ConFoBi plots, where she surveyed 18 subplots on each hectare. She analyses a set of functional traits on several biological hierarchies from leaf to whole-plant and community. Additionally, to the measurements of plant traits, she quantified the heterogeneity of forest structure by taking hemispherical photos in every subplot, creating a finer-scaled map of canopy openness and light conditions for every chosen plot. So far, she could show that there is a wide range of trait expressions between species in our study area. The variability of community – level trait variability (expressed in functional dispersion) is tightly related to the bedrock type and resource heterogeneity in the forest. Functional dispersion was highest in plots on calcareous bedrock and with high levels of habitat heterogeneity (of light and soil phosphorous and related to the number of tree species that are present). This would support the heterogeneity -diversity relationship. Plant communities on acidic bedrock did not show an effect of resource heterogeneity on functional dispersion, supporting the environmental filtering approach. Results in regard to the effect of resource heterogeneity on trait plasticity might in the next step be able to provide a more profound understanding of the mechanisms behind community assembly in managed forests that provide ample conditions for plant diversity.

 

Future projects

Next PhD project (starting 1 July 2022).

Initially (years 1-3), B2 has been focussing on the overall question of how heterogeneity in forest structure influences resource heterogeneity and availability (light and soil nutrients), and which effects this has for species diversity and composition of the understorey vegetation. In years 4-6, the effects of resource heterogeneity on trait composition and functional diversity of the understorey was analysed.

In the upcoming PhD project, B2 will:

  • Quantify the role of spatial heterogeneity of soil resources for plant community composition and performance, based on a fertilisation experiment that manipulates the spatial pattern of pH, N, P, K. This will allow separating the effect of soil heterogeneity from those of forest structure. This work will also complement the ongoing monitoring of understorey vegetation.
  • Analyse the soil seed bank adopting an observational approach, to quantify this important diversity reservoir of the understorey vegetation (in cooperation with B3). Such a combined herb layer-seed bank perspective will improve our understanding of plant community responses to environmental variability in forests.
  • Analyse the influence of habitat trees of different species, dimension, and vitality on their surroundings, in particular on herbaceous layer. This work will also be carried out at new sites outside the existing ConFoBi plots (organized by A2)

 

Skills required for PhD3 applicants in B2:

In addition to the general requirements described in the job advertisement, applicants who wish to apply for B2 should bring botanical skills including survey methodology, good knowledge of Central European plant species and forest communities, and hold a driving licence.

 

ConFoBi-publications by B2

Knuff, Anna K.; Staab, Michael; Frey, Julian; Helbach, Jan & Klein, Alexandra‐Maria (2019). Plant composition, not richness, drives occurrence of specialist herbivores. Ecol Entomol, 44, 833–843. www.doi.org/10.1111/een.12767.

Asbeck, Thomas; Sabatini, Francesco; Augustynczik, Andrey L. D.; Basile, Marco; Helbach, Jan & Jonker, Marlotte et al. (2021). Biodiversity response to forest management intensity, carbon stocks and net primary production in temperate montane forests. Scientific reports, 11, 1625. www.doi.org/10.1038/s41598-020-80499-4.

Gustafsson, Lena; Bauhus, Jürgen; Asbeck, Thomas; Augustynczik, Andrey Lessa Derci; Basile, Marco & Frey, Julian et al. (2020). Retention as an integrated biodiversity conservation approach for continuous-cover forestry in Europe. Ambio, 49, 85–97. www.doi.org/10.1007/s13280-019-01190-1.

Storch, Ilse; Penner, Johannes; Asbeck, Thomas; Basile, Marco; Bauhus, Jürgen & Braunisch, Veronika et al. (2020). Evaluating the effectiveness of retention forestry to enhance biodiversity in production forests of Central Europe using an interdisciplinary, multi-scale approach. Ecology and evolution, 10, 1489–1509. www.doi.org/10.1002/ece3.6003.