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B2) Mechanisms of Vegetation Change

Underlying mechanisms of vegetation change and diversity in retention forestry

Michael Scherer-Lorenzen
Doctoral researchers: Jan Helbach (since 2016) & Sara Klingenfuß (since 2019)

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

Background and state of research

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. 

  

Study questions and hypotheses

Here, we aim to understand the role of retention measures mechanistically 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) for plant diversity 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 focus 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 relies on two steps (papers):

  • 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

Data exploration shows that understory species richness ranges from 2 to 71 species on the 1 ha plots. Overall, 323 species were found on all plots. Further analysis showed 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.

 

Current doctoral researcher project: Functional traits of understorey plants

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 will thus quantify the role of forest structure and resource heterogeneity on trait distributions and the functional diversity of the understorey. She will also analyse plant performance, quantified on several biological hierarchies from leaf (ecophysiology: e.g. fluorescence) to whole-plant (phenotypic plasticity of leaf functional traits, such as specific leaf area, etc.) and community (functional diversity). The changes in resource heterogeneity/availability along the gradient of forest structure will thus be related to these performance measures of understorey plants. Further, both abiotic parameters and performance measures will be related to Ellenberg’s light, nutrient, and moisture-indicator values to test their usability as “functional traits” for predicting fine-scale changes in growing conditions and performance.

Sara Klingenfuß quantifies plant performance and trait distribution of all higher plants present in the six sub-plots of each of the 135 ConFoBi plots. Depending on the level of methodological complexity, some measurements can only be done with select/dominant species (e.g. for ecophysiological data). Thereby, she will make a re-inventory of the vegetation of all sub-plots.  The work also includes the quantification of canopy openness and light regime by repeating the hemispherical photos on all sub-plots.

  

Perspectives

The work planned for RTG phase I and the first half of phase II (first and second PhD cohort) focuses on the detailed quantification of forest structure, resource heterogeneity/availability, plant community composition, and plant performance and is based on a comparative methodology.

In the second half of TRG phase II (third PhD cohort), we will further quantify the soil seed band, assuming that a combined herb layer-seed bank perspective will improve our understanding of diversity changes in retention forestry. To complement these observational approaches,  we consider manipulating the spatial availability of key nutrients to experimentally test the soil nutrient heterogeneity-diversity hypothesis along gradients of structural complexity.

 Subproject B2 will thus deliver fundamental insights into central ecological concepts to explain species coexistence and diversity of forest plants.