Mechanisms of Vegetation Change (B2)

Mechanisms of Vegetation Change (B2)

B2) Underlying mechanisms of vegetation change and diversity in retention forestry

Michael Scherer-Lorenzen, Thomas Ludemann & Jan Helbach

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 and air humidity, soil temperature and moisture, litter input, and hence nutrient availability at the forest floor.

In addition, these abiotic conditions do also change across several temporal scales (daily fluctuations, seasonal changes, year-to-year variation). Particularly, the spatio-temporal variability of light intensity at the forest floor is usually increased in structurally more complex forest stands (2). Thus, the complex interplay of these changes results in altered resource availability for plants (light, nutrients, water), and hence competitive advantages of certain species over others.

Therefore, understory plant diversity in terms of species composition, species richness, and functional diversity has been shown to sensitively react to any retention measures (3). Less known are, however, the influence of the landscape context, as well as the relative contributions of the different resources axes and their variability for understory diversity.

Study questions and hypotheses

We aim to mechanistically understand the role of retention measures (with a focus on habitat trees, according to the overall design of ConFoBi) and their landscape context (forest connectivity) for plant diversity by disentangling various effects of abiotic changes on plant performance.

Specific hypothesis are:

  1. Vascular plant diversity is influenced by habitat trees, due to their complex canopy architecture, and by standing deadwood, due to reduced presence of leaves and twigs, which alter light conditions, soil water availability and organic matter input and mineralization compared to structurally more homogeneous stands.
  2. Diversity of typical forest plants profits from light heterogeneity, but only at overall low light availability, whereas simply increasing light availability will favour ruderal generalists. In addition, we will determine the relative importance of light vs. soil resource heterogeneity for understory vegetation diversity, hypothesizing a dominant effect of light.

Approach and methods

As a basic dataset, we record the composition and spatial distribution of understorey higher plants and ground-dwelling mosses within each of the 135 study plots. Light conditions using several variables (leaf area index, canopy closure, intensity of photosynthetic active radiation at forest floor, forest floor temperatures using thermography) and soil resource availability (water, N, P, cations, pH) are monitored at high spatial and temporal resolution, i.e. in all seasons and at multiple spots within each of the 135 plots. These measures are correlated to 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 and others) and community (species composition, taxonomic and functional diversity).

In each plot, we measure and map positions of up to 10 target vascular forest plants and 10 ruderal opportunists and reconstruct light conditions in these patches based on the detailed canopy and light measures. In the statistical analysis we identify light niches for those species. We then correlate the changes in abiotic parameters with performance measures of understory plants. Further, both abiotic parameters and performance measures are related to the light, nutrient and moisture indicator values of Ellenberg to test their usability for predicting fine-scale changes in growing conditions and performance.

We also relate laser-scanning data of forest structure (A1) and the spatially explicit terrestrial inventory of structural diversity (A2) to the heterogeneity in light conditions at the forest floor and soil variables, and to the spatial variability and diversity of understorey vegetation. Thereby we will be able to understand correlations between stand structure, light regime, nutrient availability and forest floor community structure and diversity.

Outputs

B2 provides baseline data on understory plant composition and diversity for all study plots, which constitute essential descriptors of forest biodiversity for the entire RTG, and which can be correlated to other aspects of forest diversity, as quantified in B3 to B6. The detailed measurements of light availability can also be used by B1 as a covariate for epiphyte diversity.

B2 is tightly linked to A1 and A2, which provide structural parameters that will be correlated with our measurements of abiotic conditions. If a relationship can be established, the LiDAR data can be used to predict resource conditions in many other forest stands. The empirical data produced by B2 will provide input for C1 and C2 to determine value and perception of plant diversity by forest practitioners, and for D2 to formulate best-practices guidelines for evidence-based biodiversity management.

Further reading

  • Kriebitzsch et al. 2013. Forest-specific diversity of vascular plants, bryophytes, and lichens In: Kraus D.,Krumm F. (eds) Integrative approaches as an opportunity for the conservation of forest biodiversity. European Forest Institute, pp. 158-169.
  • Liira, J., Sepp, T. & Parrest, O. 2007. The forest structure and ecosystem quality in conditions of anthropogenic disturbance along productivity gradient. Forest Ecology and Management, 250, 34-46.
  • Lindenmayer, D.B. et al. 2012. A major shift to the retention approach for forestry can help resolve some global forest sustainability issues. Conservation Letters, 5, 421-431.