Treatment effects on plant community composition and Dynamics – Work package 3.1

Warming and increased CO₂ may affect shoot growth and meristem activity; increase interspecific competition (Curtis and Wang, 1998) and therefore re-shape plant community composition.  Furthermore, predicted changes in growing season length and in shorter-term changes of weather event, as e.g. summer drought, are likely to affect plant growth individualistically among species (Gordon et al. 1999) resulting in changed relative abundance of species and functional groups. We believe that changes in seasonality with earlier onset of the growing season, predicted in the climate scenarios, are particularly important for structuring the plant communities. This is because the different life forms of species are adapted in different ways for starting their early-season activity depending on e.g. differences in leaf traits and physiological triggers of growth. This is likely to affect the relative abundance of plant species with consequences for microbial and animal communities and for ecosystem functions, as e.g. C sequestration.

The composition of the vegetation in the plots will be determined by point frequency analyses, with a method that allows conversion to biomass estimates (Jonasson 1988). This method is commonly employed in similar studies, as it is non-destructive and has proven very sensitive to capture changes in community structure and biomass. The measurement will give an estimate of relative abundance of all species, plant cover and height, dead and live biomass, flowering and frequency of herbivory (by registration of number of leaves with bites).

Seasonal effects of the treatments will be analyzed by registration of plant phenology and by reflectance measurements from spring to autumn. The phenological development of individually tagged plants of the most abundant species will be followed by registration of bud burst, shoot elongation and count of active meristems (Schmidt et al., in prep). Changes in overall biomass development will be measured by ground based canopy reflectance in the visible/near-infrared wavelength bands. This method measures the greenness of the vegetation, which is closely related to biomass and leaf area index (Ruimy and Saugier, 1994).

Pre-treatment conditions and status of the ecosystem, including community composition and nutrient status will be studied in 2004, before the establishment of the treatments. These data will give valuable information on the structure and variability of the site and will help establishing sampling sizes and frequencies of sampling once the experiment is running, and will also be used as a basis for data analyses. Plant species distribution above ground will be documented by vegetation analyses in all plots. However, to minimize the disturbance of the system the pre-treatment year, we will collect destructive samples in six soil profiles dug outside the treated plots in each block, in which also the vertical distribution of roots will be examined. The details of the sample collections and analyses are described below.

The vegetation will be analyzed at peak biomass (mid-July) all years, while the analyses of the seasonal dynamics will be done preferentially in the pre-treatment year of 2004 and later in 2005 and 2007, coinciding with the timing of main phenological events and measurements in the other work packages. In 2007 part of the plots will be harvested destructively at the end of the growing season as part of the ¹⁵N labeling experiment in WP4. Aboveground and belowground plant biomass will be separated by species or growth forms (e.g. evergreen shrubs, deciduous shrubs, graminoids, herbs, mosses and lichens) and biomass and nutrient content analyzed for each fraction.