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Work Package 2e “How trees in mixtures respond to environmental factors” plots

These plots were intensively managed (e.g. little weed competition) and intensively measured from year 2 and therefore, to minimise costs, they were established by Teagasc staff on Teagasc land in Kinsealy, thereby reducing the travel costs of future management and monitoring of the plots. Design of these plots was dependent on the outcome of the literature review. The plots were used to investigate the response of young trees to environmental factors when grown in mixture.

Measurements and observations

The following parameters were evaluated for samples of trees in each treatment during years 1-4 of growth: tree height, diameter, shoot dry weight (sub-samples only, year 4 only) and stem form (e.g. forks and other defects). Some physiological information, such as phenology of growth, photosynthetic rates, water-use efficiency (using pressure bomb or isotope analyses), leaf area (e.g. leaf area index) were recorded each growing season. Some root responses were also recorded, including root dry weight, rooting depth and root overlap (with other species). Tissue analyses was used to estimate effect of mixtures on nutrient uptake. 

WP2e concentrated on the effect of shading on three species. This project investigated potential implications of thinning on the growth and physiological responses of underplanted broadleaf tree species. A controlled shade (simulating different canopy densities) experiment formed the main part of the study.

The area was divided into 5 blocks with 4 split plots in each of them (randomised block design with split plots). Four different shade-treatments have been established in each block (one treatment per block): control (with no shade cloth); 47% (with shade cloth that provides 47% shade); 63% (with shade cloth that provides 63% shade); and 85% (with shade cloth that provides 85% shade). The trees were planted at close spacing (0.5 m x 0.5 m) to encourage the onset of interplant competition. The use of shade clothes ensure that the only variable affecting the trees is the amount of light incident upon them. The measurement area entailed 16 seedlings per species in each plot and it is located near the centre of the plot. There is a buffer line around the measurement area and one spare line in all the plots.

In 2011, 2 year-old seelings of ash (Fraxinus excelsior), beech (Fagus sylvatica) and pedunculate oak (Quercus robur) were planted. The shade houses were erected in August/September 2012.

Tree height, diameter and flushing date were measured in the trial before erecting shade houses. Tree height (cm) and basal stem diameter (mm, 3 cm above root collar) of the seedlings were measured before the growing season started in 2011, 2012 and 2013. Flushing date was checked from late April/early May during 2011, 2012 and 2013. Damages to trees such as dieback, death, broken stems and damages from animals or machinery were recorded at the same time. Ash seedlings had the highest increase in height and diameter between 2011 and 2012 (before erecting shade houses), while oak seedlings had the lowest increase in both variables measured. Beech and ash seedlings had a similar increase in height but the increase in diameter for ash was almost double.

Two parameters were measured in 2012 after shade houses were erected: stomatal conductance and leaf senescence. Stomatal conductance was measured (six measurements per species per plot) using a leaf porometer and on exposed and fully developed leaves during some weeks in September and October. Symptoms of leaf senescence on the different species were visually scored at mid October. Results regarding stomatal conductance showed that there were significant differences for this variable between treatments, species and species treatment interaction. Since stomatal conductance plays an important role in the plant-atmosphere water exchange and it is a key parameter in many ecological models, we can say that thinning treatment and species underplanted will affect the absolute concentration gradient of water vapour from the leaf to the atmosphere.

Quality measurements and crown architecture were recorded in April/May 2013. A general index for quality which assesses different parts of the seedling (stem, leader, form, branching, fork, shaping) was used. Number of first-order and second-order branches was recorded. Quality measurements were recorded for all the measurement area.

Dead/missing trees were replaced in the measurement area during early 2013 with trees randomly selected from the spare line.

Flushing date was recorded in all the seedlings in each plot at the end of April 2013. Flushing date (break-bud or budburst) was recorded in bottom half and top half of the plants three times per week. Flushing date is classified in three different phases: 1) Initiation of flushing; 2) Leaf form visible; 3) Leaves fully expanded.

Measurements of photosynthesis rates and response curves were made during 2013. Simultaneous gas exchange and fluorescence over the same leaf area were also assessed.

Walsh Fellow and UCD PhD student Ignacio Sevillano assessing photosynthesis rates