Growth responses of grass swards to tree-mediated temporal and spatial dynamics of light and temperature in a silvopastoral system
by Rahel Sutterlütti
Date of Examination:2023-10-10
Date of issue:2024-06-12
Advisor:Prof. Dr. Johannes Isselstein
Referee:Prof. Dr. Johannes Isselstein
Referee:Prof. Dr. Klaus Dittert
Persistent Address:
http://dx.doi.org/10.53846/goediss-10537
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Abstract
English
Agroforestry can increase the resilience of agriculture to the predicted changes in climate. Alley cropping systems are a form of agroforestry where tree lines are integrated into the agricultural area in regular distances. Combined with grassland, they increase the climate change mitigation potential by increasing carbon storage. Additionally, tree lines have a positive effect on microclimate parameters with lower potential evapotranspiration rates close to the trees due to reductions in wind speed and air temperature. The present thesis includes four studies on the interactions between trees and grassland in two willow (Salix spp.) × grassland alley cropping systems. In the first study, we quantified the tree-mediated change in microclimate parameters and photosynthetically active radiation available to the grass sward and linked them to changes in daily growth rates. In the second study, we determined the lateral extent of the tree root system as well as the vertical distribution of below-ground biomass of trees and grassland at different distances to the tree line. The third study focused on the interactions between grassland and trees under moderate and extreme drought. In the fourth study, we measured the effect of the tree lines on growth and decomposition processes. The first study was conducted in an alley cropping system with narrow spacing (9 m) of the tree lines oriented Northwest–Southeast while the other studies were conducted in an alley cropping system with wide spacing (50 m) and tree lines oriented North–South. The first study intended to connect tree–mediated changes in microclimate parameters to daily growth rates. We hypothesized that trees affect microclimate parameters more strongly at smaller distances and that the growth of the grass sward is positively influenced by favourable microclimate with lower temperatures and higher relative humidity. The experimental factor transect position lumps distance and orientation to the tree line to measure the tree influence in a gradient of distance and while considering the orientation relative to the tree line. We included three transect positions with distances of 0.5 m (with two orientations relative to the tree line) and 4.5 m (equidistant in the center between two tree lines). Additionally, cutting frequency with two levels (two or three annual cuts) and vegetation composition with two levels (diverse vs. grass-clover) were included in the experimental setup arranged in a split-plot design with three repetitions per factorial combination. Air temperature and relative humidity as well as soil temperatures were monitored in each factorial combination. In addition, we measured the reduction of photosynthetically active radiation at the transect positions compared to open grassland during individual days. Daily growth rates were calculated from sward height measurements and calibration cuts (double sampling) taken every two or four weeks, respectively. We showed that the differences in annual biomass accumulation between the single transect positions aligned well with the differences in photosynthetically active radiation. The transect position with the same distance to the tree line but a higher number of sunshine hours showed higher temperatures, lower relative humidity and higher biomass accumulation under normal weather conditions. Under dry and hot weather, the positions close to the trees were favoured over the center position with 4.5 m distance to each tree line. The cutting frequency showed no consistent pattern in the interaction with year and vegetation composition. However, the diverse vegetation composition showed significantly higher biomass accumulation in the interaction with year as well as with cutting frequency. Our results show that species diversity has a positive effect on yield stability even in a series of extreme drought years. In combination with alley cropping, we conclude that the main limiting factor on grassland growth in the present alley cropping system is the amount of photosynthetically active radiation available to the grass sward. In the second study, we quantified root biomasses of trees and grassland up to a depth of 150 cm. We hypothesized that the tree roots extend laterally into the alley and interact with the grassland roots. Furthermore, we assumed that the fine root density of trees decreases with increasing distance. We sampled cores using a hydraulic soil corer in three repetitions in distances of 25 m, 8.5 m, 5.5 m, 4.5 m, 3.5 m, 2.5 m, 1.5 m and inside the tree line as well as in > 30 m distance on a reference grassland site. Roots were extracted in defined intervals from the core and fine root density and fine root length density were determined, using the software WinRhizo for the latter. We found a vertical displacement of tree fine root densities and fine root length densities below the zone occupied by grassland roots. Furthermore, we showed that the maximum observed lateral extension of tree roots at this site is 5.5 m. The total belowground biomass was significantly higher up to a distance of 2.5 m close to the trees compared to the grassland reference. However, grassland root biomass was significantly reduced up to a distance of 3.5 m. We conclude that the main interaction between root systems in the present alley cropping system is niche differentiation. The impact of drought on productivity and forage quality of permanent grassland managed in a four-cut system was determined during the first two growth periods of two experimental years. We hypothesized that under drought, the trees have a positive influence on grassland growth processes. In this study, transect positions were placed at 2 m and 6 m distance to the trees with two orientations (West-or East-facing) relative to the tree line and 25 m in the center between two tree lines. We measured biomass accumulation, crude protein and acid detergent fiber concentrations and the share of dead herbage and herbs as well as δ13C isotopic ratios at four repetitions per transect position and growth period. Our study shows that the trees do not affect biomass accumulation systematically neither under moderately dry nor extremely dry conditions. Even close to the trees (at distances ≥ 2 m), we did not detect consistently lower biomass accumulation, while forage quality of the green biomass tended to be slighly higher. Analysis of carbon isotopes showed that the stomatal conductivity is increased under extremely dry conditions close to the tree lines compared to larger distances. This result shows that the tree lines have a positive effect on the water relations of the grassland under drought. Finally, we tested if the tree lines slow down growth and decomposition processes due to their influence on microclimate in their vicinity. We measured soil temperatures with two repetitions per factorial combination (transect position × cutting frequency) with five levels of transect position (2 m and 6 m distance with two orientations to the tree line and 25 m in the center between two tree lines) and two levels of cutting frequency (four vs. two annual cuts) during two complete years. Our study was arranged in a split-plot design with the cutting frequency assigned to the transect (main plot) and the transect position nested in the main plot. The increase of sward height was monitored every two weeks using the rising plate meter and percent soil cover was assessed visually every four weeks. Decomposition was measured with litterbags where a defined quantity of dried grassland leafy biomass is put into polyethylene bags with a defined mesh size (here 1 mm) and placed on the soil surface. The weight loss is determined after a certain time. An exponential decay function was fitted to our data and the half-life times of litter were calculated. Soil temperatures were significantly lower close to the tree lines in both cutting frequencies but soil temperatures were slightly higher under frequent cutting. We detected a slight reduction of growth, and soil cover at the transect position with the tree line adjacent in the West. Half-life times were not affected systematically by the tree line and we observed a high variation in decomposition speeds. Our results show that even though the tree lines influence the microclimate and growth processes, they do not influence decomposition of litter to a measurable degree. This thesis gives an impression on the complexity of the interactions between tree lines and alley crop and the influencing parameters like orientation and spacing of the tree lines that affect the impact on microclimate and competition for light.
Keywords: agroforestry; competition; microclimate; photosynthetically active radiation; fine roots; alley cropping; root system; root competition; fine root density; maximal rooting depth; permanent grassland; acid detergent fibre; crude protein; drought stress; dry matter yield; stomatal conductivity; litter decomposition; Histosols
