| dc.description.abstracteng | In recent times, silvopastoral systems gain increasing attention due to their various
ecological and economic benefits. They have the potential to be a more sustainable
alternative to common intensive land-use practices and monocultures and to mitigate
climate change effects in agriculture. Interactions between woody and non-woody
components in agroforestry can enhance e.g., nutrient and water cycling, microclimatic
conditions and raise productivity compared to non-agroforestry systems because of
complementary resource capture. Still, silvopastoral systems are economically unproven
land-use systems due to, inter alia, limited understanding of tree-grass sward interactions
and their development over time. Further, agricultural grassland systems face changing
growth conditions like increasing dry summers due to climate change. Hence, alternative
grasses need testing for future climate change conditions.
The present dissertation comprises three studies of which the first two aimed to reveal
interacting effects of trees, cutting frequency and sward botanical composition on
herbage production and nitrogen (N) resorption processes of the grass sward in between
tree lines in silvopastoral systems. The aim of the third study was to investigate the
effects of N availability on growth and N resorption of tall fescue (Festuca arundinacea), a
grass known for its drought tolerance, compared to perennial ryegrass (Lolium perenne)
as the most widely used grass.
The first two studies were carried out as part of the SIGNAL collaborative project as part
of the BonaRes (Soil as a Sustainable Resource for the Bioeconomy) funding program at
the University of Göttingen, which investigated the sustainable intensification of
agriculture through agroforestry. Therefore, silvopastoral systems were studied at two
sites in Central Germany in the years 2016 and 2017. Both silvopastoral systems were
arranged in the form of alley cropping systems comprising willow stripes under short
rotation coppice with grassland in the alleyway. At one site, the grassland sward can be
characterized as long-term permanent grassland with the other site consisting of two
levels of temporary grassland sown five years prior to the present studies (a standard
white clover perennial ryegrass and a diverse sward). Two defoliation frequencies were
implemented with either two cuts or three to four cuts per year. Measurements were
conducted along a transect between the tree lines at various distances (close to trees and
farther away from the trees). The main experimental factors (distance to tree line, cutting
frequency and for one site sward botanical composition) led to four and six treatments,
respectively, with six and twelve replicates. The plots were laid out in a split-plot
randomized block design and arranged in that way in order to evaluate any shade effects
of trees on the grassland sward. The experiment of the third study was conducted as a
pot experiment in a temperature-controlled greenhouse from December 2015 to June
2016. The investigated species were the conservative and more drought-tolerant grass
species Festuca arundinacea (Schreb.), the exploitative less drought-tolerant grass species
Lolium perenne (L.) and the legume Trifolium repens (L.) as reference species due to its Nfixing
ability. Two N levels were applied with either a low or a non-limiting N supply level.
The two experimental factors (species, N supply) were adjusted to a randomized block
design with twelve replications.
The first study indicates that the distance to which tree effects may occur ranges from at
least 4.5 to 6 m. Shading by trees reduced grassland growth more severely than it
increased senescence and this effect was not modified by defoliation frequency. Shorter
defoliation intervals maintained a higher proportion of live relative to dead herbage
accumulation close to trees but total herbage accumulation was generally low. In
consequence, shorter harvesting intervals close to trees are not worthwhile.
Alternatively, longer defoliation intervals at the grassland-tree interface may promote
other grassland-related ecosystem services that benefit from extensive management like
biodiversity. Additionally, 4.4% of the German grassland is at a tree interface and
potentially suitable for such extensification.
Nitrogen resorption processes in the silvopastoral grassland were affected by tree
shading. Nitrogen concentration in the grassland alley of both sward mixtures was clearly
determined by the proportion of legumes. Non-legumes dominated the area close to
trees while legumes were remarkably more present apart from trees. Consequently, N
concentrations in the live and dead herbage close to trees decreased. The N resorption
efficiency (NRE), i.e., the proportion of N that is resorbed from the senescing herbage,
showed no differences along the grassland transect due to an overruling effect of trees on
the functional composition of the grass sward irrespective of initial grass sward
composition.
According to the third study, leaf N concentrations of both grass species were greater
under high than under low N supply and higher in F. arundinacea than in L. perenne. The
grass species L. perenne showed greater N concentrations in total DM herbage than F.
arundinacea under high N supply what might be caused by dilution effects since F.
arundinacea revealed a higher total DM herbage. The NRE of both grass species varied
but it was not affected by N supply level. Generally, F. arundinceae showed a greater NRE
and a lower N concentration in the dead leaves than L. perenne under low and high N
supply.
Trees reduce herbage production and change the botanical composition with legumes
suffering most from the competition. A simultaneous provision of high yields of digestible
herbage close to trees will likely be challenging. Applying extensive management to the
border zones near trees may be a chance to support biodiversity. Nevertheless, studies
are required to evaluate potential long-term effects of the tree-induced changes in
botanical composition with respect to soil nutrient cycling. Furthermore, the results
suggest that F. arundinacea may be a suitable grass species for temperate agricultural
grasslands under future climate changes due to its drought tolerance and ability to
recycle nitrogen efficiently. | de |