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The role of large herbivores in woodland regeneration patterns, mechanisms and processes

(2009) INBO doctoraten. INBO.T.2009.3.
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Abstract
In this thesis, we focused on woodland regeneration in the presence of large herbivores. We performed field surveys, experiments and elementary modelling to gain insight into the patterns of woody species regeneration, the mechanisms at work and the natural processes that influence woodland regeneration on former agricultural land that is grazed by large herbivores. In general, we aimed to contribute to the understanding of the changing temporal and spatial patterns of the grassland-forest matrix in grazed conditions. We approached the issue of conservation management strategies and landscape openness at the scale of present day, controlled grazing management in fenced nature reserves, but results may also gain insights into mechanisms and processes that are/were active in traditional wooded pastures and wilderness landscapes. The key processes to be understood in grazed ecosystems that include woodland are plant strategies in coping with herbivory, herbivore selectivity, light and nutrient requirements, dispersal and natural disturbances. We examined the role of (1) the starting conditions of a site (arable land vs. pasture grassland, vegetation structure and abiotic conditions), (2) the surrounding landscape (seed sources), (3) associational resistance, (4) associational palatability, (5) competition of surrounding vegetation, (6) resistance traits against herbivory, (6) time gaps in grazing, (7) natural disturbances (trampling, flooding), (7) nutrient redistribution and depletion, (8) habitat proportion and (9) habitat use and foraging strategy in the regeneration process of woody species. Studies were performed at 13 study sites and one greenhouse-gardencomplex. Sites were situated on former intensively used agricultural land on nutrient rich and heavy soils (sandy loam, loam and clay). In chapter 2 we studied the earliest stages of woody species, the seed and emergence stage. We used Fraxinus excelsior and Quercus robur as model species in an experimental set up to investigate the role of vegetation structure (a successional sequence from grassland to scrub) and the interaction effects of grazing, pre-emergence flooding and trampling on emergence and early growth of seedlings. The studied species represent pioneer species with diverse recruitment strategies (differing in seed characteristics, dispersal vector and seedling light requirements) that establish naturally in grasslands with a natural disturbance regime (e.g. floodplains). Seedling emergence was enhanced in low swards and sparse vegetation types. Despite different recruitment strategies, the interaction of flooding and trampling of swards enhanced seedling emergence of both species. Grazing of soft rush and tall sedges enhanced emergence of F. excelsior. Short grass swards increased early growth of emerging Q. robur. Our results support the hypothesis that natural disturbances of soil and vegetation create microsites for seedling emergence and reduce above ground competition. For subsequent growth and survival, seedlings need protective structures such as tall herb or scrub patches. This implicates the need of a dynamic vegetation structure, offering changing patch qualities in time. Therefore, the recruitment of native tree species assemblages may not only require an appropriate grazing regime, but also the restoration of natural disturbances, such as natural floods. In chapter 3, we examined the role of different vegetation structure types as safe sites for the survival and growth of establishing trees. After dispersal and emergence, survival and growth represent the next bottlenecks in the recruitment process of trees. In an experimental set up with exclosures, we used the same species and vegetation structure types as in chapter 2. Further, we evaluated the effect of time gaps in grazing. We analysed if this offered a window of opportunity for tree saplings to grow out beyond the browse line, which is an important threshold for woodland regeneration and/or the restoration of woodpastures (i.e. open, grazed woodlands with a mosaic of grassland, shrub and tree patches). Bramble thickets were suitable safe sites for survival and growth of seedlings of both species. Tall sedges, soft rush tussocks and ruderal vegetations with unpalatable or spiny species provided temporal protection, allowing seedlings to survive. Tree regeneration in livestock grazed grassland was highly constrained, but rabbits may undo the nursing effects of bramble thickets. The first year’s survival is of major importance for the establishment of trees. Subsequent grazing affects growth rather than survival. A two year’s initial time gap before grazing, had positive effects on survival, but did not allow outgrowth of unprotected trees. In chapter 4, we described regeneration patterns of pioneer trees in woodland restoration projects on grazed former arable land. We focused on differences between functional plant groups related to herbivory, i.e. trees with different resistance traits (tolerant, defensive, non-resistant), and the facilitating role of an unpalatable or spiny herb and low shrub layer that quickly develops on the bare ground of ploughed fertile soils. We tested if grazing by large herbivores affected the possibility of the different functional groups to grow beyond the browse line and consequently determined the species composition of pioneer woodland. We further examined if large herbivores are able to increase structural heterogeneity in early pioneer assemblages, which is often a goal in conservation management. We found a massive and fast colonisation of grazing tolerant and unpalatable woody species in early successional stages. Grazing decreased frequencies and height of the competitively dominant tolerant species (mainly Salix caprea L.). After 5-7 years, frequencies of unpalatable and tolerant species were equal, but unpalatable species (mainly Betula pendula) were able to grow beyond the browse line. When the cover of unpalatable/spiny vegetation was high enough, it provided suitable nurse sites for tolerant and palatable woody species. In early assemblages, grazing increased horizontal and vertical heterogeneity, resulting in intermediate successional stages. On the longer term, the mechanism of associational resistance will also allow palatable and tolerant species to grow beyond the browse line and promote forest succession and the order of species establishment and replacement. On the longer term, facilitation tends to promote forest succession and the order of species establishment and replacement, though it may not be a conservation goal where intermediate stages are themselves desirable for biodiversity reasons. In chapter 5 we studied regeneration patterns of woody species invading grazed grasslands. We described the spatial association of establishing trees and developing vegetation patches. We tested whether these vegetation patches, consisting of grassland, tall herbs and scrub, influenced the regeneration pattern and the conditions that allow trees to grow beyond the browse line. Further, we investigated if increased susceptibility to trampling disturbance by large herbivores on wet soils in floodplains influenced sapling establishment. Sapling frequencies, with F. excelsior as the most common species, were 2-3 times higher in tall herb and scrub patches compared to grassland. Only the spiny Crataegus monogyna established in higher frequencies in grassland plots. In floodplains, we found 2-3 times more established woody saplings in wet tall herb plots, compared to moist and wet grassland plots and moist tall herb plots. Browsing reduced sapling height in grassland and tall herb patches. Only in scrub patches did average height (± 2.5m) reach above the browse line. Browsing damage to saplings in scrub was significantly lower than in grassland and tall herbs. Tree establishment occurred in spatial association with non-grassland patches that offer protection against browsing. Fast growing spiny shrubs like Rubus sp. offer protection for palatable saplings, allowing establishment and growing out beyond the browse line. Alternatively, increased establishment occurred in wet tall herb patches, where soil disturbance by moderate trampling favoured establishment. Subsequently, saplings were protected at least temporarily against browsing by unpalatable tall herbs. In chapter 6, we assesses the magnitude and direction of nitrogen redistribution in a grazed vegetation mosaic with grassland, wooded pasture and forest. We used nitrogen content of different diet classes, habitat use, foraging and defecation behavior, weight gain and nitrogen losses to explore four different habitat proportion scenarios and two different foraging strategies to calculate net nitrogen balances per habitat. We examined if the spatial redistribution of nitrogen among habitats by cattle could restore nutrient-poor conditions in preferred foraging habitats, and whether such translocation conversely could lead to a risk of eutrophication of vulnerable non-foraging vegetation types. We confronted the results with atmospheric nitrogen deposition and assessed whether the creation of nutrient-poor conditions in grassland could trigger cyclic succession and woody species regeneration. All scenarios showed a small net nitrogen transport from grassland and wooded pasture to forest habitat. We found that nitrogen redistribution strongly depends on habitat proportion. Nitrogen losses from preferred grassland habitat can be high, given its proportion is small. Depletion is only to be expected at excretion free areas and probably is of minor importance to trigger establishment of woody species, due to feedback mechanisms that promote grazed patches. In general, nitrogen transported by cattle was much lower than input by atmospheric deposition, but grazing can compensate for high N inputs in excretion free areas and maintain mesotrophic grassland types. In none of the scenarios, N transport by cattle resulted in the exceeding of critical nitrogen loads to vulnerable forest ground vegetation. In chapter 7, we examined if the introduction of large grazers is a good measure to decrease bramble cover in ancient forests that are included in grazed mosaic landscapes. Expanding bramble cover as a result of increased atmospheric nitrogen deposition and altered forest management is considered as a threat for species rich forest ground flora. We studied the effects of four years of large herbivore grazing on bramble cover, and on cover and flowering of a set of vernal flowering forest forbs. We further conducted experiments to investigate direct effects of grazing and trampling on forest ground flora. Large Rubus thickets had a clear negative impact on cover and flowering of Anemone nemorosa due to competition for light. Four years of cattle grazing reduced bramble cover by more than 50%, but then the limiting factor for A. nemorosa cover and flowering shifted to trampling damage. We also found lower cover and flowering of Hyacinthoides non-scripta in grazed plots, a direct effect of grazing. The evergreen species Vinca minor and Hedera helix totally disappeared from the grazed forest. Simulated once-only effects of grazing and trampling had a small and short term negative impact on cover of A. nemorosa, but flowering was strongly reduced. Grazing reduced biomass with 25-30 % in the following years. Year-round grazing with large herbivores is an appropriate measure for bramble control in forests, but negative effects on ground flora are possible if grazing pressure is high. A low or moderate grazing pressure should be maintained in landscape mosaics with grassland and forest; or intermittent periods of non-grazing should be provided to maintain forest ground flora diversity. Finally, in chapter 8, we made integrated analyses that summarize the conditions for woodland regeneration on grazed former agricultural land. We deduced a conceptual model for regeneration success on former arable land and grassland with different operating mechanisms and processes (resistance traits, facilitation, natural disturbances, time gaps). The integrated results were made applicable for ecological restoration and conservation management by providing guidelines for grazing pressure, grazing regimes and the use of time gaps. Further, we developed a conceptual, management oriented state-and-transition model that describes the early successional pathways to be expected on grazed former agricultural land.

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Chicago
Van Uytvanck, Jan. 2009. “The Role of Large Herbivores in Woodland Regeneration Patterns, Mechanisms and Processes.” INBO Doctoraten. Brussels, Belgium: Research Institute for Nature and Forest (INBO).
APA
Van Uytvanck, J. (2009). The role of large herbivores in woodland regeneration patterns, mechanisms and processes. INBO doctoraten. Research Institute for Nature and Forest (INBO), Brussels, Belgium.
Vancouver
1.
Van Uytvanck J. The role of large herbivores in woodland regeneration patterns, mechanisms and processes. INBO doctoraten. [Brussels, Belgium]: Research Institute for Nature and Forest (INBO); 2009.
MLA
Van Uytvanck, Jan. “The Role of Large Herbivores in Woodland Regeneration Patterns, Mechanisms and Processes.” INBO doctoraten 2009 : n. pag. Print.
@phdthesis{1989890,
  abstract     = {In this thesis, we focused on woodland regeneration in the presence of large herbivores. We performed field surveys, experiments and elementary modelling to gain insight into the patterns of woody species regeneration, the mechanisms at work and the natural processes that influence woodland regeneration on former agricultural land that is grazed by large herbivores. In general, we aimed to contribute to the understanding of the changing temporal and spatial patterns of the grassland-forest matrix in grazed conditions. We approached the issue of conservation management strategies and landscape openness at the scale of present day, controlled grazing management in fenced nature reserves, but results may also gain insights into mechanisms and processes that are/were active in traditional wooded pastures and wilderness landscapes. The key processes to be understood in grazed ecosystems that include woodland are plant strategies in coping with herbivory, herbivore selectivity, light and nutrient requirements, dispersal and natural disturbances. 
We examined the role of (1) the starting conditions of a site (arable land vs. pasture grassland, vegetation structure and abiotic conditions), (2) the surrounding landscape (seed sources), (3) associational resistance, (4) associational palatability, (5) competition of surrounding vegetation, (6) resistance traits against herbivory, (6) time gaps in grazing, (7) natural disturbances (trampling, flooding), (7) nutrient redistribution and depletion, (8) habitat proportion and (9) habitat use and foraging strategy in the regeneration process of woody species. Studies were performed at 13 study sites and one greenhouse-gardencomplex. Sites were situated on former intensively used agricultural land on nutrient rich and heavy soils (sandy loam, loam and clay).
In chapter 2 we studied the earliest stages of woody species, the seed and emergence stage. We used Fraxinus excelsior and Quercus robur as model species in an experimental set up to investigate the role of vegetation structure (a successional sequence from grassland to scrub) and the interaction effects of grazing, pre-emergence flooding and trampling on emergence and early growth of seedlings. The studied species represent pioneer species with diverse recruitment strategies (differing in seed characteristics, dispersal vector and seedling light requirements) that establish naturally in grasslands with a natural disturbance regime (e.g. floodplains). Seedling emergence was enhanced in low swards and sparse vegetation types. Despite different recruitment strategies, the interaction of flooding and trampling of swards enhanced seedling emergence of both species. Grazing of soft rush and tall sedges enhanced emergence of F. excelsior. Short grass swards increased early growth of emerging Q. robur. Our results support the hypothesis that natural disturbances of soil and vegetation create microsites for seedling emergence and reduce above ground competition. For subsequent growth and survival, seedlings need protective structures such as tall herb or scrub patches. This implicates the need of a dynamic vegetation structure, offering changing patch qualities in time. Therefore, the recruitment of native tree species assemblages may not only require an appropriate grazing regime, but also the restoration of natural disturbances, such as natural floods.
In chapter 3, we examined the role of different vegetation structure types as safe sites for the survival and growth of establishing trees. After dispersal and emergence, survival and growth represent the next bottlenecks in the recruitment process of trees. In an experimental set up with exclosures, we used the same species and vegetation structure types as in chapter 2. Further, we evaluated the effect of time gaps in grazing. We analysed if this offered a window of opportunity for tree saplings to grow out beyond the browse line, which is an important threshold for woodland regeneration and/or the restoration of woodpastures (i.e. open, grazed woodlands with a mosaic of grassland, shrub and tree patches). Bramble thickets were suitable safe sites for survival and growth of seedlings of both species. Tall sedges, soft rush tussocks and ruderal vegetations with unpalatable or spiny species provided temporal protection, allowing seedlings to survive. Tree  regeneration in livestock grazed grassland was highly constrained, but rabbits may undo the nursing effects of bramble thickets. The first year{\textquoteright}s survival is of major importance for the establishment of trees. Subsequent grazing affects growth rather than survival. A two year{\textquoteright}s initial time gap before grazing, had positive effects on survival, but did not allow outgrowth of unprotected trees.
In chapter 4, we described regeneration patterns of pioneer trees in woodland restoration projects on grazed former arable land. We focused on differences between functional plant groups related to herbivory, i.e. trees with different resistance traits (tolerant, defensive, non-resistant), and the facilitating role of an unpalatable or spiny herb and low shrub layer that quickly develops on the bare ground of ploughed fertile soils. We tested if grazing by large herbivores affected the possibility of the different functional groups to grow beyond the browse line and consequently determined the species composition of pioneer woodland. We further examined if large herbivores are able to increase structural heterogeneity in early pioneer assemblages, which is often a goal in conservation management. We found a massive and fast colonisation of grazing tolerant and unpalatable woody species in early successional stages. Grazing decreased frequencies and height of the competitively dominant tolerant species (mainly Salix caprea L.). After 5-7 years, frequencies of unpalatable and tolerant species were equal, but unpalatable species (mainly Betula pendula) were able to grow beyond the browse line. When the cover of unpalatable/spiny vegetation was high enough, it provided suitable nurse sites for tolerant and palatable woody species. In early assemblages, grazing increased horizontal and vertical heterogeneity, resulting in intermediate successional stages. On the longer term, the mechanism of associational resistance will also allow palatable and tolerant species to grow beyond the browse line and promote forest succession and the order of species establishment and replacement. On the longer term, facilitation tends to promote forest succession and the order of species establishment and replacement, though it may not be a conservation goal where intermediate stages are themselves desirable for biodiversity reasons.
In chapter 5 we studied regeneration patterns of woody species invading grazed grasslands. We described the spatial association of establishing trees and developing vegetation patches. We tested whether these vegetation patches, consisting of grassland, tall herbs and scrub, influenced the regeneration pattern and the conditions that allow trees to grow beyond the browse line. Further, we investigated if increased susceptibility to trampling disturbance by large herbivores on wet soils in floodplains influenced sapling establishment. Sapling frequencies, with F. excelsior as the most common species, were 2-3 times higher in tall herb and scrub patches compared to grassland. Only the spiny Crataegus monogyna established in higher frequencies in grassland plots. In floodplains, we found 2-3 times more established woody saplings in wet tall herb plots, compared to moist and wet grassland plots and moist tall herb plots. Browsing reduced sapling height in grassland and tall herb patches. Only in scrub patches did average height ({\textpm} 2.5m) reach above the browse line. Browsing damage to saplings in scrub was significantly lower than in grassland and tall herbs. Tree establishment occurred in spatial association with non-grassland patches that offer protection against browsing. Fast growing spiny shrubs like Rubus sp. offer protection for palatable saplings, allowing establishment and growing out beyond the browse line. Alternatively, increased establishment occurred in wet tall herb patches, where soil disturbance by moderate trampling favoured establishment. Subsequently, saplings were protected at least temporarily against browsing by unpalatable tall herbs.
In chapter 6, we assesses the magnitude and direction of nitrogen redistribution in a grazed vegetation mosaic with grassland, wooded pasture and forest. We used nitrogen content of different diet classes, habitat use, foraging and defecation behavior, weight gain and nitrogen losses to explore four different habitat proportion scenarios and two different foraging strategies to calculate net nitrogen balances per habitat. We examined if the spatial redistribution of nitrogen among habitats by cattle could restore nutrient-poor conditions in preferred foraging habitats, and whether such translocation conversely could lead to a risk of eutrophication of vulnerable non-foraging vegetation types. We confronted the results with atmospheric nitrogen deposition and assessed whether the creation of nutrient-poor conditions in grassland could trigger cyclic succession and woody species regeneration. All scenarios showed a small net nitrogen transport from grassland and wooded pasture to forest habitat. We found that nitrogen redistribution strongly depends on habitat proportion. Nitrogen losses from preferred grassland habitat can be high, given its proportion is small. Depletion is only to be expected at excretion free areas and probably is of minor importance to trigger establishment of woody species, due to feedback mechanisms that promote grazed patches. In general, nitrogen transported by cattle was much lower than input by atmospheric deposition, but grazing can compensate for high N inputs in excretion free areas and maintain mesotrophic grassland types. In none of the scenarios, N transport by cattle resulted in the exceeding of critical nitrogen loads to vulnerable forest ground vegetation.
In chapter 7, we examined if the introduction of large grazers is a good measure to decrease bramble cover in ancient forests that are included in grazed mosaic landscapes. Expanding bramble cover as a result of increased atmospheric nitrogen deposition and altered forest management is considered as a threat for species rich forest ground flora. We studied the effects of four years of large herbivore grazing on bramble cover, and on cover and flowering of a set of vernal flowering forest forbs. We further conducted experiments to investigate direct effects of grazing and trampling on forest ground flora. Large Rubus thickets had a clear negative impact on cover and flowering of Anemone nemorosa due to competition for light. Four years of cattle grazing reduced bramble cover by more than 50\%, but then the limiting factor for A. nemorosa cover and flowering shifted to trampling damage. We also found lower cover and flowering of Hyacinthoides non-scripta in grazed plots, a direct effect of grazing. The evergreen species Vinca minor and Hedera helix totally disappeared from the grazed forest. Simulated once-only effects of grazing and trampling had a small and short term negative impact on cover of A. nemorosa, but flowering was strongly reduced. Grazing reduced biomass with 25-30 \% in the following years. Year-round grazing with large herbivores is an appropriate measure for bramble control in forests, but negative effects on ground flora are possible if grazing pressure is high. A low or moderate grazing pressure should be maintained in landscape mosaics with grassland and forest; or intermittent periods of non-grazing should be provided to maintain forest ground flora diversity.
Finally, in chapter 8, we made integrated analyses that summarize the conditions for woodland regeneration on grazed former agricultural land. We deduced a conceptual model for regeneration success on former arable land and grassland with different operating mechanisms and processes (resistance traits, facilitation, natural disturbances, time gaps). The integrated results were made applicable for ecological restoration and  conservation management by providing guidelines for grazing pressure, grazing regimes and the use of time gaps. Further, we developed a conceptual, management oriented state-and-transition model that describes the early successional pathways to be expected on grazed former agricultural land.},
  author       = {Van Uytvanck, Jan},
  isbn         = {9789040303067},
  language     = {dut},
  pages        = {242},
  publisher    = {Research Institute for Nature and Forest (INBO)},
  school       = {Ghent University},
  series       = {INBO doctoraten},
  title        = {The role of large herbivores in woodland regeneration patterns, mechanisms and processes},
  volume       = {INBO.T.2009.3},
  year         = {2009},
}