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From the foothills to the crest: landscape history of the southern Manzano Mountains, central New Mexico, USA since 1800Huebner, Donald James 28 August 2008 (has links)
Not available / text
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Towards a predictive understanding of savanna vegetation dynamics in the eastern Lowveld of South Africa : with implications for effective management.Peel, Michael John Stephen. 02 December 2013 (has links)
The purpose of this study was to develop and test a predictive understanding of the
vegetation dynamics of the Lowveld of South Africa (30°35'E to 30°40'E and 24°00'S to
25°00'S). The study covered about 5000 km2 in Adjacent Private Protected Areas
(APPA) adjoining the Kruger National Park (KNP).
Data gathering (800 sites; 23 properties) commenced in 1989 and those recorded up to
2004 are reported here.
The value, both ecological and economic, of the wildlife and tourism industry dependent
on this savanna region is discussed in both historical and current perspectives. A range of
land-use objectives and anthropogenic interventions were exposed. The properties
ranged in size from 30 to 800 km2 and formed an effective and extensive manipulative
experiment for investigating interaction of bush density, animal stocking, use of fire and
landscape-scale processes. The first descriptive classification (at 1:250 000) of the area
was developed using Inverse Distance Weighted interpolations. This confirms similar
landscape/vegetation patterns in the KNP and Mocambique.
The current mode of determining stocking density or carrying capacity was interrogated
and indices suitable for complex multi-species systems developed. This was done in the
context of equilibrial/disequilibrial paradigms. Application of the original indices
resulted in drought-related declines in animal biomass of 4000 kg km2 over 20 years due
to overestimation of carrying capacity. The model proposed here uses rainfall, animal
type, biomass and vegetation parameters to determine stocking density for both coarse
(regional) and ranch-specific scales.
Principal driving determinants (rainfall, geology, soil type, tree density canopy cover,
animal numbers, feeding classes and fire) of vegetation structure and their influence on
the herbaceous layer were investigated. Groupings on ecological potential showed 'high'
potential areas are less sensitive to animal impact than those classified as 'low' potential. Sustainability, embedded in a forward-looking component viz. Strategic Adaptive
Management (SAM) with well-articulated endpoints viz. Thresholds of Potential Concern
(TPCs) was used to study fluctuations in animal populations with Connochaetes taurinus
(Blue wildebeest) as the case study.
The TPC approach provides strong pointers for proactive management aimed at
maintaining the system within bands defined by TPCs supporting operationally practical
and periodically reviewed objectives. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
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Aspects of the invasion of southern tall grassveld by Aristida junciformis subsp. junciformis Trin. et Rupr.Van Zyl, Douglas Dirk. 20 December 2013 (has links)
Aristida junciformis subsp. junciformis Trin. et Rupr. is an indigenous, densely tufted,
evergreen perennial grass associated with the degeneration of grasslands over large
areas of South Africa. More than two hundred thousand square kilometres of veld, c.
17% of the total land area of South Africa, contains A. junciformis. The aim of this study
was to improve our understanding of the mechanisms by which this species invades and
dominates grassland, specifically in this study, Southern Tall Grassveld.
Aristida junciformis also has a low nutritional value resulting in a very low grazing value.
The unpalatability of the plant is due to the high tensile strength of the leaves, whose
hard, fibrous laminas are very difficult for a grazing animal to crop once their length
attains 30 cm or more. This species lack of response to conventional grazing practices
has often resulted in a grassland in which the carrying capacity has diminished to such
an extent as to be virtually useless for grazing in a relatively short time.
The extent of encroachment of veld by A. junciformis appears dependant on the
frequency of disturbance and rest afforded to the veld. Annual burning and mowing
maintained the species composition of A. junciformis at levels <10% whilst protection
from fire, burning or grazing allows this species to dominate the herbaceous layer at
levels approaching 90%. Aristida junciformis does not become moribund and is unlikely
to die if left undefoliated. As the abundance of A. junciformis increases, veld
deterioration gradually accelerates through increased selective grazing on remaining
palatable species. Competition from adult A. junciformis plants increases the mortality of seedlings and
constrains growth of surviving seedlings. Tiller production of Aristida junciformis
seedlings declined from an average of 5.2 tillers per seedling in a no competition
situation to 2.2 tillers per seedling when subjected to full competition. Tiller production
of T. triandra seedlings decreased from 9.6 tillers per seedling free from competition to
3.3 tillers per seedling subject to full competition for resources.
Once these seedlings have become adult plants they are avoided by grazing animals
and grazing pressure on the remaining palatable species consequently increases. This
allows the A. junciformis plants to increase their size and density in the sward. Once this
density is sufficiently high, grass seedlings of either A. junciformis or more desirable
grass species such as T. triandra are unable to establish, eventually resulting in a monospecific stand of A. junciformis if left undisturbed.
Large amounts of caryopses are produced by A. junciformis - up to a 19 000 caryopses from a large mature plant (c. 38 000 caryopses/m²). Of these c. 40% is likely to be infertile but the remaining c. 60% viable caryopses are dispersed in a typically
leptokurtic distribution, the number of caryopses dispersed rapidly declining within a 10
metre radius. A large proportion of the caryopses was trapped in surrounding foliage but
in open swards caryopses had greater opportunity to be blown further distances than in
a closed sward. The density of A. junciformis caryopses on the soil surface was
positively correlated with the density of flowering adult plants in the area and varied from
400 caryopses/m² (density of parent plants c. 0.6 plants/m²) in less effected areas to 11000 caryopses/m² (density of adult plants c. 6 plants/m²) in severely encroached areas. The primary function of the three awns appears to be orientating the caryopsis correctly
for in its descent from the parent plant to expedite germination. Caryopses orientated
vertically with the awns uppermost exhibited the highest and most rapid germination
(67%) compared to caryopses lying horizontally (35%) whilst only 1% of inverted
caryopses germinated. Caryopses trapped in litter and effectively held off the soil surface
failed to germinate. Removal of the glumes from A junciformis seed greatly enhanced
the rate and overall germination of the seed except for inverted seed of which <1%
germinated.
The highest numbers of A. junciformis seedlings (32 seedlings/m²) were found in those
areas with the highest density of caryopses on the soil surface. Despite the large
amounts of caryopses produced, dispersed and landing in apparently suitable micro-sites
for germination, comparatively few A. junciformis seedlings (n=992) were found and
overall germination ranged between 1 % and 4% of the initial caryopses density on the
soil surface. Seedling survival through winter was low with only 13% surviving to the
following spring. The basal areas of A. junciformis increased overall by 66% whilst that
of other grass species increased overall by only 3% in the time monitored. These results
suggest that the primary method of encroachment of A. junciformis in the grass sward
appears to be through vegetative expansion and not seedling recruitment.
Frequent defoliation of the sward and avoidance of overgrazing to enhance the
competitive abilities of palatable species and provide as high a fuel load as possible
appear to be the most economically and logistically feasible ways to remove or at least inhibit A. junciformis veld encroachment at present. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.
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An assessment of changes in land use/cover patterns in the Albert Falls area, KwaZulu-Natal, South Africa.Yemane, Mehreteab Michael. January 2003 (has links)
The Albert Falls Area had witnessed severe competition over land use between
agriculture, recreation, conservation and other land uses. The area also has been the scene
for inefficient land management that led to degradation of land resources. For proper land
use planning and environmental management, information on land use/cover change is
vital. This study has developed a spatial and descriptive historical land use/cover
databases for the years 1944, 1967, 1989, and 2000 to provide an understanding of land
use/cover patterns in Albert Falls Area. The databases were created by interpreting
historical aerial photographs and using Geographic Information Systems. The data was
subsequently analyzed to detect relevant trends in land use/cover patterns in the study
area.
Generally land use/cover pattern of Albert Falls Area during the period 1944 to 2000 may
be described as being mainly agricultural. The results showed a marginal increase of
indigenous forests attributed to the continuous efforts by different governmental
departments and policies that focused on the protection of the remaining patches of
indigenous forests. Grassland not only decreased by more than half but it also
deteriorated in quality during 1944 to 2000. Commercial Forestry predominates the study
area and showed an increasing trend from 1944 to 1967. Cultivated Land was observed to
decrease at the expense of Commercial Forestry plantations and Waterbodies. Although
sugarcane plantations increased after 1967, total Cultivated Land showed a decreasing
trend. The construction of Albert Falls Dam and other small Farm Ponds within the
agricultural farms increased the land areas covered by Waterbodies. Residential Area
coverage generally decreased irrespective of the encroachment of informal settlements,
while that outside Non-Residential Area and Transportation Routes generally increased
with the development of commercial agriculture in the area. Barren Land decreased
continuously due to higher land demand in the study area.
The study showed that land use/cover changes in Albert Falls Area. have resulted in
habitat fragmentation, development of monoculture land use, flourishing of Farm Ponds in agricultural farms, and expansion of agricultural activities on marginal lands. Creation
of corridors/linkages between the fragmented forest patches; commencement and
implementation of the already well drafted land and land resource policies and
regulations; commencement of the holistic management plans in the area were
recommended for a sustainable land use. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.
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Monitoring serial changes in coastal grasslands invaded by Chromolaena odorata (L.) R.M. King and Robinson.Goodall, Jeremy Marshall. 17 December 2013 (has links)
The objective of this study was to describe the impacts of the density of Chromolaena odorata
(chromolaena) on species composition in coastal grasslands and to investigate serial changes in
the vegetation following the implementation of a burning programme. The thesis deals with key
ecological concepts and issues, so a comprehensive literature review is included.
Chromolaena invades coastal grasslands that are not burnt regularly (i.e. biennially). Grasslands
that were not burnt for 30 years were seral to secondary forest. The successional pathway from
open grassland to closed canopy forest varied according to soil type. Coastal grasslands on
Glenrosa soils were characterised by savanna at an intermediate stage between the grassland and
forest states. Shading ended the persistence of savanna species (e.g. Combretum molle,
Dichrostachys cinerea and Heteropyxis natalensis) in forest, whereas forest precursors (e.g.
Canthium inerme, Maytenus undata and Protorhus longifolia) only established where fire was
absent. Chromolaena infestations were characterised by multi-stemmed adult plants of variable
height (i.e. 1-3 m), depending on soil type. Regic sands did not support stratified woody
vegetation and chromolaena infestations were self-supporting, reaching a maximum height of
1.5 m. Glenrosa soils supported tree communities and chromolaena reached more than 3 m in
places. The density of chromolaena affected species composition in grasslands with moderate to
dense stands (> 5 adult plants m ¯² or >50000 shrubs ha ¯¹). Chromolaena stands became monospecific when the number of adult plants exceeded 7 m ¯². Succession to forest also ceased
once chromolaena became thicket-forming.
Fire-induced mortality of the chromolaena depended on grass fuel loads. Grass cover of 30% (c.
1 000 kg ha ¯¹) was required to achieve 80% mortality of the parent infestation after the initial
burn. Dense infestations could only be killed by running head-fires from adjacent grasslands
into thickets. Under conditions where head-fires could not be used, infestations were slashed
and burnt at the height of the dry season (July to August) to achieve an 80% kill rate. Seedlings
were killed (99%) by annual burning in sparse (≤ 10000 shrubs ha ¯¹) to moderate
< 50 000 shrubs ha ¯¹) infestations. The suppression of chromolaena and other alien species,
establishing on bare ground after clearing dense infestations, required chemical control until
grass cover was sufficient (i.e. 1 000 kg ha ¯¹) to effect uniform burning. Certain secondary alien
invaders (e.g. Lantana camara, Psidium guajava and Solanum mauritianum) persisted by
coppicing profusely after fire and herbicides need to be integrated into burning programmes
when these species occur. Grasslands on regic sands (e.g. Ischaemum fasciculatum, Panicum
dregeanum and Themeda triandra) were more resilient to the modifying effects of woody
vegetation, than grasslands on Glenrosa soils. Grasslands on Glenrosa soils did not revert to an
open state but persisted as ruderal savanna grassland (e.g. Eragrostis curvula, Hyparrhenia
tamba and Cymbopogon validus) once fire-resistant tree species (e.g. Combretum molle and
Heteropyxis natalensis) had established.
Depending the objectives for land management and the vegetation's condition, coastal
grasslands can be rehabilitated and managed in multiple states, i.e. grassland, savanna or forest
communities. A state-and-transition model based on the empirical data recorded in the study is
presented and shows chromolaena altering vegetation states from open grassland to chromolaena
dominated thicket. The model illustrates chromolaena thickets as the dominant phase of a moist
coastal forest/savanna succession, irrespective of soil type, in absence of appropriate land
management practices (e.g. control burning and integrated control of alien vegetation). This
model should aid in planning strategies for the control of chromolaena in subtropical grasslands in South Africa. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2000.
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Re-vegetation dynamics of land cleared of Acacia mearnsii (black wattle)Glaum, Melanie Jane. January 2005 (has links)
The overall aim of the study was to investigate re-vegetation of disturbed sites, using nursery grown
plugs (from seedling trays) of Themeda triandra, Heteropogon contortus and Hyparrhenia dregeana in
order to reach practical management guidelines for re-vegetation using indigenous grass plugs. A
number of field trials were set up at Kamberg Nature Reserve (29°24'S, 29°40'E) on a site that was
clear felled of A. mearnsii in October 1997. The trials were established in January 1998 and January
1999. A total of approximately 52 ,000 nursery raised plugs of T. triandra, H. contortus and
H. dregeana were planted into an area of approximately 7,000 m2
.
In the planting density trial , plugs of H. dregeana only and a combination of T. triandra/H. contortus
were planted at 15 cm and 30 cm spacings. The T. triandra/H. contortus combination at 30 spacing
showed the greatest survival and lateral plant growth (tiller number and basal area) and this
combination is thus recommended.
In the over-sowing trials, the H. dregeana and T. triandra/H. contortus combination at both 15 cm and
30 cm spacing were over-sown with E. curvula. The survival and lateral growth of the T. triandra/
H. contortus combination at 30 cm was again greater than the other treatments. Over-sowing with
E. curvula suppressed the survival and lateral growth of the planted plugs across all treatments
compared to not over-sowing. The over-sown conditions showed a significant decrease in the
diversity of the plots, both in the number of species present and the Shannon diversity index.
An area that had been cleared of A. mearnsii and sown to E. curvula 25 years previously was shown
to have a lower number of species than the neighbouring veld. Nursery raised plugs of T. triandra
were planted into the mature E. curvula in an attempt to improve the biodiversity of these areas. To
re-introduce T. triandra into these E. curvula swards the plugs must be planted into the centre of a
gap rather than around the base of an E. curvula plant. For improved survival of the plugs the
E. curvula tufts must be clipped, while for best lateral growth the E. curvula tufts must be sprayed with
a glyphosate herbicide three months prior to planting and clipping. However, the added expense of
spraying and clipping is not warranted as the clipped treatments also showed good growth.
Transplant shock is common when planting nursery raised plugs out into the field, as there is a
relatively small root volume in the plug compared to the above ground leaf biomass. Alleviation of
moisture stress at planting using a starch based polymer with high water holding capacity
(Terrasorb®) and a white, needle punched geo-fabric (Agrilen®) to provide a seven day period of
artificial shade after planting did not show significant improvements over the control with regards to
survival or plant growth. Thus these methods of moisture amelioration are not recommended in revegetation
through planting of plugs at this study site.
A trial was established to investigate the biomass production of six different treatments to determine
their potential to support a fire. The total biomass for the plots which were over-sown by E. tef and
planted to only H. dregeana were on average sufficient for a fire, but there was a discontinuous fuel
load across these plots, especially in the replications that had very low survival rates and thus these
plots could not be burnt. The control and herbicide sprayed plots also showed sufficient fuel load for
a fire, but this fuel load was made up of A. mearnsii saplings and bramble with very little grass cover
and thus a fire would not have burnt through these plots either. The T. triandra/H. contortus
combination did not produce sufficient fuel load, due to poor survival. Thus only the plots over-sown
with E. curvula were able to burn in this trial and as a burning trial per se the trial was abandoned.
Seed bearing hay (thatch) was collected in early summer (December 1997) and late summer (April
1998). Both times of year of harvesting proved to be successful in terms of grass cover, although the
early harvested thatch had a greater number of species per plot. The Shannon diversity indexes of
the two treatments were not significantly different. The multi-response permutation procedure
technique confirmed that there was a compositional difference between the treatments. By the end of
the trial Harpochloa falx and T. triandra and H. dregeana were indicators for the early and the late
harvested thatch respectively. Comparing the thatching trial and the planting density trial indicated
that the T. triandra/H. contortus combination at 30 cm spacing would be recommended to maximize
biodiversity.
The summer months have been shown to be the best time to plant the plugs, although the actual
success will be dependant on the conditions within a particular year. The plugs should not be kept in
the nursery for longer than three months and larger plugs (96 seedlings per tray) should be used.
Nursery raised plugs of T. triandra and H. contortus were planted in an equal mix in an area that was
cleared of A. mearnsii in 1996. By June 1998 661 H. contortus seedlings and 14 T. triandra seedlings
had germinated naturally. The November 1998 population consisted of 418 H. contortus seedlings
and 18 T. triandra seedlings. By May 2000 the June 1998 population showed a survival of 78.4% and
the November 1998 population showed a survival of 91 .1 %.
In the various trials, the ability of the nursery raised plugs used for re-vegetation to suppress the regrowth
of A. mearnsii was investigated by determining the number of A. mearnsii seedlings per metre
squared. The plant spacing and species of plugs used did not have a significant effect on the number
of A. mearnsii seedlings per metre squared. Over-sowing with E. curvula did, however, significantly
suppress the wattle re-growth. In the thatching trial the early harvested plots showed lower numbers
of A. mearnsii per metre squared than the late harvest plots, as they were covered with a thick layer
of thatch soon after the A. mearnsii was cleared which suppressed the A. mearnsii re-growth.
Although E. curvula is able to produce a high biomass and suppress the A. meansii seedlings, it has
a detrimental effect on the biodiversity of the area. Therefore, in conservation areas, where
biodiversity is of great importance the planted plugs (at 30 cm spacing) or seed bearing hay must be
used in preference to sowing E. curvula , although it must be remembered that greater follow up
control is likely to be needed with planted plugs or seed bearing hay. The area must be planted or
thatched as soon as possible after clear felling to provide competition for the A. mearnsii seedlings. / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
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The responses of grasses to fire and bush clearing in the Hluhluwe Game Reserve.Graham, Philip Mark. January 1992 (has links)
Contemporary and historical studies of the flora of the Hluhluwe Game Reserve (HGR), have emphasised the woody component whilst little work has been performed on the herbaceous vegetation. This is particularly true with regard to the responses of grasses to historical fire and bush clearing. This study attempted to elucidate some of these responses. Of all the variables considered in this study, woody cover, altitude, the number of fire events, geological and soil parent material are the most important affecting the abundance of grasses in this reserve. Most of these variables are not independent in their effects on grass abundance, with varying degrees of correlation between each other. Certain species appear to be restricted to particular geological substrates. Along with successional changes in the composition and cover of the woody community, due to seemingly inevitable bush encroachment, there is a parallel change in grassland communities in this reserve. In the absence of clearing, numerous fires, higher altitudes, igneous geology and soils derived from igneous parent material delays this succession, whilst sedimentary geology at lower altitudes and fire frequencies accelerates the trend to high woody cover and associated grass species. The grass communities in HGR were shown to be significantly affected by bush clearing and fire. Specifically the number of clearings and fire events, physical bush clearing during 1957 - 1963 and chemical bush clearing during 1968 - 1978. From the responses of species in relation to the various key environmental variables, viz. geological substrate, woody cover and burning and clearing, a model of species response to these variables was developed. With increased fire and bush clearing frequency, the grass communities shift from closed woodland, shade tolerant species through to more open fire climax grassland. These are also more productive communities producing palatable grasses. Validation of aspects of the model were successful - the model having a relatively high predictive capability. Further testing of the model over different substrates and under different clearing regimes is necessary. With regular fires and re-clearing in some bush cleared areas, the vegetation of this reserve should be able to be maintained as productive and diverse grasslands. In the absence of this management, the grass communities will shift towards species associated with woodlands. Bush clearing activities would appear to be most effective over sites on igneous substrate, at higher altitudes, where successional rates are slowest. This is in comparison to sites at lower altitudes over sedimentary geology. / Thesis (M.Sc.)-University of Natal, Durban, 1992.
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Vegetation change over fifty years in humid grasslands of KwaZulu-Natal (Acocks's sites)Marriott, David John. 23 December 2013 (has links)
Eighty three of Acocks's sites, originally surveyed about 50
years ago, were resurveyed in 1996 to determine the extent of
grassland change in the humid grasslands of KwaZulu-Natal. Sites
were relocated using 1:10 000 scale ortho-photos and present land
cover was determined for each site. Forty six of the sites that
were still under original grassland were further examined to
determine present species composition. A survey method was
designed that would emulate Acocks's data and comparisons were
drawn between original and present species composition. These
differences were then analysed together with some environmental
variables to try to determine the factors which had the most
influence on the change and which environment and management
factors are related to the present variation in composition among
sites.
Of the 83 sites, 26 had changed from natural vegetation to some
other form of agriculture such as forestry or cultivation. Most
of this change had occurred in the Natal Mistbelt Ngongoni Veld
where large areas are forested. Cultivation is found
predominantly in the communal areas where subsistence, cultivation
practices are employed.
The remainder of the sites had changed significantly in terms of
their species composition. The most pronounced change had
occurred in areas under communal tenure although significant
changes had occurred in the commercially farmed areas. The
direction of change was also more consistent towards species that
commonly predominate in heavily grazed areas in the communal
areas compared to the commercial areas. The exact reasons for
this were unclear but this could possibly be attributed to
heavier stocking rates in the communal areas.
Change in floristic composition was also more pronounced at lower
altitudes where the mean annual rainfall is lower and the mean
annual temperature higher. This could possibly be a result of the vegetation at lower altitudes being less stable and thus less
resistant to change.
Basal cover differed significantly between commercial and
communally grazed sites. Lower basal cover was found in the
communal sites where intensive grazing limits the growth of
individual tufts. Number of species found at each site did not
differ significantly between communal and commercially grazed
sites.
This study was also a practical implementation of the resurveying
of Acocks's sites and the original data set was found to be a
useful baseline data set to determine coarse long-term changes
in the vegetation. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1997.
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Aboveground production and growth dynamics of vascular bog plants in central SwedenBackéus, Ingvar. January 1985 (has links)
Thesis (doctoral)--Uppsala University, 1985. / Includes bibliographical references (p. [92]-98).
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Modélisation de la végétation boréale et de sa dynamique dans le modèle de surface continentale ORCHIDEE / Modeling of the boreal vegetation and its dynamics in the ORCHIDEE continental land surface schemeDruel, Arsène 23 January 2017 (has links)
L’évolution du climat sur les prochaines dizaines voire centaines d’années pose de nombreuses interrogations, du fait de l’impact de l’homme. Les émissions de gaz à effet de serre depuis le début de l’ère industrielle entrainent une augmentation des températures. Celle-ci est susceptible d’affecter les écosystèmes terrestres, notamment dans les régions boréales où les augmentations de température observées et projetées sont plus importantes. Une évolution de ces écosystèmes peut entrainer des rétroactions sur le climat. Ainsi le phénomène actuel observé de verdissement des régions boréales (ou « Arctic greening ») peut augmenter ce réchauffement via une diminution de l’albédo. Afin de répondre à ces interrogations, des modèles climatiques ont été développés, intégrant des modèles de surface continentale représentant les flux de matière et d’énergie. Le travail effectué dans cette thèse a été mené à partir de l’un d’eux, le modèle de surface continentale ORCHIDEE, qui comprend une description succincte de la végétation boréale. L’objectif de cette thèse était donc l’implémentation puis la modélisation de la végétation boréale.Afin de décrire la végétation présente au niveau des hautes latitudes, i.e. les toundras et les steppes, de nouveaux types de végétation (PFTs) ont été intégrés au modèle à partir des PFTs déjà présents. Tout d’abord, les plantes non vasculaires (NVPs) ont été introduites pour représenter les lichens et les bryophytes, ensuite les buissons pour représenter une strate intermédiaire entre les arbres et les herbacées, et enfin des herbacées C3 boréales pour distinguer la végétation considérée dans les steppes boréales et les prairies tempérées. La description de cette végétation boréale s’est accompagnée de l’intégration de nouveaux processus caractéristiques, allant de l’implémentation d’interactions nouvelles telles que la protection des buissons par la neige en hiver, au simple choix de nouveaux paramètres du PFT, en passant par la modification de processus déjà présents dans le modèle comme la conductance stomatique des NVPs. D’autres processus en lien avec la végétation ont également été mis à jour ou corrigés. Enfin, pour modéliser la dynamique de la végétation boréale, les nouveaux PFTs ont été intégrés à la description initialement présente dans le modèle.Ces modifications ont permis de modéliser la végétation boréale et ses impacts sur les autres variables du système (flux de matière ou d’énergie), soit avec une végétation prescrite (simulations de la période récente), soit avec une végétation dynamique (simulations présentes et futures, à partir des scénarios RCPs 4.5 et 8.5). Les simulations effectuées avec la végétation prescrite montrent que l’on représente mieux le comportement de la végétation avec les nouveaux PFTs. Avec les PFTs originaux la productivité et la biomasse étaient surestimées dans les régions boréales et entrainaient une sous-estimation de l’albédo et une surestimation de la transpiration. Les simulations avec une végétation dynamique ont démontré la capacité du modèle à représenter avec la nouvelle végétation boréale les biomes actuels ainsi que l’« Arctic greening ». Par contre, l’embuissonement observé dans plusieurs études n’a pas été reproduit. Globalement l’introduction des PFTs boréaux s’est traduite par une meilleure description des écosystèmes arctiques et des échanges d’énergie et de matière avec l’atmosphère. Par contre, la protection du pergélisol par les NVPs n’a pas été aussi importante qu’attendu et a été compensée par une augmentation de l’humidité du sol.L’introduction de la nouvelle végétation boréale dans le modèle ORCHIDEE semble donc pertinente et met en évidence l’importance de la représentation de ces écosystèmes. Ce travail ouvre donc des perspectives pour améliorer les simulations climatiques, tant futures que passées. Comme la modélisation de la végétation depuis l’Holocène afin de simuler la quantité de carbone contenu aujourd’hui dans le pergélisol. / Climate evolution over the next ten to hundred years involves many questions, linked to the impact of man. Indeed, greenhouse gases emissions since the beginning of the industrial era lead to an increase in temperature. The latter can affect terrestrial ecosystems, particularly in boreal regions where observed and projected temperature increase is larger than in mid-latitudes. Evolution of these ecosystems can trigger climate feedbacks. For example, the currently observed « Arctic greening » phenomenon could enhance the warming via a decrease in albedo due to the increase in vegetation cover. In order to address these questions, climate models were developped, including continental surface models taking into account the fluxes of mass and energy. In this thesis, such a model was used, the continental surface scheme ORCHIDEE, which includes a succinct description of boreal vegetation. The aim of this work was thus the implementation and the modeling of boreal vegetation.In order to describe high-latitude vegetation, i.e. toundras and steppes, new plant functional types (PFTs) were integrated into the model based on existing PFTs. First, non-vascular plants (NVPs) were integrated to represent lichens and bryophytes found in desert toundras and peatlands, then shrubs to represent an intermediate stratum between trees and grasses in toundras, and finally boreal C3 grasses to distinguish vegetation found in boreal steppes and temperate grasslands. The description of this boreal vegetation was accompanied by the integration of new charachteristic processes, from the implementation of new interactions such as the protection of shrubs by snow in winter, to the simple choice of new PFT parameters such as the lower photosynthetic capacity of boreal C3 grasses compared to temperate C3 grasses, through the modification of existing processes such as the stomatal conductance of NVPs. Other processes linked to vegetation were also updated or corrected. Finally, to model the dynamics of boreal vegetation, new PFTs were integrated into the initial description in the model.Those changes enabled the modeling of boreal vegetation and its impact on other variables (mass or energy fluxes), either using a prescribed vegetation (simulations on the recent period), or using a dynamical vegetation (recent and future simulations using RCPs 4.5 and 8.5). Simulations using the prescribed vegetation indicated that vegetation behaviour is better represented with the new PFTs. With original PFTs, productivity and biomass were overestimated in boreal regions, and lead to an underestimation of albedo and an overestimation of transpiration. Simulations using a dynamical vegetation demonstrated the ability of the model, using the new boreal vegetation, to represent current-day biomes as well as « Arctic greening ». However, the shrubification observed in several studies was not reproduced. Similarly, the impact of new PFTs on other model outputs is important, with for example a decrease in productivity or albedo in winter compared to the original vegetation. Thus, the introduction of boreal PFTs generally resulted in a better description of Arctic ecosystems and of the exchanges of energy and mass with the atmosphere. On the other hand, the protection of permafrost by NVPs was not as substantial as expected and was compensated by an increase in soil humidity (due to shrubs and boreal grasses).The introduction of the new boreal vegetation in the ORCHIDEE model thus seems relevant, and highlights the importance of representing these ecosystems. This work opens up new perspectives to improve future and past climate simulations. The next step consists in modeling vegetation since the Holocene into the future in order to simulate the current amounts of carbone in the permafrost, and to project the outcome of these stocks in the context of climate change and permafrost melt.
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