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| Titel |
Does fire frequency/intensity influence tropical vegetation structure. Or vice versa ? |
| VerfasserIn |
Jon Lloyd, Mireia Torello-Raventos, Elmar Veenendaal, Carlos Alberto Quesada |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049658
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| Zusammenfassung |
| Several theories exist as to the basis for the large variations in the degree of tree cover
observed across the savanna biome - much of which does not seem readily accountable
for in terms of variations in precipitation. These come into basically two classes:
those which invoke differential environmental and/or edaphic effects on tree as
oppossed to grass growth and/or mortality (for example, adverse affects of heavy soil
texture and/or waterlogging) and those which suppose that feedbacks associated
mostly with fire can give rise to a “steady-state” tree-grass mixtures of varying
degrees of woodiness at any given level of precipitation (at least above a certain
threshold). Of related interest is an understanding of which climatic and/or soil
factors determine the geographical location of ”zones of tension” (ZOT) where both
forest and savanna are found growing as discrete mosaics within the same area. And
which specific factors within a ZOT which determine whether forest or savanna is
found.
In order to help clarify the above issues, we analyse data from over 60 forest and savanna
sites sampled on three continents, examining the extent which variations in savanna and/or
forest structure could be accounted for by a wide range of measured variables.
Vegetation structure was analysed using a uniform protocol (see Torello-Raventos et
al.; this session) with soil measurements including texture, exchangeable cation,
total nitrogen and total phosphorus concentrations; not only in surface layers as is
usually the case, but also to 4m where possible; also noting rooting abundances
and possible/absence of physical constraints to plant growth such as hardpans or
compact layers and incorporating this information into an index of soils physical
constraints (Quesada et al., 2010). We also took into account additional environmental
factors, such as mean annual precipitation (P ), a precipitation seasonality index
(Ï) and total soil moisture holding capacity (W ), then using mixture discriminant
analysis to develop of model capable of predicting the occurrence of the 12 vegetation
formations of Torello-Raventos et al on the basis of the measured environmental/edaphic
variables.
The final model was a close to perfect predictor of vegetation class with both climatic and
edaphic factors assuming importance. For example, at the intercontinental scale, the
occurrence of ZOT at considerable lower P in Africa as compared to South America is
explained to a large degree by a lower Ï (due to a generally occurring bimodal seasonal
pattern in Africa), but with higher soil surface cations concentrations in Africa soils
also important. Some previously enigmatic observations are also accountable for
by the model. For example, large variations in woodiness of Brazilian savannas
under the same climatic regime are readily accountable for by grassier savannas
(Campo sujo) having extremely low subsoil [Ca] and an associated lack of root
development or water uptake when compared to proximal woodland vegetation
types. In other cases, soil physical conditions are clearly important. For example,
hardpan development controls vegetation structure at P of around 1 m a-1 in West
Africa.
We conclude that the edaphic controls of tropical vegetation structure may be many, deep
and complex and that cursory surface soil observations provide an inadequate means to assess
the existence or magnitude of such effects. Moreover, though elegant and mathematically
impressive it is not necessary to invoke fire/vegetation feedback models to explain
global variations in savanna structure. Indeed, we suggest that fire characteristics are
more a function of vegetation form (in turn controlled by soils and climate) than
vice versa. The more complex feedback models currently flooding the literature
would seem to be more the construct of some clever human minds, as opposed to
mechanistic explanations for what seems to be (in the absence of digging a few
holes) an enigmatic but false degree of unexplainable variation in savanna/forest
structure.
Reference
Quesada, CA, Lloyd, J, Schwarz, M, Patiño, S, Baker, TR, Czimczik, C, Fyllas, NM,
Martinelli, L, Nardoto, GB, Schmerler, J, Santos, AJB, Hodnett, MG, Herrera,
R, Luizão, F J, Arneth, A, Lloyd, G, Dezzeo, N, Hilke, I, Kuhlmann, I, Raessler,
M, Brand, WA, Geilmann, H, Moraes Filho, JO, Carvalho, FP, Araujo Filho, RN,
Chaves, JE, Cruz Junior, OF, Pimentel, TP and Paiva, R (2010) Chemical and physical
properties of Amazon forest soils in relation to their genesis. Biogeosciences 7,
1515-1541. |
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