RESEARCH: OUTCOMES, VIEWPOINTS & PERSPECTIVES

Late Quaternary palynological records from Spain highlight the influence of ecosystem properties in the long-term dynamics of mediterranean vegetation.

I have contended that traditional deterministic views of vegetation-climate response are not satisfactory to explain the patterns of Late Quaternary vegetation change in Mediterranean Spain.

A particular state of the vegetation may appear determined by its biotic history rather than by the abiotic site properties. The role of fire in shaping Mediterranean vegetation has been also underestimated in palaeoecological research, and disregarded in floristic-phytosociological and other equilibrial models.

In a contingent picture of vegetation dynamics, subtle differences in initial conditions during full glacial and lateglacial times, would have tended to cascade and affect the outcome of post-glacial events, so that it is statistically improbable to duplicate the exact sequence of vegetation types for a particular site. Phenomena such as fire, hervibory, catastrophic events, deforestation, and competitive interactions would be contingent while remaining compatible with the determinism of the climate system.

EXAMPLES:

NAVARRÉS: Pine forests dominated the glacial landscape, and there is evidence for a full glacial expansion of Pinus pinaster, Quercus faginea, Quercus ilex-rotundifolia, and Erica arborea at about 30,200-27,900 yr BP. Pine forests resisted invasion until about 5900 yr BP, even though oaks and other temperate trees occurred in the region from several thousands before. The variation of macro- and microcharcoal throughout the core demonstrates that pine forests were only replaced by evergreen-oak scrub after local fire disturbance. In sum, the sequence is explicable by (i) millennial-scale inertia of the established full-glacial pine forests, (ii) threshold response of local forests to increased fire frequency and virulence, (iii) competitively-mediated invasion of pine forests by oak scrub after disturbance and establishment of a new ecological structure. Interestingly, slight climatic changes within the pleniglacial provoked a more sensitive vegetation response than major climate fluctuations inherent to the establishment of the present interglacial
( SEE POLLEN DIAGRAMS OF NAVARRES: NAVARRÉS I, NAVARRÉS II, NAVARRÉS III, NAVARRÉS IV, NAVARRÉS INTEPRETATION, NAVARRÉS LATEGLACIAL)
( SEE SUMMARY POLLEN DIAGRAM OF NAVARRÉS).

VILLAVERDE: Pinus is dominant from about 8400 to 5100 yr BP. Pines remained dominant for more than a millennium even though climate had become more humid, and there was a rich pool of available potential free colonists. The ulterior abrupt shift towards deciduous Quercus dominance, estimated to occur within c.10-33 years, could be a threshold response ultimately mediated by climate. A change towards dominance by evergreen Quercus communities is observed within the mid Holocene over a period of 300-400 years. This change can be viewed as the consequence of competitive interactions following a trend of increased aridity, which would have been critically manifested in the pollen record at about 3200, 2600, 2300, and 1700 yr BP. The last spread of Pinus which brought about a permanent modification of the ecosystem, does not occur until c. 1680 yr BP. The fact that pine peaks since about 3200 yr BP to 1700 yr BP are preceded by charcoal increases, envisage a fire disturbance-mediated invasion of mixed and evergreen oaks forests by Pinus
( SEE POLLEN DIAGRAMS: VILLAVERDE I, VILLAVERDE II, VILLAVERDE INTEPRETATION)
( SEE SUMMARY POLLEN DIAGRAM OF VILLAVERDE).

The pollen sequence of SILES may allow insights into the vegetation history of the supramediterranean areas of the Segura mountains from c. 20,300 cal yr BP to nearly present. The increase of Zygnemataceae preceding the first spread of Pinus nigra suggest that climate warming might have occurred several centuries earlier, so that the sudden invasion by Pinus may be conceived as a threshold response to climatic change. The partial replacement of P. nigra by P. pinaster during the zone SP3 (c. 10,100-7400 cal yr BP) is preceded by increases in charcoal abundance and synchronous of high values in xerophyte pollen and indicators of lake desiccation. The invasion of pine forests by mesophilous species after c. 7400 cal yr BP is envisaged as a threshold response to increased water availability. The expansion of Mediterranean vegetation after c. 5900 cal yr BP coincides with limnological indication at SM8-SM9 towards more pronounced summer drought. As suggested by the records of abrupt shifts of Pseudoschizaea and microcharcoal during SP5, the balance between P. nigra and P. pinaster could have been episodically broken by dry spells, perhaps associated to fire disturbance. The palynological record (Sordariaceae, Riccia, Thecaphora, Trichuris, and Polygonum aviculare) coincide to suggest overgrazing in the lake catchment since c. 2400-2300 cal yr BP (SP7-SP9), and arable agriculture (Plantago, Cerealia, Vitis, Puccinia) since c. 1400 cal yr BP. The Riccia curve starts several hundred years before the grass expansion/ forest decline at SP7, and therefore, the continuous occurrences of Berberis, Rhamnus and Genisteae in phase with those anthropogenic indicators since SP8 suggest they may be related with expansion of thorny scrub following historical overgrazing on the grassland. During at least seven times throughout SP7-SP8, supramediterranean pine forests may have been pushed over a threshold by anthropogenic disturbance leading to the spread of grassland, thorny scrub, junipers, and nitrophilous communities. Plausibly, the vulnerability thresholds of supramediterranean Pinus nigra forests would be crossed by the combined action of dry climate, fire activity, and critically, pastoralism
( SEE INTERPRETATION OF THE SILES PALYNO-RECORD)
( SEE POLLEN DIAGRAM OF THE LAKE SILES)
( SEE COMPARISON OF PATTERNS OF VEGETATION CHANGE IN SEVERAL PALYNORECORDS FROM THE SEGURA MOUNTAINS REGION IN THE SOUTHERN SPAIN).

GÁDOR: The microcharcoal record envisages a picture of fire incidence broadly related with hydroclimatic variation. Microcharcoal particles are especially abundant from c. 4200 cal yr BP, reaching maxima values between c. 2100 and 1600 cal yr BP. This pathway is not altered taphonomically, that is, there is a good correlation between the curves of the two categories of microcharcoal particles. The most outstanding pattern that emerges from the microfossil charcoal records concerns the frequency of major episodes of fire. As average, these account each 300-400 years from the bottom up to c. 4100-4200 cal yr BP, then changing to each 100-200 years until the top of the sequence
( SEE SUMMARY POLLEN DIAGRAM OF THE SITE OF GÁDOR)
( SEE INTERPRETATION OF THE PALAEOECOLOGICAL RECORD OF GÁDOR).

This major change to higher fire frequency precedes a replacement of deciduous Quercus by Pinus and evergreen Quercus. This change towards more sclerophyllous vegetation occur during the Prehistoric Argaric period (c. 4250-3600-3500 cal yr BP). Archaeological literature is prone to uphold enhanced intensity of human impacts in the region respecting the former Los Millares period (c. 5400-4250 cal yr BP), including the occurrence of lowland and midland forest destruction and matorralization. The Argaric termination, in sharp contact with an ulterior, more disperse Final Bronze pattern of settlement (c. 3500-2900 cal yr BP), is viewed as socio-economic collapse.

Correlation with regional and extraregional climate proxies strengths the climatic argument. Regional vegetation during the mid Holocene attains a mesophytic optimum, maximum forest development, and xerophytic minimum. In contrast, the late Holocene emerges as a generally dry, pyrophytic period of pine forests and spread of xerophytic communities, under the context of dry spells, and shallowing and desiccation of lakes, cesation of peat accumulation and diminishing soil moisture in valley bottoms

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CARRIÓN, J.S., SÁNCHEZ-GÓMEZ, P. & MOTA, J. 2003. Fire and grazing are contingent on the Holocene vegetation dynamics of Sierra de Gádor, southern Spain. The Holocene 13: 839-849

CARRIÓN, J.S. 2001. Dialectic with climatic interpretations of Late-Quaternary vegetation history in Mediterranean Spain. Journal of Mediterranean Ecology 2: 145-156

CARRIÓN, J.S. 2001. Condicionantes de la respuesta vegetal al cambio climático. Una perspectiva paleobiológica. Acta Botanica Malacitana 26: 157-176

CARRIÓN, J.S, ANDRADE, A., BENNETT, K.D., MUNUERA, M. & NAVARRO, C. 2001. Crossing forest thresholds: inertia and collapse in a Holocene pollen sequence from south-central Spain. The Holocene 11: 635-653