On the integrated interpretation of indirect site ordinations: a case study using semi-arid vegetation in southeastern Spain

The need for rigorous methods of interpreting indirect site ordinations is discussed and the problem of oblique habitat and phytosociological trends is emphasized. A sequential approach, termed integrated interpretation, is introduced to overcome this problem. It first involves a technique termed rotational correlation a linear form of trend surface analysis, which locates the best fit between a habitat variable and the coordinates of a pluridimensional ordination. The coordinates at positions of best fit are then used to produce two-way site-species tables (seriation arrays) which summarize floristic variation in relation to each major habitat trend. The seriation arrays can also be used to identify groups of differential species. Integrated interpretation is demonstrated using semi-arid vegetation from Murcia Province, SE Spain. A two-axis site ordination of vegetation data by non-metric multidimensional scaling is shown to have two oblique trends related to aspect-induced topoclimates and types of past anthropogenic disturbance. Rotation of the configuration to maximum linear correlation achieves very high levels of explanation for each factor. Rotated ordination coordinates from the correlation analysis are used to obtain two seriation arrays and floristic noda which relate directly to the two underlying habitat trends. A floristic discontinuity is revealed on the topoclimate continuum and it is hypothesized that temperature conditions are a major determinant of floristic composition, but that moisture partly determines vegetational cover and phytomass. Several patterns of past cultivation are detected and associated with five main kinds of geomorphological unit, together with some evidence that the rate of succession is partly determined by topoclimate. The integrated approach to interpretation is compared with visual methods and other multivariate techniques. A case is made for increased interpretational rigour in site coordinate studies, especially the discontinuation of trend-seeking using individual unrotated ordination axes.Nomenclature of vascular plants follows Tutin et al. (1964–1980) Flora Europaea 5 Vols. Cambridge University Press, Cambridge. Nomenclature of syntaxa follows Rivas-Goday & Rivas-Martinez (1967).I am indebted to J. A. Michel, Dr M. A. Abdelrahman and Monica M. Dargie for field assistance. Field discussion with Dr R. L. Wright, Dr M. P. Austin and Professor F. Esteve Chueca was most helpful; the latter assisted with confirmation of vascular plant voucher specimens. Non-vascular taxonomy was determined by Dr C. Humphries (mosses and one alga) and Dr O. L. Gilbert (lichens). An anonymous referee suggested an approximation of the analytical solution to rotational correlation, and Dr N. R. J. Fieller assisted with its confirmation. The project was assisted by a grant from the University of Sheffield Research Fund.     

Mineral cycling in a tropical palm forest

Nutrient cycling and biomass characteristics of a tropical palm forest dominated byOrbignya cohune were found to be different from thsoe of hardwood dominated forests. The cohune palm forest had a high proportion of biomass in leaves (5%), a reduced sapling layer, a large amount of standing forest litter and an exceptionally low decomposition rate factor (0.1 year^–1). Mineral concentrations in palm leaves were generally lower than in hardwood species with the exception of Na, which was exceptionally high inOrbignya cohune. Biomass was estimated at 226 tons ha^–1 containing 1173 kg ha^–1 N; 126 kg ha^–1 P; 437 kg ha^–1 K; 1869 kg ha^–1 Mg; 125 kg ha^–1 Ca, and 2177 kg ha^–1 Na. Soils of cohune association did not differ significantly from those of neighbouring hardwood dominated associations with the exception of Na which occurred in higher concentration because of bioaccumulation in the dominant. The results suggest that the growth habits and physiology of a dominant can strongly influence some of the ecological parameters used to describe aforest association.     

Änderungen in der Stickstoffversorgung auf Dauerflächen im Brachland

Summary Starting in 1968 plant succession and nitrogen status in the top soil of an old field was investigated. The soil is a calcareous loam which was sterilized by heating. In the absence of human influence distinct successional stages with characteristic floristical and physiognomical features were observed. During the first six years vegetational development was characterized by competition between therophytes and hemicryptophytes. Agricultural treatments such as annual ploughing (spring, summer) and cutting (autumn) were found to change the trend of the succession.Five years after starting the experiment total nitrogen content in the top soil of the control area had increased slightly, while the ploughed plots persisted in their low values. A comparison of the nitrogen mineralisation between 1970 and 1974 showed decreasing amounts of mineral nitrogen in the later stage (1970: 67 kg N_min/ha/30 weeks; 33 kg N_min/ha/30 weeks). On the other hand, mineral nitrogen supply on the ploughed plots was not significantly different after the five-years-period. It is suggested that in old-field vegetation the external nitrogen cycle of the early therophyte stage changed to an internal one when long-lived hemicryptophytes, rhizome- and root-budding geophytes became dominant.Tables 1 to 4 show the vegetation development under different treatments; the figures represent average cover values per vegetation season using Londo's (1975) scale. Table 5 contains the total nitrogen concentration values in the uppermost of 10 cm of mineral soil, and Table 6 lists the values for soil volume, mean soil temperatures, average water content, and mineral nitrogen supplies for 1970 and 1974.

Contribution to the Symposium of the Working Group for Succession Research on Permanent Plots, held at Yerseke, the Netherlands, October 1975.     

Aspects of population dynamics in Halimione portulacoides communities

Summary Studies on sample plots inHalimione portulacoides communities show that environmental disturbances, either natural or induced by man, start a sequence of partly overlapping density maxima inSuaeda maritima, Aster tripolium andPuccinellia maritima successively, before the originalHalimione community totally recovers. When succession time before recovering is long enough, there are tendencies in redundancy of this sequence stressing the unilinear character of the succession. Minor environmental impacts induce a longer time-lag period of theSuaeda density maximum, suggesting threshold values of these impacts for the species to maintain minimal population densities or to become locally extinct. This sequence of interim species starting after an environmental disturbance, suggests also a gradient character in various biological attributes, for instance in life-time, propagation, nutrient and genetic plasticity strategies. The mechanism described can therefore be interpreted as a complex of mostly well-adapted and well-integrated inherent species strategies capable of absorbing environmental shocks. It is suggested that in the salt-marsh ccosystem the pattern of spatial variation in densities and that of temporal variation in fluctuations of the three species populations under natural conditions reflect corresponding patterns of environmental disturbances in the vegetation taking into account a timelag associated with the magnitude of the impact concerned.Contribution to the Symposium on Plant species and plant communities held at Nijmegen, 11–12 November 1976, on the occasion of the 60th birthday of Professor Victor Westhoff.Nomenclature follows Heukels-van Ooststroom. Flora van Nederland, 18e druk, 1975. Wolters-Noodhoff, Groningen.The authors are greatly indebted to Dr K.F. Vaas (Yerseke) for reviewing the English text.Communication Nr. 160.     

Post-glacial vegetation in the South Patagonian territory of the giant ground sloth, Mylodon

By comparison with the major modern plant communities of southern Patagonia, the changing post-glacial vegetation of the region is reconstructed from macrofossils derived from deposits in the Cueva del Mylodon, Ultima Esperanza, S. Chile. The oldest deposits, carbon-dated as c. 12,400 B.P., comprise dung of the extinct ground sloth Mylodon darwinii and show the animal to have fed entirely on Cyperaceae, Gramineae and species associated with these in the modern cool, wet sedge-grasslands of western Patagonia, communities which would be expected after the retreat of the ice. Overlying deposits of well-preserved, wind-blown leaf-litter permitted some quantitative analyses which show a rise of evergreen forest dominated by Nothofagus betuloides that reached its maximum c. 7000 B.P. and then declined as it was replaced by deciduous N. pumilio forest. A break in the fossil plant record, covering the human occupation about 5643 B.P. and the subsequent final appearance of Mylodon remains, is followed by evidence of mixed evergreen/deciduous forest in which Nothofagus pumilio gradually increases in importance to give deciduous forest some 2500 years ago similar to that found in the environs of the cave in historical times. Comparison with pollen diagrams shows that the modern climatic and vegetation difference between E. Fudgia and Ultima Eeperanza has persisted throughout the post-glacial period.     

江苏省海滩植被演替的研究

江苏省海滩植被可分为滨海盐土植被、盐沼植被及海滩沙生植被三个基本类型。本文论述了这些植被类型的演替规律。滨海盐土植被与盐沼植被的演替,外因于土壤盐分含量递减与有机质含量的递增;海滩沙生植被的演替,外因于土壤沙颗粒大小及其相应的土壤含水量的变化,所以海滩植被演替为外因动态演替。     

Seasonal and spatial patterns of S,Ca, and N dynamics of a Northern Hardwood forest ecosystem

Seasonal dynamics of S, Ca and N were examined at the Huntington Forest, a northern hardwood ecosystem in the central Adirondacks of New York for a period of 34 months (1985–1988). Solute concentrations and fluxes in bulk precipitation, throughfall (TF) and leachates from the forest floor, E horizon and B horizon were quantified. Both above and below-ground elemental fluxes mediated by vegetation (e.g. uptake, litter inputs, and fine roots production) were also determined. The roles of abiotic and biotic processes were ascertained based on both changes in solute concentrations through the strata of the ecosystem as well as differences between dormant and growing seasons. Concentrations of SO₄ ²⁻, NO₃ ⁻, NH₄ ⁺ and Ca²⁺ were greater in TF than precipitation. Forest floor leachates had greater concentrations of SO₄ ²⁻, NO₃ ⁻ + NH₄ ⁺ and Ca²⁺ (9, 6 and 77 μeq L⁻¹, respectively) than TF. There were differences in concentrations of ions in leachates from the forest floor between the dormant and growing seasons presumably due to vegetation uptake and microbial immobilization. Concentrations and fluxes of NO₃ ⁻ and NH; were greatest in early spring followed by a rapid decline which coincided with a demand for N by vegetation in late spring. Vegetation uptake (44.7 kg N ha⁻¹ yr⁻¹ ) could account for the low leaching rates of N0₃ ⁻. Within the mineral soil, changes with soil depth and the absence of seasonal patterns suggest that cation exchange (Ca⁺) or anion sorption (SO₄ ²⁻) are primarily responsible for regulating solute concentrations. The increase in SO₄ ²⁻ concentration after leachates passed through the mineral soil may be attributed to desorption of sulfate that was adsorbed during an earlier period when SO₄ ²⁻ concentrations would have been greater due to elevated S inputs.     

Microsite differentiation in a Mediterranean oak savanna

Abstract. Microscale variation of vegetation was studied in a Mediterranean oak savanna dominated by annual plants at Neve Ya'ar Experimental Station, in the lower Galilee of Israel. Species presence/absence data were recorded in 20 cm x 20 cm quadrats on a 500-quadrat transect. These data were ordinated with Detrended Correspondence Analysis. Soil nitrate, phosphorous, potassium, pH, soil moisture and salinity were determined at 2-m intervals on the 100-m transect, at monthly intervals from November 1981 to April 1982. Litter cover and canopy shade were surveyed in October 1981 and April 1982. A sharp rise in available nitrate occurred in November, following the first winter rains. Patterns in herbaceous vegetation, as expressed by sample ordination scores, correlated best with litter cover in October. This suggested that vegetation pattern in this community may be strongly influenced by environmental conditions at the time of seedling germination and establishment.     

Shell-banding polymorphism of the land snail Xeropicta vestalis along the coastal plain of Israel

Along the coastal plain of Israel, shell darkness of the polymorphic land snail Xeropicta vestalis is positively related to the extent of perennial vegetation. It is not related to rain, temperature, geographic position or darkness of the ground.
White shells reflect more radiation than dark ones do, and therefore in Israel's coastal plain, where solar radiation is very strong, they are favoured. Perennial vegetation, where it occurs, shields the snails from the sun by absorbing radiation, so that snails amongst them can afford to be darker, and thus more cryptic. Hence, the more perennial vegetation in the habitat, the darker the shells.
Also during the Pleistocene, when temperatures were 5–10°C lower than today, the shells of the coastal plain were as pale as recent ones are. The distribution of a snail-predator, Gerbillus allenbyi , which is restricted to sandy biotopes where perennial bushes occur, and which was absent from Israel during the Pleistocene, could perhaps explain the distribution of X. vestalis morphs, both today and in the past.