Detection of critical densities associated with pinon-juniper woodland ecotones

The interfaces between piñon-juniper canopies and grasslands in the south-western USA present an opportunity to use the modern theory of spatial phase transitions as a formal characterization of ecotone structure. The theory requires an estimation of a critical value of tree cover at which the woodl...

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Vydáno v:	Ecology (Durham) Ročník 77; číslo 3; s. 805 - 821
Hlavní autoři:	Milne, Bruce T., Johnson, Alan R., Keitt, Timothy H., Hatfield, Colleen A., David, John, Hraber, Peter T.
Médium:	Journal Article
Jazyk:	angličtina
Vydáno:	Washington, DC Ecological Society of America 01.04.1996
Témata:	aerial photography Animal, plant and microbial ecology Biological and medical sciences CANOPY climate COUVERT CUBIERTA DE COPAS Density Ecological modeling Ecology Ecotones Environmental aspects environmental factors Forest ecology Fractals Fundamental and applied biological sciences. Psychology General aspects. Techniques grasses GRASSLANDS HERBAGE Juniper JUNIPERUS JUNIPERUS MONOSPERMA Landscape ecology Landscapes NEW MEXICO NOUVEAU MEXIQUE NUEVO MEXICO PINUS EDULIS Pinyon pines pinyon-juniper plant density PLANT POPULATION Plant spacing POBLACION VEGETAL POPULATION VEGETALE PRADERAS Southwestern United States SPATIAL VARIATION Teledetection and vegetation maps Trees Woodlands United States New Mexico North America America USA, Southwest Southwestern United States Grassland Forests Critical value Phase transitions Fragmentation Vegetation map Ecotone Edge effect Gymnospermae Softwood forest tree Percolation Spermatophyta Coniferales Plant cover Population density
ISSN:	0012-9658, 1939-9170
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Shrnutí:	The interfaces between piñon-juniper canopies and grasslands in the south-western USA present an opportunity to use the modern theory of spatial phase transitions as a formal characterization of ecotone structure. The theory requires an estimation of a critical value of tree cover at which the woodlands switch abruptly from a fragmented collection of small patches of trees to a network of connected canopies. Presumably, this transition is associated with critical environmental conditions that regulate the ecologies of trees vs. grasses. We developed and tested a new method to estimate the critical cover value of woodlands on complex terrain. The method was based on multiscale assessments of the associations between local tree coverage and two types of patch edge. Tests on artificial gradient percolation maps revealed an interaction between the type of edge used ("hull edge," which is based on only the orthogonal connections between canopy-occupied cells, vs. "accessible edge," which is based on both orthogonal and diagonal connections between canopy-occupied cells) and the neighborhood rule used to define a cluster (von Neumann 5-cell or Moore 9-cell). When applied to digitized, geographically referenced aerial photographs, the method indicated that areas ≤ 6.6 ha exhibited the theoretical critical value of 59.3% cover predicted for square lattices and the 5-cell neighborhood. Construction of both edge types on a given map can reveal locations of steep environmental gradients that may be buffered against modest climate fluctuations. The edges can be used in the calibration of independent variables to predict tree cover. The agreement between the expected and observed critical densities will motivate extensions of phase transition theory to studies of ecotones in real landscapes.
Bibliografie:	9616575 F40 SourceType-Scholarly Journals-1 ObjectType-Feature-1 content type line 14 ObjectType-Article-2 content type line 23 ObjectType-Article-1 ObjectType-Feature-2
ISSN:	0012-9658 1939-9170
DOI:	10.2307/2265503