86 Natural systems and greenspaces
Natural communities
Anatural communityis the assemblage of interacting species in an ecosys-
tem(Krebs1994,Morin1999). Communities dominated by different plant species
arevegetation types(or ecosystem types), which are determined primarily by cli-
matic and secondarily by soil conditions. Across the globe we recognize broad
vegetation types orbiomes,suchastundra, boreal forest (taiga), temperate decid-
uous forest, grassland, and tropical rain forest. Within each of these, vegetation
types are often associated with microclimatic conditions and differ less dra-
matically, such as on the north vs. south sides of mountains and coastal vs.
inland areas. Also soil conditions frequently differentiate fine-scale vegetation
differences, such as calcareous vegetation on limestone and acidity-dependent
vegetation of a former peat bog. Mapping microclimatic variations and soil types
within an urban greenspace is a key step in planning and designing for habitat
heterogeneity or a diversity of vegetation types.
Urban areas are extremely rich in vegetation or habitat types (Gilbert 1991,
Sukopp et al. 1995, Goddeet al. 1995, Wheater 1999, Boada and Capdevila
2000). One of the many ways to classify urban vegetation is a simple division
into three groups (Hough 2004): (1) native plant community, such as remnant
woods, dominated by native plants; (2) cultivated plant group, such as a garden
area dominated by plants developed by horticulture for urban conditions; and
(3) naturalized plant community, dominated by plants which acclimated or
adapted to urban conditions without human action (e.g., breeding or plant-
ing). Many in the last group are from warmer climes or other countries, and are
favoured by disturbance. Indeed the number of natural communities or vegeta-
tion types is an important component of nature’s richness (biodiversity), and is
also a basis for the richness of experience available to the public.
Vertical, horizontal, and species dimensions describe three types of commu-
nity structure.Ver ticalcommunity structuretypically refers to the distribution or
stratification of layers of vegetation, which are primarily determined by light
intensity. Consider a forest with canopy, subcanopy, understory, shrub, and herb
layers. Each layer might receive about 10 % of the sunlight received by the layer
above it, so plants of the herb or ground layer are adapted to thrive in very low
light conditions. Animals in the forest have different food and cover require-
ments and are relatively different in each vertical layer. Also, dead wood in
standing trunks and branches is a major habitat for many animal groups. In
maintaining high faunal biodiversity, e.g., in a semi-natural park area, the num-
ber of vegetation layers is generally considered primary and the diversity of
plants secondary. The relative loss of vertical community structure is a useful
measure of habitat degradation.