15.4.2 Open forests

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Open forests are commonly dominated by eucalypts with a canopy cover of between 30–70%. They occur in both tropical and temperate zones, and most frequently in wetter coastal areas. In tropical open forests the understorey is most frequently dominated by grasses (Section 15.4.1); in temperate zones the understorey can range from dense shrub to grasses. Canopy height and canopy cover increase with increasing plant-available moisture.

Growth of any plant community and thus its structure, productivity and biodiversity depends on the relationship between soil water and evaporative demand. More leaves means greater transpiration and more water required to maintain that community. In any canopy, upper exposed foliage is the most critical determinant of transpiration and experiences the strongest leaf to air vapour pressure difference. Community dynamics are thus governed by evaporative conditions and the amount of foliage in direct contact with the atmosphere (foliage projective cover, FPC %, of the community; sensu Winkworth and Goodall 1962).

Potential water loss from a plant community is driven by the gradient of water potential in the soil–plant–atmosphere continuum (from soil to atmosphere, ΔΨSPAC). Actual water loss is modified by canopy resistance (canopy boundary layer, rbl) plus canopy leaf resistance (rl × FPC). Thus, water flux through the soil–plant–atmosphere continuum (represented here by actual evapotranspiration (ETa) over time t becomes:

where ΔΨSPAC = overall decrease in water potential within the soil–plant–atmosphere continuum (mean over time t);
rbl = boundary-layer resistance (mean over time t);
rl = leaf resistance (mean over time t);
FPC = foliage projective cover (scale 0 to 1).

Annual flow of water through the soil–plant–atmosphere continuum is represented by ETa (Specht and Specht 1989). ETa approaches potential evaporation, Epan (evaporation of water from a class A pan), in humid environments during the wettest months of the year (Epan is considered the maximum for any area and is taken as an annual constant within a particular location). The ratio between actual evapotrans-piration and potential evaporation (ETa/Epan) can be ex-pressed as the product of the available water (Wavail) and an evaporative coefficient (ke). Moisture index (MI) as defined by Specht (1972) and Specht and Specht (1989) then becomes ETa/Epan, thus:

The evaporative coefficient, ke, therefore represents the rate at which actual evapotranspiration is able to approach potential evaporation under the conditions of available water. Values for ke generally range between 0.0 and 0.1.

Climate type            Evaporative coefficient (ke)
Perhumid climate >0.075
Humid climate 0.055–0.075
Subhumid climate 0.045–0.055
Semi-arid climate 0.035–0.045
Arid climate <0.035




Figure 15.26 Foliage projective cover (%) recorded for the overstorey of 56 Australian plant communities plotted against the evaporative coefficient (ke), an inverse measure of evaporative demand. Mesic condition favour canopy development, and that relationship is apparent here as a highly significant correlation (n = 63, r2 = 0.80) between ke and foliage projective cover. (Based on original data compiled by A. Specht)



Figure 15.27 Specific leaf area (SLA, cm-2 g-1 dry mass) of discs punched from the lamina of dominant Eucalyptus spp. Is related to the evaporative coefficient (ke) of the sampling site. Slopes are consistent across species groups, implying a coming mechanism underlying SLA:ke relationships, but intercepts differ according to tropical, subtropical and temperate climatic zones. As the atmosphere gets drier, average SLA declines because average leaves get thicker.

Tropical SLA = 1183.2 × ke + 9.8 (n = 15, r2 = 0.96)
Subtropical SLA = 1257.8 × ke – 15.3 (n = 27, r2 = 0.93)
Temperate SLA = 1087.1 × ke – 19.25 (n = 15, r2 = 0.93)

(Based on original data compiled by A. Specht)



Figure 15.28 Annual leaf litter fall (kg ha-1 year-1, oven dry mass) from the overstorey of mature evergreen plant communities shows a curvilinear relationship with the evaporative coefficient (ke). Leaf litter is an indicator of canopy mass and turnover, and thus productivity. This index declines with atmospheric dryness. Annual leaf litter fall = 295.6 × 1012.2ke (n = 35, r2 = 0.80)

Values for ke are directly correlated with FPC % of the overstorey trees or shrubs (Figure 15.26). This clearly expresses the first effect of water availability and balance on a plant community; without an adequate water supply, leaf development is restricted.

Sclerophylly is also directly related to water flux through the soil–plant–atmosphere continuum. Overstorey species of eucalypt forests become more mesophytic with increased water flux as expressed by the evaporative coefficient (ke) (Figure 15.27). This effect is temperature dependent, with more mesophytic leaves in the tropics than in the temperate zone.

Interception of sunlight determines annual canopy pro-duction (Section 12.3) so that annual leaf litter fall increases exponentially with an increasing flux of water through the plant community (Figure 15.28). Production of canopy biomass is clearly related to the foliage projective cover of the overstorey and hence total photosynthetic production. Interestingly, the increase in production with increasing foliage projective cover (linearly related to evaporative coefficient (ke) in Figure 15.26) is exponential rather than linear (Specht and Specht 1989).

In a forest, photoassimilate is distributed between many individuals. The number of individuals, their stand density and their species richness can all be related to the amount of water flowing through the plant community. Just as annual canopy productivity is exponentially related to the flux of water through the plant community and hence foliage projective cover, so the number of stems per unit area, and potentially biodiversity, is also related. In moist environments, stem density is highest, while in drier environments fewer stems survive (cf. Figure 15.22). This effect intensifies with annual temperature regime as the growing season is prolonged (Specht and Specht 1993, 1994). This is reflected in the species diversity of the community; wetter and warmer conditions (and a long growing season) predispose towards more ‘niches’ (sensu Grime 1979).