Genetically conditioned mechanisms which restrict or limit gene exchange between coexisting species or population systems permit the preservation of their genetic integrity, and afford evolutionary independence In the absence of genetically controlled isolation, coexisting populations would fuse into a single variable population The fact that biotically sympatric populations which differ in adaptive mode almost invariably maintain their identity is prima facie evidence for their genetic isolation Genetic isolation is evident in some form in essentially every genus where it has been sought The degree of isolation need not be a function of genetic divergence or taxonomic distance, although in most instances a positive correlation may be found

The Origin of Isolating Mechanisms in Flowering Plants

Decision tree analysis was used to predict the distribution of forest communities in an area on the south coast of New South Wales, Australia. The analysis was carried out using a geographical information system environmental data base of those topographic and geological variables thought to influence the distribution of vegetation and derived from cartographic sources. The resulting maps of forest communities are of a resolution sufficient to delimit individual forest stands and contain much ecological information.

A new method for predicting vegetation distributions using decision tree analysis in a geographic information system

Postglacial forest patterns associated with till and outwash in northcentral Upper Michigan

Distributions of common species of Eucalyptus in south-eastem Australia are related to gradients in temperature and rainfall. To determine whether intrinsic water-use efficiency (as indexed by carbon isotope discrimination, Δ) or other leaf attributes were related to climate of native habitats, we sampled 17 populations representing 14 species of Eucalyptus growing in a common garden in south- central New South Wales. Phreatophytes were clearly distinguished from populations that are totally dependent upon soil moisture derived from rainfall by having higher Δ at a particular level of rainfall. Among 12 non-phreatophytic populations (11 species), Δ was positively correlated with mean annual precipitation (r = 0.75, P = 0.005), December-March precipitation (r = 0.79, P = 0.002), an index of annual soil moisture (r = 0.81, P = 0.001) and seasonality of precipitation (r = 0.85, P < 0.001). There were similarly strong but negative correlations between Δ and potential evaporation during the summer months, but Δ was not correlated with annual potential evaporation of the source sites. Leaf mass per unit area (ρe) was negatively correlated with indices of water availability, positively correlated with nitrogen per unit leaf area (r = 0.90, P < 0.001), and negatively correlated with Δ (r = -0.73, P = 0.007). A was negatively correlated with area-based leaf nitrogen (r = -0.79, P = 0.002). These complementary correlations among Δ, ρe, and nitrogen per unit leaf area indicate that variation in Δ may stem largely from variation in photosynthetic capacity. The results provide strong evidence that variation in Δ and ρe reflect genetic adaptations to native habitats.

Correlations Between Carbon Isotope Discrimination and Climate of Native Habitats for Diverse Eucalypt Taxa Growing in a Common Garden

Few direct tests of phosphorus (P) limitation on highly-weathered soils have been conducted, especially in mature, native Eucalyptus stands. We tested whether growth in a mature >80-year old stand of Eucalyptus tereticornis in Cumberland Plain Woodland was limited by P, and whether this P-limitation affected leaf photosynthetic capacity. P was added to trees at the native woodland site at 50 kg ha-1 year-1 in each of 3 years, and stem and leaf responses were measured. Leaf P concentrations before fertilisation were < 1 mg g-1 and N:P ratios ranged between 16 and 23. Addition of 50 kg ha-1 year-1 of P increased leaf P concentration significantly (+50 %) compared to non-fertilised trees, for two but not for the 3 years. Despite higher leaf P in fertilised trees, photosynthetic capacity was unaffected. However, there was a 54 % increase in tree stem basal area growth during the first and second years of P fertilisation, statistically significant in the second year of the experiment. Our evidence shows that E. tereticornis is P-limited on Cumberland Plain soils. This has implications for forest responses to rising atmospheric [CO2], because photosynthesis in elevated [CO2] may become further constrained by required phosphate pools within the photosynthetic apparatus.

Is phosphorus limiting in a mature Eucalyptus woodland? Phosphorus fertilisation stimulates stem growth

The chemical and physical soil characteristics of the Eucalyptus associations in Benandra State Forest are described. Changes in species associations occur over a gradient of increasing moisture and nutrient status of soils from ridge to gully sites. Concentrations of major nutrients in leaves and bark of dominant trees on these sites generally follow the soil gradients. In particular, leaf phosphorus and bark calcium are correlated with the corresponding soil nutrients. Soil calcium and magnesium are also correlated with leaf phosphorus, indicating a possible interaction between these soil nutrients and phosphorus uptake by the trees. Soil magnesium is similarly correlated with bark calcium. Aluminum is the major exchangeable cation in soils of the poorer ridge sites which are the most acid. It is hypothesized that soil aluminum competes with other nutrient cations such as calcium and magnesium for sites of absorption on plant roots. In addition to the effect of soil pH on phosphate solubility, competiti...

https://www.jstor.org/stable/info/1933883

Soil‐Plant Relationships in a Eucalyptus Forest on the South Coast of New South Wales

Growth of Eucalyptus gummifera and E. maculata seedlings is statistically correlated with the concentrations of a number of soil nutrients. Step—wise multiple linear regression models of seedling growth are presented with soil nitrogen, phosphorus, calcium, and aluminum as independent variables. Regression models with soil calcium and phosphorus as independent variables give the best estimates of seedling growth. See full-text article at JSTOR

J. G. McColl

Papers

Soil‐Plant Relationships in a Eucalyptus Forest on the South Coast of New South Wales

Regression models relating soil nutrients and growth of Eucalyptus gummifera and Eucalyptus maculata seedlings.