Methods

The point of this research is to understand the consequences of various agricultural and management practicies on the population growth rates of the four species. To predict population growth rates, we need four rates for each species: recruitment, growth, mortality, and fecundity. The first three requir repeated observations of the same trees over the course of several years. By observing trees, then returning and observing them again, we can observe whether they have survived the time interval and how much they have grown. Repeated observations also allow us to identify the appeareance of new individuals that had not previously been in present in the population, which we refer to as recruitment. Finally, fecundity can be observed directly on Eucalypts, as they retain their fruit for extended periods. Thus, with binoculars from the ground, we can make an estimate of the number of fruit any tree set as we census the plot.

To understand the population dynamics of the four key tree species that make up the canopy of our two threatened ecological communities, we established a set of 25 permanent plots across the New England Tablelands. Each of them was 50 x 50m. In them, we mapped, measured, and identified every tree greater than 1 m in height. In two 2 x 50m strips that crossed through the center of the plot, we additionally marked, measured and identified all seedlings. In total, we marked 3046 individual trees, with 488 E. dalrympleana, 596 E. nova-anglica, 806 E. pauciflora, 849 E. viminalis, and 307 individuals of other Eucalyptus species.

Thus far, we have established the 25 plots, and surveyed them a single time. As a consequense, we do not yet have observations of growth, survival or recruitment. However, we can share with you our results about the abundance and distribution of trees, as well as their fecundity.

How big does a tree need to be in order to set fruit?

This asks how large a tree needs to be before it begins to set fruit.

In these figures, tree size, as diameter at breast height (i. e., 130 cm above the ground, DBH). is on the horizontal axis, and the probability of whether a tree was fruiting or not is shown on the vertical axis. The raw data, in other words, whether a particular individual tree bore fruit, is shown as a pair of hitograpms for each species. The histogram below shows the distribution of sizes of trees that did not fruit, whereas the upper 'hanging' histogram shows the distribution of sizes of trees that did bear fruit. The curve represents the best estimate of the trend between tree size and the probability that it will set fruit.

These values vary enormously among species. E. pauciflora is very likely to set fruit by the time it reaches 80 cm in diameter, while E. dalrympleana trees larger than 80 cm DBH are almost all fruiting. Fewer trees of E. nova-anglica and E. viminalis, on the other hand, were observed with fruit. For these species, even the largest individuals we observed were not all setting fruit.

As a tree grows, how does the number of fruit it makes change?

Here, we ask whether and how the amount of fruit made by a tree changes as it grows.

The interpretation of these graphs is similar to the previous ones: size is on the horizontal axis. But here, the Y axis show s the number of fruit we estimated to be in the canopy of each tree. Note that the Y axis is log-transformed. The number of fruit varied enormously among individuals, and log-transformtion makes it much easier to display. Each point represents an observation on an individual tree, and the curve represents the best estimate of the trend between tree size and the number of fruit it sets. Similar to the previous graph, there's a big difference between E. pauciflora and E. dalrympleana, on one hand, and E. nova-anglica and E. viminalis on the other. The former species attain their maximal number of fruit around a diameter of 60 cm DBH, whereas the latter species continue to increase in fruit set as trees grow larger.

Together, these figures indicate the great importance of large trees to reproduction. This is true for all four species, but especially for E. nova-anglica and E. viminalis.