Logging effects on liana diversity and abundance in Central Guyana

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Pre-logging diversity 25 20 15 Pibiri, pre-harvest observed logseries S 10 5 0 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 Abundance class, upper boundary Figure 3.2 Distribution of liana species abundances in the pre-harvest census of Pibiri (data from all 15 plots combined), Sample A. S is the number of species per abundance class. The dots and line give the distribution predicted by the log series. S total = 101. Sample B also followed a log series. The distribution of species over abundance classes in the pre-harvest plots in Pibiri is adequately described by a log series. It is an underlying assumption for the calculation of Fisher’s α, that species are distributed as in a log series. The distribution expected in a log series was not statistically different from the observed distribution (α=15.95; x=0.99823; χ 2 10=11.34, n.s.). The expected log series distribution is shown by a line in Figure 3.2. Pibiri, pre-harvest 100 Relative abundance (%) 10 1 0.1 Connarus perrottetii 0.01 0 20 40 60 80 100 Species Rank Figure 3.3 Rank-abundance plot of the 101 species present in Sample A in the pre-harvest plots at Pibiri. Abundance (in %) is plotted on a log scale, and is fitted by log(relative abundance) = 0.52 - 0.066*(rank); R 2 =0.952. Connarus perrottetii is by far the commonest species in Pibiri. Essentially the same information as in Figure 3.2 is provided by the rank-abundance plot, in which species are ranked from highest to lowest abundance and their proportion in the total population plotted on a log scale (Figure 3.3). Of 101 species found in the 8989 individuals of Sample A in Pibiri prior to harvest, eighteen species each accounted for more than 1% of the individuals, while 21 species were represented by a single individual. 3.1.4 Size class distribution Most natural populations show a highly skewed distribution of individuals over the various size classes. This was no different for the liana communities near Mabura Hill. Undisturbed populations (pre-logging and control plots) showed a high abundance of small individuals (Figure 3.4), whereas large lianas were scarce. On average, fewer than 20 lianas with a diameter of >10 cm were present per hectare. The number of seedlings (height 50% of correlations between pairs of plots was >0.99), the number of individuals per size class varied markedly between plots. Apart from seedling numbers, which are always variable due to local 25
<strong>Logging</strong> <strong>effects</strong> on liana diversity and abundance in Central Guyana variations in seed dispersal and germination, the highest variability was found among lianas 3-4 cm dbh. Further analysis showed that differences in population size distribution were not geographically determined, i.e. plots that were located close together were as different from each other as plots further apart (data not shown). Differences in response to logging are therefore probably not systematically related to differences in pre-harvest size class distributions. Figure 3.4 Size class distribution of lianas in undisturbed rain forest in Pibiri. Means and standard deviation of 15 one ha plots, 10 5 Pibiri, pre-harvest 10 4 Number of Individuals 10 3 10 2 10 1 10 0 H0.5 H1 H2 1 2 3 4 5 10 20 50 Size Class based on all data. Note log scale on Y-axis. Classes preceded by ‘H” are based on height 3.1.5 Diversity-size trends Species differ in maximum size, which suggests that diversity in the larger size classes would be different from lower size classes. Indeed, species density peaks at c. 1 cm dbh, after which it decreases with increasing size (Figure 3.5). Because of the decreasing trend in liana abundance with increasing size (Figure 3.4), the reduced number of species in the large size classes can be explained as a density or sample effect rather than a real decrease in species richness. As a result, Fisher’s α fluctuates a bit without trend (Figure 3.5, middle). In all size classes based on dbh, Fisher’s α is approximately constant between 11 and 15 (Sample A, calculated for the aggregated pre-harvest data from Pibiri), but smaller size classes (based on height and Sample B) appear to be less diverse than the dbh-classes. This reduced diversity in smaller size classes is somewhat counterintuitive, as these classes are very abundant and are, in principle, expected to contain individuals of each regenerating species found in the plot. Their lesser diversity can be understood if the species-abundance pattern of small individuals is very much different from large individuals, if seedling populations are ephemeral: irregular regeneration events followed by rapid mortality and growth into larger size classes and thus have a low “detection probability” in a single census, or if species are missed in the census due to identification problems. The trend in Simpson’s index, which is a measure of dominance, shows much stronger fluctuations. Classes with a low value of the index (Figure 3.5, right) are highly dominated by Connarus perrottetii. (This is illustrated by its abundance in Figure 3.5, right hand panel). Curiously, this dominance shows a dip in diameter classes less than 0.5 cm, coinciding with a near absence of C. perrottetii. A low Simpson’s index, i.e., a high dominance of C. perrottetii, in the 1-2 cm classes might be due to an accumulation of shrub phase individuals of C. perrottetii. This species grows up as a freestanding shrub before it starts climbing. If climbing opportunities (suitable supports, trellises) are limiting for the growth of C. perrottetii into larger size classes, then individuals may accumulate in the shrub-class and thus depress Simpson’s index. 26
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Logging effects on liana diversity and abundance in Central Guyana

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