Changes in liana density over 30 years in a Bornean rain forest supports the escape hypothesis

Newbery, D. M.; Zahnd, C. (4 August 2020). Changes in liana density over 30 years in a Bornean rain forest supports the escape hypothesis (bioRxiv). Cold Spring Harbor Laboratory 10.1101/2020.08.03.234500

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In two permanent plots of lowland dipterocarp forest at Danum a liana census in 1988 was repeated in 2018 using the same method. Trees ≥ 30 cm gbh (girth at breast height) were recorded for number of lianas ≥ 2 cm gbh on their stems. The forest was evidently in a late stage of recovery from a large natural disturbance. Over the interval, mean number of lianas per tree decreased by 22 and 34 % in plots 1 and 2. By 2018 there were relatively more trees with few lianas and relatively fewer trees with many lianas than in 1988. The redistribution was strongest for overstorey trees of the Dipterocarpaceae (more with no lianas by 2018) and understorey trees of the Euphorbiaceae (many losing high loads in plot 2). Proportion of trees with lianas rose overall by 3.5%. Number of lianas per tree showed a quadratic relationship with tree size (ln[gbh]): maximal for large trees, fewer for smaller and very large trees. Tree survival and stem growth rate were significantly negatively related to number of lianas, after accounting for spatial autocorrelation. Monte Carlo random samples of half of trees in 1988 were compared with the other half of trees in 2018. Relative frequency distributions differed significantly over time, but dipterocarps and euphorbs varied noticeably in their liana dynamics between plots. Regressions achieved best significant fits when number of lianas was a function of date, ln(gbh) and ln(gbh)2, but differently in the plots reflecting complicated host-liana dynamics. Analysis of most abundant trees species, individually, highlighted a group of emergent dipterocarps with low liana counts decreasing with time. Building on an earlier hypothesis, these trees lose their lianas with branch shedding, as they move into, and emerge from, the main canopy. They escape from the parasite. The process may in part explain the uneven nature of the forest canopy at Danum. Change in liana density was contingent on forest history and site succession, and plot-level structure and dynamics. Liana promotion in intermittent dry periods was seemingly being offset by closing of the forest and continued dominance by the Dipterocarpaceae.

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