Testing heterogeneity-diversity relationships in tropical forest restoration

Environmental Studies Department, University of California, Santa Cruz, CA, 95064, USA, .
Oecologia (Impact Factor: 3.09). 03/2013; 173(2). DOI: 10.1007/s00442-013-2632-9
Source: PubMed


Restoring small-scale habitat heterogeneity in highly diverse systems, like tropical forests, is a conservation challenge and offers an excellent opportunity to test factors affecting community assembly. We investigated whether (1) the applied nucleation restoration strategy (planting tree islands) resulted in higher habitat heterogeneity than more homogeneous forest restoration approaches, (2) increased heterogeneity resulted in more diverse tree recruitment, and (3) the mean or coefficient of variation of habitat variables best explained tree recruitment. We measured soil nutrients, overstory and understory vegetation structure, and tree recruitment at six sites with three 5- to 7-year-old restoration treatments: control (no planting), planted tree islands, and conventional, mixed-species tree plantations. Canopy openness and soil base saturation were more variable in island treatments than in controls and plantations, whereas most soil nutrients had similar coefficients of variation across treatments, and bare ground was more variable in control plots. Seedling and sapling species density were equivalent in plantations and islands, and were substantially higher than in controls. Species spatial turnover, diversity, and richness were similar in island and plantation treatments. Mean canopy openness, rather than heterogeneity, explained the largest proportion of variance in species density. Our results show that, whereas canopy openness and soil base saturation are more heterogeneous with the applied nucleation restoration strategy, this pattern does not translate into greater tree diversity. The lack of a heterogeneity-diversity relationship is likely due to the fact that recruits respond more strongly to mean resource gradients than variability at this early stage in succession, and that seed dispersal limitation likely reduces the available species pool. Results show that planting tree islands facilitates tree recruitment to a similar degree as intensive plantation-style restoration strategies.

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Available from: Karen Holl, Feb 27, 2014
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    • "Comparisons of cross-treatment differences in Ecosynth canopy openness were similar to those quantified using field-based densiometer measurements (Holl et al., 2013), and a strong correlation between Ecosynth and field-measured data was found. Ecosynth canopy roughness comparisons showed that island treatments have more heterogeneous canopy cover than passive or plantation treatments, which is consistent with results of field-based measures (Holl et al., 2013). Despite similar treatment level results, Ecosynth data correlated relatively poorly with the standard deviation of height, which is often used as a field-based proxy indicator of canopy roughness. "
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    • "This could be because large, bat-dispersed seeds were spatially clumped below feeding roosts (Melo et al. 2009) and thus difficult to detect with passive seed rain monitoring. A purported advantage of applied nucleation is that it is a light-handed intervention compared to tree plantations (Corbin and Holl 2012), and it promotes more heterogeneous habitat conditions (Reis et al. 2010, Holl et al. 2013). Because only one-third as many trees were initially planted in applied nucleation treatments, these plots may have fewer unintended consequences as a result of species selection. "
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