Testing heterogeneity–diversity relationships in tropical forest restoration

Abstract

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, mixedspecies 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.

Cluster planting facilitates survival but not growth in early development of restored tropical forest

“…they should be outplanted close to pioneer trees). Furthermore, outplanting clusters may enhance spatial heterogeneity in the system (Holl, Stout, Reid, & Zahawi 2013), which may promote an increase in diversity in late successional stages.…”

Soil phosphorus heterogeneity promotes tree species diversity and phylogenetic clustering in a tropical seasonal rainforest

“…The mean and coefficient of variation (CV =  SD / M , SD  = standard deviation, M  = mean) of AN and EP were calculated to represent the availability and heterogeneity of these nutrients within each plot (Baer, Blair, Collins, & Knapp, 2004; Douda et al., 2012; Holl et al., 2013). The Shapiro–Wilk test was first implemented to evaluate the normal distribution of all variables; all the soil variables within each plot were log‐transformed to promote normality.…”

Neotropical rainforest restoration: comparing passive, plantation and nucleation approaches

“…Holl et al (2013) used only four species (two fast-growing and N-fixing species and two long-lived species) to facilitate recovery rather than try to plant several of the original species. Lamb (2011), using the ''framework species method '' and Chazdon (2012) suggested 20-30 species as the reference number; but Lamb (2011), in the ''method of maximum diversity'' recommended 80-100 species for highly degraded landscapes.…”