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.


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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    ABSTRACT: Large areas of tropical lands are being removed from agriculture and restored to address conservation goals. However, monitoring the ecological value of these efforts at the individual landowner scale is rare, owing largely to issues of cost and accessibility. Traditional field-based measures for assessing forest recovery and habitat quality can be labour intensive and costly. Here we assess whether remote sensing measurements from lightweight unmanned aerial vehicles (UAV) are a cost-effective substitute for traditional field measures. An inexpensive UAV-based remote sensing methodology, ''Ecosynth'', was applied to measure forest canopy structure across field plots in a 7–9-yr tropical forest restoration study in southern Costa Rica. Ecosynth methods combine aerial images from consumer-grade digital cameras with computer vision software to generate 3D 'point cloud' models of vegetation at high spatial resolutions. Ecosynth canopy structure measurements were compared to field-based measures and their ability to predict the abundance of frugivorous birds; key seed dispersers that are sensitive to canopy structure. Ecosynth canopy height measurements were highly correlated with field-based measurements (R 2 P 0.85), a result comparable in precision to LiDAR-based remote sensing measurements. Ecosynth parameters were also strongly correlated with above-ground biomass (R 2 P 0.81) and percent canopy openness (R 2 = 0.82). Correlations were weaker with proportion-based measures such as canopy roughness (R 2 = 0.53). Several Ecosynth metrics (e.g., canopy openness and height) predicted frugivore presence and abundance at levels of accuracy similar to those of field-based measurements. Ecosynth UAV remote-sensing provides an effective alternate methodology to traditional field-based measures of evaluating forest structure and complexity across landscapes. Furthermore, given the volume of data that can be generated in a single flight plan, as well as the ability to use the technology in remote areas, these methods could expand the scope of studies on forest dynamics and recovery when combined with field-based calibration plots.
    Biological Conservation 06/2015; 186:287–295. DOI:10.1016/j.biocon.2015.03.031 · 3.76 Impact Factor
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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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    ABSTRACT: Past studies have shown that tropical forest regeneration on degraded farmlands is initially limited by lack of seed dispersal, but few studies have tracked changes in abundance and composition of seed rain past the first few years after land abandonment. We measured seed rain for 12 months in 10 6–9-year-old restoration sites and five mature, reference forests in southern Costa Rica in order to learn (1) if seed rain limitation persists past the first few years of regeneration; (2) how restoration treatments influence seed community structure and composition; and (3) whether seed rain limitation is contingent on landscape context. Each restoration site contained three 0.25-ha treatment plots: (1) a naturally regenerating control, (2) tree islands, and (3) a mixed-species tree plantation. Sites spanned a deforestation gradient with 9–89% forest area within 500 m around the treatment plots. Contrary to previous studies, we found that tree seeds were abundant and ubiquitous across all treatment plots (585.1 6 142.0 seeds�m�2�yr�1 [mean 6 SE]), indicating that lack of seed rain ceased to limit forest regeneration within the first decade of recovery. Pioneer trees and shrubs comprised the vast majority of seeds, but compositional differences between restoration sites and reference forests were driven by rarer, large-seeded species. Large, animal-dispersed tree seeds were more abundant in tree islands (4.6 6 2.9 seeds�m�2�yr�1) and plantations (5.8 6 3.0 seeds�m�2�yr�1) than control plots (0.2 6 0.1 seeds�m�2�yr�1), contributing to greater tree species richness in actively restored plots. Planted tree species accounted for ,1% of seeds. We found little evidence for landscape forest cover effects on seed rain, consistent with previous studies. We conclude that seed rain limitation shifted from an initial, complete lack of tree seeds to a specific limitation on large-seeded, mature forest species over the first decade. Although total seed abundance was equal among restoration treatments, tree plantations and tree islands continued to diversify seed rain communities compared to naturally regenerating controls. Compositional differences between regenerating plots and mature forests suggest that large-seeded tree species are appropriate candidates for enrichment planting.
    Ecological Applications 06/2015; 25(4):1072-1082. DOI:10.1890/14-1399.1 · 4.09 Impact Factor
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    • "This commonly applied strategy accelerates forest recovery by encouraging animal seed dispersal, reducing cover of light-demanding pasture grasses, ameliorating microclimatic conditions, and enhancing nutrient availability (Chazdon, 2008; Lamb, 2011). Planting large areas of land with trees, however, can be costly (Lamb et al., 2005; Kanowski et al., 2008), and result in more homogeneous abiotic conditions than natural recovery (Holl et al., 2013). Moreover, the planted species selected can strongly influence biomass accumulation rates, nutrient cycling, and composition of naturally establishing species (Cusack and Montagnini, 2004; Celentano et al., 2011), particularly since fast-growing, low wood density species, including some N-fixers, are often selected for restoration plantings (Lamb, 2011). "
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    ABSTRACT: Secondary forests comprise an increasing area of the tropics and play an important role in global carbon cycling. We compare above-ground biomass accumulation of both planted and naturally regenerating trees, as well as C in the top soil layer, in three restoration treatments replicated at 14, six to eight year old restoration sites in southern Costa Rica. Restoration strategies include: control (no planting), planting tree islands, and conventional, mixed-species tree plantations. We evaluate the importance of past land-use, soil nutrients, understory cover, and surrounding forest cover in explaining variation in above-ground biomass accumulation (ABA) rate across sites. Total ABA and planted tree ABA rate were highest in plantations, intermediate in islands, and lowest in control treatments, whereas ABA rate of naturally regenerating trees did not differ across treatments. Most ABA in plantations (89%) and islands (70%) was due to growth of planted trees. Soil carbon did not change significantly over the time period of the study in any treatment. The majority of across-site variation in both total and planted tree ABA rate was explained by duration of prior pasture use. Tree growth in the first two years after planting explained approximately two-thirds of the variation in ABA rate after 6–8 years. Soil nutrient concentrations explained relatively little of the variation in planted or naturally recruiting ABA rate. Our results show that planting trees substantially increases biomass accumulation during the first several years of forest recovery in former agricultural lands and that past-land use has a strong effect on the rate of biomass accumulation. Planting tree islands is a cost-effective strategy for increasing ABA and creating more heterogeneous habitat conditions than tree plantations. We recommend small scale planting trials to quickly assess potential biomass accumulation and prioritize sites for ecosystem service payments for carbon sequestration.
    Forest Ecology and Management 05/2014; 319:36–43. DOI:10.1016/j.foreco.2014.01.024 · 2.66 Impact Factor
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