Questions: Natural regeneration is increasingly recognized as a potentially costeffective strategy to reach ambitious forest landscape restoration targets, but rates of recovery are notoriously variable. We asked how well initial habitat conditions after cessation of agriculture predict forest recovery after nearly a decade. We aimed to provide land managers with general rules of thumb to assess when it is necessary to invest resources in active restoration, such as tree planting, to accelerate forest recovery.Location: Coto Brus County, Puntarenas, Costa Rica. Methods:We compiled data on initial vegetation structure, soil nutrients, prior landuse history and surrounding forest cover at 13 sites. After 8.5 years, we measured vegetation indicators commonly used to assess forest recovery, namely amount of canopy closure and number and diversity of woody recruits.Results: Two variables, grass cover and canopy closure, measured 1.5 years after site abandonment, explained 47-87% of five of the six response variables after 8.5 years; recovery was faster in sites with lower grass cover and higher canopy closure initially.Waiting an additional year to measure initial vegetation variables did not improve model fit. Time since the original forest was cleared explained 62% of change in canopy cover, whereas percentage of surrounding forest cover, length of pasture use and soil variables explained minimal additional variation. Conclusions:Our results suggest that two easily measurable vegetation variables can provide guidance to land managers and policy makers about where to invest scarce restoration resources to facilitate forest recovery. K E Y W O R D Scanopy closure, forest recovery indicators, forest restoration, grass cover, natural regeneration, secondary succession, seedling recruitment S U PP O RTI N G I N FO R M ATI O NAdditional supporting information may be found online in the Supporting Information section at the end of the article. Appendix S1. Study and site locations Appendix S2. Site characteristics Appendix S3. Vegetation sampling design Appendix S4. Correlations between initial site variables Appendix S5. Δ recruit density 8.5-1.5 as a function of canopy closure including outlier site Appendix S6. Recruit species list How to cite this article: Holl KD, Reid JL, Oviedo-Brenes F, Kulikowski AJ, Zahawi RA. Rules of thumb for predicting tropical forest recovery.

Fires are an important and increasingly common driver of habitat structure in the intermountain West. Through an ongoing study of burned and adjacent unburned areas along the John D. Rockefeller, Jr. Memorial Parkway, we examine the long-term effects of the 1988 fire season on community assembly, succession, and ecological processes. We collected mark/recapture data on rodents, removal data for insectivorous mammals and invertebrates, and habitat measurements on four grids in 2014 and combined these results with previous survey data. In 2014, 4,800 trap nights yielded 13 species of small mammals, comprising 618 individuals. Macroarthropod abundance was higher on burned grids, but diversity was higher on unburned grids. In contrast, springtail (Collembola) diversity was higher on burned grids, but abundance was highest in unburned grids. Since the beginning of this long-term study, the total number of mammal species has increased across all sites, and relative abundance in burned areas has shifted from early successional species (Peromyscus maniculatus) to those more associated with old growth forests (such as Myodes gapperi). Other than in 1991, the burned grids have harbored more diverse small mammal communities than the unburned control grids. Significant, long-term differences in vegetation based upon burn history were observed, including different ground cover, less canopy cover, and more coarse woody debris in burned sites. This work provides a unique long-term picture of the interrelationships of small mammal and invertebrate communities and correlated habitat variables as these ecosystems undergo post-fire succession.

Forest restoration is increasingly heralded as a global strategy to conserve biodiversity and mitigate climate change, yet long-term studies that compare the effects of different restoration strategies on tree recruit demographics are lacking. We measured tree recruit survival and growth annually in three restoration treatments—natural regeneration, applied nucleation and tree plantations—replicated at 13 sites in southern Costa Rica—and evaluated the changes over a decade. Early-successional seedlings had 14% higher survival probability in the applied nucleation than natural regeneration treatments. Early-successional sapling growth rates were initially 227% faster in natural regeneration and 127% faster in applied nucleation than plantation plots but converged across restoration treatments over time. Later-successional seedling and sapling survival were similar across treatments but later-successional sapling growth rates were 39% faster in applied nucleation than in plantation treatments. Results indicate that applied nucleation was equally or more effective in enhancing survival and growth of naturally recruited trees than the more resource-intensive plantation treatment, highlighting its promise as a restoration strategy. Finally, tree recruit dynamics changed quickly over the 10-year period, underscoring the importance of multi-year studies to compare restoration interventions and guide ambitious forest restoration efforts planned for the coming decades. This article is part of the theme issue ‘Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration’.

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