What Does a "Natural" Forest Look Like?

The Vital Role of Tree Diversity in Forest Recovery

It’s often alleged that artificial restoration of former forests - i.e. planting trees - is unnecessary in most instances. Forests will come back on their own, through a process known as natural regeneration. But how successful is natural regeneration? Specifically, is this beneficial for the biodiversity of an ecosystem? What trees will come back?

This study in north-eastern Costa Rica on the changes in tree species diversity during natural regeneration over 12–20 years is a critical contribution to understanding forest recovery and the effectiveness of restoration efforts. The methodology employed, including the use of diversity profiles and subsampling techniques, highlights important aspects of assessing biodiversity in regenerating forests. 

Critical Time for Biodiversity Assessment

This study challenges a few key assumptions that people may have about naturally regenerating forests - most interestingly in our view, forests that regenerate naturally see many ups and downs in terms of biodiversity in their first decades of growth. Their findings challenge the traditional view of linear recovery in forest ecosystems, presenting a more nuanced narrative of ecological succession. Rare species do not necessarily come back unassisted, at least based on the sample plots shown in this study. 

The authors posit that the health of a regenerating forest cannot in fact be measured in terms of the prevalence of rare plants. Areas that regenerate naturally should actually have their success measured in terms of the growth of already abundant species. 

A further point of interest from the data shown here is the unreliability of monitoring plots that have a small sample size. For the Costa Rican forests sampled here, the authors concluded that any plot smaller than 0.5 ha is most likely unrepresentative of the larger forest ecosystem. Samples as large as 1 ha may miss out on the rarer species blooming in a naturally regenerating forest. 

By identifying the successional trajectories of tree species diversity in regenerating forests, the study offers guidance for restoration practices. It highlights the importance of considering the age and previous use of the forest in restoration efforts, as these factors can significantly influence the recovery of biodiversity.

Current Challenges in Forest Restoration

Despite significant progress in forest restoration techniques, accurately monitoring and assessing biodiversity remains a formidable challenge. The study identifies key methodological hurdles, such as the need for larger plot sizes to capture comprehensive biodiversity data, especially for rare species. These insights are crucial for enhancing the quality of data used for assessing restoration projects.

Innovating Biodiversity Monitoring with Data-Driven Approaches

Chazdon et al. advocate for the integration of sophisticated biodiversity metrics and statistical models to improve the reliability of ecological assessments. Their use of diversity profiles and Hill numbers, which account for species richness and evenness, provides a more detailed understanding of species distribution and community structure in restored forests.

Open Data & Code

All data used in this study are available at https://doi.org/10.5061/dryad.ncjsxksvr.