By Leland Werden
Leland Werden, PhD, is a Senior Scientist at ETH Zurich. lwerden@gmail.com
I remember starting my PhD in 2012 with a strong desire to develop tools for recovering tropical forests around the world. My background was in ecosystem ecology, which had led me to spend time thinking about soil nutrient cycling and other belowground processes in different temperate forests across the Northeastern United States. What I didn’t realize at the time was that these forests were themselves an incredible regeneration success story.
As agriculture moved westward to more suitable land between the 1870s and 1920s, New England farmers abandoned their fields and pastures, and forests naturally regenerated at scale. This century of intensive agriculture left a patchwork of land-use legacies that continued to influence forest composition and recovery in fascinating ways. I stumbled into this living laboratory as an undergraduate student at the Harvard Forest Long Term Ecological Research site, measuring forest recovery across this patchwork.
One patch has stuck in my head since: a grove of the largest striped maples (Acer pensylvanicum) I’ve ever seen. Striped maple is typically an understory shrub that rarely reaches more than six meters (20 feet) tall. But in this tiny patch, about 20 individuals had ascended to the canopy and completely dominated the overstory. In chatting with one of the landscape ecologists on staff over lunch one day about this pattern, I learned that there used to be a farmstead there and the striped maples thrived on the elevated nutrients still present in the soil more than a century after the farm was abandoned.
Despite having collected hundreds of soil cores in my life, sieved roots out of soil for months on end, and counted tiny soil fauna, I had never fully grasped how strongly soil type and land-use legacy could shape plant communities. That grove of striped maples taught me to look belowground to understand plant composition. Years later, when I began working in tropical forest restoration, some colleagues and I grew to understand that we were missing the same understanding on a large scale — focusing almost entirely on what grows aboveground while largely ignoring what happens beneath our feet when restoring tropical forests.
This disconnect isn’t unique to tropical restoration. Across forest restoration projects worldwide, we count trees, measure canopy cover, and get excited to see the return of birds and dung beetles. These are important signs of recovery, but soils, roots, and the communities of organisms that live within are often overlooked. These living organisms drive water and nutrient cycling, contribute strongly to climate mitigation capacity, and also to long-term resilience that determine whether restored forests can withstand decades of droughts and other stresses to come.
The recognition of this blind spot led some worlds to collide. A group of academics passionate about tropical root dynamics (TropiRoot) began to collaborate with multiple research groups working on restoration science across Europe, Latin America, tropical Africa, and the United States. Our core question was: How can resource-limited tropical forest restoration projects measure belowground recovery in a scientifically robust way? In the fall of 2023, my colleague Dr. Laura Toro and I, along with several others, started a working group to tackle this challenge. We met for three days in October of 2023 at the Yale School of the Environment and brainstormed a project that aimed to distill decades of research on soil science into priority indicators that capture the essence of soil recovery.
A clear belowground monitoring gap
As we dug into global restoration monitoring frameworks and the scientific literature, the scope of this oversight became even clearer. A recent worldwide stocktake catalogued more than 4,500 indicators that restoration projects use to measure “success”. Of the 61 indicators chosen as highest priority, just one focused on belowground processes: soil carbon (Gann et al. 2022). Soil carbon is a notoriously difficult indicator to detect changes in, especially over the short timeframes most projects monitor.
We then systematically reviewed almost 200 scientific studies on tropical forest restoration. Only 28 — fewer than one in seven — directly compared above and belowground properties at the same sites. Without these paired measurements, it’s really difficult to understand whether the recovering vegetation reflects holistic ecosystem recovery or if it might be masking soils struggling to recover from previous degradation.
The problem extends beyond academic research. When we surveyed restoration practitioners across 14 Latin American countries, we heard a similar story: projects typically have only resources to evaluate aboveground recovery, and often only for the first few years after planting. This is despite having ambitious goals of restoring entire ecosystem processes and recovering biodiversity (Cole and Werden et al. 2024).The appetite to track recovery holistically is there, but accessible tools and protocols often aren’t, so we developed a short list of six indicators and some rules of thumb for belowground monitoring, lowering the barrier to capture belowground recovery (Toro and Werden et al. 2025). We summarize the indicators below.
Six indicators to track belowground recovery
The strength of these indicators is their simplicity, paired with the ability to robustly summarize belowground recovery over time. Most indicators can be measured with equipment that can be purchased at a hardware store, or analyzed by a basic agricultural soil lab, and the priority indicators selected all change over timescales that matter for restoration
Physical –
- Bulk density tells you how compacted the soil is – or how much soil is packed into a given space. When forests are cleared, machinery and/or grazing animals can compress the soil, making it hard for water to soak in and plant roots to penetrate. We measure this by pushing a metal cylinder of known volume into the soil, drying the sample in an oven, and dividing the dry weight by the cylinder’s volume. High bulk density means compacted soil; low bulk density means less dense, healthy soil with space for air and water that roots can grow into more easily.
- Aggregate stability captures how well soil particles stick together in clumps, which determines whether rain soaks in or runs off, carrying topsoil with it. In the past, measuring this required laboratory equipment, but now there’s a smartphone app called SLAKES that guides you through the process. You simply just take photos of soil chunks before and after soaking them, and the app calculates an index of how well they held together.
Chemical –
- Soil organic matter is often called the engine of soil fertility — decomposed plant and animal material that holds nutrients and water. It’s what gives soil its color and smell. When land is cleared and intensively used, the organic matter is often burned away or gets washed off. As forests regrow, leaves fall, roots die and decompose, and organic matter slowly builds back up. You can measure it by sending dried soil samples to a lab.
- Soil pH can be thought of as a whole bunch of soil chemistry compressed into a single number — it determines which nutrients plants can access. Low pH (acidic soils) can lock up nutrients and sometimes reach toxic levels of aluminum. You can test pH easily in the field by mixing soil with water and using a simple pH meter, or sending a sample to a lab.
Biological –
- Decomposition rate gives you a window into how active the soil’s living community is, all microbes that break down dead material and release nutrients for plants to use again. We suggest measuring this using “decomposition bags” — mesh bags filled with a standard amount of leaf litter (or even tea bags if available) that get buried in the soil and retrieved after a set time to see how much material decomposed. Faster decomposition usually means a healthier, more active soil community.
- Macrofauna abundance is the count of larger soil animals such as earthworms, termites, beetle larvae, and others. These soil animals create tunnels that let water and air penetrate, mix organic matter through the soil layers, and their castings can contain lots of incredibly fertile nutrients. To measure this, you dig a small trench, carefully sort through the soil and litter, and count all the macrofauna you find. It can take time, but these animals can be excellent indicators of soil recovery.
A simple way to get started
As part of our paper we also developed some recommendations for practitioners to start implementing belowground monitoring:
- When to sample: Before restoration begins, set a baseline, then measure every 5 years or more frequently if possible.
- Where to sample: The top 10 cm of soil captures most early changes.
- How many samples: 5-10 cores per plot usually gives a reliable average.
- How to report: Use standard units so your data can be compared across sites and integrated into broader monitoring efforts.
- Other measurements: If you already measure above ground recovery indicators, keep doing that. Some aboveground patterns offer clues about what’s happening below, but in many cases, especially for soil carbon or microbial activity, what you see aboveground doesn’t tell the full story.
Why below recovery matters for restoration
Understanding soil recovery has immediate practical implications for understanding restoration outcomes. Healthy soils speed up seedling establishment, improve drought resistance, and support robust carbon storage that can make tropical forest restoration a viable natural climate solution. But soil recovery can also help projects make better management decisions in real time.
If bulk density measurements reveal severe compaction, you could adjust your species selection to favor deep-rooted trees that can help break up hardpan layers. If organic matter is low, you could experiment with compost additions or mulching. But, without measuring this belowground information at the outset and as your project progresses, these management decisions can become educated guesswork.
An invitation to what’s next
Through the SNAPP Monitoring Restoration Effectiveness (MoRE) working group, we’re now building a collaborative network that connects researchers with practitioners on the ground. Among other things, our goal is to develop straightforward protocols for monitoring belowground indicators. These protocols will be tested in real-world conditions and adapted based on feedback from practitioners. We’re also compiling longitudinal data from any tropical restoration project that has measured an indicator of below- or above-ground recovery for a global synthesis that aims to develop best practices for restoration monitoring.
If you’re working to refine your tropical soil monitoring practices, or if you just generally want to get involved, we’d love to hear from you.
Take our questionnaire on monitoring practices – Here
Learn more about our data synthesis – On our website
Get in touch with the SNAPP MoRE team – snapp-more@umn.edu