Why forests can look healthy while silently losing value?

Did you know a forest can stay green while its growth engine is already slowing down?

If we only react when the canopy starts browning, we may already be managing the late stage of the problem. Field walks remain essential, but on their own they rarely show what has been quietly trending down for years.

That is the core challenge behind forest value loss. In practice, the problem often begins not with obvious dieback, but with reduced growth, weaker functional performance, rising stress, and a lower ability to cope with future disturbance. By the time damage is visually obvious, options are often fewer, more expensive, and more reactive.

Why this matters economically and operationally? How indices like NDRE and NDMI help detect early warning signs at the stand level? The goal is not to replace foresters or field assessment. It is to support better prioritization, sharper inspection planning, and more defensible decisions across a forest portfolio.

Why forests can decline while still looking green

A forest can look stable while already losing productive potential, because decline starts as a functional problem before it becomes visible. Early signals include reduced photosynthesis, chlorophyll activity, drought stress, or slower growth, all while the canopy can still appear green.

Lenka Foltýnová, Ph.D.

ASITIS.cz, Climate Data Analyst

This matters because managers do not make decisions based only on appearance. They make them based on timing, budgets, risk, intervention sequencing, and expected future outcomes. A stand that still looks acceptable today may already be moving toward lower performance tomorrow.

The research logic behind this is strong. Growth decline often precedes mortality. In a synthesis of dendrochronological evidence, radial growth decline occurred before tree death in about 84 percent of cases. In other words, long before a stand is visibly failing, its growth history may already be telling a different story.

Hidden decline drivers are rarely single cause

Decline tends to be multi factor. Drought legacies, heat, pest pressure, competition, soil conditions, and disturbance interactions can all contribute. A stand may not have one clean diagnosis. It may have a growing imbalance between what the site demands and what the stand can sustain.

That is why it is useful to treat stress as risk, not as a single cause. The practical question is which stands now deserve closer attention because their signals no longer look stable.

The cost of late detection

The consequences of late detection are not only ecological. They are operational, financial, logistical, and sometimes reputational.

Modeled estimates also suggest that climate related disturbance losses in European forestry could rise sharply. Under high impact scenarios, annual losses may approach 15 percent of the current gross value added of the European forestry sector. Some studies also report modeled losses in timber based forest value of up to EUR 19,885 per hectare. These figures are scenario based and context dependent, not direct forecasts for every forest. Still, they make one point clearly: forest value loss can be economically material, and waiting for obvious damage is rarely the cheapest moment to act.

The Czech bark beetle crisis shows how quickly hidden stress can escalate into large scale losses. Annual damage in spruce stands rose from about 0.2 – 1.4% (2003 -2016) to 3.1- 5.4 % (2017 – 2019), with salvaged timber reaching nearly 23 million m³ in 2019. That scale of disruption does not stay confined to the stand. It affects public budgets, timber markets, logistics, workforce availability, regeneration planning, and political scrutiny.

Miloslav Kaláb

ASITIS.cz, Climate Resilience Specialist

What NDRE reveals about vitality and canopy stress

NDRE, or Normalized Difference Red Edge, is a vegetation index based on NIR and red edge bands. In forest monitoring, its value lies in sensitivity to changes related to chlorophyll and leaf or needle physiological condition.

Bark beetle and the invisible phase

Bark beetle pressure makes this especially relevant. The so called green attack phase is the clearest example of why appearance can mislead. During this phase, biochemical and biophysical changes are underway, but the stand may still look green to a field observer at first glance.

In Berchtesgaden National Park, a Sentinel 2 time series approach showed that NIR and SWIR provided the best separability, and water stress related indices were especially sensitive.

Why chlorophyll and water are a useful pair

Chlorophyll related change and water related change are not the same thing. That is why combining NDRE and NDMI is so useful.

A weakening NDRE signal may indicate changing canopy condition. A weakening NDMI signal may indicate a worsening water regime or moisture stress. Together, they create a more informative risk picture than either one alone.

This pairing matters because forest decline often does not begin with spectacular defoliation. It may begin with reduced water availability, lower assimilation, less robust photosynthetic function, and a gradual reduction in growth performanc

From scattered observations to portfolio oversight

This is where scale changes the conversation. A forester may know certain compartments well. But across a large municipal or institutional portfolio, decisions often depend on where to inspect first, where not to waste time, and where intervention delay is becoming riskier.

That is the role of stand level analytics. UpGreen Forest is designed to assess the functional condition of forest stands as a whole, not individual trees. It combines satellite data and available inventories to evaluate vitality, water stress, growth trends, and survival capacity, helping identify where forest value may be increasing, stagnating, or declining.

The real value of remote sensing is not that it gives you more maps. It is that it helps you use limited field capacity more intelligently. Start by identifying stands showing declining vitality signals, worsening water stress signals, or both.

Petr Klimeš

ASITIS.cz, Climate Data Analyst

How UpGreen Forest turns early signals into insight: survival, productivity, stress

One of the key layers in UpGreen Forest is productivity. This should not be understood as a direct measurement of timber yield from satellite data alone. Rather, it is an analytical view of how the stand is functioning over time, based on signals related to canopy vitality and, where available, linked inventory context.

In practice, this helps answer a question that matters to owners and managers: which stands are still showing healthy functional performance, and which ones are beginning to lose momentum?

A second core layer is stress. Here, the goal is not to claim a single diagnosis, but to highlight where the stand is under elevated pressure.
This stress layer draws on indicators related to water balance and canopy condition, including signals such as NDMI and LST (surface temperature), to show where moisture related pressure or broader physiological strain may be increasing. In management terms, it helps flag stands where the current condition may be less robust than visual appearance suggests.

UpGreen Forest also uses a survival capacity layer. It is an interpretation of how well a stand may be positioned to maintain function and value over time, based on the combination of vitality signals, water stress indicators, and longer term development patterns.

It helps separate stands that appear more stable and resilient from those where repeated stress and declining condition suggest a higher likelihood of future deterioration.

„For portfolio management, that matters because not every hectare carries the same urgency. Some stands may justify continued monitoring. Others may deserve priority inspection, adaptation planning, or earlier intervention. UpGreen Forest helps make those differences more visible.“

Miloslav Kaláb

ASITIS.cz, Climate Resilience Specialist

References:

Cailleret, M. et al. Tree mortality mechanisms and the prevailing role of growth decline. Trends in Plant Science.

Čada, V. et al. Long-term growth decline precedes sudden crown dieback of European beech. Trees, Forests and People / Research study on pre-decline growth trajectories in beech.

Hlásny, T. et al. Bark beetle outbreak in the Czech Republic, drivers, impacts, and management implications. Forest Ecology and Management / related forestry research.

Patacca, M. et al. A global and European perspective on forest disturbance records, including the European disturbance database for 1950 to 2019. Nature Climate Change / related dataset publication.

Mohr, S. et al. Economic losses from climate-related forest disturbances in Europe, including timber-based forest value estimates. Ecological Economics / related economic assessment.

Eitel, J.U.H. et al. Broadband red-edge information from satellites improves early stress detection in a New Mexico conifer woodland. Remote Sensing of Environment.

Mandl, F., Lang, S. Early detection of bark beetle green attack using Sentinel-2 time series in Berchtesgaden National Park. Remote sensing based forest disturbance study.

Huo, L. et al. Early detection of spruce bark beetle attack using Sentinel-2 and the NDRS index. Frontiers in Forests and Global Change.

Candotti, D. et al. Multi-temporal Sentinel-2 monitoring of windthrow and bark beetle disturbances in Alpine forests. Remote Sensing.

Laštovička, J. et al. Time-series trajectories of NDMI and related indices for disturbance and recovery monitoring in Low Tatras and Šumava forests. Remote sensing based disturbance study.

Verbesselt, J. et al. Detecting trend and seasonal changes in satellite image time series, BFAST and BFAST Monitor. Remote Sensing of Environment.

Sturm, M. et al. Satellite data reveal differential forest responses to the 2018 drought in Switzerland. Global Change Biology / related drought response study.

U.S. Geological Survey. Normalized Difference Moisture Index, NDMI. Landsat Missions, USGS.

European Space Agency. Sentinel-2 User Handbook. ESA.