The impact of logging on vertical canopy structure across a gradient of tropical forest degradation intensity in Borneo

Forest degradation through logging is pervasive throughout the world’s tropical forests, leading to changes in the three-dimensional canopy structure that have profound consequences for wildlife, microclimate and ecosystem functioning. Quantifying these structural changes is fundamental to understanding the impact of degradation, but is challenging in dense, structurally complex forest canopies.

We exploited discrete-return airborne LiDAR surveys across a gradient of logging intensity in Sabah, Malaysian Borneo, and assessed how selective logging had affected canopy structure (Plant Area Index, PAI, and its vertical distribution within the canopy).

LiDAR products compared well to independent, analogue models of canopy structure produced from detailed ground-based inventories undertaken in forest plots, demonstrating the potential for airborne LiDAR to quantify the structural impacts of forest degradation at landscape scale, even in some of the world’s tallest and most structurally complex tropical forests.

Plant Area Index estimates across the plot network exhibited a strong linear relationship with stem basal area (R2 = 0.95). After at least 11–14 years of recovery, PAI was ~28% lower in moderately logged plots and ~52% lower in heavily logged plots than that in old-growth forest plots. These reductions in PAI were associated with near-complete lack of trees >30-m tall, which had not been fully compensated for by increasing plant area lower in the canopy. This structural change drives a marked reduction in the diversity of canopy environments, with the deep, dark understorey conditions characteristic of old-growth forests far less prevalent in logged sites. Full canopy recovery is likely to take decades.

Effective management and restoration of tropical forests requires detailed monitoring of the forest and its environment. We demonstrate that airborne LiDAR can effectively map the canopy architecture of the complex tropical forests of Borneo, capturing the three-dimensional impact of degradation on canopy structure at landscape scales, therefore facilitating efforts to restore and conserve these ecosystems.

Milodowski (2021) Journal of Applied Ecology


Recovery of logged forest fragments in a human-modified tropical landscape during the 2015-16 El Niño

It is unclear whether tropical forest fragments within plantation landscapes are resilient to drought. Here the authors analyse LiDAR and ground-based data from the 2015-16 El Niño event across a logging intensity gradient in Borneo. Although regenerating forests continued to grow, canopy height near oil palm plantations decreased, and a strong edge effect extended up to at least 300 m away.

Nunes (2021) Nature Communications

Carbon flux and forest dynamics: Increased deadwood decomposition in tropical rainforest tree‐fall canopy gaps

Tree mortality rates are increasing within tropical rainforests as a result of global environmental change. When trees die, gaps are created in forest canopies and carbon is transferred from the living to deadwood pools. However, little is known about the effect of tree‐fall canopy gaps on the activity of decomposer communities and the rate of deadwood decay in forests. This means that the accuracy of regional and global carbon budgets is uncertain, especially given ongoing changes to the structure of rainforest ecosystems. Therefore, to determine the effect of canopy openings on wood decay rates and regional carbon flux, we carried out the first assessment of deadwood mass loss within canopy gaps in old‐growth rainforest. We used replicated canopy gaps paired with closed canopy sites in combination with macroinvertebrate accessible and inaccessible woodblocks to experimentally partition the relative contribution of microbes vs. termites to decomposition within contrasting understorey conditions. We show that over a 12 month period, wood mass loss increased by 63% in canopy gaps compared with closed canopy sites and that this increase was driven by termites. Using LiDAR data to quantify the proportion of canopy openings in the study region, we modelled the effect of observed changes in decomposition within gaps on regional carbon flux. Overall, we estimate that this accelerated decomposition increases regional wood decay rate by up to 18.2%, corresponding to a flux increase of 0.27 Mg C ha−1 year−1 that is not currently accounted for in regional carbon budgets. These results provide the first insights into how small‐scale disturbances in rainforests can generate hotspots for decomposer activity and carbon fluxes. In doing so, we show that including canopy gap dynamics and their impacts on wood decomposition in forest ecosystems can help improve the predictive accuracy of the carbon cycle in land surface models.

Griffiths (2021) Global Change Biology

Monitoring ash dieback in British forests using hyperspectral remote sensing

Fungal ash dieback (Hymenoscyphus fraxineus) is posing an imminent threat to forest health in Europe. Using airborne hyperspectral imagery trained against 422 tree crowns of known species and ash dieback severity, we built PLS-DA and RF models that classified individual tree crowns (ITCs) into five species (>90% OA) and ash crowns into three disease severity classes (77% OA) respectively. Dark pixel filtering was found to improve the accuracy of species (+6%) but not disease classification. By incorporating automatic ITC segmentation and the classification models, we further demonstrated how species and fungal ash dieback can be mapped at a region scale for forest management and epidemiological research.

Remote Sensing in Ecology and Conservation, 2020

Link to Abstract and video presentation

Riparian buffers act as microclimatic refugia in oil palm landscapes

There is growing interest in the ecological value of set‐aside habitats around rivers in tropical agriculture. These riparian buffers typically comprise forest or other non‐production habitat, and are established to maintain water quality and hydrological processes, while also supporting biodiversity, ecosystem function and landscape connectivity.

We investigated the capacity for riparian buffers to act as microclimatic refugia by combining field‐based measurements of temperature, humidity and dung beetle communities with remotely sensed data from LiDAR across an oil palm dominated landscape in Borneo.

Riparian buffers offer a cool and humid habitat relative to surrounding oil palm plantations, with wider buffers characterised by conditions comparable to riparian sites in continuous logged forest.

High vegetation quality and topographic sheltering were strongly associated with cooler and more humid microclimates in riparian habitats across the landscape. Variance in beetle diversity was also predicted by both proximity‐to‐edge and microclimatic conditions within the buffer, suggesting that narrow buffers amplify the negative impacts that high temperatures have on biodiversity.

Synthesis and applications: Widely legislated riparian buffer widths of 20–30 m each side of a river may provide drier and less humid microclimatic conditions than continuous forest. Adopting wider buffers and maintaining high vegetation quality will ensure set‐asides established for hydrological reasons bring co‐benefits for terrestrial biodiversity, both now, and in the face of anthropogenic climate change.

Williamson (2020) Journal of Applied Ecology

Do tall trees have a higher risk of wind damage?

When an intense tropical storm passes over a forest it leaves destruction in its wake. Post-damage surveys often show that the tall trees are disproportionately killed in these events. However, it is very difficult to attribute the cause of death of a large tree after the event. A tree may be snapped and lying on the ground, but it could have been killed years before the storm by lightning, or fundamentally weakened by disease. Therefore, we set out to answer a simple question: do tall trees have a higher risk of wind damage?

Tall trees are exposed to higher wind speeds, but they also have wider trunks. The resistance to bending for a cylindrical beam is proportional to the cube of its diameter (this is why hollow scaffolding poles are so strong). We didn’t know a priori which of these two factors would prove the most important, so we took some measurements.

Figure 1 – The experimental setup. Left – Unding bin Jami attaching wind sensors to trees. Top right – looking up a 95 m tall tree (1 m shorter than Big Ben!). Bottom right – strain gauge sensor attached to the trunk of a large tree + an unwelcome visitor.

We measured local wind speeds and the bending stress on the trunks of 17 large trees (you have to start somewhere!). These two measurements were highly correlated and related to each other by a power law. We then compare the maximum bending stress to the breaking stress, which is different

for each species but is well known for the tall timber species in our study. This gave us a measure of wind damage risk for each tree.

Sure enough, we found that the taller trees did have a higher risk of wind damage. This means that the wider trunks of tall trees were not sufficient to compensate for their increased exposure to strong winds. This agrees with previous work in conifer forests and suggests that wind is likely to be an important cause of death for tall trees. The next steps are (1) to increase the sample size and test in different forests and (2) track tall trees over time to see how many are killed during storms.

Link to the paper:

Link to the nature highlight:

Resilience of Spanish forests to recent droughts and climate change

Time-series of canopy greenness derived from satellite imagery can be
analysed alongside environmental factors, species composition and
management regimes, to better understand forest resilience to drought.
In Spain, forests are on average greening despite drying trends. This
resilience manifests in the short-term with native species activating
drought tolerance and avoidance mechanisms observable from space (i.e.
losing and gaining little greenness like chestnuts to losing and gaining
a lot of greenness like maritime pines). The non-native eucalypt
dominated forests reveal a low short-term resilience (i.e. do not
recover enough after droughts) and hence have a higher percentage of
declining pixels. Factors such as water balance, elevation, and
protection status greatly influence these drought response patterns.

Khoury S, Coomes DA. (2020) Global Change Biology

Treetops protect forest life from global warming

The forest canopy mitigates peak summer temperatures for the understorey. When that shade disappears, the organisms living there suffer.

The cooling leaf canopy protects forest organisms from extreme temperatures and has a significant influence on their adaptation to global warming, according to this study which was published in the journal Science.

The climate in the forest is not the same as the climate outside the forest. That much is clear to anyone who has taken to the woods to seek respite from the heat on a hot summer day. However, scientific studies consider ‘climate warming’ to be the warming measured by many thousands of standardised weather stations all over the world; these are generally located in open spaces and measure the temperature at a height of 1.5 to 2 metres from the ground. Yet the majority of all land-based species live in forests, with many dwelling in the understorey and the soil. As a result, climate data gathered from open spaces are only of limited relevance to them.

Beech forest in France. (Photo: Jonathan Lenoir)

An international research team led by Florian Zellweger at the University of Cambridge and the WSL in Switzerland has now come up with the first concrete figures on climate warming under the forest canopy, thus also showing how warming in the forest differs from warming in open spaces. To obtain these results, and colleagues measured the temperature in the forest interior at 100 sites and combined these measurements in a computer model with up to 80 years of data on the density of the forest canopy. This latter series comprised data taken from almost 3,000 locations within the framework of long-term observation programmes.

Writing in the journal Science, the researchers report that climate warming measurements taken in open spaces do not sufficiently reflect changes in temperature under the leaf canopy. If the tree canopy is denser, it buffers climate warming for the organisms living beneath it. If it becomes sparser, the temperature below it rapidly surges.

Hemispherical photograph of a beech forest. The denser the canopy, the greater the cooling effect in the understorey and on the forest floor. (Photo: Pieter de Frenne)

Lag in climate adaptation

The forest canopy creates locally different climatic conditions, which is vitally important for forest life. Anemones in the Lehnflue near Oensingen in the canton of Solothurn. (Photo: Markus Bolliger)

All organisms have an optimum temperature at which they thrive best. When the climate warms, warm-affinity species benefit and displace cold-affinity species, which may, for instance, move to higher mountain areas. However, the optimum temperature for forest organisms is significantly lower than actual measured temperatures: these organisms are therefore lagging behind when it comes to climate adaptation. “In the context of global climate change, many species live in an increasingly suboptimal temperature range,” notes David Coomes, senior author of the study.

Consequently, if the protective tree canopy is lost – whether naturally or as a result of human intervention – the plants living beneath it experience additional drastic warming for which they are ill prepared. Their previously cool, shady and generally more humid habitat is suddenly exposed to the intense heat far more often and for longer periods, and the soil also dries out. Many species cannot adapt quickly enough, are displaced by warm-affinity species and may die out locally. Given the expected increase in summer heatwaves in Europe, this is likely to transform forest biodiversity and may “spell trouble for individual species” according to Zellweger. Forest managers should therefore take account of the effects of forestry work on the climatic conditions in the forest interior and of the work’s impact on biodiversity.

Links to paper:

Standardizing Ecosystem Morphological Traits from 3D Information Sources

3D-imaging technologies provide measurements of terrestrial and aquatic ecosystems’ structure, key for biodiversity studies. However, the practical use of these observations globally faces practical challenges. First, available 3D data are geographically biased, with significant gaps in the tropics. Second, no data source provides, by itself, global coverage at a suitable temporal recurrence. Thus, global monitoring initiatives, such as assessment of essential biodiversity variables (EBVs), will necessarily have to involve the combination of disparate data sets. We propose a standardized framework of ecosystem morphological traits – height, cover, and structural complexity – that could enable monitoring of globally consistent EBVs at regional scales, by flexibly integrating different information sources – satellites, aircrafts, drones, or ground data – allowing global biodiversity targets relating to ecosystem structure to be monitored and regularly reported.

Valbuena (2020) Trends in Ecology & Evolution

Protecting biodiversity and economic returns in resource‐rich tropical forests

In pursuit of socioeconomic development, many countries are expanding oil and mineral extraction into tropical forests. These activities seed access to remote, biologically rich areas, thereby endangering global biodiversity. Here we demonstrate that conservation solutions that effectively balance the protection of biodiversity and economic revenues are possible in biologically valuable regions. Using spatial data on oil profits and predicted species and ecosystem extents, we optimise the protection of 741 terrestrial species and 20 ecosystems of the Ecuadorian Amazon, across a range of opportunity costs (i.e. sacrifices of extractive profit). For such an optimisation, giving up 5% of a year’s oil profits (US$ 221 million) allows for a protected area network that retains of an average of 65% of the extent of each species/ecosystem. This performance far exceeds that of the network produced by simple land area optimisation which requires a sacrifice of approximately 40% of annual oil profits (US$ 1.7 billion), and uses only marginally less land, to achieve equivalent levels of ecological protection. Applying spatial statistics to remotely sensed, historic deforestation data, we further focus the optimisation to areas most threatened by imminent forest loss. We identify Emergency Conservation Targets: areas that are essential to a cost‐effective conservation reserve network and at imminent risk of destruction, thus requiring urgent and effective protection. Governments should employ the methods presented here when considering extractive led development options, to responsibly manage the associated ecological‐economic trade‐offs and protect natural capital.

Ball (2020) Conservation Biology