Author: Yi Zhang

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

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

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

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

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: https://onlinelibrary.wiley.com/doi/10.1111/btp.12850

Link to the nature highlight: https://www.nature.com/articles/d41586-020-02911-3

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