Methodology matters for comparing coarse wood and bark decay rates across tree species

Post provided by Chenhui Chang

 

落红不是无情物,化作春泥更护花。 –龚自珍(清)
The fallen petals are not as cruel as they seem; they fertilize those in full bloom instead.”Gong Zizhen (Qing Dynasty)

log

A decaying Douglas fir log

This picture shows a decomposing log of Douglas fir, Pseudotsuga menziesii (Mirb.) Franco, in Schovenhorst, The Netherlands, which is one of the deadwood incubation sites of the LOGLIFE “tree cemetery” project. 25 angiosperm and gymnosperm species covering a diverse range of functional traits were selected and incubated in the “common garden experiment”. This project was founded in 2012, aiming to disentangle the effects of different species’ wood traits and site-related environmental drivers on decomposition dynamics of wood, and its associated diversity of microbial and invertebrate communities.

Why did we do this study?

Both bark and wood decomposition processes are important for global carbon and nutrient cycles, and these processes may be coordinated because of their shared ontogenetic origin from the cambium between the innermost bark and (sap)wood. Whether tree species with fast (versus slow)-decomposing wood also have fast-decomposing bark matters for predicting the consequences of changing tree species composition for forest carbon turnover.

However, decay rates of large logs in the field are often quantified as loss in tissue density, which can give large underestimations of bark and wood volume depletions. Here, we present new methods for quantifying volumetric loss of bark and wood, and percentage reduction of bark cover during decomposition.

Two types of bark mass loss

wood

Volumetric loss of Douglas fir bark. Left: Bark after four years of decay. Right: newly dead bark.

1. Minimal bark mass loss

We quantified mass per unit stem surface area at two harvests to calculate percentage bark mass loss (∆g/cm2). The area of each bark sample was measured after scanning the bark outline with a flatbed scanner. The dry mass was measured after oven drying, and the oven dry mass and area yielded mass per area. The percentage bark mass loss equals the proportional difference of the area-based mass between the two harvests, which we name minimum bark mass loss.

 

 

 

2. Maximum bark mass loss

bark 2

Bark cover loss. For this log, more than 50% of bark cover had been lost.

Pieces of bark can fall off during decay, leading to an underestimation if using minimal bark mass loss. While the detached fragments may represent bark mass loss from the perspective of the log, the extent to which they represent bark mass loss depends on their subsequent decomposition rates, which may differ from those of attached bark.

Thus, we contrast minimum bark mass loss, (i.e. assuming bark coverage was 100% at the second harvest), with maximum bark mass loss, considering losses of bark fragments and assuming all the fragments decomposed immediately. To calculate the percentage of bark loss, we estimated the fraction of bark cover of each log at the second harvest visually.

The actual bark mass loss should always lie between the minimum and maximum bark mass loss.

 

log 2

Wood volume correction. The missing part was reconstructed artificially.

Volume correction of wood

To reconstruct the initial volume, we drew the outlines of the top and bottom cross-section for each log on paper. For the missing parts, based on visual observation, we reconstructed the original outline on the paper based on the intact or least decomposed part of the log. Then we scanned the reconstruction of the outline and measured the area inside the outline from the scan. We obtained the log volume as averaged cross-sectional areas (i.e. top and bottom cross-section of each log) times by length of the log.

What did we find?

The area-based method generally showed a more than three times higher bark mass loss than the density-based method (even higher if considering bark cover loss), and volume-corrected wood mass losses were 1.08-1.12 times higher than density-based mass loss. The deviation of bark mass loss between the two methods was higher for tree species with thicker inner bark. Bark generally decomposed twice as fast as wood across all species, and faster- decaying bark came with faster decaying wood (R2=0.26, P=0.006).

graph

Comparison of minimum bark mass loss (BML2) and volume-corrected wood mass loss (WML4) across species over four years of decomposition.

Benefits of our new methods

Compared with previous approaches, our easy-to-use methods can be applied to field studies and common garden experiments for various research purposes. The methods also showed an excellent ability to estimate mass loss of highly-decayed coarse and fine woody debris, and log stems without circular cross section. These methods also allow researchers to quantify the relationship between the traditional decay class system and density, making it possible to broadly understand woody debris decay dynamics across species at regional and global scales.

To find out more about bark and wood mass loss, read our Methods in Ecology and Evolution article “Methodology matters for comparing coarse wood and bark decay rates across tree species”.

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