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The little things matter: microbes of the boreal bryosphere

The little things matter: microbes of the boreal bryosphere

Project aims

The aim of this project is characterise the microbial communities of the boreal bryosphere, i.e., the bacteria and fungi that live in the moss layer that blankets huge swathes of the boreal forest. The research focuses on a) temporal development, specifically microbial community assembly following forest fires and b) fine-scale spatial differences e.g. how the microbial communities on the growing tips of the moss stems differ from those on older, senescent stem sections.

Background

The boreal forest is the world's second largest biome (after the tropical rainforests) and a major terrestrial carbon sink. This habitat is experiencing rapid environmental change that is likely to lead to more frequent ecological disturbances. In particular, the frequency and intensity of forest fires will probably increase in a warmer world. Forest fires are known to disrupt the cycling of nitrogen (N), primary productivity and carbon storage. In order to predict the ecological consequences of more frequent burning, it is essential understand the microbial communities that live on the forest floor. These communities play a key role in N-cycling in boreal forests, as most of the nitrogen in this habitat is fixed by cyanobacteria that live in the moss layer (DeLuca et al. 2002). They are also important in carbon (C) cycling, as they decompose dead organic matter. Despite its importance, the microbial component of boreal ecosystems has received little attention and is usually regarded as a 'black box'. Crucially, it is not known how quickly microbial communities recover from disturbance by forest fires, or precisely how N fixed in the upper parts of the moss layer is transferred to the soil and, ultimately, tree roots.

Study area

The research utilises moss samples collected from five sites in Arctic Sweden. The sites are characterised by the dominance of Scots Pine (Pinus sylvestris) and a feather moss ground layer (primarily Pleurozium schreberi). Stems of P. schreberi were analysed. The six study areas form a fire chronosequence i.e. a series of sites that only differ in terms of the period since they were last burnt (time-since-fire, or TSF). The 'youngest' site was burnt 14 years ago, the 'oldest' site last experienced a forest fire 366 years ago (Fig. 1).

Methods

Molecular (DNA-based) methods were used establish the identity and relative abundance of the microbes in the moss layer. The study focuses on bacteria and fungi, two groups that decompose dead organic matter and thus underpin the ecology of soils. A sequence of post-fire development was established by comparing microbial communities on sites at different stages of recovery. Amplicon pyrosequencing was used to establish community composition on sites of different ages and on different parts of the moss stem (i.e. green, living tissue vs brown, senescent stems).

Early findings and implications of research

The first results from this study revealed unexpectedly diverse microbial communities (Cutler et al. 2016). The structure of the communities could be largely explained by just two factors, time-since-fire and pH. The microbial communities on the youngest site (TSF = 14 yrs) were clearly different from older locations (TSF > 100 yrs), suggesting relatively rapid post-fire recovery. A shift towards Proteobacterial taxa on older sites, coupled with a decline in the relative abundance of Acidobacteria, suggested an increase in resource availability with TSF, at least in the green parts of the moss layer (Fig. 2).

The fungi were slower to recover than the bacteria (probably several decades). Saprotrophs (organisms that feed on dead organic matter) dominated the fungal community on all sites. Mycorrhizal fungi, which form symbiotic relationships with trees and play a key role in nutrient acquisition, appeared to decline in abundance with TSF, possibly due to changing soil fertility over time.

As well as temporal variation, the microbial communities also varied according to stem position i.e. there were clear differences in community structure on the scale of a few centimetres. The highest fungal diversity was found on brown, senescent tissue (Fig. 3). Conversely, the highest bacterial diversity occurred on green stem tips.

Evidence for the decadal-scale legacy of burning has implications for the ecology of boreal forests if, as predicted, the return period of wildfires decreases over the next century. Most notably, reduced fire return periods could keep the microbial communities of the bryosphere (particularly fungi) at an early successional stage, with a knock-on effect on the cycling of recalcitrant C.

Figure 1

Fig. 1: The Swedish wildfire chronosequence, showing the site codes (bold) with the number of years since burning (TSF) in parentheses.

Figure 2

Fig. 2: The relative abundance of the major bacterial phyla, showing the general increase in Proteobacterial reads with time-since-fire (TSF), and the accompanying decrease in Acidobacteria. Each pie represents a different site; TSF increases from left to right.

Figure 3

Fig. 3: Microbial diversity on 'young' (green) and 'old' (red) sites; bacterial diversity figures are in the left panel; the equivalent figures for fungi are on the right; diversity is indicated for new, mature and senescent sections of the moss stem.

Funding and collaborators

This project is funded by the Royal Geographical Society (ref: SRG 13:13) and conducted in collaboration with Dr María Arróniz-Crespo (University of Bangor) as an adjunct to the NERC-funded project The Boreal Nitrogen Gap: Size, fate and impacts of nitrogen fixation in Fennoscandia forest ecosystems (ref: NE/I027150/1). The molecular analyses were carried out at the Centre of Ecology and Hydrology (CEH), Wallingford, UK, with the kind permission of Dr Anna Oliver.

References

  • Cutler, N.A., Arróniz-Crespo, M., Street, L.E., Jones, D.L., Chaput, D.L. and DeLuca, T.H., (2016) Long-term recovery of microbial communities in the boreal bryosphere following fire disturbance. Microbial Ecology (doi:10.1007/s00248-016-0832-7).
  • De Luca, T.H. et al. (2002) Quantifying nitrogen-fixation in feather moss carpets of boreal forests. Nature, 419, 917-920.