Woody encroachment slows decomposition and termite activity in an African savanna

Abstract

Woody encroachment can lead to a complete switch from open habitats to dense thickets, and has the potential to greatly alter the biodiversity and ecological functioning of grassy ecosystems across the globe. Plant litter decomposition is a critical ecosystem process fundamental to nutrient cycling and global carbon dynamics, yet little is known about how woody encroachment might alter this process. We compared grass decay rates of heavily encroached areas with adjacent nonencroached open areas in a semi-arid South African savanna using litterbags that allowed or excluded invertebrates. We also assessed the effect of woody encroachment on the activity of termites- dominant decomposer organisms in savanna systems. We found a significant reduction in decomposition rates within encroached areas, with litter taking twice as long to decay compared with open savanna areas. Moreover, invertebrates were more influential on grass decomposition in open areas and termite activity was substantially lower in encroached areas, particularly during the dry season when activity levels were reduced to almost zero. Our results suggest that woody encroachment created an unfavourable environment for invertebrates, and termites in particular, leading to decreased decomposition rates in these areas. We provide the first quantification of woody encroachment altering the functioning of African savanna ecosystems through the slowing of aboveground plant decomposition. Woody encroachment is intensifying across the globe, and our results suggest that substantial changes to the carbon balance and biodiversity of grassy biomes could occur.

Wood decomposition is more rapid on than off termite mounds in an African savanna

“…It seems the decomposition rates associated with active Macrotermes mounds are not dependent on moisture, consistent with Veldhuis et al (2017). On a larger scale, Leitner et al (2018) showed that shade, resulting from increasing tree cover, was associated with reduced decomposition. This was attributed to reduced termite activity in the wooded areas.…”

“…On a larger scale, Leitner et al. () showed that shade, resulting from increasing tree cover, was associated with reduced decomposition. This was attributed to reduced termite activity in the wooded areas.…”

Teatime in the Serengeti: macrodetritivores sustain recalcitrant plant litter decomposition across human-modified tropical savannahs

“…Moreover, on both agricultural and pastoral land-uses in drier regions, macrodetritivore communities were able to sustain recalcitrant litter decomposition during periods of water scarcity. Continued macrodetritivore foraging during the dry season has been observed in other tropical ecosystems Jouquet et al 2011;Veldhuis et al 2017), but is not exhibited by all savannah macrodetritivore species (Davies et al 2015;Davies et al 2013;Leitner et al 2018). In contrast, litter decomposition by microbes and microdetritivores in our study was constrained by regional and seasonal water availability across all land-use types, as also seen in other tropical ecosystems (Anaya et al 2012;Becker and Kuzyakov 2018;Ngatia et al 2014).…”

“…This, in turn, could further constrain decomposition by microbes and negatively influence macrodetritivores that would seek moist refuges and minimise foraging due to higher risk of desiccation (Cornelius and Osbrink 2010;Joly et al 2019;Woon et al 2019). Additionally, dry season foraging is not a universal trait to all litter feeding macrodetritivore species, and several termite and isopod species are seemingly dormant during the dry season months resulting in little or no litter foraging (Davies et al 2015;Davies et al 2013;Leitner et al 2018). As such, there remains ambiguity over the contribution of macrodetritivores to litter decomposition across temporally and spatially variable rainfall in tropical savannah ecosystems.…”

Land productivity dynamics in and around protected areas globally from 1999 to 2013

“…Europe is the continent with the lowest percentage of land with stable 31 productivity levels in PAs (38%) and with the largest share of protected land with increasing land 32 productivity (32%), which may be related to the high population density and share of agricultural 33 land within PAs as well as to rural land abandonment processes in many regions of Europe. In 34 conclusion, we provide a relevant indicator and assessment of land productivity dynamics that 35 contributes to characterise the state, pressures and changes in and around protected areas 36 globally. Further research may focus on more detailed analyses to disentangle the relative 37 contribution of specific drivers (from climate change to land use change) and their interaction 38 with land productivity dynamics and potential land degradation in different regions of the world.…”