Matthew Krna: Decomposition rates of leaf litter are temperature-independent

In this new post, Matt Krna—then a PhD student at Massey University, New Zealand—discusses his recently published paper in Functional Ecology: Temperature dependency of litter decomposition is not demonstrated under reciprocal transplantation of tussock leaves along an altitudinal gradient.

This research follows on from his publication on clarifying carbon sequestration (Krna and Rapson, 2013). In his study of decomposition across a 700 m altitudinal gradient (as a proxy for a temperature gradient) on Mount Tongariro, Matt found rates of leaf litter decomposition to be independent of temperature changes. This finding held true regardless of leaf-litter origin (where litter was obtained) or leaf-litter destination (where litter was deposited in nylon-mesh bags for decomposition) across a full-reciprocal transplant. His experience shows the important role of serendipity in provoking paradigm shifts in science.

About the paper

Functional Ecology likes its blogs to be “good detective stories”, but that is difficult here because we (my supervisors and I) didn’t set out to find what we did. Instead, we stumbled across something interesting while trying to measure productivity to decomposition ratios, which we don’t even discuss in this publication (see my thesis for the gruesome details! Krna, 2015). Our discovery shouldn’t have come as a surprise; however, science, like the mainstream media, likes positive and confirmatory stories—sometimes more so than reality—and lack of clarity is the enemy of science.

Site of decomposition experiment, with Matt hiking up Mt Tongariro with field gear for research-plot establishment, leaving his beloved base camp behind. Plots 1-3 are in the background (credit: Jill Rapson)

The big surprise was that the original question my thesis was addressing was marginalised by these findings. I found NO temperature dependence of litter decomposition, despite the small differences in litter quality, and random variations in trends along the environmental gradient. This took some credulity, even for my supervisors! Not unexpectedly, science journals have also had difficulty accepting this, and this work was repeatedly rejected for publication by a string of (prestigious) journals. These journals all felt that the lack of temperature dependence was an unfortunate quirk of the data and/or site and/or species and/or anything except reality!

It was thanks to the independent (but curiously similar) advice of esteemed scientists Ken Thompson and Hans Cornelissen (BES Eminent Ecologist of the Year for 2022) that we removed the (complex) productivity measurements from the paper in order to enable us to focus on the decomposition results—a clear case of international collegiality being in the long-term interests of science! This is also a case where lack of perseverance with publishing would have resulted in the loss of these important findings. Consequently, we hope as many ecologists and C-sequesterers read this paper as possible (and take it seriously!), as we believe a refocus of international decomposition research is now clearly needed.

Research often answers questions (including, hopefully, the question asked!) but it also leads to even more questions. Questions such as: is there ANY reliable evidence for temperature dependence of decomposition of non-labile carbon over ambient temperature ranges?; What useful information on other matters can be extracted from those experiments which may have claimed such dependences for non-labile C?; What warning signs are there that science is off-topic in an existing paradigm? What lessons can be drawn from this situation for the way we conduct modern scientific research?

About the research

The information in this paper provides a better understanding of how litter decomposition rates are NOT influenced by changes in temperature (a long-held legacy of plant-decomposition research). The implication is that carbon sequestration in terrestrial systems—under elevated temperatures projected by future climate change scenarios—will be driven more by plant productivity, as well as biotic and abiotic soil characteristics.

Tussock grass, Chionochloa rubra, on the slopes of Mount Tongariro, below Te Maari crater, still venting steam after its eruption. The crater is approximately 1 km east of my research plots (which were undamaged; shame about Ketetahi Hut!) (credit: Matthew Krna)

For my PhD, I studied snow tussock grasses in the genus Chionochloa, as these are long-lived perennials that dominate New Zealand’s mountains and span large altitudinal ranges. Initially I hypothesised that litter from more harsh environments (i.e., from higher altitudes) would possess greater concentrations of recalcitrant compounds (e.g., lignin and phenolic compounds), making it more resistant to decomposition compared to litter from less harsh environments (i.e., at lower altitudes). I also thought litter decomposition would be greater, but not necessarily linearly so, with higher temperatures, as per much of the then available literature. Ultimately my thinking was warming temperatures would increase decomposition rates and become a negative feedback for climate change. Therefore, I investigated how the origin and destination of litter—through a full-reciprocal transplant across an altitudinal gradient—would influence litter decomposition rates.

Home away from home; getting back to base camp near research Plot 3 at 1162 m elevation. Plots 4-8 are uphill on the flank of Mt Tongariro, the uppermost above the cloud’s shadow (credit: Jill Rapson)

At Mt Tongariro I used Chionochloa rubra (red tussock) in a reciprocal transplant approach to cancel out small, random variations which otherwise add noise to gradient-based experiments. I used a 700 m altitudinal gradient of montane tussock grasslands in the heart of New Zealand’s North Island—on the north-facing slope of Mount Tongariro—with 8 plots spaced approximately 100 m apart in elevation. This resulted in a grid of 8 origin sites by 8 destination sites (i.e., a 64–cell grid), of which into each cell I inserted about 22 related measurements. It’s pretty hard to argue with a data file this size!

Matthew Krna taking a break (tho’ still in the forest) from the back-breaking work of packing supplies for a week of field work (credit: Patrick Johnson)

Accessing the research plots involved hiking for several hours to get through the forest, above the treeline, and into the montane tussock grasslands. I established a base camp near Plot 3 (1162 meters in elevation)—needed because of long days in the field (sunup to sundown)—for multiple days on end throughout the year and over the years. 

Numerous volunteers aided in experimental setup, data collection, and field sites’ remediation. Close bonds and life-long friendships were formed, likely owing to shared common interests in environmental, ecological, and field research. These bonds were also likely formed because of our isolation and the mind-numbing repetitive nature of our tasks where conversations helped to pass the time and create friendships. Other portions of my research looked at the growth and productivity of Chionochloa at this and other field sites. A recurring joke I would tell volunteers was that portions of this research are as much fun as “watching the grass grow”. A special thanks to all those kind souls who helped me in the field—without you I could never have completed this work!

[Supervisor GLR adds that Matt has an almost incredible ability to talk unsuspecting friends and colleagues, and even acquaintances, into spending weeks at a time in the field in usually very basic conditions, whilst doing careful and meticulous (and pretty boring) work on uncomfortably steep slopes, mostly regardless of the weather. And those volunteers (inexplicably) often come back for more!]

Data analysis consisted of analysing and reanalysing and further reanalysing the data, because the results did not support our two primary hypotheses and preconceived notions that: (1) litter from higher altitude plots would have greater concentrations of phenolics, fibre, and lignin-compounds known to be upregulated to combat stress and are also recalcitrant to decomposition; and (2) decomposition would be greater at lower altitudes with warmer temperatures. No significant origin by destination interactions were observed with litter decomposition, indicating each litter type decomposed at the same rate as all others at each plot in spite of any differences in microenvironments or litter quality between plots. Our findings—as well as others utilizing space-for-time substitutions and meta-analyses investigating decomposition across environmental gradients—are contrary to the popular conception regarding temperature dependency of litter decomposition.

Field work is not always as easy as planning suggests, resulting in difficulties arising due to unforeseen circumstances needing to be overcome in the field (and at the computer in my case, overcoming my preconceived notions of temperature’s influence on litter quality and decomposition). One major difficulty with this research was re-locating the decomposition bags, though we ultimately achieved a 91% success rate in finding them. Another difficulty was working on an active volcano. A small vent erupted at Te Maari Crater on August 6th, 2012, approximately 1 km from my research plots, which resulted in access being delayed for a month until it was deemed safe to return. Weather conditions in montane regions also change unexpectedly and adverse conditions of snow, rain, fog, cold, and heat are often encountered. Unfortunately, as much as humanity is altering Earth’s climate, we cannot control it to our liking, and getting cold, wet, and sunburnt is all part of the job.

About the author

Matthew Krna at the top of Mount Tongariro (the north aspect of which was used for research plots) with Mount Ngauruhoe—AKA Mount Doom to Lord of the Rings‘ fans—in the background (credit: Matthew Krna)

My interest and passion for nature, the environment, and ecology started early, likely owing to outdoor activities such as hiking, camping, and canoe trips with family and friends. Science classes and field trips in elementary school further piqued my interest in the natural world, an interest that continued throughout my academic pursuits to the current day. During my undergraduate years I was torn between pursuing a degree in biology or in art. I often used art (in forms of ceramics, drawing, and photography) to express my concerns and love of nature, giving nature a “face.” I knew I could continue art on the side, so I opted to become a “starving student” in biology to give nature a “voice” through dissemination of research and findings.

As a biology undergraduate at the University of Missouri—Columbia my passion for research grew with involvement in greenhouse and laboratory experiments with Dr. Timothy Holtsford. We investigated the effects of nutrient manipulations and population density on the relative fitness of two closely related plant species. This, in conjunction with an undergraduate course on climate change and its impacts on nature, fuelled my passion for plant ecology. I wanted to investigate climate change on both the micro- and macro-aspects of plants and pursued my master’s thesis at Minnesota State University—Mankato with an emphasis on plant ecological physiology, supervised by Dr. Christopher Ruland. It was here I was granted the opportunity to perform my field research in the Antarctic tundra researching terrestrial plant growth and litter decomposition, as well as carbon and nutrient cycling. Knowing polar and montane systems are the terrestrial systems most threatened by climate change led me to pursue my PhD at Massey University in Palmerston North, New Zealand with Dr Gillian (Jill) Rapson. Here I investigated plant productivity, litter decomposition, and carbon sequestration of New Zealand tussock grasses using altitudinal gradients as proxies for climate change. I continue to explore ways of fulfilling my love and passion for saving the planet.

Umm .. Not a bad planet .. z .. Must think about .. zz to .. zzz .. save it! (credit: Jill Rapson)

The greatest scientific inspirations for me are the supervisors from my undergraduate, MSc, and PhD mentioned above. There were times where I doubted myself as a scientist and they continued to believe in me and push me to achieve what they knew I was capable of and help me achieve my goals. Aside from self-doubt being a slight barrier to progress in my academic and scientific career, I feel as though mental health is too often overlooked in graduate students. Over one third of respondents to a 2019 global survey of PhD students reported seeking assistance for depression and/or anxiety caused by their studies (Woolston, 2019). Having experienced anxiety and depression throughout portions of my studies, and having reached out for help, one piece of advice I want to offer to up-and-coming scientists is to seek help for mental health issues as needed. Sure, the pursuit of higher degrees is stressful, but these are also a learning experience, and not just about science, but also about one’s self as a scientist and as a human being.

Enjoyed the blogpost? Read the research here!

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