1. Where has all the leaf litter gone?....
Charlotte Buehler, Chad Byers, and Summer Drake, and Vanessa Artman
Biology Department, DePauw University, Greencastle Indiana
Are Earthworms the Culprit?
Earthworms are known to cause rapid rates of leaf litter disappearance. We speculated that we would find high numbers of earthworms at our sites, especially QS. We measured abundance of earthworms using the hot mustard extraction method in 0.25 m2 quadrats during July Yes, earthworms are present at our sites, but in low numbers, nowhere near the high numbers (20 to 25 per 0.25 m2) observed in other forests lacking leaf litter (Suarez et al. 2006).
What Affects the Amount of Leaf Litter in a Forest?
Decomposition rates of leaf litter?
Abiotic soil conditions (pH, calcium content, other nutrients)?
Abundance of earthworms?
Abundance of millipedes?
Abundance of macroinvertebrates (beetles, crickets, spiders)?
Forest composition (maples, oaks, elms)?
Other abiotic factors (microclimate, microtopography, moisture)?
Leaf chemistry (tannin content, lability, recalcitrance)?
Other biotic factors (bacteria, fungi, mycorrhizae)?
What about Macroinvertebrates?
Macroinvertebrates with a key role in litter decomposition include beetles, springtails, centipedes, millipedes, and ants. We used pitfall traps to survey macroinvertebrates during summer Each of these types of macroinvertebrates is present at our sites, but there were no differences in abundance or biomass among our sites.
What about Decomposition Rates?
We hypothesized that decomposition rates would be higher at QS than QH or ARB. We ran an experiment using mesh bags, each containing 10 grams of mixed leaf litter. We set out a total of 60 bags at each site and collected a subsample of bags after different time periods. There were no differences in decomposition rates among our sites. Decomposition rates seemed to be low; only 5% of the litter decomposed after 6 weeks, and over 60% remained after 58 weeks. Something strange seemed to occur at the end of the experiment also, with more mass remaining after 58 weeks than 52 weeks. This increase in mass may have been due to fungi colonizing the bags.
Study Sites: Our three sites, ARB, QH, and QS, are located at the DePauw Nature Park in west-central Indiana. The Nature Park was established in 2004; its central feature is an old limestone quarry that was abandoned about 35 years ago.
ARB QH QS
abandoned limestone quarry
DePauw Nature Park boundary
What about the Trees?
Forest composition has a huge effect on soil dynamics and nutrient recycling. Some types of litter, such as oaks, are “recalcitrant”, containing large amounts of lignin and tannins. Other types of litter, such as maple and ash, are more “labile”, containing more calcium and digestible carbon compounds. The forest composition is quite different among our three sites. QS has large proportions of slippery elm, black cherry, and walnut, which may be more labile compared to the dominant trees at ARB and QH.
What about Millipedes?
During the earthworm surveys, we noticed a bunch of millipedes. It was only after these earthworm surveys, that we realized millipedes could be the culprit. Millipedes are known to consume decaying leaves (it’s true!…. we’re serious…. ask us if you don’t believe us! ). And abundance of millipedes was negatively correlated with the amount of leaf litter at our sites; both more millipedes and less leaf litter were observed at QS.
Our Next Experiments
So where has all the leaf litter gone? Our two most promising hypotheses are (1) forest composition and (2) millipedes.
We set up another long-term experiment with mesh bags to look at the role of forest composition. We set out a total of 810 bags with air-dried leaves from 9 tree species. We hypothesize that slippery elm, black cherry, and black walnut will decompose faster than sugar maple and oaks.
We are also planning another experiment to look at the role of millipedes. There were more millipedes and less leaf litter at QS – pattern or coincidence? We will set out 720 packages of leaf litter, half of which are accessible to millipedes and half of which are not. We will collect the bags after different time periods and look at the decomposition rates. Who knows, maybe millipedes are the answer to the questions we posed over two years ago! Our research continues indefinitely…
ARB
Baseline Data
The amount of leaf litter is consistently higher at ARB and QH than at QS. We initially observed these patterns in 2004, and since then, we have been testing various hypotheses to explore why these patterns exist.
Leaf litter depth, measured during summer, is very low overall (only 2-4 mm) and differs among the three sites with less leaf litter at QS than ARB or QH. QH
What about the Soil?
We hypothesized that the soil at QS would have a higher pH and contain more calcium than the other sites. Limestone rubble from the quarry operation may have been deposited at QS, thus contributing more calcium to the soil. Higher calcium content and higher pH both contribute to higher rates of litter decomposition (Reich et al. 2005). Our analysis of soil samples show no differences in soil among the sites, except for phosphorus content. Soil pH is neutral and calcium content is high at all three sites.
Percent cover of leaf litter, measured during the summer, is also very low (only 15 to 45%) and is lower at QS than ARB or QH.
Acknowledgments
Financial support for this project was provided by DePauw University through the Science Research Fellows program, the Faculty Development program, and the Office of Academic Affairs. We thank Neal Abraham for supporting our research. We thank the students, faculty, and staff at DePauw who have contributed time and energy to our research, and we thank our friends and families for their support. QS
Leaf litter biomass, measured during autumn after leaf fall, is comparable to other forests, but is lower at QS than ARB or QH.
The team, Summer Drake, Chad Byers, and Charlotte Buehler, celebrating the end of the field season.