Decomposition rates of native versus non-native litter in a northwest
temperate forest
Jane Q. Sample
Hypothetical University, Planet Earth
Introduction:
Soils of the earth harbor much more carbon than is contained in the earth’s atmosphere
(Davidson and Janssens 2006). Fallen litter provides pulses of nutrients for forest plants, soil
invertebrates, and microbes and creates stocks of organic carbon in the soil. Current literature
suggests that an increase in temperature of 1°C in areas with an annual mean temperature of 5°C
will result in a loss of over 10% soil organic carbon. The amount of loss at an area with a mean
annual temperature of 30°C will only be 3% (Kirschbaum 1995). Tropical forests, dominated by
fleshy deciduous leaves, experience extreme spikes in available nutrients in the fall when its
leaves senesce. These leaves decompose rather quickly, resulting in long periods of low
nutrients during the spring and summer months. Conversely, our forests in the temperate
Northwest are dominated by coniferous trees. As a result, our forests accumulate deep layers of
duff through the shedding of needles, but don’t experience the nutrient spikes of the tropics.
Climate change is expected to create a positive feedback in the global carbon cycle through
decreases in soil organic carbon levels (Kirschbaum 1995).
In this study our aim was to measure decomposition rates of native versus introduced leaf litter.
Biological invasion theories assert that success rates of invasions are determined in part by the
invader being released from natural predators. In our case, we consider these predators to
include shredding insects and soil microbes that decompose leaf material in order to obtain the
carbohydrates and sugars contained within. Because decomposition rates depend on the triangle
of soil fauna/soil moisture/soil temperature, we also consider climate to be an important abiotic
“predator”. We therefore predict that native leaf litter, remaining under pressure from their
native predators, will decompose at a faster rate than introduced litter, released from the
predators of its native range.
Materials and Methods:
Litter bags were constructed as recommended in the protocol provided by the Ecoplexity Project
(http://ecoplexity.org/node/617). We randomly selected bags to receive treatments of Alder, live
Douglas fir needles, and dead Douglas fir needles from the H. J. Andrews Research Forest in
Oreong; Cecropea from the Luquillo Tropical Forest in Puerto Rico; grass from the Shortgrass
Steppe in Colorado, mesquite from the Chihuahuan desert of New Mexico, and paper from an
urban site in Pheonix, Arizona. All litter was dried to a constant dry weight at 10⁰C prior to
placement in the bags.
At the H. J. Andrews Research Forest in Blue River, Oregon, we constructed two strings of bags
per litter type, five bags per string, with the bags six inches apart. We placed one string of each
litter type above ground (Fig. 1), and one string of each just below the surface of the forest floor
soil. Fourteen strings were constructed in this manner for a total of 70 bags. We allowed the
litter bags to remain undisturbed from 2/1/2008 through 6/24/2008. At the termination of the
experiment, we collected the bags and again weighed to a dry constant weight.
Figure 1. Example of “above ground” litter bag filled with leaf litter. (Image courtesy of S.
Kirkpatrick)
Results:
While the bags of paper placed above and below ground experienced equal weight loss, all others
placed below the soil surface lost a greater proportion of their starting weights (Fig. 2). The bags
containing paper lost the greatest proportion of weight and the Cecropea samples lost the least
amount (Table 1). All non-native litter samples, with the exception of Cecropea, experienced the
greatest amount of mass lost during the experiment (Fig. 3).
Mean Weight Loss Percentage
0.60
0.50
0.40
0.30
0.20
0.10
0.00
Alder
Live Doug Dead Cecropea
fir
Doug fir
Grass
Mesquite
Paper
Figure 2. Percentage of beginning weight lost during experiment. Values are the collective
mean of the five bags above ground and five bags below ground for each litter type.
Species
Alder
Live Doug fir
Dead Doug fir
Cecropea
Grass
Mesquite
Paper
Above
Donor Ground
Region Loss (%)
HJA
12
HJA
32
HJA
12
LUQ
0
COL
15
NM
41
AZ
52
Below
Ground
Loss (%)
23
34
15
5
25
47
52
Table 1. Mean percentage of weight lost in litter bags above and below ground level. Donor region
codes are as follows: HJA = H. J. Andrews Research Forest, Temperate Rain Forest; LUQ = Luquillo
Research Forest, Puerto Rico, Tropical Rain Forest; COL = Colorado, Shortgrass Steppe; NM = New
Mexico, Desert; and AZ = Arizona, Urban.
Percent Weight Loss‐ Plants Native to H. J. Andrews
0.60
0.50
0.40
0.30
above ground
0.20
below ground
0.10
0.00
Alder
Live Doug fir
Dead Doug fir
Percent Weight Loss‐ Plants Not Native to H. J. Andrews
0.60
0.50
0.40
0.30
above ground
0.20
below ground
0.10
0.00
Cecropea
Grass
Mesquite
Paper
Figure 3. Weight loss relative to litter type and nativity. Because initial weights
varied slightly from species to species, proportion of weight loss is reported here.
Discussion:
Litter decomposition rates are determined by the soil fauna present, soil moisture content, and soil
temperatures. It was our expectation that due to having its native predators present, native leaf litter
would decompose at a faster rate than non-native litter. Contrary to our expectation, however, all
but one non-native litter type lost a greater proportion of their starting weights than did native
litters. There are a number of possible explanations for this finding, only a couple of which we will
discuss here. First, Douglas fir needles have thick, waxy cuticles that prevent the plant from
desiccating during the hot and dry summers that are typical at the H. J. Andrews Forest. By design,
this cuticle is slow to break down. Cecropea, grass, Mesquite, and paper do not have waxy surfaces
and would therefore decompose at faster rates. This does not explain the unexpected results of the
alder sample, as alder is a deciduous tree without a thick leaf cuticle. It is important to note,
however, that alder is a nitrogen-fixing species. While this may at first appear irrelevant, we
believe this to be a very real explanation of why the alder did not lose as much mass as was
expected. Alders indirectly benefit Douglas firs in that the alder fixes nitrogen which feeds the
native mycorrhizal fungus that dramatically increases the Douglas firs’ root surface area, allowing
increased nutrient and water uptake. It might very well be the case that the alder litter itself
decomposed more than appears and the hyphae of the mycorrhizal fungus is masking that fact by
contributing to the final weight.
As discussed above, Micorrhizae obtain sugars they need in part by developing mutualistic
relationships with deciduous trees in our temperate forests, chiefly alder trees. A second possible
explanation for the unexpected results involves the fact that the introduced types of litter, with the
exception of the paper, are deciduous. Leaves of deciduous plants are typically fleshier and contain
higher levels of sugar than do coniferous leaves. The Micorrhizae may show a preference for the
fleshy introduced litter, extracting the sugars and carbohydrates and dramatically decreasing their
masses. Future studies that test native deciduous against native coniferous litter are needed to
investigate this hypothesis further.
The final litter mass may not be fully representative of processes in the field due to the construction
of the litter bags themselves. As is typical in leaf litter decomposition studies, the underside of the
bags consisted of fine mesh so as to not lose litter material out of the bottom of the bag. While not
as fine as the underside, the top of the bags were also made of fairly fine mesh, most likely to the
exclusion of shredding insects. During the decomposition process, it is these shredding insects that
remove the greatest proportion of leaf material, either using it for themselves or breaking the larger
litter into more usable units for other detritivores. We recommend similar experiments be
conducted that assign bag mesh size as a treatment level in order to elucidate the effects on
variability.
Due to climate change we are seeing plants growing at higher elevations and in habitats once
considered novel. It is not unreasonable to assume that over time, our temperate coniferous forests
will contain a greater number of deciduous trees introduced into the system as a result. Studies such
as this are important in understanding the effects this will have on decomposition rates, and
therefore the organic carbon supply, of our forests. This study should be viewed as only a starting
point upon which to build.
References:
Davidson, Eric A. and Ivan A. Janssens. 2006. Temperature sensitivity of soil carbon
decomposition and feedbacks to climate change. Nature 440: 165-173.
Miko U. F. Kirschbaum. 1995. The temperature dependence of soil organic matter decomposition,
and the effect of global warming on soil organic C storage. Soil Biology and Biochemistry
27(6): 753-760.