Production, accumulation, and decomposition of Melaleuca quinquenervia litter biomass in time and space

Authors: Rayamajhi, Min; Pratt, Paul; Center, Ted

Submitted to: Florida Exotic Pest Plant Council
Publication Type: Abstract Only
Publication Acceptance Date: 4/24/2009
Publication Date: 4/24/2009
Citation: Rayamajhi, M.B., Pratt, P.D., Center, T.D. Production, accumulation, and decomposition of Melaleuca quinquenervia litter biomass in time and space. Florida Exotic Pest Plant Council.

Interpretive Summary: Melaleuca quinquenervia (melaleuca) is highly invasive in many parts of southern Florida. It has invaded over 200,000 hectares of land. Some areas have become closed-canopy monocultures with thick layers of leaf litter covering the forest floor and limiting other plants from being established. We assumed that the dramatic accumulation of thick layers of litter was due to large amounts of annual litterfall (leaves and twigs) that decomposed slowly compared to the litter of other native plants such as sawgrass and hence was able to build a thick cover on the forest floor. To test our assumption we documented litterfall rates and accumulated litter biomass (leaves and twigs) on the floor of mature melaleuca forests. We learned that melaleuca litterfall rate in our study site was up to 8 metric tons per hectare per year and the existing accumulated litter was up to 26 metric tons per hectare of which up to 16 metric tons was still undecomposed. Of the total litterfall, 50-70% was leaf material that covered the forest floor. Therefore, we wanted to know the time it takes for melaleuca and native sawgrass leaves to decompose so that the mineral soils are exposed for other plants to establish. It turns out that melaleuca leaves decompose at faster rates in melaleuca stands than in relatively more wet sawgrass stands. Also, melaleuca leaves decompose quickly on forest floors with organic soils than on sandy soils. Similarly, melaleuca leaves appeared to decompose at a slower rate than the sawgrass leaves. These findings indicate that melaleuca litterfall decomposition is relatively slower than sawgrass and hence may have helped impede the germination and establishment of other plants on the forest floor.

Technical Abstract: Melaleuca quinquenervia (melaleuca) is highly invasive in various habitats of southern Florida where it has invaded over 200,000 hectares. Some areas have become closed-canopy monocultures with thick layers of leaf litter covering the forest floor. We hypothesized that the dramatic accumulation of thick layers of litter was due to large amounts of annual litterfall coupled with slower decomposition rates. To test this we documented litterfall rates and standing litter biomass in melaleuca stands. During 1997-1998 total mean annual litterfall values were 4.4 and 7.8 mt/ha within small- and large-tree stands, respectively; over 70% of the total annual litterfall biomass in these stands was comprised of melaleuca leaves. In 1997, total standing litter biomass on forest floors averaged 12.3 mt/ha in small-tree plots and 25.6 mt/ha in large-tree plots; of this 6.1 and 15.6 mt/ha, respectively, was comprised of undecomposed litter (leaves and < 2.5 diameter stems). Over 50% of the undecomposed standing litter biomass consisted of melaleuca leaves. Therefore, we investigated melaleuca leaf-mass loss trends over a 6-year period. We collected freshly senesced fallen melaleuca leaves from mature melaleuca-forests and senescing Cladium jamaicense (sawgrass) leaves from sawgrass stands. We inserted a known mass of each species into separate nylon pouches, placed the pouches on the soils of melaleuca and sawgrass stands, harvested the bags periodically, washed off any decomposed organic matters, dried the remaining biomass to a constant weight and weighed the remaining contents of the pouches. The rate of mass loss in the pouches between sites (dry vs wet) and stands (melaleuca vs sawgrass) was determined based on the time it took to incur 50% loss of the original amount of leaf tissue. Results revealed that the melaleuca leaf mass loss in melaleuca stands was faster in organic soils than in sandy soils. The loss of melaleuca leaf biomass on organic soils in melaleuca stands was 1.5 times faster than in sawgrass stands. Melaleuca leaves decomposed at 1.8 and 2.9 times more slowly than sawgrass leaves under melaleuca and sawgrass stands, respectively. These finding indicate that melaleuca litterfall decomposition is slower than sawgrass regardless of the site. This study also provides insight to the role of rapid accumulation of melaleuca litter on the tree’s invasion potential and negative effects on recruitment and establishment of other plants under healthy canopies.