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27 Year Chronology Of Litter Production In A Puerto Rican Moist Tropical Forest Heather E. Erickson, Consulting Research Ecology, Portland, OR Grizelle González, International Institute for Tropical Forestry, USDA Forest Service, Rio Piedras, PR Context Litterfall in forest ecosystems is a major source of C and nutrients to soils and serves as an index of primary production. Previous research in hurricane-prone regions shows a significant effect of hurricanes on litterfall, and in the subtropical moist region of Puerto Rico, a lesser effect of short term (2-4 month) drought (Beard et al. 2005), with both disturbances showing a positive effect on litterfall inputs as their intensity increases. This is interesting as both drought and hurricane intensities will likely increase in the Caribbean with global warming. However few studies have examined long-term trends in litter production where hurricanes and drought are influential, and may interact. Here, we present 27 years (1989-2015) of litterfall production in a moist subtropical forest in eastern Puerto Rico.
Methods Forest litter was collected bi-weekly from 60 litterfall traps placed in the Bisley Watersheds in the Luquillo Experimental Forest. There is a slight dry season from January – March/April where rainfall is about half that of the remaining months. Litter was separated into leaves, woody material, fruits and flowers together, and a miscellaneous category prior to drying and weighing. Samples were analyzed for a suite of nutrient elements – only mass inputs are presented. Reliable (i.e., mostly complete) climate data for the site began in 1997.
Results and Discussion Major peaks in litter production over the 27 years corresponded to the occurrence of hurricanes and tropical storms. The most destructive hurricane was Hugo in 1989; leaf litterfall appeared to have recovered by Hurricanes Luis and Marilyn in 1995 (see also Scatena et al. 1996). Additional hurricanes in the late 1990s continued to impact litterfall inputs. The relatively quiet years from 1999-2009 show leaf fall had mostly stabilized, while wood and fruit and flower inputs continued to increase. Secondary peaks in litter production occurred in early summer, typically in early June, after the start of the rainy season. Bi- or multi-modal peaks are common in broad-leaved evergreen tropical forests (Zang et al. 2014).
Fruits and Flowers
Mean total litterfall over the 27 years = 8.97 Mg ha-1 y-1 (for old-growth tropical rainforests across South America = 8.61 Mg ha-1 y-1 , Chave et al. 2010). Annual Litterfall Total Litterfall
Fruits and Flowers
Annual Rainfall (cm)
Preliminary Conclusion and Next Steps: 1. The visual pattern across the chronology suggests full recovery of litterfall from a major hurricane can take years and may mask more subtle effects due to changes in climate. 2. Complete the climate record and climate analyses by cross-correlating rainfall with a nearby site for years with missing data. 3. Examine the roles of wind speed and solar radiation, the latter thought to trigger leaf abscission (Chave et al. 2010). 4. Address questions related to individual litter components (e.g., does drought impact woody litterfall more than leaf litterfall in this moist forest?)
g m-2 y-1 or cm of rainfall
Annual trends: Large inputs of litterfall corresponded to high rainfall years, but litterfall was also relatively high during the dry years of 2002, 2014, and 2015. (Although not shown, 1991, 1994, and 1996 were also relatively dry.) Woody litterfall appears to contribute most to the rising trend in total litterfall since 1999, including the final dry years, suggesting the effects of long term drought may be difficult to separate from the effects of stand dynamics (i.e., increased woody litterfall production with stand age).
1000 800 600 400 200 0 1989
References: Beard et al. 2005 Ecological Monographs; Chave et al. 2010 Biogeosciences; Scatena et al. 1996 Biotropica; Zang et al. 2014 Ecological Complexity