Total, foliar, and reproductive fine litter production for up to 38 years for four old-growth forests in central Panama

Forests sequester a substantial portion of anthropogenic carbon emissions. Many open questions concern how. We address two of these questions. Has leaf and fine litter production changed? And what is the contribution of old-growth forests? We address these questions with long-term records (≥10 years) of total, reproductive, and especially foliar fine litter production from 32 old-growth forests. We expect increases in forest productivity associated with rising atmospheric carbon dioxide concentrations and, in cold climates, with rising temperatures.

We evaluate the statistical power of our analysis using simulations of known temporal trends parameterized with sample sizes (number of years) and levels of interannual variation observed for each record. Statistical power is inadequate to detect biologically plausible trends for records lasting less than 20 years. Modest interannual variation characterizes fine litter production. More variable phenomena will require even longer records to evaluate global change responses with sufficient statistical power. 

Just four old-growth forests have records of fine litter production lasting longer than 20 years, and these four provide no evidence for increases. Three of the four forests are in central Panama, also have long-term records of wood production, and both components of aboveground production are unchanged over 21 to 38 years. The possibility that recent increases in forest productivity are limited for old-growth forests deserves more attention.

This data package contains previously unpublished data from four old-growth forests in central Panama. Data compiled from the published literature for another 28 forests and the R scripts required to recreate our analyses can be found here: https://smithsonian.dataone.org/view/urn:uuid:8bbcd334-059b-45b1-9b83-94b52abbd6f8.  

Methods
The data are dry masses of material collected by litter traps. The litter traps are open, 1-mm mesh bags supported 80 to 100 cm above the ground by PVC frames. We collect fallen material from traps each week at three sites (named BCI_Inceptisol, BCI_FDP, and BPSL) and each month at the final site (Gigante), Table 1 below provides site descriptions. We oven dry the collected material to constant mass at 60°C. We then separate and determine the dry mass of leaves, reproductive material, fine wood (< 2 cm diameter), and unidentifiable material or "dust".   

We summed the data over censuses within each year for each trap and present the sums by year and trap. Units are Mg ha-1 yr-1. Years begin with the first census at each site and run from November 1985 through October 2023 for the BCI_Inceptisol site, May 1997 through April 2024 for the BPSL site, July 1998 through June 2023 for the Gigante site, and May 2013 through April 2024 for the BCI_FDP site. Fine litter fall is at its annual low during the rainy season from May through November in central Panama. There are 59 0.25 m2 traps at the BCI_Inceptisol site, 64 0.5 m2 traps at the BCI_FDP site, 40 0.5 m2 traps at the BPSL site, and 12/20 0.58 m2 traps at the Gigante site, where 12/20 indicates there were 12 traps from 1998 to February 2007 and 20 traps thereafter. 

Rarely, fallen objects or vandals compromised traps. These rare occassions accounted for 0.02%, 0.06%, 0.19%, and 0.13% of the trap-census combinations for the BCI_Inceptisol, BCI_FDP, BPSL, and Gigante sites, respectively. The lower values for the two BCI sites probably reflect the near total exclusion of potential vandals from Barro Colorado Island. When a trap was compromised, its value for that census was replaced by the mean value for functioning traps for the same census. 

Table 1. Site descriptions. MAP refers to mean annual precipitation. Mean annual temperature is 26°C. Soil pH (in water), resin phosphorus (resin P), and total bases (sum of potassium, calcium, and magnesium) are for the upper 10 cm. 

Site name

Latitude

Longitude

Elev. (m)

MAP (mm)

Soil Order

pH in water

Resin P (mg P kg-1)

Total bases (cmolc kg-1)

Years since disturbance

BCI Inceptisol

9°8'30"N

79°51'05"W

30

2620

Inceptisol

6.2

2.50

25.0

~ 600

BCI FDP

9°9'20"N

79°51'15"W

180

2620

Oxisol

5.9

1.86

9.75

~ 600

Gigante

9°6'31"N

79°50'37"W

50

2450

Oxisol

5.4

0.85

13.0

~ 600

BPSL

9°16'48"N

79°58'30"W

120

3421

Oxisol

4.7

0.24

1.81

~ 200

Additional site-specific methodological details can be found in the following articles:

Puerta-Piñero C, Muller-Landau HC, Calderon O, Wright SJ. 2013. Seed arrival in tropical forest tree fall gaps. Ecology 94: 1552-1562.

Wright, SJ, O Calderon, HC Muller-Landau. 2019. A phenology model for tropical species that flower multiple times each year. Ecological Research 34: 20-29.

Wright SJ, Turner BL, Yavitt JB et al. 2018. Plant responses to fertilization experiments in lowland, species-rich, tropical forests. Ecology 99: 1129-1138.

Wright, SJ, O Calderon. 2025. Statistical power and the detection of global change responses: The case of leaf production in old-growth forests. Ecology 106: in press.