Status: 

Ongoing

Title

Canopy Trimming Experiment (CTE) Plot Treatments
Summary

Short name: 

LUQMetadata165

Data set ID: 

165

Abstract: 

This data set is updated every time a trim is performed on the plots. The last trim was in 2014.

The Luquillo Experimental Forest LTER program in Puerto Rico was initially focused on the understanding of the effects that two major hurricanes, Hugo in 1989 and George in 1998, had on the structure and function of a tropical mountain forest and how the biota responded to these disturbances. This focus provided insights into the key characteristics of disturbance that alter forest function over long time scales.  After several years of research through the LEF-LTER program, it became clear that one primary effect of disturbances associated with the impact of a hurricane is to redistribute organic matter from live biomass compartments to the detrital pool.  Then a combination of biotic and abiotic processes, all modified by the disturbance, contributes to the decomposition of detritus and to the subsequent fate of associated C and nutrients. These critical regulating processes define detrital dynamics and play a central role in the recovery of forest structure and function by regulating decomposition and therefore carbon and nutrient storage and flow.

Our understanding of hurricane impacts comes from measurements of the effects of naturally occurring hurricanes on tabonuco forest and comparisons with similar disturbances in other forests (Walker et al. 1991, 1996a).  Base on the evaluation of long-term measurements after the impact of hurricane Hugo and George it becomes clear that the two primary effects of hurricane disturbance are changes in microclimate and redistribution of biomass, and that the interaction of both factors propagate through the system in complex ways.  These measurements are informative but cannot tease apart the effects of various aspects of hurricane disturbance and suffer from the lack of a control or reference condition.  From this assessment the LUQ-LTER principal investigators jointly identified the need for an experimental manipulation to decouple the effect of shifts in resource availability due to redistribution of biomass and altered microclimate conditions due to canopy opening on community and ecosystem processes and forest recovery.

As a long term experiment, the CTE is also designed to help evaluating predictions regarding the effects of an increased intensity and rate of hurricane disturbance on tabonuco forest (Sanford et al. 1991) as predicted by climate change models for Caribbean hurricanes (Emmanuel 1987, Goldenberg et al. 2001).  This long-term experiment will increase the frequency of simulated hurricane effects above background levels to once every six years. The experiment will determine effects of repeated disturbance of the forest canopy and increased detrital inputs to the forest floor on germination, growth, survival, nutrient cycling, soil conditions, and trophic structure.

Dates
Date Range: 
2004-10-26 00:00:00 to 2014-11-30 00:00:00
People

Additional Project roles: 

Name: Diana Garcia-Montiel Role: Associated Researcher
Methodology

Methods: 

CTE Site Location

The location of blocks was identified on the basis of similar slope, soil characteristics, forest canopy species composition and similar land-use history, with aspect selected as the blocking variable.  The plot size was chosen to provide sufficient space for long-term monitoring of plot responses as well as the biotic manipulations.  Each plot was at least 20 m distance from the edge of adjacent plots.  In order to minimize edge effects associated with the treatments, each 30 x 30 m plot had a 20 x 20 m interior measurement area.  Additionally, the 20 x 20 m measurement area was divided into a grid of 16 quadrats (ca. 4.7 x 4.7 m) with established walking trails bisecting adjacent quadrats to minimize disturbance.  To plan for the large number of both destructive and non-destructive measurements for this long-term experiment, the 16 quadrats were randomly assigned to soil related measurements (e.g., root, soil, soil gas sampling plots) or non-destructive sampling (e.g., seedlings, litter arthropods; litter decomposition).

CTE Experiment

Canopy Trimming Procedure
The canopy trimming is designed to simulate the opening of the canopy and the deposition of debris that result from a hurricane.  Six plots, each about 30 x 30 m, will be trimmed.  The trimming procedure will be as follows:

  • All trees greater than or equal to 15 cm dbh will have limbs and stems less than 10 cm diameter removed, meaning that large limbs and stems will be cut at the point where they become about 10 cm diameter.
  • All trees between 10 and 15 cm dbh will be trimmed at 3 m height and all limbs <10 cm diameter will be removed.
  • Palms will have leaves reaching above 3 m cut, leaving the apical meristem
  • Vegetation below 3 m will not be cut
  • No trees will be cut down 
  • Vine cutting will be minimal
  • Limbs on trees rooted in the plot but extending outside the 30 x 30 m will not be trimmed.  Limbs on trees rooted outside the plot but extending inside the 30 x 30 m will be trimmed.  Some adjustments to the 30 x 30 m trimmed area (e.g., reduced on the north, extended on the south) may be necessary to achieve the 4 hours of direct sunlight needed.
  • The number of trees greater than or equal to 10 cm dbh in a 30 x 30 m plot is about 74, including about 46  greater than or equal to  15 cm dbh. The total number of trees to be trimmed is therefore about 444.
  • Cutting will be clean, within the limits of practicality.  Wounds will not be painted (our forest mycologist advises that painting seals in moisture and promotes rot).
  • Debris will be allowed to fall to the ground but not in areas not inventoried for plants, wildlife, etc, for the EA.  (As stated above, limbs on trees rooted in the plot but extending outside the 30 x 30 m will not be trimmed).

At the field
To asses the effect of canopy trimming on community structure and ecosystem processes, the experimental manipulations involved four different treatments applied in a completely randomized block design. We established a total of three blocks and within each block the four treatments where randomly assigned to 30 x 30 m plots for a total of three replicate by treatment (16 plots totals). The four treatments involved:
1) Canopy trimmed and cut biomass distributed on forest floor (Trim+Debris). This simulates the changes in microclimate and redistribution of biomass created by a hurricane,
2) Canopy trimmed and cut biomass removed from the plot (Trim&clear). This simulates the changes in microclimate openness created by the hurricane without the associated redistribution of biomass.
3) Canopy not trimmed but canopy biomass from a trimmed plot distributed on the forest floor (NoTrim+Debris). This simulates the changes in redistribution of biomass created by the hurricane without the associated change in microclimate.
4) Canopy not trimmed and no canopy biomass added to forest floor (Control). This maintains the forest unmodified by simulated hurricane disturbance.

At the lab
The trimmed detritus was sorted into three categories:  wood (branches >1.5 cm diameter), leaves and twigs (branches <1.5 cm diameter and all non-palm foliar material), and palm fronds.  The detritus was immediately weighed to establish wet weight, and then subsampled, weighed, dried at 45o C, and reweighed to establish wet-dry weight ratios.  All detritus was then piled by category outside respective treatment plots (Drop or Addition) until trimming and weighing in both plots within a block was completed.  Approximately 11,157 ± 362 kg of (wet weight) detritus (mean ± SE; 6530 ± 186 kg dry-mass) was removed from each of the six trim plots (Drop and Removal).  Detritus was added into the Drop and Addition plots by spreading it evenly across each 30 x 30 m area.  Subsamples of detritus were weighed just prior to addition to determine the mass lost since the trimming occurred, and to estimate the total detritus added into each plot.  The amount of mass lost from the detritus piles (resulting from natural decomposition) during the time period from trimming the canopy to the detritus addition was as follows: 11.6% for wood, 27.5% for leaves and twigs, 16.1% for palm fronds.  Treatments within a block had equal portions of detritus added.  Similarly, the amount of detritus (kg) from each category for treatments among block were matched as closely as possible, and it was necessary to trim additional detritus from a nearby location (a telephone line ca. 60 m distance outside the nearest plot) for block A in order to more closely match the total detritus added to treatment plots in the other two blocks.  In total, the amount of detritus added to each of the six detritus addition plots (Drop and Addition) was 5,408 ± 143 kg (mean ± SE; dry-weight basis), and was represented by 67% wood, 29% leaves and twigs, and 4% palm fronds.

Additional information: 

  • CTE experimental blocks are located in El Verde (18"19'N, 65'49'W), the site of a 70-ha research forest that is 350-500m a.s.1. and receives a mean annual precipitation of 3460 mm; mean monthly temperatures range between 21 and 25 oC (Brown et al. 1983). The vegetation is classified as subtropical wet forest (Ewe1 & Whitmore 1973) and lies in the tabonuco zone which is dominated by Dacryodes excelsa Vahl. (tabonuco) and Prestoea montana (R. Grah.) Nichols (sierra palm).
  • See sketch of Blocks and plots with treatment: CTETreatmentsSketch.jpg
  • At the end of each plants and seedlings census, field technicians (J. Bithorn and M. Matta) generate a Census Report. The Seedlings report has two parts: 1) A summary of the New ("Nuevas") and dead ("Muertas") plants by year and the time taken to survey each Block (A, B, and C) by year and 2) A series of tables describing the time and people surveying each Block's Plots (1, 2, 3 and 4) and a summary of the New and dead plants found in them in the last Census.

    See the 2013 CTE Seedlings Census Report.

    The Plants Report provides a summary of the time taken to do the survey and reports the new and dead plants found by Block-Plot. A general summary by year is also provided.

    See the 2013 CTE PLant Census Report.

Status: 

Ongoing

Time Period: 

Long-Term

Periodicity: 

Every 5 years