Understanding the key mechanisms of tropical forest responses to canopy loss and biomass deposition from experimental hurricane effects

Abstract

To date, it is not clear which are the factors that most influence tropical forest recovery from hurricanes. Increased canopy openness and increased detritus (debris) deposition are two of the most likely factors, but due to their simultaneous occurrence during a hurricane, their relative effects cannot be separated without a manipulative experiment. Hence, in the Luquillo Experimental Forest (LEF) of Puerto Rico, the Luquillo Long-Term Ecological Research Program (LTER) has undertaken experimental manipulations in replicated 30×30m plots to simulate the major effects of hurricane disturbance–increased canopy openness and debris addition to the forest floor. Using a factorial experiment enabled investigation of the separate and combined effects of canopy opening and debris on this wet tropical forest; the experimental outcomes may help direct forest management decisions in similar disturbance-prone environments. In this first article of the special issue, we (1) provide details of the design and methodology for this manipulative experiment (the Canopy Trimming Experiment, CTE), (2) report some principal abiotic responses to treatments, and (3) introduce the subject areas of the 12 additional CTE manuscripts in this special issue. The physical conditions created by canopy and understory treatment and the amounts of debris added to CTE plots were similar to the LEF’s conditions following Hurricane Hugo (a category 4 storm) in 1989; although more wood and a 37% (1.5cm) deeper litter layer was present in the CTE. Our selective cutting and removal of the forest canopy above 3m, which included trimming 234 palm trees and 342 non-palm trees, greatly altered the understory micro-environment by increasing light levels and decreasing litter moisture for 18months; throughfall and soil moisture were elevated in trim plots for 3months. In plots where the canopy was trimmed and the debris (6kgm−2) was added to the forest floor, the canopy debris persisted on the forest floor for at least 4years; debris decomposed more quickly in plots with intact canopies. The diverse collection of papers in this special issue provide mechanistic understandings of response patterns of tropical forest biota (microbes, plants, animals) and processes (decomposition, herbivory, nutrient cycling, primary production) to canopy and understory disturbance that resembles a major (⩾category 3) hurricane. Although measurements for this experiment are on-going to further identify the mechanisms of long-term forest change resulting from hurricanes, we include findings up to the first seven years post-treatment at this time.