Abstract
The Hawaiian Islands are an ideal location to study the response of tropical forests to climate variability because of their extreme isolation in the middle of the Pacific, which makes them especially sensitive to El Niño-Southern Oscillation (ENSO). Most research examining the response of tropical forests to drought or El Niño have focused on rainforests, however, tropical dry forests cover a large area of the tropics and may respond very differently than rainforests. We use satellite-derived Normalized Difference Vegetation Index (NDVI) from February 2000-February 2009 to show that rainforests and dry forests in the Hawaiian Islands exhibit asynchronous responses in leaf phenology to seasonal and El Niño-driven drought. Dry forest NDVI was more tightly coupled with precipitation compared to rainforest NDVI. Rainforest cloud frequency was negatively correlated with the degree of asynchronicity (ΔNDVI) between forest types, most strongly at a 1-month lag. Rainforest green-up and dry forest brown-down was particularly apparent during the 2002–003 El Niño. The spatial pattern of NDVI response to the NINO 3.4 Sea Surface Temperature (SST) index during 2002–2003 showed that the leeward side exhibited significant negative correlations to increased SSTs, whereas the windward side exhibited significant positive correlations to increased SSTs, most evident at an 8 to 9-month lag. This study demonstrates that different tropical forest types exhibit asynchronous responses to seasonal and El Niño-driven drought, and suggests that mechanisms controlling dry forest leaf phenology are related to water-limitation, whereas rainforests are more light-limited.
Highlights
The response of terrestrial ecosystems to climate variability such as El Nino-Southern Oscillation (ENSO) serves as an important analogy for ecosystem responses to projected future climate change
Most research examining the response of tropical forests to drought or El Nino have focused on rainforests in the Amazon [4,5,6,7], Panama [8,9,10] or Southeast Asia [11,12], even though tropical dry forests cover a large area of the tropics and may respond very differently compared to rainforests
The strongest relationship between precipitation and rainforest Normalized Difference Vegetation Index (NDVI) was between the yearly cumulative precipitation and mean NDVI, whereas precipitation minimum and maximum were not significantly correlated to any measure of rainforest NDVI
Summary
The response of terrestrial ecosystems to climate variability such as El Nino-Southern Oscillation (ENSO) serves as an important analogy for ecosystem responses to projected future climate change. Most research examining the response of tropical forests to drought or El Nino have focused on rainforests in the Amazon [4,5,6,7], Panama [8,9,10] or Southeast Asia [11,12], even though tropical dry forests cover a large area of the tropics and may respond very differently compared to rainforests. One of the few studies that included different forest types was by Condit et al [10]. They assessed the growth and mortality of tree species across a precipitation gradient in a Panamanian tropical forest. Tree growth was observed to be higher during 1998 and this was hypothesized to be related to increased photosynthetically active radiation (PAR) during the 1998 El Nino, consistent with the relationship between light and forest phenology proposed by Wright and van Schaik [13]
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