Tropical Island Forests and Climate Change

Preparers

Christian Giardina, Pacific Southwest Research Station ; Tana Wood, International Institute of Tropical Forestry

Issues

United States has formal research, management and technology transfer responsibilities for a dramatic range of tropical forests. These forests support highly endemic and critically endangered flora and fauna, and are home to a remarkable diversity of host cultures. Critically, most of these US and US affiliated forests occur on Islands, including: the state of Hawaii; the Territories of Puerto Rico, Virgin Islands, Guam and American Samoa; the Commonwealth of the Northern Mariana Islands; and three Compact of Free Association States that are the Republic of Palau, the Republic of the Marshall Islands and the Federated States of Micronesia. These geographies span half the globe and support forests that formed under widely ranging climates – from equatorial lowland wet forests and coastal mangrove swamps to montane peat lands, subalpine dry forests and alpine deserts. As a result of both the high biological diversity as well as geographic isolation, these ecosystems also support a high number of indigenous cultures that have millennial scale traditional knowledge relationships with local resources. Also, because of this tremendous geographic, topographic, ecological, life zone, biotic, and cultural diversity, US and US affiliated tropical islands have very complex and widely ranging sensitivities and vulnerabilities to climate change, especially when considering interactions with biological invasions, altered disturbance regimes, and land use changes (1; 2).

Climate change itself is expressed differently across and even within tropical islands. None-the-less, several factors can be stated about climate change in tropical island ecosystems:

1. It is now established that climate is changing rapidly for tropical islands;
2. Climate change is expressed as a wide diversity of threats, with drivers of change including sea level rise, altered rainfall, the frequency and intensity of big storms (e.g., cyclones, also known as typhoons or hurricanes), warming, and interactions with disturbances such as fire and invasive species;
3. As with most other tropical regions, baseline capacity to understand and mitigate climate change threats and drivers is often limited by access to economic resources for expensive monitoring efforts and associated technologies and highly-trained expertise; and
4. A mismatch exists between mitigation/adaptation approaches developed for large-scale applications and the often small-scale of local needs. Conversely, the communities of people that make tropical islands their home often rely on traditional knowledge, tried and tested tools, nimble and often informal institutions, and locally based ownerships, and so these communities also have some adaptation advantages for dealing with change.

Likely Changes

The distribution of vegetation communities on tropical islands is controlled strongly by local topography, temperature and rainfall, with often dramatic changes over short distances. Currently tropical regions are warming at rates that exceed the global average (3; 4; 5), while projections for rainfall vary regionally, even for leeward versus windward sides of an island. In Hawaii, temperatures have increased 0.30 ^oC per decade (6), while stream gauging studies have shown 10 percent reductions over the past 100 years (7). Dynamical downscaling for the Pacific region forecasts continued warming, and reductions in total rainfall as well as a shift to more intense rain events with longer rain-free periods (8, 9, 10; 11). Because air temperatures track sea surface temperatures (SST) on tropical islands (12), projected increases in SST for the Caribbean suggest that surface air temperatures also will rise. In Puerto Rico, a 62-yr record of temperature shows a significant increase in minimum and annual maximum temperatures (0.09 and 0.24 ^oC per decade, respectively; 13; 14). Total precipitation in lowland areas of Puerto Rico also has decreased significantly over the past century, and modeling suggests that local to regional land-use change will further exacerbate declines because vegetation cover affects cloud development (15; 14). Some recent research however suggests an increasing trend in rainfall rather than a decrease in Puerto Rico (16). Furthermore, global models indicate that Puerto Rico may experience changing patterns of hurricane activity, decreases in soil moisture, and increases in dry season length (17; 18). In summary, the increase in atmospheric carbon dioxide in the past century has already detectably altered temperature and rainfall amount and distribution in Pacific and Caribbean regions. By the end of the century, models project accelerated warming and continued alteration of rainfall, with additional severe impacts from sea level rise including increased sensitivity to storms, salt water intrusion and contamination of fresh water resources, and complete inundation for low lying regions and even whole islands.

In addition to the direct effects of climate change on tropical islands, the progressively warmer and increasingly altered rainfall in the US and US Affiliated Tropical Islands, will promote the fitness of non-native, drought-adapted and often fire-promoting plant species (e.g., 19) that have been introduced to many of these islands. The combination of climate change and species invasions will alter and in some cases displace the distribution of forest types dominated by native species. Further, warming trends will likely affect timing of breeding and phenology of flowering and fruiting plants and subsequently, the organisms that depend upon them (20). In tropical islands, alterations to phenology also impact traditional cultures that for centuries have used phenological cues to manage natural resources. Intensive land development, high coastal population density, and the effects of tourism-based industry all intensify the vulnerability of US and US Affiliated Tropical Islands to climatic change and increased variability, and there is limited human and capital infrastructure to address such problems (21; 2). This coupling of human and natural systems and resulting efforts to develop climate action plans will require a new generation of ecologically informed social research.

Options for Management

For tropical islands, management for climate change adaptation and mitigation must be comprehensive because climate change impacts are expressed in such a wide diversity of threats (1). A critical first tier of adaptation and mitigation work focuses on coastal zone management. Conservation measures directed at coastal plant communities, such as mangrove communities, can reduce local sensitivity to sea level rise. Conserving lowland forests can minimize the regional effects of climate change associated with deforestation (e.g., decreases in precipitation – minor benefit), and simultaneously augment the survival of lowland species as they colonize higher elevations (22; 2 – major benefit). More active strategies may include considering climate projections in restoration efforts, for example using new research on drought-tolerance to select drought-resistant genotypes of native species, or trait-based research on native and non-native plants to build climate resilient hybrid ecosystems. Similarly, conservation and restoration activities must now take place at landscape scales (23). Modeling-based approaches can support these efforts by helping to prioritize strategic projects that support larger landscape goals an increase efficiencies.

Addressing the independent and interactive effects of invasive species will also be central to effective management. Fire is often a novel and entirely human-caused disturbance regime on tropical islands, and non-native species may create fire regimes that displace fire-sensitive native species. Managing for fire prevention will include the management of fire-adapted and fire-prone non-native species. The climate benefits of fire prevention are many fold, including reduced erosion and sedimentation, especially to near shore habitat, enhanced watershed health, and increased access to forest resources.

Many adaptation and mitigation efforts are new, and significant knowledge gaps persist – especially in the prediction of natural system response to climate change and in the interaction of social-ecological knowledge systems. Considering the strongly human dimensions of climate change (5) and of effective climate change adaptation and mitigation in tropical islands (24), there is a need for expanded social and economic research and demonstration – a need being filled currently by universities, multi-agency partnerships, and non-governmental organizations.