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Drought is a prominent feature of Hawaiʻi’s climate. However, it has been over 30 years since the last comprehensive meteorological drought analysis, and recent drying trends have emphasized the need to better understand drought dynamics and multi-sector effects in Hawaiʻi. Here, we provide a comprehensive synthesis of past drought effects in Hawaiʻi that we integrate with geospatial analysis of drought characteristics using a newly developed 100-year (1920–2019) gridded Standardized Precipitation Index (SPI) dataset. The synthesis examines past droughts classified into five categories: Meteorological, agricultural, hydrological, ecological, and socioeconomic drought. Results show that drought duration and magnitude have increased significantly, consistent with trends found in other Pacific Islands. We found that most droughts were associated with El Niño events, and the two worst droughts of the past century were multi-year events occurring in 1998–2002 and 2007–2014. The former event was most severe on the islands of O’ahu and Kaua’i while the latter event was most severe on Hawaiʻi Island. Within islands, we found different spatial patterns depending on leeward versus windward contrasts. Droughts have resulted in over $80 million in agricultural relief since 1996 and have increased wildfire risk, especially during El Niño years. In addition to providing the historical context needed to better understand future drought projections and to develop effective policies and management strategies to protect natural, cultural, hydrological, and agricultural resources, this work provides a framework for conducting drought analyses in other tropical island systems, especially those with a complex topography and strong climatic gradients.

The area burned annually by wildland fire in Hawaii has increased fourfold in recent decades. The archipelago's novel fuel types and climatic heterogeneity pose significant challenges for fire risk assessment and fire management. Probability-based fire occurrence models using historical wildfire records provide a means to assess and attribute fire risk in regions of the world like Hawaii where investment in fire science is limited. This research used generalized additive models to 1) assess the relative contribution of vegetation, climate, and human-caused ignitions to the probability of fire in the northwest quadrant of Hawaii Island and 2) compare how landscape flammability varies due to interannual rainfall variability versus projected changes in mean annual rainfall (MAR) and temperature. Annual fire probability was highest for grasslands and peaked at drier conditions (0.04 at 450 mm MAR) when compared with shrublands (0.03 at 650 mm MAR) and forest (0.015 at 600 mm MAR). Excess rainfall the year prior to fire occurrence increased fire risk across grasslands, and thus overall fire probability, more so than drought the year that fire occurred. Drying and warming trends for the region under projected climate change increased maximum values of fire probability by as much as 375% and shifted areas of peak landscape flammability to higher elevation. Model predictions under future climate also indicate the largest changes in landscape flammability will happen by mid-Century. The influence of antecedent wet years on fire risk can improve near-term predictions of fire risk in Hawaii while climate projections indicate that fire management will need to be prioritized at upper elevations where high value natural resources are concentrated.

Drought is a prominent feature of Hawaii's climate, however, the biological, ecological, cultural, and socioeconomic impacts of drought in Hawaii are not well understood. This paper provides a comp...

Identifying management actions required to maintain desired ecological conditions in response to high intensity disturbance events remains a critical question, especially as disturbance regimes and species composition shift due to human activities and climate change. Feedbacks between novel fire disturbance and invasive species on islands, in particular, have resulted in the degradation and conversion of native ecosystems into more alien-species-dominated communities. Further, monitoring and evaluation of management responses to these events remains sparse. We draw on 14 years of monitoring to assess the effects of fire and management response (weed removal and experimentally controlled outplanting of native plants), on the structure, composition and diversity of a native, mesic tropical forest in Hawai‘i. Natural regeneration of the native, disturbance-adapted tree, Acacia koa was rapid and abundant and formed the dominant canopy component after 14 years. However, naturally recruited native small trees and understory plants all declined in abundance over the course of the study. In contrast, nonnative, invasive species were highly successful at establishing at the site and currently dominate the understory. Outplanted native species (Acacia koa, Dodonea viscosa, and Alyxia stellata) had relatively high survival and significantly increased native species abundance and richness at the site. We conclude that despite high natural recruitment of a native overstory tree (Acacia koa), restoration management intervention enhanced the native component of post-fire vegetation. However, continued intervention will likely be required over the long-term to facilitate the recovery of vegetation to the pre-fire, native-dominant conditions. As high intensity disturbances increase across island ecosystems, understanding and monitoring the successional trajectory of both nonnative and native forest species can improve the efficiency of management response and allow managers to adapt to changes in conditions as well as prioritize actions across larger disturbance events.

Resource managers increasingly seek to implement cost-effective watershed restoration plans for multiple ecosystem service benefits. Using locally adapted ecosystem service tools and historical management costs, we quantified spatially explicit management costs and benefits (in terms of groundwater recharge and landscape flammability) to assist a state agency in evaluating cobenefits for a predefined restoration scenario (focused on biodiversity benefits) and to prioritize an expanded restoration scenario in the state-managed Pu‘u Wa‘awa‘a watershed ( Hawai‘i) now and under the Representative Concentration Pathway (RCP) 8.5 midcentury climate scenario. Restoring all available areas increases recharge by ∼1.74 million m3/yr (5% of recharge over the entire watershed) under the current climate and does not meaningfully change recharge under RCP 8.5 midcentury, whereas climate change decreases recharge by ∼50%. For landscape flammability, climate change increases the median and maximum probability o...

Well-managed rangelands provide important economic, environmental, and cultural benefits. Yet, many rangelands worldwide are experiencing pressures of land-use change, overgrazing, fire, and drought, causing rapid degradation. These pressures are especially acute in the Hawaiian Islands, which we explore as a microcosm with some broadly relevant lessons. Absent stewardship, land in Hawaiʻi is typically subject to degradation through the spread and impacts of noxious invasive plant species; feral pigs, goats, deer, sheep, and cattle; and heightened fire risk. We first provide a framework, and then review the science demonstrating the benefits of well-managed rangelands, for production of food; livelihoods; watershed services; climate security; soil health; fire risk reduction; biodiversity; and a wide array of cultural values. Findings suggest that rangelands, as part of a landscape mosaic, contribute to social and ecological health and well-being in Hawaiʻi. We conclude by identifying important knowledge gaps around rangeland ecosystem services and highlight the need to recognize rangelands and their stewards as critical partners in achieving key sustainability goals, and in bridging the long-standing production-conservation divide.

Spatially explicit, wall-to-wall rainfall data provide foundational climatic information but alone are inadequate for characterizing meteorological, hydrological, agricultural, or ecological drought. The Standardized Precipitation Index (SPI) is one of the most widely used indicators of drought and defines localized conditions of both drought and excess rainfall based on period-specific (e.g., 1-month, 6-month, 12-month) accumulated precipitation relative to multi-year averages. A 93-year (1920–2012), high-resolution (250 m) gridded dataset of monthly rainfall available for the State of Hawai‘i was used to derive gridded, monthly SPI values for 1-, 3-, 6-, 9-, 12-, 24-, 36-, 48-, and 60-month intervals. Gridded SPI data were validated against independent, station-based calculations of SPI provided by the National Weather Service. The gridded SPI product was also compared with the U.S. Drought Monitor during the overlapping period. This SPI product provides several advantages over currently available drought indices for Hawai‘i in that it has statewide coverage over a long historical period at high spatial resolution to capture fine-scale climatic gradients and monitor changes in local drought severity.

Wildfire is a major threat to natural resources and native species in Hawai‘i, but the frequency and extent of wildfires across the archipelago has not been well quantified. Our objective was to summarize the available wildfire data for Hawai‘i and synthesize the social and ecological dimensions of wildfire drivers, impacts, and management responses. We constructed a 110-yr span of wildfire records for the state of Hawai‘i to examine historical trends (1904–2011) and summarized relationships between contemporary wildfire occurrence (2005–2011) and land use/land cover types and human population. Total area burned statewide increased more than fourfold from 1904 to 1959 to peaks in the 1960s–1970s and mid-1990s to present. From 2005 to 2011, on average, 1,007 wildfires were reported across the state per year (±77 SE), burning an average of 8,427 ha yr-1 (±2,394 SE). Most fires (95%) were

Hawai‘i has served as a model system for studies of nutrient cycling and conservation biology. The islands may also become a laboratory for exploring new approaches to forest restoration because of a common history of degradation and the growing number of restoration projects undertaken. Approximately half of the native ecosystems of Hawai‘i have been converted to non-native conditions. Many restoration projects have focused on intensively managed out plantings of native plants with emphasis on threatened and endangered species. While these projects have been effective in stabilizing plant populations, this model is often prohibitively expensive for restoration at the scale needed to protect watersheds and provide habitat for rare bird species. Here we suggest ways of rethinking ecological restoration that are applicable across the tropics, particularly on islands and fire-prone grasslands. First, we suggest making use of non-native, non-invasive species to help reclaim degraded or invaded sites or as long-term components of planned restoration outcomes. Second, we suggest incorporating remote sensing techniques to refine where restoration is carried out. Finally, we suggest borrowing technologies in plant production, weed control, and site preparation from industrial forestry to lower restoration costs. These suggestions would result in ecosystems that differ from native reference systems in some cases but which could be applied to much larger areas than most current restoration efforts while providing important ecosystem services. We also stress that community involvement is key to successful restoration, as a major goal of almost all restoration projects is to re-connect the community with the forest.

Despite the challenges wildland fire poses to contemporary resource management, many fire-prone ecosystems have adapted over centuries to millennia to intentional landscape burning by people to maintain resources. We combine fieldwork, modeling, and a literature survey to examine the extent and mechanism by which anthropogenic burning alters the spatial grain of habitat mosaics in fire-prone ecosystems. We survey the distribution of Callitris intratropica, a conifer requiring long fire-free intervals for establishment, as an indicator of long-unburned habitat availability under Aboriginal burning in the savannas of Arnhem Land. We then use cellular automata to simulate the effects of burning identical proportions of the landscape under different fire sizes on the emergent patterns of habitat heterogeneity. Finally, we examine the global extent of intentional burning and diversity of objectives using the scientific literature. The current distribution of Callitris across multiple field sites suggested long-unburnt patches are common and occur at fine scales (

Climatic conditions that challenge human thermoregulatory capacity currently affect around a quarter of the world’s population annually. Such conditions are projected to increase in line with CO2 emissions particularly in the humid tropics. Climate change can increase the risk of conditions that exceed human thermoregulatory capacity1,2,3,4,5,6. Although numerous studies report increased mortality associated with extreme heat events1,2,3,4,5,6,7, quantifying the global risk of heat-related mortality remains challenging due to a lack of comparable data on heat-related deaths2,3,4,5. Here we conducted a global analysis of documented lethal heat events to identify the climatic conditions associated with human death and then quantified the current and projected occurrence of such deadly climatic conditions worldwide. We reviewed papers published between 1980 and 2014, and found 783 cases of excess human mortality associated with heat from 164 cities in 36 countries. Based on the climatic conditions of those lethal heat events, we identified a global threshold beyond which daily mean surface air temperature and relative humidity become deadly. Around 30% of the world’s population is currently exposed to climatic conditions exceeding this deadly threshold for at least 20 days a year. By 2100, this percentage is projected to increase to ∼48% under a scenario with drastic reductions of greenhouse gas emissions and ∼74% under a scenario of growing emissions. An increasing threat to human life from excess heat now seems almost inevitable, but will be greatly aggravated if greenhouse gases are not considerably reduced.

Lifestyle-related, non-communicable diseases, such as diabetes, hypertension, and obesity have become critical concerns in the Pacific islands of Micronesia. We investigated the relationship between the diminution of traditional lifestyle practices and the decline in the health of the population in the State of Pohnpei, Federated States of Micronesia. To assess this, our interdisciplinary team developed two scales, one to rank individuals on how traditional their lifestyles were and one to rank individuals on the healthiness of their lifestyles. Participants' locations were categorized as living on a remote atoll, living on the main island, or as a transitional population. Pohnpeians living in transitional communities (e.g. recently moved from a remote atoll to the main island, or the reverse) ranked lowest on both the tradition and health scales, rather than ranking intermediate between the remote and main island groups as we had hypothesized. As predicted, individuals residing on the remote atolls were living the most traditional lifestyles and also had the healthiest lifestyles, based on our rating system. The higher an individual scored on the tradition scale, e.g. the more traditional life they lived, the higher they scored on the health scale, suggesting the importance of traditional lifestyle practices for maintaining health. These findings have significant implications for promoting health and longevity of Micronesians and other Pacific Island peoples. We suggest the process of transition be recognized as a significant lifestyle and health risk and be given the attention we give to other risk factors that negatively influence our health. Based on our findings, we discuss and recommend the revitalization of particular traditional lifestyle practices, which may advance healthy aging among Pohnpeians.

Patches of fire-sensitive vegetation often occur within fire-prone tropical savannas, and are indicative of localized areas where fire regimes are less severe. These may act as important fire refugia for fire-sensitive biota. The fire-sensitive tree Callitris intratropica occurs in small patches throughout the fire-prone northern Australian savannas, and is widely seen as an indicator of low-severity fire regimes and of good ecosystem health. Here, we address the question: to what extent do Callitris patches act as refuges for other fire-sensitive biota, and therefore play a broader conservation role? We contrast floral and faunal species composition between Callitris patches and surrounding eucalypt savanna, using three case studies. In the first case study, a floristic analysis of 47 Callitris patches across Western Australia's Kimberley region showed that woody species in these patches were overwhelmingly widespread, fire-tolerant savanna taxa. No species of special conservation concern occurred disproportionately within Callitris patches. Similarly, there was no concentration of fire-sensitive fauna or flora in five Callitris patches in the East Kimberley. Finally, there was no difference in ant species composition among 12 Callitris patches and surrounding eucalypt savannas in Kakadu National Park, Northern Territory, and there were no fire-sensitive ant species in Callitris patches. Our three case studies from throughout the northwestern Australia provide no evidence that Callitris patches act as important refuges for fire-sensitive flora or fauna within fire-prone eucalypt savannas. This calls into question the notion that Callitris is a strong indicator of general ecosystem health.

1. Callitris intratropica is a long-lived, obligate-seeding, fire-sensitive overstorey conifer that typically occurs in small groves (

The planting of non-timber forest products (NTFPs) in the understory of tropical forests is promoted in many regions as a strategy to conserve forested lands and meet the economic needs of rural communities. While the forest canopy is left intact in most understory plantations, much of the midstory and understory vegetation is removed in order to increase light availability for cultivated species. We assessed the extent to which the removal of vegetation in understory plantations of Chamaedorea hooperiana Hodel (Arecaceae) alters understory light conditions. We also examined how any changes in light availability may be reflected by changes in the composition of canopy tree seedlings regenerating in understory plantations. We employed a blocked design consisting of four C. hooperiana plantation sites; each site was paired with an adjacent, unmanaged forest site. Hemispherical canopy photographs were taken and canopy tree seedlings were identified and measured within 1 23 × 2m randomly placed plots in each site for a total of 96 plots (4 blocks × 2s ites× 12 plots). Plantation management did not affect canopy openness or direct light availability but understory plantations had a higher frequency of plots with greater total and diffuse light availability than unmanaged forest. Comparisons of canopy tree seedling composition between understory plantations and unmanaged forest sites were less conclusive but suggest that management practices have the potential to increase the proportion of shade-intolerant species of tree seedlings establishing in plantations. Given the importance of advanced regeneration in gap-phase forest dynamics, these changes may have implications for future patterns of succession in the areas of forest where NTFPs are cultivated.

The planting of non-timber forest products (NTFPs) in the understory of tropical forests is promoted in many parts of the world as a strategy to conserve forested lands and meet the economic needs of rural communities. While many studies of NTFP management have focused on the effects of harvesting on wild populations, the impacts of understory NTFP plantations, or enrichment plantings, on forest community composition and structure have not been investigated. We assessed the effects of understory plantations of the palm Chamaedorea hooperiana Hodel, on community composition and structure of an old growth tropical rain forest in the Los Tuxtlas Biosphere Reserve in southeastern Mexico. A blocked design consisting of four C. hooperiana plantation sites each paired with an adjacent site of unmanaged forest was used to compare the plant species richness and diversity, stem density, basal area and size class structure in plantations versus unmanaged forest. In each site, 12 10 m diameter plots were established for a total of 96 plots (4 blocks × 2 sites × 12 plots). Although the stem density, diversity, richness and basal area of large overstory trees (≥20 cm dbh) were unaffected by the establishment and management of understory palm plantations, the stem density, species richness and basal area of woody species in smaller size classes (

We use the fire ecology and biogeographical patterns of Callitris intratropica, a fire-sensitive conifer, and the Asian water buffalo (Bubalus bubalis), an introduced mega-herbivore, to examine the hypothesis that the continuation of Aboriginal burning and cultural integration of buffalo contribute to greater savanna heterogeneity and diversity in central Arnhem Land (CAL) than Kakadu National Park (KNP). The ‘Stone Country’ of the Arnhem Plateau, extending from KNP to CAL, is a globally renowned social–ecological system, managed for millennia by Bininj-Kunwok Aboriginal clans. Regional species declines have been attributed to the cessation of patchy burning by Aborigines. Whereas the KNP Stone Country is a modern wilderness, managed through prescribed burning and buffalo eradication, CAL remains a stronghold for Aboriginal management where buffalo have been culturally integrated. We surveyed the plant community and the presence of buffalo tracks among intact and fire-damaged C. intratropica groves and the savanna matrix in KNP and CAL. Aerial surveys of C. intratropica grove condition were used to examine the composition of savanna vegetation across the Stone Country. The plant community in intact C. intratropica groves had higher stem counts of shrubs and small trees and higher proportions of fire-sensitive plant species than degraded groves and the savanna matrix. A higher proportion of intact C. intratropica groves in CAL therefore indicated greater gamma diversity and habitat heterogeneity than the KNP Stone Country. Interactions among buffalo, fire, and C. intratropica suggested that buffalo also contributed to these patterns. Our results suggest linkages between ecological and cultural integrity at broad spatial scales across a complex landscape. Buffalo may provide a tool for mitigating destructive fires; however, their interactions require further study. Sustainability in the Stone Country depends upon adaptive management that rehabilitates the coupling of indigenous culture, disturbance, and natural resources.