A Hawaiian Tropical Dry Forest Regenerates: Natural Regeneration of Endangered Species under Biocultural Restoration

Abstract

Tropical dry forests (TDFs) are among the most at-risk ecosystems globally. In Hawai‘i, more than 45% of TDF species are threatened or endangered. Despite decades of active TDF restoration, there remains limited information on the potential for long-term success, since there are few studies of natural regeneration. We assess natural regeneration of endangered plants at Ka‘ūpūlehu dryland forest, a Hawaiian biocultural restoration initiative. Drawing on 6 annual censuses we (1) assess rates and patterns of natural regeneration across species and as a function of rainfall and (2) identify bottlenecks. Our surveys document natural recruits of 11 of the 12 endangered species first outplanted 15–20 years ago. Higher annual rainfall increased the number of new recruits per year and growth, but decreased survival of larger recruits. The total number of natural recruits increased three-fold over the study period and varied across species. For nearly half of the species, we documented a second generation of recruits. Successes appear to be a function of time, including a changing microclimate and adaptive management practices. Remaining bottlenecks include lack of seed dispersal, and seed predation and herbivory by introduced species. The success at Ka‘ūpūlehu highlights the potential for TDF restoration and the value of a biocultural approach.

A Near Four-Decade Time Series Shows the Hawaiian Islands Have Been Browning Since the 1980s

“…In this study, time series results can be used by natural resource managers to identify if landscapes, native ecosystems, or protected areas have 1) been stable across the time series (thus suggesting healthy vegetation and appropriate management strategies), or 2) experienced significant declines across the time series or monthly trends (suggesting vegetation health may be declining and the causes and appropriate management strategies need to be identified) (Alcaraz-Segura et al 2009 ). For example, Kaʿūpūlehu on Hawai’i is a well-known tropical dry forest remnant site that has experienced a decline in NDVI since 1982 for 11 out of 12 months of the year with 6 of those months showing a significant decline (Online Resource 2: SI 5a ) (Litton et al 2006 ) despite highly successful restoration efforts (Libby et al 2022 ). Low NDVI values at Kaʿūpūlehu directly track prolonged drought periods (1998–2003 and 2007–2014), and the overall decline is likely the result of the combination between a significant decrease in precipitation across the landscape over the past century (Frazier and Giambelluca 2017 ), the removal of non-native trees, and a slow loss of canopy trees that was accompanied by little regeneration prior to restoration efforts.…”

“…Auwahi, on Maui, (Online Resource 2: SI 5b ) is a well-known tropical dry forest that has been restored from pasture and has experienced a significant increase in NDVI as native canopy trees replace non-native grasses. Species population trend data for endangered and threatened species can be compiled and compared to trends of NDVI or ecosystem health (Craven et al 2018 , Libby et al 2022 ). Indeed, trends in NDVI can also be compared to trends in bird populations, with most of the endangered bird species restricted to nine or fewer AVHRR CDR NDVI cells (Barton et al 2021 ).…”

Tropical Dry Forest Restoration in an Era of Global Change: Ecological and Social Dimensions

“…Socioeconomic indicators are rarely used to evaluate restoration outcomes [41], and there are few studies including both ecological and socioeconomic indicators in TDF restoration practice [42]. Several papers in this Special Issue explore a broader range of social and environmental limitations for the effective restoration of TDFs in Mexico [33,43] and Hawaii [44], and review the opportunities for the integration of social science dimensions in TDF restoration [45]. These three papers of the present Special Issue stress the urgent need to integrate local people in order to not only develop better TDF restoration programs, but also to guide TDF restoration at regional and global scales.…”

Multi-Stemmed Habit in Trees Contributes Climate Resilience in Tropical Dry Forest

“…Tropical dry forests (TDFs) are some of the most endangered ecosystems on the planet [20][21][22], and the forest fragments that remain in Hawaii are of high conservation concern [23]. Most TDFs have distinct wet and dry seasons [24,25], but Hawaiian dry forests tend to be dry (i.e., <100 mm rain per month) all year long [26,27].…”