Ann. For. Res. 60(2): 279-295, 2017 ANNALS OF FOREST RESEARCH DOI: 10.15287/afr.2017.805 www.afrjournal.org Stable isotope ratios and reforestation potential in Acacia koa populations on Hawai’i S.S. Lawson, C.C. Pike Lawson S. S., Pike C. C. 2017. Stable isotope ratios and reforestation potential in Acacia koa populations on Hawai’i. Ann. For. Res. 60(2): 279-295. Abstract. Stable carbon and nitrogen isotopes can be influenced by a mul- titude of factors including elevation, precipitation rate, season, and tem- perature. This work examined variability in foliar stable carbon (δ13C) and nitrogen (δ15N) isotope ratios of koa (Acacia koa) across 17 sites on Ha- wai’i Island, delineated by elevation and precipitation gradients. Sites were identified and grouped with respect to mean annual precipitation (MAP), mean annual temperature (MAT) and position along three elevation tran- sects. Analysis of resultant δ13C and δ15N isotope ratios from multiple in- dividuals across ecoregions indicated certain regions exhibited a correlation between carbon and/or nitrogen content, isotope ratios, precipitation, and elevation however, many comparisons showed no correlation. We used pub- licly available temperature and moisture data to help eliminate confounding effects by climatic drivers and capture possible points of contention. At sites where the temperature, precipitation, and elevation data were not sig- nificantly different, we compared stable isotope information to determine if additional variables might have contributed to a lack of definitive data. Our results identified several areas within the Waiakea Forest Reserve and Vol- canoes National Park where, based on isotope analyses, reforestation efforts could be most successfully initiated. Keywords Acacia koa, elevation, Hawaiian, isotopes, plasticity Authors. Shaneka S. Lawson ([email protected]), USDA Forest Service, Northern Research Station, Hardwood Tree Improvement and Regeneration Center (HTIRC), Purdue University, 715 West State Street, West Lafayette, IN USA 47907; Carolyn C. Pike, USDA Forest Service, Northeastern Area State and Private Forestry, Purdue University, 715 West State Street, West Lafayette, IN USA 47907. Manuscript received June 15, 2017; revised July 7, 2017; accepted July 14, 2017; online first July 21, 2017. Introduction all islands within the Hawaiian archipelago but only exists in populations of sufficient size Koa (Acacia koa) is a tree species endemic to to be called forests on the islands of Hawai’i, 279 Ann. For. Res. 60(2): 279-295, 2017 Research articles Kauai, Maui, and Oahu. Koa trees are legumes, across locations (Ozolincius et al. 2014, Juday a vital representative of tropical forests due to et al. 2015). their ability to fix nitrogen. Several examples Temperatures are increasing across many exist regarding the preference or prevalence sites on and near the Hawai’i Island chain of nitrogen-fixing trees on disturbed sites with (Murakami et al. 2013, Safeeq et al. 2013) limited nitrogen, as opposed to undisturbed and tropical forests worldwide (Marcott et sites (Roggy et al. 1999, Ometto et al. 2006). al. 2013, Vlam et al. 2014). While these tem- This majestic tree is found across a wide range perature data effectively show overall chang- of elevation (300-2200 m) and temperature es across several sites, the resultant effects on (11-23 °C) gradients. Important to the timber native tree populations have not been thor- industry and closely tied to Native Hawaiian oughly examined. The effects of changes to culture, koa populations have suffered decline temperature and precipitation on tree growth following unchecked free-range cattle grazing, and development may be positive or negative introduction of invasive grasses, as well as depending on site, morphological and physio- other abiotic and biotic threats. Because of the logical characteristics of the species’ (Vitasse significant timber and ecological value of koa, et al. 2009, Vitasse et al. 2014, Bussotti & studies of its natural plasticity and potential for Pollastrini 2015). On the Mediterranean is- range expansion are needed to help formulate land of Mallorca, Gulías et al. (2009) reported scenarios directed towards halting population correlations between meteorological variabil- decline through assisted restoration of the spe- ity and both growth form and environmental cies. adaptation. It was also noted that physiologi- Recent forest cover data collected across cal characteristics can be used in estimations both tropical and alpine indicator species high- of ecosystem responses. Similar results were lighted population shifts in response to envi- found in studies of tropical forest species from ronmental variability (Rannow 2013, Juday et Xishuangbanna Botanical Garden in Yunnan, al. 2015). Aided by analyses of temperature China (Cai et al. 2008). and precipitation gradients, tropical plant re- Stable isotopes have been used for decades searchers in Hawai’i have also noted subtle in both plant and animal studies to investigate shifts in the range of native Hawaiian plant nutrient cycling and environmental compati- species in response to climate change (Kru- bility. Carbon isotopes are used in research to shelnycky et al. 2013) but this work did not provide insight on intrinsic water-use efficien- extend to forest tree species. Krushelnycky cy as well as how efficiently plants photosyn- (2014) noted that a decline in plant numbers thesize. Plants occurring across a wide range would result in increased distances between of microhabitats, ranging from sloping hills host plants and altered behavior of insect pol- to alluvial valleys, exhibit plasticity in their linators. These data highlight the importance rooting patterns. At high elevations roots are of maintaining population sizes and decreased shallow, compared to valleys with deep soils, habitat fragmentation on future reproductive and therefore have less access to soil water at success. Nearly a decade ago, Aitken et al. the high elevations (Liu et al. 2014). These (2008) proposed that tree species capable of a moisture shortages can effectively be reflect- greater degree of adaptation, were most like- ed in foliar carbon (δ13C) and nitrogen isotope ly to survive future climatic change trees may (δ15N) data (Casper et al. 2012, Liancourt et experience range expansion or contraction de- al. 2013). Initial research using carbon isotope pending on inherent plasticity and availability values allow for general comparative analyses of suitable soils and site conditions in novel across sites within an area and may not allow locations, versus less plastic species that will for differences in plant micro-distributions, or exhibit decreased population representation alterations of population substructure that have 280 Lawson & Pike Stable isotope ratios and reforestation potential in Acacia koa ... become more prevalent due to climate change increased temperatures globally. Both carbon (Peterson & Fry 1987, Ward et al. 2005). and nitrogen isotope readings have been linked Isotopes such as δ13C and δ15N have been to climate and land-use change and are consid- used extensively to understand numerous eco- ered excellent tools to help understand the true system processes. Plants discriminate against implications of these environmental shifts. the heavier δ13C molecules when CO2 is assim- The use of stable isotope comparisons for tree ilated allowing for calculation of subsequent populations differing in elevation, temperature of δ13C to δ12C ratios. The expression of this and moisture gradients have widely been ac- ratio is an indication of environmental fac- cepted as a means used to determine overall tors contributing to the identification of those physiological adaptation (Soolanayakanahally plants under greater water stress. Detection of et al. 2015, Bell et al. 2017). Fractionation and carbon isotope ratios differences in stomatal measurement of stable isotopes such as δ13C/ conductance and carboxylation that under- δ12C and δ15N/ δ14N provided reliable means score photosynthetic machinery and plant re- for evaluation of these and numerous addition- sponses to varied environmental conditions. al physiological processes in plants. Use of Also, carbon isotope ratios vary with plant stable isotopes for determination of water-use type. For example, C3 plants typically exhibit efficiency (WUE) (Hansen & Steig 1993, Ares lower ratios (-33 to -23‰) than C4 plants (-16 et al. 2000) and abiotic stress (Hansen et al. to -10‰) hence, these values can be used to 1996, Craven et al. 2010) conditions has been differentiate between the photosynthetic path- previously reported. Other researchers suggest ways of known and unknown plant species that, providing no confounding temperature, (Lynott et al. 1986, Kohn & Thiemens 2010, precipitation, or light data; overall δ13C also Magdas et al. 2014). Nitrogen isotopes have decreases at higher altitudes (Heitz 1998, Zhu also been used to describe many phenomena et al. 2010). ranging from climate change effects on nutri- Vitousek et al. (1994) and Idol et al. (2007) ent cycling to relative density of mycorrhizal examined overall koa ecosystem productivity fungi in the soil. Craine et al. (2009) conduct- and noted that leaf nutrient concentrations in ed a meta-analysis on nitrogen isotopes (δ15N) dry regions may increase, relative to wet areas, from independent studies conducted around in part because of reduced growth in water-lim- the world to examine potential patterns in ter- ited environments. Generally, δ13C