Mapping habitat suitability for at-risk plant species and its implications for restoration and reintroduction

The conservation of species at risk of extinction requires data to support decisions at landscape to regional scales. There is a need for information that can assist with locating suitable habitats in fragmented and degraded landscapes to aid the reintroduction of at-risk plant species. In addition, desiccation and water stress can be significant barriers to the success of at-risk plant reintroduction programs. We examine how airborne light detection and ranging (LiDAR) data can be used to model microtopographic features that reduce water stress and increase resource availability, providing information for landscape planning that can increase the success of reintroduction efforts for a dryland landscape in Hawaii. We developed a topographic habitat-suitability model (HSM) from LiDAR data that identifies topographic depressions that are protected from prevailing winds (high-suitability sites) and contrasts them with ridges and other exposed areas (low-suitability sites). We tested in the field whether high-suitability sites had microclimatic conditions that indicated better-quality habitat compared to low-suitability sites, whether plant-response traits indicated better growing conditions in high-suitability sites, whether the locations of individuals of existing at-risk plant species corresponded with our habitat-suitability classes, and whether the survival of planted individuals of a common native species was greater in high-suitability, compared to low-suitability, planting sites. Mean wind speed in a high-suitability field site was over five times lower than in a low-suitability site, and soil moisture and leaf wetness were greater, indicating less stress and greater resource availability in high-suitability areas. Plant height and leaf nutrient content were greater in high-suitability areas. Six at-risk species showed associations with high-suitability areas. The survival of planted individuals was less variable among high-suitability plots. These results suggest that plant establishment and survival is associated with the habitat conditions identified by our model. The HSM can improve the survival of planted individuals, reduce the cost of restoration and reintroduction programs through targeted management activities in high-suitability areas, and expand the ability of managers to make landscape-scale decisions regarding land-use, land acquisition, and species recovery.

Native trees show conservative water use relative to invasive trees: results from a removal experiment in a Hawaiian wet forest

While the supply of freshwater is expected to decline in many regions in the coming decades, invasive plant species, often 'high water spenders', are greatly expanding their ranges worldwide. In this study, we quantified the ecohydrological differences between native and invasive trees and also the effects of woody invasive removal on plot-level water use in a heavily invaded mono-dominant lowland wet tropical forest on the Island of Hawaii. We measured transpiration rates of co-occurring native and invasive tree species with and without woody invasive removal treatments. Twenty native Metrosideros polymorpha and 10 trees each of three invasive species, Cecropia obtusifolia, Macaranga mappa and Melastoma septemnervium, were instrumented with heat-dissipation sap-flux probes in four 100 m(2) plots (two invaded, two removal) for 10 months. In the invaded plots, where both natives and invasives were present, Metrosideros had the lowest sap-flow rates per unit sapwood, but the highest sap-flow rates per whole tree, owing to its larger mean diameter than the invasive trees. Stand-level water use within the removal plots was half that of the invaded plots, even though the removal of invasives caused a small but significant increase in compensatory water use by the remaining native trees. By investigating the effects of invasive species on ecohydrology and comparing native vs. invasive physiological traits, we not only gain understanding about the functioning of invasive species, but we also highlight potential water-conservation strategies for heavily invaded mono-dominant tropical forests worldwide. Native-dominated forests free of invasive species can be conservative in overall water use, providing a strong rationale for the control of invasive species and preservation of native-dominated stands.

Remote analysis of biological invasion and the impact of enemy release

Escape from natural enemies is a widely held generalization for the success of exotic plants. We conducted a large-scale experiment in Hawaii (USA) to quantify impacts of ungulate removal on plant growth and performance, and to test whether elimination of an exotic generalist herbivore facilitated exotic success. Assessment of impacted and control sites before and after ungulate exclusion using airborne imaging spectroscopy and LiDAR, time series satellite observations, and ground-based field studies over nine years indicated that removal of generalist herbivores facilitated exotic success, but the abundance of native species was unchanged. Vegetation cover <1 m in height increased in ungulate-free areas from 48.7% +/- 1.5% to 74.3% +/- 1.8% over 8.4 years, corresponding to an annualized growth rate of lambda = 1.05 +/- 0.01 yr(-1) (median +/- SD). Most of the change was attributable to exotic plant species, which increased from 24.4% +/- 1.4% to 49.1% +/- 2.0%, (lambda = 1.08 +/- 0.01 yr(-1)). Native plants experienced no significant change in cover (23.0% +/- 1.3% to 24.2% +/- 1.8%, lambda = 1.01 +/- 0.01 yr(-1)). Time series of satellite phenology were indistinguishable between the treatment and a 3.0-km2 control site for four years prior to ungulate removal, but they diverged immediately following exclusion of ungulates. Comparison of monthly EVI means before and after ungulate exclusion and between the managed and control areas indicates that EVI strongly increased in the managed area after ungulate exclusion. Field studies and airborne analyses show that the dominant invader was Senecio madagascariensis, an invasive annual forb that increased from < 0.01% to 14.7% fractional cover in ungulate-free areas (lambda = 1.89 +/- 0.34 yr(-1)), but which was nearly absent from the control site. A combination of canopy LAI, water, and fractional cover were expressed in satellite EVI time series and indicate that the invaded region maintained greenness during drought conditions. These findings demonstrate that enemy release from generalist herbivores can facilitate exotic success and suggest a plausible mechanism by which invasion occurred. They also show how novel remote-sensing technology can be integrated with conservation and management to help address exotic plant invasions.

Hadway L. Towards restoration of Hawaiian tropical dry forests: the Kaupulehu outplanting programme

Hawaiian tropical dry forests contain diverse assemblages of woody canopy species, including many endemic and endangered species that warrant conservation attention before completely disappearing. Today, tropical dry forests in Hawaii are not viable ecosystems. Poor land use practices, fragmentation, non-native plant invasions, and inadequate native vegetation regeneration are all factors that have contributed to their endangerment. Only an ambitious restoration programme that includes non-native ungulate exclusion, weed control, fire management, and the outplanting of seeds and seedlings will be sufficient to enhance Hawaiian tropical dry forests. We selected a 25 ha preserve within the Kaupulehu Dry Forest Preserve, located in North Kona on the Island of Hawaii, to test dry forest restoration strategies. In 1997, the preserve was fenced and all non-native ungulates were removed. Altogether, 4892 outplants were planted from 1999?2006. In 2007, we surveyed all of the outplants. The survey found 1487 live plants, 3357 dead, and 48 plants missing. This equates to an overall survival rate of 30%. Survival by vegetation type indicated that vines had the highest rate of survival (63%) followed by trees (34%). Herbs had the lowest rate of survival (12%). Twelve of a total of 35 species that were outplanted in the Kaupulehu Dry Forest Preserve accounted for more than 90% of the total surviving plants species, while five federally listed species represent almost 60% of the total. The outplanting of dry forest species into the Kaupulehu Dry Forest Preserve considerably increased the population of many federally listed endangered species. However, the high mortality of many common and important plant species of tropical dry systems highlights the importance of an outplanting programme that emphasizes ecosystem sustainability rather that species success. In equal measure, the successes and failures of the Kaupulehu outplanting project have enhanced our ability to begin to restore this unique and endangered ecosystem.

Functional diversity of carbon-gain, water-use, and leaf-allocation traits in trees of a threatened lowland dry forest in Hawaii

We examined carbon-gain, water-use, and leaf-allocation traits for six tree species of a Hawaiian dry forest to better understand the functional diversity within this threatened ecosystem. Tropical dry forests are among the most endangered ecosystems on Earth, and in Hawaii, as elsewhere, declining biodiversity threatens ecosystem processes that may depend on forest functional diversity. We found broad variation among species including a two-fold difference for mean photosynthetic rate, a greater than three-fold difference for predawn water potential, and a nearly three-fold difference for leaf life span. Principal component analysis showed a clear separation of species based on carbon-gain vs. water-use related axes, and δ(13)C analysis revealed differing limitations (supply vs. demand) on carbon assimilation. The broad functional variation not only spanned traditional classifications (avoiders vs. tolerators), but also included unusual strategies (e.g., fast growth with drought tolerance). Correlations among traits, including leaf life span, leaf mass per area, and %N, followed typical global patterns, but some exceptions appeared as a result of unique life-history characteristics, such as latex-rich sap and root parasitism. Elucidating functional variation provides important information that can be used to link plant biodiversity with ecosystem processes and also facilitate the management and preservation of tropical dry forests and other threatened communities.

Wood vessel diameter is related to elevation and genotype in the Hawaiian tree Metrosideros polymorpha (Myrtaceae)

We tested the hypothesis that trees growing at high elevations with occasional freezing temperatures have smaller diameter xylem vessels than trees of the same species growing at lower and warmer elevations. The young branch wood of the wide-ranging Hawaiian tree species Metrosideros polymorpha (Myrtaceae) was examined in three natural field populations (high, middle, and low elevations: 2469, 1280, and 107 m a.s.l., respectively) and contrasted with seedlings from these populations that were grown in a common garden at middle elevation (1190 m). Previous studies showed that these populations have some genetic differences and have distinctive leaf structure and ecophysiological traits. Vessel diameter was significantly smaller in the high elevation field and common garden plants than in middle elevation plants. However, high elevation vessels were wider in common garden plants compared to field plants, indicating that vessel diameter is determined both by genotype (parental populations) and environment (growing conditions different from those of parents). Reduced vessel diameter has implications for resistance to cavitation induced by freezing and/or drought in plants growing near tree line in Hawaii.

Morphological and physiological adjustment to N and P fertilization in nutrient-limited Metrosideros polymorpha canopy trees in Hawaii

Leaf-level studies of Metrosideros polymorpha Gaud. (Myrtaceae) canopy trees at both ends of a substrate age gradient in the Hawaiian Islands pointed to differential patterns of adjustment to both nutrient limitation and removal of this limitation by long-term (8-14 years) nitrogen (N), phosphorus (P) and N + P fertilizations. The two study sites were located at the same elevation, had similar annual precipitation, and supported forests dominated by M. polymorpha, but differed in the age of the underlying volcanic substrate, and in soil nutrient availability, with relatively low N at the young site (300 years, Thurston, Hawaii) and relatively low P at the oldest site (4,100,000 years, Kokee, Kauai). Within each site, responses to N and P fertilization were similar, regardless of the difference in soil N and P availability between sites. At the young substrate site, nutrient addition led to a larger mean leaf size (about 7.4 versus 4.8 cm2), resulting in a larger canopy leaf surface area. Differences in foliar N and P content, chlorophyll concentrations and carboxylation capacity between the fertilized and control plots were small. At the old substrate site, nutrient addition led to an increase in photosynthetic rate per unit leaf surface area from 4.5 to 7.6 micromol m(-2) s(-1), without a concomitant change in leaf size. At this site, leaves had substantially greater nutrient concentrations, chlorophyll content and carboxylation capacity in the fertilized plots than in the control plots. These contrasting acclimation responses to fertilization at the young and old sites led to significant increases in total carbon gain of M. polymorpha canopy trees at both sites. At the young substrate site, acclimation to fertilization was morphological, resulting in larger leaves, whereas at the old substrate site, physiological acclimation resulted in higher leaf carboxylation capacity and chlorophyll content.

Supercooling Capacity Increases from Sea Level to Tree Line in the Hawaiian Tree Species Metrosideros polymorpha

Population-specific differences in the freezing resistance of Metrosideros polymorpha leaves were studied along an elevational gradient from sea level to tree line (located at ca. 2500 m above sea level) on the east flank of the Mauna Loa volcano in Hawaii. In addition, we also studied 8-yr-old saplings grown in a common garden from seeds collected from the same field populations. Leaves of low-elevation field plants exhibited damage at -2 degrees C, before the onset of ice formation, which occurred at -5.7 degrees C. Leaves of high-elevation plants exhibited damage at ca. -8.5 degrees C, concurrent with ice formation in the leaf tissue, which is typical of plants that avoid freezing in their natural environment by supercooling. Nuclear magnetic resonance studies revealed that water molecules of both extra- and intracellular leaf water fractions from high-elevation plants had restricted mobility, which is consistent with their low water content and their high levels of osmotically active solutes. Decreased mobility of water molecules may delay ice nucleation and/or ice growth and may therefore enhance the ability of plant tissues to supercool. Leaf traits that correlated with specific differences in supercooling capacity were in part genetically determined and in part environmentally induced. Evidence indicated that lower apoplastic water content and smaller intercellular spaces were associated with the larger supercooling capacity of the plant's foliage at tree line. The irreversible tissue-damage temperature decreased by ca. 7 degrees C from sea level to tree line in leaves of field populations. However, this decrease appears to be only large enough to allow M. polymorpha trees to avoid leaf tissue damage from freezing up to a level of ca. 2500 m elevation, which is also the current tree line location on the east flank of Mauna Loa. The limited freezing resistance of M. polymorpha leaves may be partially responsible for the occurrence of tree line at a relatively low elevation in Hawaii compared with continental tree lines, which can be up to 1500 m higher. If the elevation of tree line is influenced by the inability of M. polymorpha leaves to supercool to lower subzero temperatures, then it will be the first example that freezing damage resulting from limited supercooling capacity can be a factor in tree line formation.