Site characteristics of the Pōhakuloa substrate age gradient.

<p>Site characteristics of the Pōhakuloa substrate age gradient.</p

Primary Succession on a Hawaiian Dryland Chronosequence

<div><p>We used measurements from airborne imaging spectroscopy and LiDAR to quantify the biophysical structure and composition of vegetation on a dryland substrate age gradient in Hawaii. Both vertical stature and species composition changed during primary succession, and reveal a progressive increase in vertical stature on younger substrates followed by a collapse on Pleistocene-aged flows. Tall-stature <i>Metrosideros polymorpha</i> woodlands dominated on the youngest substrates (hundreds of years), and were replaced by the tall-stature endemic tree species <i>Myoporum sandwicense</i> and <i>Sophora chrysophylla</i> on intermediate-aged flows (thousands of years). The oldest substrates (tens of thousands of years) were dominated by the short-stature native shrub <i>Dodonaea viscosa</i> and endemic grass <i>Eragrostis atropioides</i>. We excavated 18 macroscopic charcoal fragments from Pleistocene-aged substrates. Mean radiocarbon age was 2,002 years and ranged from < 200 to 7,730. Genus identities from four fragments indicate that <i>Osteomeles spp</i>. or <i>M</i>. <i>polymorpha</i> once occupied the Pleistocene-aged substrates, but neither of these species is found there today. These findings indicate the existence of fires before humans are known to have occupied the Hawaiian archipelago, and demonstrate that a collapse in vertical stature is prevalent on the oldest substrates. This work contributes to our understanding of prehistoric fires in shaping the trajectory of primary succession in Hawaiian drylands.</p></div

Airborne imaging spectroscopy and LiDAR can be used to quantify the composition and fractional cover of vegetation.

<p>The top image is a true color composite overlaid on the digital terrain model (DTM). The bottom image is an RGB composite of the same area processed to quantify barren substrate (B, <b>r</b>ed), photosynthetic vegetation (PV; <b>g</b>reen) and non-photosynthetic vegetation (NPV; <b>b</b>lue). Changes in the lateral distribution of PV, NPV, B, and height are apparent across the substrate age gradient. For example, <i>M</i>. <i>polymorpha</i> woodland (MPW) on the 750–1200 year-old substrate is dominated by B and tall-statured NPV, indicated by reds and blues in the bottom panel. In contrast, <i>D</i>.<i>viscosa</i> shrubland (DVS) on a Pleistocene aged substrate is dominated by short-statured NPV and PV, indicated by blues and greens in the bottom panel. The areas shown are a 1 km² sample of the 23.1 km² study area mapped at 2.2 m resolution.</p

Relationships among types of lateral vegetation cover.

<p>PV = photosynthetic vegetation; NPV = non-photosynthetic vegetation; B = barren substrate. The youngest substrates are dominated almost exclusively by B, but accumulate NPV and PV during primary succession and ecosystem development. The rate of development is faster on pāhoehoe (right column) than on aʻa (left column). Pāhoehoe and aʻa lava types are not distinguishable on the 65 ky Pleistocene aged substrates.</p

Distributions of lateral vegetation cover on substrates of the Pōhakuloa substrate age gradient.

<p>Distributions of lateral vegetation cover on substrates of the Pōhakuloa substrate age gradient.</p

Mean distance (m) 2,000 randomly selected locations on substrates of the Pōhakuloa substrate age gradient and the nearest location with at least 25%, 50% or 75% lateral cover of NPV.

<p>Standard deviation provided in parentheses. Because NPV in this landscape is mainly fine fuels, these numbers demonstrate that a gradient in susceptibility to fire is associated with substrate age.</p><p>Mean distance (m) 2,000 randomly selected locations on substrates of the Pōhakuloa substrate age gradient and the nearest location with at least 25%, 50% or 75% lateral cover of NPV.</p

¹⁴C age frequency distribution for 18 macroscopic charcoal fragments excavated from Pleistocene- aged soils.

<p>Black vertical lines indicate charcoal ages. Red vertical lines indicate the timing of arrival of Europeans (solid) and Polynesians (dashed) to the Hawaiian archipelago.(Kirch 2010).</p