Forest Structure in Low-Diversity Tropical Forests: A Study of Hawaiian Wet and Dry Forests

<div><p>The potential influence of diversity on ecosystem structure and function remains a topic of significant debate, especially for tropical forests where diversity can range widely. We used Center for Tropical Forest Science (CTFS) methodology to establish forest dynamics plots in montane wet forest and lowland dry forest on Hawai‘i Island. We compared the species diversity, tree density, basal area, biomass, and size class distributions between the two forest types. We then examined these variables across tropical forests within the CTFS network. Consistent with other island forests, the Hawai‘i forests were characterized by low species richness and very high relative dominance. The two Hawai‘i forests were floristically distinct, yet similar in species richness (15 vs. 21 species) and stem density (3078 vs. 3486/ha). While these forests were selected for their low invasive species cover relative to surrounding forests, both forests averaged 5–>50% invasive species cover; ongoing removal will be necessary to reduce or prevent competitive impacts, especially from woody species. The montane wet forest had much larger trees, resulting in eightfold higher basal area and above-ground biomass. Across the CTFS network, the Hawaiian montane wet forest was similar to other tropical forests with respect to diameter distributions, density, and aboveground biomass, while the Hawai‘i lowland dry forest was similar in density to tropical forests with much higher diversity. These findings suggest that forest structural variables can be similar across tropical forests independently of species richness. The inclusion of low-diversity Pacific Island forests in the CTFS network provides an ∼80-fold range in species richness (15–1182 species), six-fold variation in mean annual rainfall (835–5272 mm yr⁻¹) and 1.8-fold variation in mean annual temperature (16.0–28.4°C). Thus, the Hawaiian forest plots expand the global forest plot network to enable testing of ecological theory for links among species diversity, environmental variation and ecosystem function.</p></div

Invasive species cover distribution.

<p>Map showing percent cover and locations of invasive species in the MWF. Each grid square represents one 5×5-m subquadrat white: absent, light grey: present to <5%, medium grey: 5–25%, dark grey: 25–50%, black: >50% cover).</p

Statistics on abundance, basal area, and frequency of the species in the Laupāhoehoe (montane wet forest) plot, with data displayed on an absolute and a relative basis.

<p>Statistics on abundance, basal area, and frequency of the species in the Laupāhoehoe (montane wet forest) plot, with data displayed on an absolute and a relative basis.</p

Aboveground biomass listed by species for the two Hawai‘i forest plots; species abbreviations as in Table S2 in File S2.

<p>Aboveground biomass listed by species for the two Hawai‘i forest plots; species abbreviations as in Table S2 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103268#pone.0103268.s002" target="_blank">File S2</a>.</p

Distinctive structural and demographic features of Hawaiian forests.

<p>Superscripts refer to references listed in Table S5 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103268#pone.0103268.s002" target="_blank">File S2</a>.</p><p>Distinctive structural and demographic features of Hawaiian forests.</p

Combined invasive species cover.

<p>In each 5×5 m subquadrat a cover score from 0–4 was given based on cover classes (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103268#s2" target="_blank">Methods</a>). The y axis represents the average cover class across the 400 subquadrats, separated by life form: grasses, herbaceous, or woody (shrubs and trees). The combined cover represents the species shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103268#pone-0103268-g005" target="_blank">Figure 5</a>.</p

Map showing percent cover and locations of invasive species in the LDF.

<p>Each grid square represents one 5×5-m subquadrat (white: absent, light grey: present to <5%, medium grey: 5–25%, dark grey: 25–50%, black: >50% cover).</p

Contour map of the two 4-ha forest plots on Hawai‘i Island.

<p>Pālamanui site in west Hawai‘i is lowland dry forest (LDF; left panel showing the dominant canopy tree <i>Diospyros sandwicensis</i> and the open canopy and understory structure of small trees and shrubs); Laupāhoehoe plot in east Hawai‘i is montane wet forest (MWF; right panel showing <i>Metrosideros polymorpha</i> tree and <i>Cibotium</i> spp. tree fern understory).</p

Diversity and forest structure characteristics of plots in the Center of Tropical Forest Science global plot network, including the Hawaiian plots, arranged in order of descending species richness.

<p>Southern hemisphere latitudes are negative; land types are island (I) and mainland (M); dry season months are as those with <100 mm precipitation (Richards 1996). Data are from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103268#pone.0103268-Losos1" target="_blank">[104]</a> and ctfs.si.edu unless indicated by footnotes.</p>1<p>Mean annual rainfall data for the nearby city of Songkhla, Thailand (<a href="http://www.world-climates.com" target="_blank">www.world-climates.com</a>)</p>2<p>Kira T (1998) NPP Tropical Forest: Khao Chong, Thailand, 1962–1965. Data set. Available on-line [<a href="http://www.daac.ornl.gov" target="_blank">http://www.daac.ornl.gov</a>] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.</p>3<p>Average of 4 plots (2 monodominant forest, 2 mixed forest)</p>4<p>Divided into four 10-ha plots</p>5<p>Data from Ferreira de Lima RA, Oliveira AAD, Martini AMZ, Sampaio D, Souza VC, Rodrigues RR (2011) Structure, diversity, and spatial patterns in a permanent plot of a high restinga forest in Southeastern Brazil. Acta Botanica Brasilica 25: 633–645.</p>6<p>Basal area including tree ferns; 36.1m²/ha without tree ferns</p>7<p>Chave J and 37 others (2008) Assessing evidence for a pervasive alteration in tropical tree communities. PLoS Biology 6: 455–462.</p><p>Diversity and forest structure characteristics of plots in the Center of Tropical Forest Science global plot network, including the Hawaiian plots, arranged in order of descending species richness.</p

A reverse-cumulative distribution of basal area by size class.

<p>Size classes are: ≥1 cm, ≥10 cm, ≥30 cm, and ≥60 cm. Data shown for the Hawaiian montane wet forest (LAU) and lowland dry forest (PLN) (top row) and for selected other CTFS plots. Tree ferns (found only at LAU) are symbolized by the gray bars. Island sites are open bars and continental sites are filled bars. Abbreviations as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103268#pone-0103268-t002" target="_blank">Table 2</a>. Data from Losos and Leigh, Jr. (2004) and <a href="http://www.ctfs.si.edu" target="_blank">www.ctfs.si.edu</a>.</p