Response to Comment on “Plant diversity increases with the strength of negative density dependence at the global scale”

Hülsmann and Hartig suggest that ecological mechanisms other than specialized natural enemies or intraspecific competition contribute to our estimates of conspecific negative density dependence (CNDD). To address their concern, we show that our results are not the result of a methodological artifact and present a null-model analysis that demonstrates that our original findings—(i) stronger CNDD at tropical relative to temperate latitudes and (ii) a latitudinal shift in the relationship between CNDD and species abundance—persist even after controlling for other processes that might influence spatial relationships between adults and recruits

Climate sensitive size-dependent survival in tropical trees

Survival rates of large trees determine forest biomass dynamics. Survival rates of small trees have been linked to mechanisms that maintain biodiversity across tropical forests. How species survival rates change with size offers insight into the links between biodiversity and ecosystem function across tropical forests. We tested patterns of size-dependent tree survival across the tropics using data from 1,781 species and over 2 million individuals to assess whether tropical forests can be characterized by size-dependent life-history survival strategies. We found that species were classifiable into four ‘survival modes’ that explain life-history variation that shapes carbon cycling and the relative abundance within forests. Frequently collected functional traits, such as wood density, leaf mass per area and seed mass, were not generally predictive of the survival modes of species. Mean annual temperature and cumulative water deficit predicted the proportion of biomass of survival modes, indicating important links between evolutionary strategies, climate and carbon cycling. The application of survival modes in demographic simulations predicted biomass change across forest sites. Our results reveal globally identifiable size-dependent survival strategies that differ across diverse systems in a consistent way. The abundance of survival modes and interaction with climate ultimately determine forest structure, carbon storage in biomass and future forest trajectories

Global importance of large-diameter trees

Aim: To examine the contribution of large‐diameter trees to biomass, stand structure, and species richness across forest biomes. Location: Global. Time period: Early 21st century. Major taxa studied: Woody plants. Methods: We examined the contribution of large trees to forest density, richness and biomass using a global network of 48 large (from 2 to 60 ha) forest plots representing 5,601,473 stems across 9,298 species and 210 plant families. This contribution was assessed using three metrics: the largest 1% of trees ≥ 1 cm diameter at breast height (DBH), all trees ≥ 60 cm DBH, and those rank‐ordered largest trees that cumulatively comprise 50% of forest biomass. Results: Averaged across these 48 forest plots, the largest 1% of trees ≥ 1 cm DBH comprised 50% of aboveground live biomass, with hectare‐scale standard deviation of 26%. Trees ≥ 60 cm DBH comprised 41% of aboveground live tree biomass. The size of the largest trees correlated with total forest biomass (r2 = .62, p < .001). Large‐diameter trees in high biomass forests represented far fewer species relative to overall forest richness (r2 = .45, p < .001). Forests with more diverse large‐diameter tree communities were comprised of smaller trees (r2 = .33, p < .001). Lower large‐diameter richness was associated with large‐diameter trees being individuals of more common species (r2 = .17, p = .002). The concentration of biomass in the largest 1% of trees declined with increasing absolute latitude (r2 = .46, p < .001), as did forest density (r2 = .31, p < .001). Forest structural complexity increased with increasing absolute latitude (r2 = .26, p < .001). Main conclusions: Because large‐diameter trees constitute roughly half of the mature forest biomass worldwide, their dynamics and sensitivities to environmental change represent potentially large controls on global forest carbon cycling. We recommend managing forests for conservation of existing large‐diameter trees or those that can soon reach large diameters as a simple way to conserve and potentially enhance ecosystem services

Regeneration Ecology of Native-Dominated Hawaiian Forests

Several hypotheses have been proposed to explain the maintenance of tropical forest diversity. Three frequently investigated hypotheses focus on the seed and seedling stages when plants are most vulnerable to environmental factors. These hypotheses propose that either niche differentiation, negative density dependence, and/or neutral process maintain biodiversity in tropical forests. The niche differentiation hypothesis proposes that plant species are specialized to microhabitats, as evidenced by differential performance (germination, growth, and or survival) of each species. The second, also termed the Janzen-Connell hypothesis, posits that negative density dependence (i.e., higher pathogen- and predator-induced mortality near conspecifics) regulates the density of common species. The neutral theory maintains that stochastic factors and limited seed dispersal contribute to avoidance of competitive interactions by functionally equivalent species. To investigate these hypotheses in low-diversity tropical forest, I measured seed/seedling dynamics and microhabitats (understory irradiance and substrate) in 4-ha plots in Hawaiian wet and dry forests in which all adult trees were mapped. I found evidence in support of all three hypotheses. Overall, recruitment limitation was the strongest driver of seedling dynamics in Hawaiian wet forest. Recruitment limitations and habitat specialization varied more among species within Hawaiian wet forest than among forests with comparable data. In Hawaiian wet forest, I also found evidence of differential performance among species across microhabitats and striking differences in allometric relationships, suggesting the existence of niche differentiation, though some species-pairs appeared to be functionally equivalent and there was substantial niche overlap in seedling distribution across microhabitats. In both wet and dry Hawaiian forest, density dependence was largely positive, thus it does not appear to maintain coexistence. Altogether, these results show that Hawaiian forest recruitment patterns are complex and are more similar than expected to mainland tropical forests. The results of this study will be useful for identifying and predicting the effects of factors that may be important for tree recruitment at the seedling stage and how these factors vary across species and forest types

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

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

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