Are endemics functionally distinct? Leaf traits of native and exotic woody species in a New Zealand forest

<div><p>Recent studies have concluded that native and invasive species share a common set of trait relationships. However, native species in isolated regions might be functionally constrained by their unique evolutionary histories such that they follow different carbon capture strategies than introduced species. We compared leaf traits relating to resource investment, carbon return, and resource-use efficiency in 16 native (endemic) and three non-native (invasive) species in a temperate forest in Canterbury, South Island, New Zealand. Trait differences were more closely associated with leaf habit than nativity. Deciduous species (including invaders) exhibited greater maximum photosynthetic rates at similar resource costs, which resulted in greater nitrogen- and energy-use efficiencies than evergreen natives. Leaf area was the only trait that differed significantly by nativity (over two-fold larger in invaders). Invaders and deciduous natives both occupied the ‘fast return’ end of the leaf economics spectrum in contrast to the native evergreens which had comparatively slow return on investment. Dominant woody invaders in this forest are physiologically distinct from many New Zealand endemic species, which are overwhelmingly evergreen. It remains unclear whether these trait differences translate to an ecological divergence in plant strategy, but these results suggest that ecophysiological tradeoffs are likely constrained by biogeography.</p></div

Mean values (± 1 SE) of photosynthetic, biochemical, structural, and resource-use efficiency leaf traits among native and invasive species.

<p>Mean values (± 1 SE) of photosynthetic, biochemical, structural, and resource-use efficiency leaf traits among native and invasive species.</p

Woody species measured, including invasive status, biogeographic origin, growth form, and number of replicate individuals.

<p>Woody species measured, including invasive status, biogeographic origin, growth form, and number of replicate individuals.</p

Are endemics functionally distinct? Leaf traits of native and exotic woody species in a New Zealand forest - Fig 4

<p><b>Standardized major axis (SMA) relationships for area-based light-saturated maximum photosynthetic rate (A</b>_{<b>max,area</b>}<b>) and leaf resource cost traits (a, e) dark respiration rate (R</b>_{<b>d,area</b>}<b>), (b, f) nitrogen concentration (N</b>_<b>area</b><b>), (c, g) construction cost (CC</b>_<b>area</b><b>), and (d, h) specific leaf area (SLA).</b> Points refer to species- (a-d) or individual-level (e-h) estimates. Native deciduous species are denoted by black text and open triangles, invasive species by closed triangles, and native evergreen species (open circles) are shown in grey. Light gray error bars denote 95% credible intervals on posterior means from light response curve models. Only significant SMA lines are drawn (deciduous, solid black line; evergreen, dashed grey line). SMA analyses were performed only for relationships showing at least moderate correlation (R²>0.1, <i>P</i><0.1). Significance tests indicate differences in slope, elevation (y-intercept), or shift along common slope. +<i>P</i><0.1; * <i>P</i><0.05; <i>**P</i><0.01<i>; ***P</i><0.001. Note axes are on log scale.</p

Species-level average modeled light response curves for 3 invasive (non-grey) and 15 native (grey) species.

<p>Curves estimate each species’ area-based net photosynthetic rates (A_net) response to irradiance (photosynthetic photon flux density, PPFD), using all data with random effects for species. Only deciduous species are labeled, following codes listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0196746#pone.0196746.t001" target="_blank">Table 1</a>. Corresponding parameter estimates for each species are illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0196746#pone.0196746.g002" target="_blank">Fig 2</a>.</p

Are endemics functionally distinct? Leaf traits of native and exotic woody species in a New Zealand forest - Fig 5

<p><b>Standardized major axis (SMA) relationships between (a) N</b>_<b>mass</b><b>and SLA and (b) N</b>_<b>area</b><b>and SLA.</b> Deciduous individuals are denoted by triangles (native closed, invasive open points) and native evergreen species (open circles) are shown in grey. +<i>P</i><0.1; * <i>P</i><0.05<i>; ***P</i><0.001. Note axes are on log scale.</p

Means and 95% credible intervals by species grouped by nativity and leaf habit.

<p>(a) area-based maximum photosynthetic rate (A_max,area), (b) area-based dark respiration rate (R_d,area), (c) apparent quantum yield (ϕ), (d) light compensation point (LCP), (e) mass-based maximum photosynthetic rate (A_max,mass), and (f) mass-based dark respiration rate (R_d,mass). Vertical lines show group level averages.</p

Partial contributions to Observed vs. Fitted tree occurrences within the simplified BRT model.

<p>The graphs show Observed vs. Fitted tree occurrences (A) and smoothed partial contributions within the simplified BRT model for (B) mean annual temperature (C) percentage woody cover in a 25 m radius and (D) distance to nearest forest. The smoothed partial contribution plots reflect the influence of a predictor variable when all other variables are held constant. CVROC is the cross-validated receiver operator curve (ROC) for the final boosted regression tree model. ROC is a measure of discrimination accuracy when predicting a binary response.</p

Map of survey plots used in boosted regression tree modeling.

<p>Vegetation classes for New Zealand survey plots are mapped based on a reclassification of Dymond and Shepherd [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0075219#B58" target="_blank">58</a>]. ‘Other’ is all non-forest vegetation except subalpine scrub.</p

Functional Traits Reveal Processes Driving Natural Afforestation at Large Spatial Scales

<div><p>An understanding of the processes governing natural afforestation over large spatial scales is vital for enhancing forest carbon sequestration. Models of tree species occurrence probability in non-forest vegetation could potentially identify the primary variables determining natural afforestation. However, inferring processes governing afforestation using tree species occurrence is potentially problematic, since it is impossible to know whether observed occurrences are due to recruitment or persistence of existing trees following disturbance. Plant functional traits have the potential to reveal the processes by which key environmental and land cover variables influence afforestation. We used 10,061 survey plots to identify the primary environmental and land cover variables influencing tree occurrence probability in non-forest vegetation in New Zealand. We also examined how these variables influenced diversity of functional traits linked to plant ecological strategy and dispersal ability. Mean annual temperature was the most important environmental predictor of tree occurrence. Local woody cover and distance to forest were the most important land cover variables. Relationships between these variables and ecological strategy traits revealed a trade-off between ability to compete for light and colonize sites that were marginal for tree occurrence. Biotically dispersed species occurred less frequently with declining temperature and local woody cover, suggesting that abiotic stress limited their establishment and that biotic dispersal did not increase ability to colonize non-woody vegetation. Functional diversity for ecological strategy traits declined with declining temperature and woody cover and increasing distance to forest. Functional diversity for dispersal traits showed the opposite trend. This suggests that low temperatures and woody cover and high distance to forest may limit tree species establishment through filtering on ecological strategy traits, but not on dispersal traits. This study shows that ‘snapshot’ survey plot data, combined with functional trait data, may reveal the processes driving tree species establishment in non-forest vegetation over large spatial scales.</p> </div