Novel forests maintain ecosystem processes after the decline of native tree species

Abstract

Abstract. The positive relationship between species diversity (richness and evenness) and critical ecosystem functions, such as productivity, carbon storage, and nutrient cycling, is often used to predict the consequences of extinction. At regional scales, however, plant species richness is mostly increasing rather than decreasing because successful plant species introductions far outnumber extinctions. If these regional increases in richness lead to local increases in diversity, a reasonable prediction is that productivity, carbon storage, and nutrient cycling will increase following invasion, yet this prediction has rarely been tested empirically. We tested this prediction in novel forest communities dominated by introduced species (;90% basal area) in lowland Hawaiian rain forests by comparing their functionality to that of native forests. We conducted our comparison along a natural gradient of increasing nitrogen availability, allowing for a more detailed examination of the role of plant functional trait differences (specifically, N 2 fixation) in driving possible changes to ecosystem function. Hawaii is emblematic of regional patterns of species change; it has much higher regional plant richness than it did historically, due to .1000 plant species introductions and only ;71 known plant extinctions, resulting in an ;100% increase in richness. At local scales, we found that novel forests had significantly higher tree species richness and higher diversity of dominant tree species. We further found that aboveground biomass, productivity, nutrient turnover (as measured by soil-available and litter-cycled nitrogen and phosphorus), and belowground carbon storage either did not differ significantly or were significantly greater in novel relative to native forests. We found that the addition of introduced N 2 -fixing tree species on N-limited substrates had the strongest effect on ecosystem function, a pattern found by previous empirical tests. Our results support empirical predictions of the functional effects of diversity, but they also suggest basic ecosystem processes will continue even after dramatic losses of native species diversity if simple functional roles are provided by introduced species. Because large portions of the Earth's surface are undergoing similar transitions from native to novel ecosystems, our results are likely to be broadly applicable.

Biodiversity inventory of trees in a neotropical secondary forest after abandonment of shaded coffee plantation

“…According to these authors, "novel ecosystems" arise from altered biotic elements, such as soil fertility, degradation, and the introduction of species. Many characteristics of our study area are similar to those observed for "novel ecosystems" in abandoned coffee plantations in Puerto Rico (Lugo & Helmer 2004), e.g., the increase in density of large individuals (DBH > 55 cm) of P. gonocantha in comparison with the surrounding forest communities, or the presence of dominant species in the community (Mascaro et al 2012). In this study, only two native species (E.edulis and P. gonacantha) accounted for 48.4% of individuals.…”

Simple plant traits explain functional group diversity decline in novel grassland communities of Texas

“…While our earlier work demonstrated drops in species diversity after 2 years (Wilsey et al 2009(Wilsey et al , 2011, this new work concludes that novel ecosystems have less functional group diversity after the 1st year and that the simple plant traits that we measured were associated with the declines. Although further work is necessary in other systems, as well as with natives and exotics in mixed situations, our findings that functional group diversity remains higher in pure native than pure exotic communities (this study), and that spring greenup is earlier in exotic species and communities (Wilsey et al 2011), implies that grasslands restored to native assemblages will function differently from those dominated by exotic species (Wilsey et al 2009;Isbell and Wilsey 2011a, b;Mascaro et al 2012).…”

“…Exotic grasslands can persist for many years even when native seed sources are nearby (e.g., Christian and Wilson 1999;Wilsey and Polley 2003;Kulmatiski 2006). Observational and experimental studies in the Great Plains have found that species diversity at the site-level declines as exotic communities replace native ones in modern landscapes (e.g., Christian and Wilson 1999;Wilsey et al 2011;Isbell and Wilsey 2011b), a result that is in contrast with the increased diversities associated with invasions of remote-island forested systems (Mascaro et al 2012), but consistent with the view that dominant exotic plant species possess traits that separate them ecologically from natives. For example, Firn et al (2011) found that grasses were more abundant and forbs were less abundant in their introduced ranges than their home ranges, suggesting that there are important ecological differences between exotic grasses and forbs.…”

Does the terrestrial biosphere have planetary tipping points?

“…Consequently, on the scale at which the diversity-functioning mechanisms operate, species richness has, in many cases, increased, even as it becomes more globally homogenized, and the logical implication of the positive diversity-functioning relationship is therefore that ecosystem functioning would, on average, have benefited from this process-provided that introduced species can fulfill similar roles to the extirpated taxa [51]. This hypothesis is lent support by a meta-analysis of over 1,000 field studies, showing that while regional native species richness has often declined, primary production and several ecosystem processes were usually maintained or enhanced as a result of species introductions [52].…”