Patterns in understory woody diversity and soil nitrogen across native- and non-native-urban tropical forests

Abstract

Urban expansion is accelerating in the tropics, and may promote the spread of introduced plant species into urban-proximate forests. For example, soil disturbance can deplete the naturally high soil nitrogen pools in wet tropical soils, favoring introduced species with nitrogen-fixing capabilities. Also, forest fragmentation and canopy disturbance are likely to favor high-light species over shade-adapted rainforest species. We measured understory woody diversity, the abundance of introduced species, and soil nitrogen and carbon in urban, suburban, and rural secondary forests in Puerto Rico with canopies dominated by (1) native species, (2) introduced Fabaceae (potential nitrogen-fixers), and (3) introduced non-Fabaceae species. We hypothesized that forest stands with introduced Fabaceae in the canopy have higher soil nitrogen levels than stands with other introduced canopy species, and that this higher nitrogen is linked to increased native woody species diversity in the understory. We also predicted that more open canopies and smaller fragment sizes would be positively related with introduced species in the understory, and negatively related with total understory diversity. We found that stands with introduced Fabaceae in the canopy had significantly higher soil nitrogen levels than stands with other non-nitrogen fixing introduced species, and understory woody diversity in Fabaceae stands approached similar diversity levels as stands with native-dominated canopies. As predicted, aboveground stand structure and fragment size were also significantly associated with understory woody diversity across stands. These results suggest that introduced nitrogen-fixing trees may improve recruitment of native woody species in degraded tropical sites where native soil nitrogen is naturally high, particularly as Fabaceae stands mature and canopies close.

Tropical Deforestation and Recolonization by Exotic and Native Trees: Spatial Patterns of Tropical Forest Biomass, Functional Groups, and Species Counts and Links to Stand Age, Geoclimate, and Sustainability Goals

“…Although it is a macronutrient, adding nitrogen to secondary tropical forests is thought to enhance tropical forest growth, but it can also inhibit growth [120,121]. In addition, increasing the basal area of N-fixing trees, including exotic species or species other than legumes, increases nitrogen levels in leaf litter, forest floor organic matter, or soils, and soils of secondary tropical forests release more NOx to the atmosphere [28,87,104,109,122], and NOx's are potent greenhouse gases. The maps here illustrate where N-fixing species are most abundant.…”

Global change effects on humid tropical forests: Evidence for biogeochemical and biodiversity shifts at an ecosystem scale

“…In a Puerto Rican study, secondary forests with substantial numbers of exotic trees in the canopy (i.e., novel species composition) had less aboveground biomass than native secondary forests of similar age, although much of the difference was attributed to past land use [ Lugo and Helmer , ]. Another Puerto Rican study found no significant relationship between stem biomass and the proportion of exotic tree species in the canopy [ Cusack and McCleery , ]. Because of the limited number of studies on biomass in exotic‐dominated versus native‐dominated secondary forests on abandoned lands, and because of the many site and land use history factors affecting rates of plant growth and biomass accumulation, it is difficult to generalize whether invasive tropical trees generally decrease C stocks in novel secondary forests relative to native secondary forests.…”

“…For example, secondary forests along an urban‐rural gradient in Puerto Rico showed very high variability in soil N levels among nearby secondary forest stands [ Cusack , ]. In these sites, some of the spatial variability in soil N was related to varying abundances of exotic N‐fixing trees across secondary forests [ Cusack and McCleery , ], with additional variability likely caused by differences in land use history.…”

Exotic grasses and nitrate enrichment alter soil carbon cycling along an urban–rural tropical forest gradient

“…All forest stands were secondary regeneration following widespread agricultural abandonment in Puerto Rico starting in the 1930s (Kennaway & Helmer, 2007). Both the Rio Piedras and Cubuy forests contained novel mixtures of native and non-native tree species (Helmer, 2004;Lugo, 2004;Kennaway & Helmer, 2007;Cusack & McCleery, 2014). Abundant exotic grass species in the urban watershed included Panicum maximum Jac q. and Pennisetum purpureum Schumach., which are both African grasses and can establish in open areas (Lugo et al, 2001).…”

“…However, the potential for recovery of lost soil C, even after long-term forest regeneration, varies greatly across sites with no clear trends (Powers, 2004;Jimenez et al, 2008;Marin-Spiotta & Sharma, 2013). Also, many abandoned lands are relatively near urban centers and are within the range of current urban expansion, such that secondary forests are subject to ongoing disturbances such as clearing, exotic species establishment, and N deposition (Chytry et al, 2008;Cusack & McCleery, 2014).…”