The establishment of new botanic gardens in tropical regions highlights a need for weed risk assessment tools suitable for tropical ecosystems. The relevance of plant traits for invasion into tropical rainforests has not been well studied. 

Working in and around four botanic gardens in Indonesia where 590 alien species have been planted, we estimated the effect of four plant traits, plus time since species introduction, on: a) the naturalization probability and b) abundance (density) of naturalized species in adjacent native tropical rainforests; and c) the distance that naturalized alien plants have spread from the botanic gardens.  

We found that specific leaf area (SLA) strongly differentiated 23 naturalized from 78 non-naturalized alien species (randomly selected from 577 non-naturalized species) in our study. These trends may indicate that exotics with high SLA benefit from at least two factors when establishing in tropical forests: high growth rates and occupation of forest gaps. We also found that height was unrelated to naturalization probability, but naturalized aliens were having high SLA and were short. 

Exotic species that were present in the gardens for over 30 years and those with small seeds also had higher probabilities of becoming naturalized, indicating that garden plants can invade the understorey of closed canopy tropical rainforests, especially when invading species are shade-tolerant and have sufficient time to establish.

On average, exotic species that were not animal dispersed spread 78 m further into the forests than animal-dispersed species. We did not detect relationships between the measured traits and estimated density of naturalized exotics in the adjacent forests.

Synthesis: Traits were able to differentiate exotic species from botanic gardens that naturalized in native forest from those that did not; this is promising for developing trait-based risk assessment in the tropics. To limit the risk of invasion and spread into adjacent native forests, we suggest tropical botanic gardens avoid planting exotic species with fast carbon capture strategies and those that are shade tolerant.

Trait data collection involved a combination of measurements in the field and extracting data from the following repositories: Global Biodiversity Information Facility (gbif.org), Hawaiian Ecosystem at Risk (hear.org), efloras.org, and Biodiversity Heritage Library (biodiversitylibrary.org) (plant description only, did not include herbarium collections).

We measured SLA for every individual of the naturalized alien species we detected (a total of 23 species and 913 individuals) and for non-naturalized aliens (195 individuals with the average and minimum number of individuals per species being 6 and 2 respectively). SLA values at the individual level were averaged (without prior transformation) and then we calculated the mean value within each species to obtain mean SLA values at the species level. For SLA data of non-naturalized aliens, we sampled shaded leaves instead of fully sun-exposed as suggested by Pérez-Harguindeguy et al. (2013) to avoid bias toward environmental differences between shaded forests and open areas of botanic gardens.

Plant height (m) was measured following the method suggested by Pérez-Harguindeguy et al. (2013). Not all height data were obtained from direct measurement during surveys due to measurement difficulties in the field. Direct measurements were conducted for 40% of the species (i.e. 40/102 species). All of the 40 directly measured species were mature individuals, and were exist in relatively common environmental conditions. For the remaining 62 species, we collected median height data from botanical descriptions (e.g. http://www.efloras.org/ and http://hear.org), and then calibrated these data using data we collected in the field for the other 40 species.  We did this calibration using the following process: First, we plotted the average of heights from direct measurement (39 species) against the median height value from the databases of the same 39 species  (Figure S9). Then, we fitted a model to describe the relationship between these two sources of information (direct measured height data and database height data). We chose the model with best fit based on r2 values (Table S10). Then, we ‘calibrated’ the height of the remaining 62 alien species based on the fitted model, using the median height data from databases as the predictor variable. The scatter plot between the model prediction and the real data presented in Figure S11.

We did not measure seed mass directly because of limited fruit availability in the study sites during the survey period. Secondary seed mass data (mg) for naturalized and non-naturalized aliens were obtained from the Kew Seed Information Database (<http://data.kew.org/sid/>). When we did not find seed mass data for a species in the databases (i.e. 33/102 species), we used the average data of the corresponding genus (29 species) or family (4 species) from the same database, using a minimum of 30 other species.

We included dispersal method, origin and growth-form as categorical variables. We categorized dispersal method as whether the plants are dispersed by animals or not. We focused on animal dispersal because animals are suggested as important vectors for tropical invasion (Swarbrick, 1993; Dawson et al., 2011). The origin of aliens denotes whether the species is native to a tropical or non-tropical region. We classified species into tropical vs non-tropical categories because we expected that tropical plant species would be more likely to naturalize in tropical forests than non-tropical species because of habitat suitability (i.e. the study region is tropical). Finally, we classified growth-form of alien plants into herbs, shrubs, and trees (we excluded other growth forms, including ferns and vines, due to very low detection rates). Data on dispersal method, origin, and growth-form were collected from databases, including <http://data.kew.org/sid/>, <http://www.ars-grin.gov/>, <http://www.gbif.org>, <http://www.pfaf.org/>, and <http://www.hear.org/pier/>. 

Information on minimum residence time, i.e. the number of years elapsed since the species was first known to be present in the botanic gardens to the date of the survey was obtained from botanic gardens’ catalogue collections (Cibodas: 1930, 1963, 1977, and 1988; Bali: 1989, 1999, and 2006) and planting date official records (Kuningan and Baturraden). We used minimum residence time in our analyses to account for the potential lag time between the introduction and establishment of an alien species.

We used ImageJ in this study to measure leaf area from leaf photo data.