Introduction
Soil and vegetation cover degradation resulting from anthropogenic practices such as cattle, agriculture, and urbanization have impacted the dynamics and functioning of forests worldwide (Jie et al. Reference Jie, Jing-Zhang, Man-Zhi and Zi-Tong2002). Land degradation has been extreme in tropical dry forests (TDF), which used to represent 42% of all the world’s tropical forests (Brown & Lugo Reference Brown and Lugo1982), with more than 50% left in South America (Miles et al. Reference Miles, Newton, Defries, Ravilious, May, Blyth, Kapos and Gordon2006). In Colombia, around 90% of its cover was replaced by the end of the 20th century (Etter et al. Reference Etter, McAlpine and Possingham2008; García et al. Reference García, Corzo, Isaacs, Etter, Pizano and García Martínez2014), and only 8% of TDF’s original cover is left in land mosaics of early successional stages (González-M et al. Reference González-M, Garcia, Isaacs, Cuadros, López-Camacho, Rodriguez, Pérez, Mijares, Castano-Naranjo, Jurado, Idárraga-Piedrahita, Rojas, Vergara and Pizano2018). In these degraded landscapes, natural regeneration is challenging, and planting trees is necessary for re-establishing local biodiversity and ecosystem functioning (Rodrigues et al. Reference Rodrigues, Lima, Gandolfi and Nave2009; Garbowski et al. Reference Garbowski, Avera, Bertram, Courkamp, Gray, Hein, Lawrence, McIntosh, McClelland, Post, Slette, Winkler and Brown2020; Werden et al. Reference Werden, Zarges, Holl, Oliver, Oviedo-Brenes, Rosales and Zahawi2022). However, water limitation imposed by the pronounced dry season (3–6 dry months precipitation <100 mm⋅month−1, Portillo-Quintero and Sánchez-Azofeifa, Reference Portillo-Quintero and Sánchez-Azofeifa2010; González-M. et al. Reference González-M, Norden, Posada, Pizano, García, Idárraga-Piedrahita, López-Camacho, Nieto, Rodríguez-M, Torres, Castaño-Naranjo, Jurado, Franke-Ante, Galindo-T, Hernández R, Barbosa and Salgado-Negret2019), which is exacerbated by soil compaction due to livestock grazing (Batey Reference Batey2009; Tracy et al. Reference Tracy, Black, Roberts and Mooney2011), represents a barrier to the successful restoration of degraded TDF. Under this scenario, exploring the variation and coordination of belowground traits among species will be fundamental for selecting species that can overcome the soil barriers dominant in dry degraded ecosystems.
Fine roots, which are the most distal orders of fine root systems (typically first- to third-order roots), are specialized in nutrient and water acquisition (McCormack et al. Reference McCormack, Dickie, Eissenstat, Fahey, Fernandez, Guo, Helmisaari, Hobbie, Iversen, Jackson, Leppälammi-Kujansuu, Norby, Phillips, Pregitzer, Pritchard, Rewald and Zadworny2015) and therefore critical for species performance in degraded ecosystems (Paz et al. Reference Paz, Pineda-García and Pinzón-Pérez2015; Garbowski et al. Reference Garbowski, Avera, Bertram, Courkamp, Gray, Hein, Lawrence, McIntosh, McClelland, Post, Slette, Winkler and Brown2020; Werden et al. Reference Werden, Zarges, Holl, Oliver, Oviedo-Brenes, Rosales and Zahawi2022). Recent studies have identified two main functional belowground gradients expressing globally (Bergmann et al. Reference Bergmann, Weigelt, van der Plas, Laughlin, Kuyper, Guerrero-Ramirez, Valverde-Barrantes, Bruelheide, Fresche, Iversen, Kattge, McCormack, Meier, Rillig, Roumet, Semchenko, Sweeney, van Ruijven, York and Mommer2020). One gradient encompasses plants that optimize resource uptake by investing carbon in thin roots that efficiently explore the soil themselves (“do-it-yourself”) to those that “outsource” resource acquisition via mycorrhizal associations, whereby an expanded cortical area of roots provides a wider intraradical habitat for their fungal partners (Raven & Edwards Reference Raven and Edwards2001; Brundrett Reference Brundrett2002; Kong et al. Reference Kong, Ma, Zhang, Li, Chen, Zeng and Guo2014; Ma et al. Reference Ma, Guo, Xu, Lu, Bardgett, Eissenstat, McCormack and Hedin2018; Brundrett & Tedersoo Reference Brundrett and Tedersoo2018). The acquisition-conservation gradient, orthogonal to the collaboration gradient, represents a trade-off between traits linked to high metabolic activity (e.g., root nitrogen) and those associated with the costs of root construction (e.g., root tissue density [RTD] or root dry matter content [RDMC]) (McCormack & Iversen Reference McCormack and Iversen2019; Bergmann et al. Reference Bergmann, Weigelt, van der Plas, Laughlin, Kuyper, Guerrero-Ramirez, Valverde-Barrantes, Bruelheide, Fresche, Iversen, Kattge, McCormack, Meier, Rillig, Roumet, Semchenko, Sweeney, van Ruijven, York and Mommer2020). These gradients have been reported across natural ecosystems worldwide (Bergmann et al. Reference Bergmann, Weigelt, van der Plas, Laughlin, Kuyper, Guerrero-Ramirez, Valverde-Barrantes, Bruelheide, Fresche, Iversen, Kattge, McCormack, Meier, Rillig, Roumet, Semchenko, Sweeney, van Ruijven, York and Mommer2020). However, the exploration of root traits covariation and its importance for the belowground niche partitioning among species is infrequently studied in degraded habitats. For example, the dry compacted soils typical of degraded TDFs may affect root exploration capabilities (Alameda & Villar Reference Alameda and Villar2012), their morphology, and root partnership with mycorrhizae as they are mostly involved in P but not in water acquisition (Smith & Read Reference Smith and Read2009). These factors may limit belowground niche partitioning along the collaborative more than across the conservative gradient as water acquisition may critically depend on root morphology and mass deployment.
In the present study, we used six fine root traits (root diameter [D], specific root length [SRL], RDMC, RTD, branching intensity [BI], and percentage of arbuscular mycorrhizal colonization [%M]) measured in young individuals of 11 TDF tree species growing in sympatry for 9 years in a degraded TDF landscape. Our aims were (1) to define covariation between fine root traits and (2) to explore the belowground niche partitioning among 11 species coexisting under the same compacted soil conditions.
Methods
Study site and species
The study area is located in the restoration area of El Quimbo in the department of Huila, Colombia (75°41’ W, 2°18’ N), between 700 and 800 m of altitude. The zone is classified as a TDF with a mean annual temperature of 24°C and an annual mean precipitation of 1036 mm in a bimodal regime, with dry seasons from June to August and from December to January (Avella-M. et al. Reference Avella-M, García-G, Fajardo-Gutiérrez, González-Melo, Avella-M, García-G, Fajardo-Gutiérrez and González-Melo2019). We took advantage of a restoration experiment established 9 years ago in abandoned pasture lands in which 11 common tree species (Table 1) were transplanted to plots containing individuals of all species randomly distributed, thus controlling for individual age and soil conditions (Torres-Rodríguez et al. Reference Torres-Rodríguez, Díaz-Triana, Villota, Gómez and Avella-M2019). Soil bulk density varies between 0.8 and 1.2 (1.1 on average; unpubl. data) grams per cm3, slopes vary between 0.1° and 14°, and the soils are classified as inceptisols and mollisols with low P, K, and organic matter content due to cattle ranching (Torres-Rodríguez et al. Reference Torres-Rodríguez, Díaz-Triana, Villota, Gómez and Avella-M2019).
Tree species studied in a tropical dry forest in Huila, Colombia. Family, common name, code, leaf habitat, and wood density (WD) per species. Range (min-max) and mean (in parenthesis) for height (H) and diameter at breast height (DBH) of trees sampled per species. *Wood density was obtained from Gonzalez et al. (Reference González-M, Posada, Carmona, Garzón, Salinas, Idárraga-Piedrahita, Pizano, Avella, López-Camacho, Norden, Nieto, Medina, Rodríguez-M, Franke-Ante, Torres, Jurado, Cuadros, Castaño-Naranjo, García and Salgado-Negret2021)
Functional traits
Five individuals of each species were sampled in 2021, nine years after they were planted as saplings. Individual height varied between 1.7 m (Casearia corymbosa) and 6 m (Guazuma ulmifolia and Celtis iguanaea). For each individual tree, fine roots were sampled by following lateral roots growing radially from the main shoot until fine roots (2 mm or less in thickness) were found. Then, following the order-based classification (McCormack et al. Reference McCormack, Dickie, Eissenstat, Fahey, Fernandez, Guo, Helmisaari, Hobbie, Iversen, Jackson, Leppälammi-Kujansuu, Norby, Phillips, Pregitzer, Pritchard, Rewald and Zadworny2015), the fine roots were separated into individual root orders, prioritizing the collection of the first three orders. The diameter of our fine roots ranged from 0.25 to 0.71 mm (Table S1). The fresh weight of the sampled roots for each individual varied between 0.8 and 1.0 g.
In the field, fine roots were carefully collected, washed, and stored in plastic bags filled with water inside a cooler for transportation to the field station laboratory. Upon arrival at the laboratory, the fresh roots were cleaned, weighed, and scanned at 300 dpi (Epson Perfection V700 Photo scanner). Then, the roots were dried in an oven at 70°C for 48 hours and weighed to estimate the dry weight. The images were analyzed with RhizoVision Explorer, an open-source software for root trait measurement (Seethepalli et al. Reference Seethepalli, Dhakal, Griffiths, Guo, Freschet and York2021). We obtained six morphological traits (Table 2) using the weights and the measurements from the scanned roots: (1) average root diameter (D, mm) computed as the mean of all diameters in each skeleton pixel, (2) Specific root length (SRL, m g−1) calculated by dividing total length by the dry weight, (3) root dry matter content (RDMC, mg g− 1) calculated by dividing the dry weight by the fresh weight, and (4) root tissue density (RTD, g cm−3) calculated by dividing the dry weight by root volume, assuming root tissues to be cylindrical in shape (Freschet et al. Reference Freschet, Pagès, Iversen, Comas, Rewald, Roumet, Klimešová, Zadworny, Poorter, Postma, Adams, Bagniewska-Zadworna, Bengough, Blancaflor, Brunner, Cornelissen, Garnier, Gessler, Hobbie, Meier, Mommer, Picon-Cochard, Rose, Ryser, Scherer-Lorenzen, Soudzilovskaia, Stokes, Sun, Valverde-Barrantes, Weemstra, Weigelt, Wurzburger, York, Batterman, Gomes de Moraes, Janeček, Lambers, Salmon, Tharayil and McCormack2021). (5) Branching intensity (BI, mm−1) was calculated as the total number of root branching points (“links,” where a lateral root is linked to its parent root) divided by the total length (Eissenstat et al. Reference Eissenstat, Kucharski, Zadworny, Adams and Koide2015; Seethepalli et al. Reference Seethepalli, Dhakal, Griffiths, Guo, Freschet and York2021). (6) The percentage of arbuscular mycorrhizal colonization (%M) was calculated by clearing fine roots with 3% KOH and 3% H2O2 acidified with 3% HCl, washed with distilled water and stained with 0.05% aniline blue in lactoglycerol solution (Zangaro et al. Reference Zangaro, Bononi and Trufen2000). Stained root segments of each harvested plant were then mounted on a slide, and the presence/absence of Arbuscular mycorrhizal fungi (AMF) was recorded at 100 intersect points for each slide (McGonigle et al. Reference McGonigle, Miller, Evans, Fairchild and Swan1990). Although RDMC and D are claimed to be tightly correlated with each other and with SRL, we decided to include all of them because for the TDF, these relationships remain unclear. There, species with high SRL have been reported to exhibit either high diameter but succulent low tissue density or the opposite (Sanaphre-Villanueva et al. Reference Sanaphre-Villanueva, Pineda-García, Dáttilo, Pinzón-Pérez, Ricaño-Rocha and Paz2022).
Functional traits measured and their functional significance
Data analysis
To explore the patterns of covariation among fine root traits (first aim), we performed Pearson correlation with Bonferroni correction using the R Stats package (R Core Team 2021). To explore the belowground niche partitioning among 11 species (second aim), we followed 2 approaches (univariate and multivariate) using trait probability density (TPD) (Carmona et al. Reference Carmona, de Bello, Mason and Lepš2016). The TPD approach is based on estimating Gaussian kernel density functions around each observation (Carmona et al. Reference Carmona, de Bello, Mason and Lepš2016, Reference Carmona, de Bello, Mason and Lep S2019). The TPD function of a given species represents the probability of observing different trait values in that species considering all sampled individuals (Carmona et al. Reference Carmona, de Bello, Mason and Lepš2016). For the multivariate approach, we first generated a multivariate functional space among species by applying a Principal Component Analysis (PCA) on all plants measured per species and selecting the first two PCA axes as suggested by the Horn’s test for our data (Horn Reference Horn1965; Dinno Reference Dinno2018). Then taking the scores of each plant of every species along the first and second PCA axes, we calculated the multivariate functional niche of each species by using the TPD (Carmona et al. Reference Carmona, de Bello, Mason and Lepš2016), implemented in the TPD’s function of the R package “TPD” (Carmona et al. Reference Carmona, de Bello, Mason and Lep S2019). To explore belowground niche partitioning among species, we used the dissimilarity index (dissim function provided by the TPD package and TPD dissimilarity <0.05; Carmona et al. Reference Carmona, de Bello, Mason and Lep S2019) that measures the overlapping probability in the functional space between two species (TPD distributions). In addition, to better understand which functional traits differed most between species (univariate approach), we calculated functional niches based on single traits and compared them, following the same procedures described above. All statistical analyses were performed using R statistical software (R Core Team 2021).
Results
We found a negative correlation between root diameter (D) and all root traits except for %M which was not significantly correlated with D (Figure 1, Table S2). Additionally, we found a positive correlation between traits associated with construction root costs, RTD or RDMC, and BI (Figure 1, Table S2).
Pearson correlation coefficients with Bonferroni correction for pairwise relationships among six fine root traits. Significant correlations are indicated as red (negative) and blue (positive) ovals. Trait abbreviations: D: root diameter, %M: mycorrhizal colonization, RDMC: root dry matter content, RTD: root tissue density, BI: branching intensity, SRL: specific root length.
The bivariate relationships between traits were supported by the multivariate approach (Figure 2). The belowground trait space of the 11 tree species was summarized by the first 2 dimensions of the PCA (Figure 2a, Table S3), which together explained 73.3% of the among-individuals variation and were the only 2 significant components according to Horn’s test (Horn’s test adjusted eigenvalues greater than 1: 2.657 and 1.004, respectively). The first PCA axis (PC1 explaining 52.5%) suggested a trade-off between high BI and SRL (loading positively on PC1) and a high D (loading negatively on PC1). The contribution of %M to the first axis was low (Figure 2a, Table S3). The second PCA axis (PC2 explaining 20.8%) separated species with high tissue densities (i.e., high values of RTD and RDMC) located on the negative side from species with high SRL on the positive side (Figure 2a). However, it is important to note that both SRL and tissue density also partially contributed to PC1. The multivariate approach showed strong belowground niche partitioning among species, mainly across the D and BI gradient (PC1) (Figure 2b, Table S4). Species such as Celtis iguanaea (Ce.ig), Casearia corymbosa (Ca.co), and Guazuma ulmifolia (Gu.ul) with high scores on PC1 were significantly different from species with high values of D, such as Ceiba pentandra (Ce.pe), Tabebuia rosea (Ta.ro), Chloroleucon mangense (Ch.ma), and Ochroma pyramidale (Oc.py) (Figure 2b). We did not find differences between trees of different species along the PC2 (Figure 2b, Table S4).
Multivariate trait analysis. (a) PCA of six fine root traits. Each point in the PCA is an individual of a given species; color codes are shown in panel (b). Trait abbreviations: D: root diameter, BI: branching intensity, SRL: specific root length, RDMC: root dry matter content, RTD: root tissue density, %M: mycorrhizal colonization. (b) Trait probability densities (TPD) showing the functional trait combinations for eleven tree species. Species abbreviations are in Table 1; each species has a distinctive color in the PCA and TPD plots. Different letters in parenthesis next to the species abbreviation indicate significant differences between species using the dissimilarity index (>95% non-shared probability).
The multivariate divergences for the individual traits among species were supported by the univariate approach (Figure 3, Table S5–S10). The species differentiation was mainly explained by D, with Ca.co and Ce.ig showing the thinnest roots and differing significantly from the other species (except Gu.ul and Pse.sep) (Figure 3a). Regarding BI, Ca.co showed the highest values and were statistically different from the other species (except for Ce.ig, Sapindus Saponaria (Sa.sa) and Gu.ul and Jacaranda caucana (Ja.ca) (Figure 3b). Ta.ro showed the lowest BI values and varied from Sa.sa, Ce.ig, and Ca.co, which had the highest values (Figure 3b). Concerning SRL, Ca.co showed the highest values and significantly differed from Oc.py, Ce.pe, Ch.ma, Ja.ca, and Machaerium capote (Ma.ca) (Figure 3c). We detected little differentiation among species for RDMC, RTD, and %M, probably associated with strong functional intraspecific variability (Figure 3d–f, Table S8–S10).
Trait probability density for each of the 6 fine root traits and for the 11 tree species. The Y axis is the density. Trait abbreviations: D: root diameter, BI: branching intensity, SRL: specific root length, RDMC: root dry matter content, RTD: root tissue density, %M: mycorrhizal colonization. Species abbreviations are in Table 1. Different letters in parenthesis next to species abbreviations indicate significant differences between species according to the similarity index.
Discussion
Using 6 fine root traits measured in young individuals of 11 TDF tree species growing in degraded pastures in Colombia, we found ample variation in root traits that does not resemble the expectations from the collaboration gradient. The percentage of mycorrhizal colonization, an essential trait associated with the collaboration gradient, was unrelated to any fine root trait. Furthermore, we found strong species’ belowground differentiation mainly determined by the gradient between D versus BI and SRL rather than the acquisitive-conservative gradient. The differences in belowground niche among species highlighted their multiple strategies to deal with drought and penetrate compacted soils, essential information to understand species’ responses to climate change and to prioritize species for restoration purposes.
In principle, D and %M are expected to be tightly associated due to the limited intrinsic ability to acquire soil resources of thick roots (Bates & Lynch Reference Bates and Lynch2001); however, in our study, such traits varied independently. The lack of such correlation is not exclusive to our study, though no clear explanations have been claimed (Siqueira & Saggin-Júnior Reference Siqueira and Saggin-Júnior2001; Lugli et al. Reference Lugli, Andersen, Aragão, Cordeiro, Cunha, Fuchslueger, Meir, Mercado, Oblitas, Quesada, Rosa, Schaap, Valverde-Barrantes and Hartley2020). One possibility is that under habitats with dry soils, since mycorrhizal association enhances mainly P capture but not water (Gavito et al. Reference Gavito, Pérez-Castillo, González-Monterrubio, Vieyra-Hernández and Martínez-Trujillo2008; Smith & Read Reference Smith and Read2009, but see Bahadur et al. Reference Bahadur, Batool, Nasir, Jiang, Mingsen, Zhang, Pan, Liu and Feng2019), the root’s abilities to forage water may be more dependent on root morphology itself than on microbial associations. Alternatively, under low phosphorous availability, typical from old and weathered tropical soils (Vitousek Reference Vitousek2004), even finely branched roots may not ensure sufficient nutrient uptake by themselves, making them very responsive to and dependent on AMF (Siqueira & Saggin-Júnior Reference Siqueira and Saggin-Júnior2001). For instance, in our study, the species with the thinnest roots (Celtis iguanaea and Casearia corymbosa) exhibited mycorrhizal colonization percentages exceeding 42% on average. It is essential to recognize that the informative value of %M may be reduced, and identifying specific fungal groups through molecular techniques that better capture the functional nature of the symbiosis is needed (Cusack et al. Reference Cusack, Addo-Danso, Agee, Andersen, Arnaud, Batterman, Brearley, Ciochina, Cordeiro, Dallstream, Diaz-Toribio, Dietterich, Fisher, Fleischer, Fortunel, Fuchslueger, Guerrero-Ramírez, Kotowska, Lugli, Marín, McCulloch, Maeght, Metcalfe, Norby, Oliveira, Powers, Reichert, Smith, Smith-Martin, Soper, Toro, Umaña, Valverde-Barrantes, Weemstra, Werden, Wong, Wright, Wright and Yaffar2021). As we did not report a positive relationship between D and %M and, therefore, did not find a collaboration gradient, the negative relationship between D and SRL may reflect a mathematical autocorrelation more than ecological meaning (Ostonen et al. Reference Ostonen, Püttsepp, Biel, Alberton, Bakker, Lõhmus, Majdi, Metcalfe, Olsthoorn, Pronk, Vanguelova, Weih and Brunner2007). However, in TDFs, the previous observations of species with high SRL and high root diameters with low-density succulent tissues (Sanaphre-Villanueva et al. Reference Sanaphre-Villanueva, Pineda-García, Dáttilo, Pinzón-Pérez, Ricaño-Rocha and Paz2022) call for a functional role of both traits that still need to be studied in depth. Given the importance of mycorrhizal symbiosis for promoting the survival and growth of trees and shrubs within restoration experiments (Asmelash et al. Reference Asmelash, Bekele and Belay2019), an in-depth exploration of their role under degraded and dry soils becomes imperative.
The second gradient was related to resource acquisition-conservation, supported by a high RTD and RDMC (although we did not measure root N concentration). These results are consistent with other studies (Bergmann et al. Reference Bergmann, Weigelt, van der Plas, Laughlin, Kuyper, Guerrero-Ramirez, Valverde-Barrantes, Bruelheide, Fresche, Iversen, Kattge, McCormack, Meier, Rillig, Roumet, Semchenko, Sweeney, van Ruijven, York and Mommer2020; Arrieta-González et al. Reference Arrieta-González, Paez, Dominguez-Haydar and Salgado-Negret2021; Weemstra et al. Reference Weemstra, Valverde-Barrantes, Fortunel, Oblitas Mendoza, Prata, Vásquez Pilco, Vicentini, Vleminckx and Baraloto2023). Surprisingly, conservative traits such as RTD and RDMC were negatively related to D (Werden et al. Reference Werden, Zarges, Holl, Oliver, Oviedo-Brenes, Rosales and Zahawi2022), challenging the traditional idea that D increases with RTD for carbon conservation (McCormack et al. Reference McCormack, Adams, Smithwick and Eissenstat2012; Weemstra et al. Reference Weemstra, Mommer, Visser, Van Ruijven, Kuyper, Mohren and Sterck2016). This negative correlation may reflect that D increases through the root cortex, a parenchymatous tissue with low carbon content and dry weight, faster than the stele area, a tissue specialized in transporting nutrients and water through lignified cells (Kong et al. Reference Kong, Ma, Zhang, Li, Chen, Zeng and Guo2014; Valverde-Barrantes et al. Reference Valverde-Barrantes, Freschet, Roumet and Blackwood2017). The common observation of thick roots with low tissue density in other TDFs suggests a potential role of water storage in fat roots in maintaining tissue hydration within a dry soil matrix (Paz et al. Reference Paz, Pineda-García and Pinzón-Pérez2015; Sanaphre-Villanueva et al. Reference Sanaphre-Villanueva, Pineda-García, Dáttilo, Pinzón-Pérez, Ricaño-Rocha and Paz2022). Interestingly, according to the PCA, in our study, SRL was an important trait contributing to both root gradients, which has already been reported by other studies (Cusack et al. Reference Cusack, Addo-Danso, Agee, Andersen, Arnaud, Batterman, Brearley, Ciochina, Cordeiro, Dallstream, Diaz-Toribio, Dietterich, Fisher, Fleischer, Fortunel, Fuchslueger, Guerrero-Ramírez, Kotowska, Lugli, Marín, McCulloch, Maeght, Metcalfe, Norby, Oliveira, Powers, Reichert, Smith, Smith-Martin, Soper, Toro, Umaña, Valverde-Barrantes, Weemstra, Werden, Wong, Wright, Wright and Yaffar2021; Weemstra et al. Reference Weemstra, Valverde-Barrantes, Fortunel, Oblitas Mendoza, Prata, Vásquez Pilco, Vicentini, Vleminckx and Baraloto2023; but only in pairwise correlations in Bergmann et al. Reference Bergmann, Weigelt, van der Plas, Laughlin, Kuyper, Guerrero-Ramirez, Valverde-Barrantes, Bruelheide, Fresche, Iversen, Kattge, McCormack, Meier, Rillig, Roumet, Semchenko, Sweeney, van Ruijven, York and Mommer2020). The longer and thinner fine roots can increase overall absorptive capacity (“do-it-yourself” strategy), but the negative correlation with conservative traits could be explained mathematically (as long as the volume is filled with dry mass and not water or air), as high RTD implies more root mass per unit root volume and generally decreases the root mass per unit root length (Weemstra et al. Reference Weemstra, Valverde-Barrantes, Fortunel, Oblitas Mendoza, Prata, Vásquez Pilco, Vicentini, Vleminckx and Baraloto2023).
Although our species were planted in a pasture with similar soils and drought conditions, we found strong belowground differentiation among species mainly through the D-BI gradient rather than the acquisitive-conservative gradient. The compacted soils, typical of degraded pastures in dry ecosystems, could promote strong differentiation in traits associated with resource acquisition strategies (D vs BI and SRL). Under compacted soils, cortical cells tend to become broader and shorter as a strategy to penetrate soils, protecting the stele and causing thicker roots (Atwell Reference Atwell1993). Conversely, mechanical damage caused by soil compaction may also alter the root system architecture in some species, forcing lateral roots to alter their direction of growth and resulting in an increase in lateral root branching (Chen et al. Reference Chen, Palta, Clements, Buirchell, Siddique and Rengel2014), which is particularly important in those species unable to develop thick roots. Additionally, thick roots have higher storage space for accumulating water and starch (cortical cells, Lux et al. Reference Lux, Luxová, Abe and Morita2004); both resources are fundamental for drought responses under stomatal closure (Sala et al. Reference Sala, Piper and Hoch2010). On the contrary, the absence of belowground niche differentiation among species in conservative traits (RDMC and RTD) and %M could be explained by the high intraspecific variability in these traits. This variability may result from the high spatial heterogeneity of soil resources (Weemstra et al. Reference Weemstra, Mommer, Visser, Van Ruijven, Kuyper, Mohren and Sterck2016). For example, the spatial distribution of root-mycorrhizal associations is expected to be driven by P content, a highly variable nutrient in the soil (Cui & Caldwell Reference Cui and Caldwell1996). Furthermore, compaction may exert strong selective pressure, leading a reduced intraspecific variability in D and BI (as mentioned earlier). However, it appears that compaction does not influence conservative traits (RDMC and RTD), which are more associated with resource conservation. As a result, there is no single optimal phenotype of conservative traits (with low intraspecific variability) that responds to compacted soils.
More information is needed to better understand how variations in root traits within and between species contribute to niche differentiation and prioritization of restoration efforts in TDF communities. For example, species such as Ochroma pyramidale (Oh.py), Pseudobombax septenatum (Pse.sep), and Ceiba pentandra (Ce.pe), known for their low wood density and fast growth rates, exhibited thick roots with low density. This efficient strategy allows them to store water and carbohydrates, which is advantageous in dry forest environments (Pineda-García, Paz & Tinoco-Ojanguren, Reference Pineda-García, Paz and Tinoco-Ojanguren2011; Sanaphre-Villanueva et al. Reference Sanaphre-Villanueva, Pineda-García, Dáttilo, Pinzón-Pérez, Ricaño-Rocha and Paz2022). However, it is worth noting that dense wood species like Celtis iguanaea (Ce.ig) and Tabebuia rosea (Ta.ro) also exhibited a similar combination of traits. This suggests that further exploration and study of a wide functional space of species are needed to fully understand the patterns of variation in root traits.
Root traits such as root diameter and branching intensity may be useful for prioritizing species to restore abandoned pastures of TDF regions. For instance, in Ceiba pentandra (Ce.pe), thick roots and low tissue density were associated with water storage. This may be important for the maintenance of root xylem hydration and the water stream from roots to leaves, allowing photosynthesis and growth rates during dry periods at the seasonal or daily scale, during the hours of greatest transpiration (Čermák et al. Reference Čermák, Kučera, Bauerle, Phillips and Hinckley2007; Poorter & Markesteijn Reference Poorter and Markesteijn2008). Several studies have suggested that the large diameter of growing tips helps to deform and separate soil particles, enhancing mechanical force and soil penetration ability (Materechera et al. Reference Materechera, Alston, Kirby and Dexter1992; Atwell Reference Atwell1993; Clarkl et al. Reference Clarkl, Whalleyl and Barraclough2003; Bengough et al. Reference Bengough, Bransby, Hans, McKenna, Roberts and Valentine2006; Weemstra et al. Reference Weemstra, Mommer, Visser, Van Ruijven, Kuyper, Mohren and Sterck2016), which makes species with thick roots promising candidates for establishing at initial stages of forest cover. A perfect example is Chloroleucon mangense (Ch.ma), with thick roots and high fiber and structural carbohydrate contents (Paz obs. Pers.). Additionally, Ch.ma is a nitrogen-fixing legume (Sprent Reference Sprent2001), which may restore soil fertility in degraded drylands, frequently seen in TDF (Avendaño-Yáñez et al. Reference Avendaño-Yáñez, De La, López-Ortiz, Perroni and Pérez-Elizalde2018). Conversely, Casearia corymbosa (Ca.co), Celtis iguanaea (Ce.ig), and Guazuma ulmifolia (Gu.ul), with branched, thin, and high-density roots, are predicted to be efficient in exploring soils and acquiring resources, allowing them to supply the high-water demand of the leaves, as these species have low water potentials during drought periods (Werden et al. Reference Werden, Becknell and Powers2018). However, those species may not develop in compacted soils because their roots cannot easily penetrate dense soils (Clarkl et al. Reference Clarkl, Whalleyl and Barraclough2003); therefore, they are good candidates for restoring sites with low soil compaction.
Here, we highlight these results as essential insights about what traits should be used to select and what can be the possible relations with restoration effectiveness. However, other belowground traits need to be explored. Root depth, elongation rate, and other physiological traits have been identified as critical determinants of plant performance under drought due to allowing access to unexploited soil moisture (Garbowski et al. Reference Garbowski, Avera, Bertram, Courkamp, Gray, Hein, Lawrence, McIntosh, McClelland, Post, Slette, Winkler and Brown2020). Additionally, in TDF, the introduction of exotic grasses and legumes to promote and improve animal forage (Vasquez-Valderrama et al. Reference Vasquez-Valderrama, González-M, López-Camacho, Baptiste and Salgado-Negret2020) may reduce the establishment probability of tree species. Therefore, to explore traits associated with the competitive exclusion of those exotic species may be essential to improve the restoration success (Garbowski et al. Reference Garbowski, Avera, Bertram, Courkamp, Gray, Hein, Lawrence, McIntosh, McClelland, Post, Slette, Winkler and Brown2020). Finally, tree species selection for restoration should not rely only on root traits analysis, but these results must be confirmed by demographic analyses that relate these strategies to species’ performance in the field.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0266467424000129.
Acknowledgments
We acknowledge Enel Colombia and Natura Foundation and its local team from the “Ecological restoration plan of the tropical dry forest of the El Quimbo Hydroelectric plant” (“Plan de restauración ecológica de bosque seco tropical de la Central Hidroeléctrica El Quimbo”) for all of their support in carrying out this investigation.
Financial support
This research was supported by the project “¿Cómo optimizar los esfuerzos de restauración del bosque seco? un análisis desde el paisaje y los rasgos funcionales de las especies” funded by the “Convocatoria nacional para el fomento de alianzas interdisciplinarias que articulen investigación, creación, extensión y formación”, grant of the Universidad Nacional de Colombia 20192021. Rosa Arrieta was supported by the “Colombia Biodiversa” grant no 09-2021 provided by the Alejandro Angel Escobar Foundation.
