Functional traits are morpho-physio-phenological characters that impact fitness through their effects on growth, reproduction, and survival (Violle et al. 2007). Variation in functional traits influence plant life history strategies, resource allocation for growth and defense (Coley et al. 1985), niche differentiation, and environmental filtering (Westoby et al. 2002; Poorter et al. 2008; Boukili and Chazdon 2017). Leaf economics spectrum theory has shown the existence of a universal trade-off between two opposing life history strategies (Wright et al. 2004). On one hand, species exhibiting an “acquisitive” strategy are characterized by high photosynthetic rates, thinner and low-cost leaves, with high nitrogen and phosphorus content and high specific leaf area (SLA) that exploit high-resource environments. On the other hand, species with “conservative” strategies have thicker, tougher, low SLA leaves, with low photosynthetic rates, high construction costs, low nitrogen content, and long lifespans, and exploit low-resource environments (Westoby et al. 2002; Wright et al. 2004). These strategies are consistent across biomes demonstrating the universality of this trade-off (Díaz et al. 2004, 2016).
Palms (Arecaceae) are one of the most diverse and widely distributed groups of plants in tropical and subtropical areas, with more than 2,600 species and 181 genera (Baker and Dransfield 2016) that dominate many tropical ecosystems (Mejia and Kahn 1990; Myers 2013). Palms are “hyperdominant” elements in the Amazon lowlands (ter Steege et al. 2013), with 6 out of the 10 most abundant species being palms. Although palms have a limited contribution to carbon stocks in diverse tropical rainforests (Fauset et. 2015), they influence forest function (Boukili and Chazdon 2017), play a crucial role in food webs, provide habitat and food to a multitude of animal species (Zona and Henderson 1989; Howard et al. 2001; Onstein et al. 2017), and are invaluable to many human groups who use them as raw materials for construction, food, drink, clothing, fuel, medicine, and fibers (Jones 1995; Henderson 2002; Dransfield et al. 2008; Sylvester et al. 2012). To improve our understanding of the ecological role of palms of different forest strata and growth forms it is essential to improve the knowledge on their inter and intraspecific variation in functional traits (Westerband et al. 2021).
Many palm species start their lives as light-suppressed seedlings and juveniles in the understory, until they accumulate enough resources to develop appropriate mechanical support around the base of the stem to grow in height. Since light (Montgomery and Chazdon 2001; Sylvester and Avalos 2013) and nutrients (Wright et al. 2018; Wright 2019; Collins et al. 2022) can limit the survival of understory plants, the selective pressure to acquire and timely invest resources in growth are greater in the understory than in the canopy, or under higher light conditions. Species that spend their entire life cycle, or spend significant time in in the understory, have evolved to adapt to the shade (Avalos 2019). Understory palms produce long-lived fronds rich in structural defenses and increase their slenderness ratio (stem height divided by stem diameter) as they grow (Avalos and Fernández Otárola 2010; Otárola and Avalos 2014; Avalos 2022 in rev). Palm seedlings have thinner leaflets with high SLA to capture more light per unit of invested carbon (Avalos 2022, in rev). This trend reverses as the palms that start in the understory transition from small seedlings to adults and move to the canopy producing thicker, low SLA, long-lived leaves (Avalos 2019; Avalos 2022 in rev). Still little is known about the variation in functional traits in palms, and tropical plants in general, as they traverse different ontogenetic and successional stages (but see Hérault et al. 2011; Lasky 2015; Boukili and Chazdon 2017). Such research is fundamental to understand community organization specially in highly diverse tropical forests (Dayrell et al. 2018; Trujillo et al. 2022).
Palms have been excluded from most inventories of functional traits in tropical forests (DeWalt and Chave 2004; Chave et al. 2005; Lorenz and Lal 2010). As monocots, they have a different structure, allometry, and strategies of resource use relative to trees (Tomlinson 2006, 2011). With a few exceptions, palms are monopodial and lack aerial branching, have only one shoot meristem, and lack dormancy and secondary growth. In palm species where stem diameter and stem height show a significant relationship, diameter increases through sustained primary growth (i.e., through the division, lignification, and expansion of parenchyma cells, which also differentiate into fibers, Henderson 2002; Tomlinson 2011). In addition, leaf longevity and leaf construction costs are higher in palms than in dicotyledonous trees (Renninger and Phillips 2016), which have smaller leaves and could drop leaflets rather than the entire compound leaf to acclimate to new light conditions. Within the family Arecaceae there is considerable morphological variation which is reflected in niche differentiation and habitat filtering (Henderson 2002).
There is an incomplete inventory of functional traits for tropical plants, and especially for palms (Göldel et al. 2015). Functional traits such as tissue density (Rich 1986, 1987), dry mass fraction (dmf), slenderness ratio, leaf toughness, and SLA, and even stem height, as well as gas exchange parameters, are rarely documented for palms as a group, or are limited to a few species (i.e., Chazdon 1986a, 1986b; Araus and Hogan 1994; da Silva et al. 2015; Renninger and Phillips 2016). Much less is known about how these traits vary with ontogenetic stage and palm size (but see Chazdon 1986a, 1986b). Tissue density, for instance, varied with position along the stem, since sclerotized tissue is denser closer to the base and periphery of the stem, and decreases in abundance close to the top of the stem (Niklas 1992). The functional trait databases (Perez-Harguindeguy et al. 2013) are still data-deficient for palms, and for tropical species in general, although Kissling et al. (2019) provide a very comprehensive compilation. Still, these databases are based on a few individuals (e.g., http://db.worldagroforestry.org), and often do not provide metadata. It is necessary to incorporate more species, a larger sample size per species, a greater range of sizes, and phylogenetic bias corrections.
Our main objective is to analyze the interspecific variation in nine functional traits related to biomass allocation and tissue quality (tissue density, dmf, slenderness ratio, carbon content, diameter, height, leaf area, and root:shoot ratios based on biomass and carbon content) in seven palm species from three forest strata (understory, subcanopy and canopy). We tested the hypothesis that the dominant light environment in each stratum (understory, subcanopy and canopy) that a palm species typically exploits, will determine the strategy of resource use, and thus, the degree of functional trait similarity among species. Therefore, palms that complete their life cycle in the understory will show a conservative resource-use strategy that emphasizes efficient biomass distribution (dominant traits in this strategy will be tissue density, slenderness ratio, root:shoot ratios, and dmf), in contrast to palms exploiting better lit environments (subcanopy and canopy species) that will show an acquisitive strategy, and therefore will be characterized by functional traits favoring large size and leaf area (i.e., diameter and height, total carbon content, leaf area). Augmenting our knowledge about the variation in palm functional traits is relevant not only to expand the database of functional traits in tropical plants, but also to understand how different resource allocation strategies regulate plant growth in contrasting light environments, and ontogenetic niche shifts in functional traits as the palm grows into the canopy, or within a given stratum (Dayrell et al. 2018; Westoby et al. 2022). Exploring the variation of functional traits within palms, one of the most abundant life forms in tropical forests, could significantly expand our understanding of how plants adapt to environmental gradients.