Of the 22 traits measured, variation in only a few of these was explained by differences in microclimate across the sites (Figs. 2, 3 & 6). In contrast, we found substantial variation driven by a continuum of trait combinations expressed by more succulent versus less succulent species (Fig. 5, 9). Such trade-offs indicate differences in ecological strategies relating to drought avoidance and drought tolerance in the epiphyte community. Below, we address the questions presented in the Introduction.
1) How do traits and trait combinations vary from warmer and drier to cooler and wetter microclimates?
There are clear differences in microclimate across the six study sites. In general, the higher sites have more moisture in the system exemplified by lower VPD but higher leaf wetness and canopy soil moisture (Table 1, Gotsch et al., 2017). This variation in microclimate appears to have a substantial effect on community composition of vascular epiphytes in the Monteverde region, where epiphyte communities in cloud forest sites at higher elevations have higher species richness and epiphyte abundance compared with drier sites below the cloud base (Gotsch et al., 2017; Amici et al., 2019). Thus, we expected to find significant variation across sites in at least some traits relating to water relations and the leaf economics spectrum. Although variation in most traits appeared to be driven only by trait relationships underlying drought tolerance or avoidance in this community, variation in C and N isotopes were most notably influenced by microclimate.
δ13C varied predictably across the gradient—plants in drier sites (i.e., below the current cloud base) had more positive δ13C values (Fig. 3, Supplemental Table 2). These results are consistent with observations that stomatal conductance tends to be greater over the lifespan of leaves at wetter sites, providing RuBisCO with more opportunities to discriminate against the heavier isotope (Farqhuar et al. 1989). Although the δ13C values are consistent with C3 photosynthesis, the range of values (-28‰ to -32‰) is at the low end of the distribution of plants with C3 photosynthesis and may be a result of within-crown recycling of soil-respired carbon, especially in the wetter sites (Da Silveira et al., 1989). It is possible that the high biomass of epiphytes and their associated canopy soils in wetter sites create a buffer from atmospheric CO2, which could cause more localized C-recycling and more negative δ13C values. In contrast, in the drier sites, tree crowns are more open and the epiphyte abundance is much lower (Gotsch et al., 2017; Amici et al., 2019), which could facilitate greater mixing between in- and above-canopy air masses.
We also found significant differences in δ15N across sites (Fig. 3), which could be due to atmospheric sources of N being preferred over N in canopy soils. The range of δ15N values we documented was similar to those found in studies on vascular epiphytes (Heitz et al., 1999; Heitz et al., 2002; Reich et al., 2003; Cardelus and Mack 2010; Craine et al., 2015). For example, in Monteverde, epiphytes living on small branches had more negative δ15N values, whereas higher values were found for plants rooted in canopy soil (Heitz et al., 2002). Smaller individuals that receive a greater proportion of their N from atmospheric sources had more negative δ15N values compared with larger bromeliads that received a greater proportion of their N from canopy soil and had more positive δ15N values (Reich et al., 2003). The δ15N values in our three premontane rain forest sites below the current cloud base were significantly higher than sites located above the cloud base (Fig. 3). In our study, all individuals were in the interior of the canopy and had root access to canopy soil, with greater access to canopy soils in the upper sites. Nonetheless, we found that δ15N values were more negative in higher elevation sites where inputs from wet deposition, which are presumed to have lower δ15N values, were greater (Cornell et al., 1995; Heaton et al., 1997; Koopmans et al., 1997; Heitz et al., 2002). This pattern suggests that atmospheric sources of N are preferred over N in canopy soils, even in sites where canopy soils are abundant. Alternatively, it is possible that epiphytes in wetter sites receive a large proportion of their N from canopy soils, but that the wetter canopy soil conditions promote mineralization, which lowers the δ15N. Additional measures of δ15N in canopy soils across our gradient are needed to disentangle these two patterns.
Although functional traits in several plant communities vary along environmental gradients, substantial trait variation has also been documented within communities (Wright and Westoby, 1999; Wright et al., 2004; Wright et al., 2005; ter Steege et al., 2006; Ordoñez et al., 2009). Variation within communities can be due to niche partitioning whereby different species express unique trait combinations resulting in alternative ecological strategies (Ludlow, 1989; Reich et al., 1997; Kobe, 1999; Grime, 2001; Diaz et al., 2004; Ackerly and Cornwell, 2007; Kraft et al., 2008). Such partitioning can provide alternate solutions in response to a common limiting resource and can also promote diversity in communities. We are aware of only two studies that have examined sources of trait variation in epiphytes, and these have also found substantial within-site variation (Petter et al. 2016, Costa et al., 2018). In both studies, the target traits related most closely related to leaf carbon and nutrient allocation rather than water relations, even though water limitation is a ubiquitous feature of the epiphytic life form. In fact, it was through the inclusion of a number of water relations traits in this study (e.g., TLP, LWS, LWC, LT, C, SD, & gmin) that a clear understanding of the continuum of trait patterns relating to succulence emerged.
2) Do ecological trade-offs, such as the degree of succulence or drought tolerance underlie variation in functional traits within epiphyte communities?
In contrast to variation across sites, we found substantial variation within sites that relate to functional groups and to an even greater degree, leaf succulence. Despite substantial variation in leaf size, shape and species-level diversity in this system, most of the traits we measured underlie trade-offs in trait allocation between more succulent and less succulent epiphytes (Fig. 4, 5, & 9). In general, epiphytes with more succulent leaves expressed traits conferring avoidance of rather than tolerance to low water potentials. These plants have high water storage and area-normalized hydraulic capacitance, and also have thick and tough leaves with thick cuticles. These species also tended to have higher TLPs and may be more vulnerable to cavitation than non-succulent epiphytes. The notion of tradeoffs in trait allocation between more vs. less succulent epiphytes is supported by a tendency for herbaceous epiphytes -- many of which are succulent -- to avoid water stress during extended dry periods by greatly reducing sap flow (Gotsch et al., 2017). On the other hand, epiphytic shrubs that generally do not have succulent leaves, maintained higher water use during a drought and subsequently took longer to recover (Gotsch et al., 2017).
In general, succulents also had less structural carbon and greater allocation to water storage tissue (Figs. 4 & 5). In this system, leaf toughness can be conferred either by succulence or high allocation to structural carbon. Counterintuitively, succulent plants had a greater minimal leaf conductance while simultaneously having lower stomatal density (Figs. 4 & 5). Hydrenchymal cells tend to have greater elasticity, which allows these cell layers to release water with minimal resistance to the photosynthetically active cell layers when needed (Ogburn and Edwards 2010). It is likely that succulent plants also have higher minimal conductance values since this loosely held water in hydrenchyma may be more easily lost from leaf surfaces. The lower stomatal density of succulent plants may minimize water loss from the hydrenchyma. However, in our study, the lower stomatal density of succulent epiphytes did not seem to be sufficient to reduce the gmin of succulent plants to below that of their non-succulent counterparts. Conversely, non-succulent epiphytes exhibit traits that confer greater drought resistance. These plants had higher stomatal density and more structural carbon while leaf water storage and gmin were lower (Figs. 4 & 5).
These trade-offs in trait allocation align with and extend upon results from 11 species in the same TMCF, where foliar water uptake capacity and foliar water uptake in field sap flow trials were negatively correlated with traits relating to succulence (Gotsch et al., 2015). For example, epiphytes with high foliar water uptake tended to have thinner leaves with thinner cuticles and hydrenchymal layers, and they had a lower leaf toughness and turgor loss points (Gotsch et al., 2015). Succulent leaves on the other hand specialize in storing water, but the traits that tend to maximize storage capacity (i.e., thick cuticles and hydrenchymal layers) also limit the ability to directly absorb cloud water via leaf surfaces (Gotsch et al., 2015). The current study expanded on previous work to increase the number of species, sites, and traits we measured, which has resulted in our insights that trait relationships along a continuum of leaf succulence is likely driven by trade-offs between drought avoidance and tolerance (Fig. 9).
Our analyses revealed few significant correlations between functional traits and stable isotopes (four for δ13C and two for δ15N), which may be an indication of the strong within-site variation for most traits since the isotopes varied significantly across sites (Figs. 3 & 6). However, we found a significant relationship between δ13C and SD that was pronounced at the wet end of the gradient (Fig. 7). Our δ13C data suggest that stomata tended to be more closed in lower and drier environments and that epiphytes in these sites may be limited in the range of SD they possess. In contrast, epiphyte communities at the wetter end of the gradient supported species exhibiting higher stomatal density. Species with higher stomatal density are likely able to better use cloud water via foliar water uptake (Gotsch et al., 2015; Berry et al., 2018). In the wetter sites, species also exhibited traits indicating a succulent strategy; however, species with a high stomatal density were restricted to sites with abundant precipitation and cloud cover.
Climate change projections for this region and for other TMCFs predict increases in the cloud base height, air temperature, and the number of consecutive dry days (Pounds et al., 1999; Still et al., 1999; Lawton et al. 2001; Pounds et al. 2006; Ray et al., 2006, Helmer et al. 2019). As water limitation intensifies in this system, functional diversity will likely decrease in the lower elevation edge of the TMCF where epiphyte taxonomic and functional diversity is lowest, whereas there will likely be some continued redundancy across sites that experience a more moderate microclimate. We suggest future research on the impacts that shifts in functional diversity will have on epiphyte community processes and the resultant changes in ecosystem services in the TMCF. This is especially important given the role that epiphytes play in the storage and cycling of water and nutrients as well as the food and habitat resources the community provides.