Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

1. The question whether the strategies of above-and belowground plant organs are coordinated as predicted by the plant economics spectrum theory is still under debate. We aim to determine the leading dimensions of tree trait variation for above-and belowground functional traits, and test whether they represent spectra of adaptation along a soil fertility gradient in tropical Andean forests. 2. We measured leaf, stem and fine root functional traits, and individual-level soil nutrient availability for 433 trees from 52 species at three elevations between 1000 and 3000 m a.s.l. 3. We found close coordination between above– and belowground functional traits related to the trade-off between resource acquisition and conservation, whereas root diameter and specific root length formed an independent axis of covarying traits. The position of a tree species along the acquisition–conservation axis of the trait space was closely associated with local soil nitrogen, but not phosphorus, availability. 4. Our results imply that above-and belowground plant functional traits determine at which edaphic microhabitats coexisting tree species can grow, which is potentially crucial for understanding community assembly in species-rich tropical montane forests.

Abstract:

With increasing elevation, trees in tropical montane forests have to invest larger frac-tions of their resources into their fine roots in order to compensate for increasingly unfavorable soil conditions. It is unclear how elevation and related edaphic changes influence the variability in tree fine root traits and belowground functional diversity. We measured six fine root traits related to resource acquisition on absorptive fine roots of 288 trees from 145 species along an elevational gradient from 1000 m to 3000 m a.s.l. in tropical montane forests of the Ecuadorian Andes. We analyzed trait relation-ships with elevation and soil nutrient availability, and tested whether root functional diversity varied along these gradients. Fine roots at higher elevations and at more nutrient-poor sites were thicker, had higher tissue densities, and lower specific root length and nutrient concentrations than at lower elevations. These trends were diluted by the coexistence of tree species with a broad range of different root traits within communities particularly towards lower elevations, where root functional diversity was significantly higher. We conclude that nutrient limitation and potentially further adverse conditions at higher elevations are strong environmental filters that lead to trait convergence towards a conservative resource use strategy, whereas different trait syndromes are equally successful at lower elevations.

Abstract:

Plant–animal interactions are fundamentally important in ecosystems, but have often been ignored by studies of climate-change impacts on biodiversity. Here, we present a trait-based framework for predicting the responses of interacting plants and animals to climate change. We distinguish three pathways along which climate change can impact interacting species in ecological communities: (i) spatial and temporal mismatches in the occurrence and abundance of species, (ii) the formation of novel interactions and secondary extinctions, and (iii) alterations of the dispersal ability of plants. These pathways are mediated by three kinds of functional traits: response traits, matching traits, and dispersal traits. We propose that incorporating these traits into predictive models will improve assessments of the responses of interacting species to climate change.

Abstract:

Although fine root traits are crucially important for plants and ecosystems, little is known about the mechanisms driving their variation. According to the theory of the plant economics spectrum, plants have to adopt a consistent strategy for all organs on a one-dimensional axis from slow to fast nutrient acquisition. The different strategies are supposed to be reflected in fine root traits, and to vary with resource availability. This master’s thesis aimed to investigate the variation of tree fine root traits along a topographical gradient in southern Ecuador, and their coordination with aboveground traits, in order to find evidence for a plant economics spectrum. In a tropical montane forest near Loja, fine root systems (diameter < 2 mm) of 179 adult trees were sampled from 18 plots at different slope positions. The steep slopes in this megadiverse ecosystem are characterized by decreasing fertility from the lower to the upper slope, and a high turnover of tree species on small scales along the slopes. The traits root diameter, specific root length (SRL), root tissue density (RTD), root branching intensity (RBI) and nitrogen content (N root ) were measured on each root system. The root traits were tested for being correlated with a topography index, edaphic factors, and aboveground traits. These relationships were analyzed both on the level of plot means and individual trees. Moreover, functional dispersion of the root traits was calculated for each plot and related to the topographical gradient. Pagel’s ? was calculated as an indicator of phylogenetic signal for each root trait. Furthermore, mixed effect models and principal component analyses were calculated in order to better understand the complex relationships between environment, plant traits, and phylogeny. Fine roots tended to be thicker and less intensely branched, contain less nitrogen and have lower SRL with increasing distance from the lower slope. Functional dispersion decreased from lower to upper slope. Especially SRL and N root were closely linked to aboveground traits. Trees with high SRL and N root tended to have softer wood and leaves, higher specific leaf area and higher leaf N and P concentrations. These trends were also apparent when comparing plot means instead of data from tree individuals. This indicates that trees evolved consistent resource economic strategies that manifest in their fine roots, wood, and leaves. Species with a slow, conservative strategy are typical for the upper slopes, whereas acquisitive species assembled at the lower slopes. However, RTD was independent from the topographical gradient and most aboveground traits. SRL, N root and root diameter had strong phylogenetic signals. Rubiaceae and Moraceae were typical families with acquisitive traits growing at the lower slope, whereas the early-diverged Lauraceae occurred predominantly at the upper slope and had very thick roots of the conservative type. These findings provide valuable new insights into patterns of fine root trait variation. They confirm the hypothesis of a plant economics spectrum where root traits are integrated, and demonstrate that tree species in tropical montane forests assemble along topographical gradients according to their resource economics strategies and the associated trait combinations.

Abstract:

Global changes in nutrient deposition rates are likely to have profound effects on plant communities, particularly in the nutrient-limited systems of the tropics. We studied the effects of increased nutrient availability on the seedlings of six tree species in montane forests of southern Ecuador in situ. After five years of continued N, P, or N+P addition, naturally grown seedlings of each of the two most common species at each elevation (1000, 2000, and 3000 m asl) were harvested for analyses of leaf morphology, nutrient content, herbivory, and tissue biomass allocation. Most species showed increased foliar N and P concentrations after addition of each respective element. Leaf tissue N:P ratios of >20 in the control plants of all species suggest that P is more growth-limiting in these forests than N. Leaf morphological responses to nutrient addition were species and nutrient specific, with some species (Hedyosmum purparescens, Graffenrieda emarginata) exhibiting increased specific leaf area (SLA), and others (Graffenrieda harlingii) increased leaf area ratios (LAR). Pouteria torta (1000 m) had lower SLA and LAR after P addition. Increased herbivory was only evident in G. emarginata (after N and N+P addition). Only the species from 3000 m asl modified biomass allocation after nutrient addition. In general, N and N+P addition more strongly affected the species studied at the upper elevations, whereas P addition had a similar range of effects on the species at all elevations. We conclude that the responses of the studied tropical montane forest tree seedlings to chronic N and P addition are highly species-specific and that successful adaptation to increased nutrient availability will depend on species-specific morphological and physiological plasticity.

Abstract:

Trait response-effects are critical to forecast community structure and biomass production in highly diverse tropical forests. Ecological theory and few observation studies indicate that trees with acquisitive functional traits would respond more strongly to higher resource availability than those with conservative traits. We assessed how long-term tree growth in experimental nutrient addition plots (N, P and N+P) varied as a function of morphological traits, tree size and species identity. We also evaluated how trait-based responses affected stand scale biomass production considering the community structure. We found that tree growth depended on interactions between functional traits and the type or combination of nutrients added. Common species with acquisitive functional traits responded more strongly to nutrient addition, mainly to N+P. Phosphorous enhanced the growth rates of species with acquisitive and conservative traits, had mostly positive effects on common species and neutral or negative effects in rare species. Moreover, trees receiving N+P grew faster irrespective of their initial size relative to control or to other treatment plots. Finally, species responses were highly idiosyncratic suggesting that community processes including competition and niche dimensionality may be altered under increased resource availability. We found no statistically significant effects of nutrient additions on aboveground biomass productivity because acquisitive species had a limited potential to increase their biomass, possibly due to their generally lower wood density. In contrast, P addition increased the growth rates of species characterized by more conservative resource strategies (with higher wood density) that were poorly represented in the plant community. We provide the first long-term experimental evidence that trait-based responses, community structure, and community processes modulate the effects of increased nutrient availability on biomass productivity in a tropical forest.