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Understanding how biotic interactions influence the distribution of epiphytic orchids in tropical forests: An integrated network approach.

Periodic Reporting for period 1 - EpiNet (Understanding how biotic interactions influence the distribution of epiphytic orchids in tropical forests: An integrated network approach.)

Reporting period: 2019-09-02 to 2021-09-01

Understanding the factors that limit species distribution is a longstanding question in ecology. In land plants, species distribution is simultaneously limited by multiple factors, including climate, dispersal ability, competition, and species' physiological tolerance. The ecological requirements of obligate inter-specific interactions impose additional, often overlooked, limitations. How complex interactions influence plant species distribution has received little attention despite its relevance for species conservation in the face of global change. EpiNet addressed this gap by investigating how two obligate partners, interacting with climate, influence plant distribution in mega-diverse tropical assemblages.
Tropical epiphytic orchids provide an exceptional study model because they depend on two partners to germinate and grow: a host tree and mycorrhizal fungi. I proposed to study tripartite interaction networks of tropical epiphytic orchids with their partners under different ecological conditions. I had three specific goals. First, I investigated whether the diversity of mycorrhizal fungi affect the distribution of orchid species across different types of tropical forests (cloud, temperate and dry forests). Since those forests differ in their climate (temperature, rainfall, and sunlight), I expected them to harbor different communities of orchids, host trees and mycorrhizal fungi. Furthermore, climate can also have an effect on species' traits that are important for survival. For example, roots of epiphytes need to take water from the atmosphere. Therefore, the roots of species growing in dry forests should be especially adapted to drought, and those growing in cloud forests are not expected to be adapted to water scarcity. To assess whether root traits differed significantly between forests, I investigated several root traits related to water conservation and water transport.
Finally, we considered the possibility that orchids might be able to switch their mycorrhizal partners over their lifetimes. Such switches (or temporal turnover) might afford a considerable acclimation potential to the plant. This could enable fairly rapid responses to short‐term fluctuations in growth conditions as well as lasting responses to long‐term climatic trends. Moreover, mycorrhizal replacement is an understudied variable that may influence plant establishment success. I found that we were lacking not only empirical evidence of turnover, but also a unifying conceptual framework to tackle the issue. Therefore, I put forward a theoretical framework that will assist researchers in assessing whether orchids replace mycorrhizal partners or retain them for a lifetime.
The main research has been conducted according to the plan, with some delays related to the pandemic. For the first six months, I conducted fieldwork and labwork in Colombia. Back in Belgium, I started to work on the molecular data and statistical analyses. Meanwhile, I developed a conceptual article that explored patterns of partner switching in mycorrhizal interactions. There are three more manuscripts in different stages of completeness.
After the project, we expected to attain an unprecedented, comprehensive understanding of the ecological factors influencing the distribution of tropical epiphytic orchids. Preliminary data analyses suggest that the identity of mycorrhiza is important, but not a major limitation to orchid distribution across forests. Indeed, our theoretical work indicates that temporal turnover of mycorrhiza might be highly frequent in nature, which would make it easier for orchids to establish working symbioses with the fungi that are locally available. In addition, based on root water conservation traits, orchids from cloud forests seem to be more vulnerable to the forecasted heating of mountain areas in the tropics. When the dataset is fully analyzed, we will be able to make an indirect assessment of the impact of climate on natural orchid communities in relation with their mycorrhizal and host trees. This information will be useful to inform orchid conservation actions in the near future.
We provided the first conceptual and methodological framework for evaluating whether orchid mycorrhizal partners are replaced or retained over an individual's lifetime. This framework represents a major contribution to the field because studies on the subject were being done without a unifying conceptual framework. This contribution provides researchers on the topic with standard protocols for designing studies and interpreting results. Overall, this work will increase the comparability of studies and considerably improve our understanding of temporal shifts in mycorrhizal interactions in the next years.
We discovered that root traits of epiphytic orchids display a large variation over altitudinal ecological gradients in response to solar radiation, temperature, and vapor pressure deficit. Specifically, orchids from lowland drier forests had root traits that enhance water retention capacity and water transport. An important novel finding is that increasing solar radiation was a major predictor of traits linked to enhanced water retention capacity. A high water retention capacity enables orchids to reduce water losses during periods of high evaporative demand. This suggests that orchids from dry forests are better equipped to tolerate climate warming than orchids from cloud forests. This is one of the few studies addressing the response of root functional traits over ecological gradients, and the first to confirm the climatic factors that better predict such variation.

The work carried out has the potential to address climate change-related issues. Specifically, an overarching aim was to assess how obligate partners of orchids affect orchid distribution over a climatic gradient. This will allow to make an indirect assessment of the impact of climate on mycorrhizal communities and the mycorrhizal interaction, as well as the expression of root traits that are critical for orchid survival. Our findings will open new avenues for research, conservation efforts and risk assessment of epiphytic orchid communities in the face of predicted drier and warmer climates in tropical mountains.
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