Effects of simulated nitrogen deposition and a stable isotopic assessment for the neotropical epiphytic orchid Laelia speciosa

Abstract

The accelerated increase of nitrogen deposition is the third cause of biodiversity loss, as a result of saturation of ecosystems worldwide. The effects of nitrogen deposition on the endemic and endangered neotropical epiphytic orchid, Laelia speciosa, were evaluated via a dose-response experiment and a stable isotopic field assessment for individuals from a city and from an oak forest, in order to evaluate the potential risk facing this orchid, and record the history of the nitrogen deposition of series of consecutive annually produced pseudobulbs. Lower doses of nitrogen of up to 20 kg N ha yr -1 the dose that led to optimal performance of plants, acted as fertilizer. For instance, chlorophyll content and chlorophyll fluorescence (Fv/Fm) peaked at 0.66 ± 0.03 g m -2 and 0.85 ± 0.01, respectively. In contrast, toxic effects were observed at the higher doses of 40 and 80 kg N ha yr -1 , leading a decrease of 38% of the chlorophyll content and 23% of the chlorophyll fluorescence. For the field assessment, a tissue nitrogen content of 1.2 ± 0.1% (dry mass basis) for the orchids suggested non-toxic deposition rates both at the city and the oak forest. However, their respective isotopic signatures revealed different sources of N at each site.Indeed, in the oak forest δ 15 N amounted -3.1 ± 0.3‰, typical of places with low industrial activities, while in the city the δ 15 N reached 5.6 ± 0.2‰, typical of sites with some degree of industrial and automobile activity. Laelia speciosa would be an adequate bioindicator of nitrogen deposition because its ability to take up nitrogen from the atmosphere while preserving its isotopic signature and showing a clear physiological response to increasing inputs of nitrogen.However, its limited geographical distribution precludes the orchid as an ideal candidate for biomonitoring. Thus other vascular epiphytes should be considered for this purpose.

Physiological diversity of orchids

“…The δ 15 N values measured from the leaves, pseudobulbs, and roots of Laelia speciosa are higher at sites exposed to industrial and vehicular activities than in oak forests (Díaz-Álvarez et al., 2016). Performance by those plants is optimal at doses of up to 20 kg N ha yr −1 , but toxic effects are observed at doses of 40 and 80 kg N ha yr −1 (Díaz-Álvarez et al., 2015). However, few studies have examined the responses of orchids to nitrogen-deposition and it remains unclear whether the continuous rise in those depositions is beneficial or harmful to epiphytic orchids.…”

High-Temperature Tolerance by the Endangered Mexican Mayflower Orchid, Laelia Speciosa

“…On the one hand, the oak forests of central Mexico, to which L. speciosa is restricted, are among the most endangered ecosystems in the country under various climate change scenarios [3,[13][14][15][16]. On the other hand, epiphytes such as the Mayflower are especially exposed to changing environmental factors, such as insolation, changes in temperature, severe and frequent droughts, and even atmospheric pollution [11,12,[17][18][19]. Higher exposure leads to increased risk unless a species is tolerant of stress, but studies of environmental stress tolerance are scant for non-timber species from oak forests.…”