Effects of altitude, land use and microsites on early life performance of a high mountain tree: Insights from an in situ sowing experiment

Abstract

Aim: Understanding the forces that drive range shifts in forest landscapes is imperative for predicting species distributions under anthropogenic climate and land use change. However, empirical studies exploring how these components jointly influence critical early life stages of mountain tree species across environmental gradients are scarce. We used the high mountain tree Polylepis australis as model species to investigate the relative importance of altitude and associated climatic conditions, land use for livestock and microsite characteristics on early life performance.Location: Córdoba Sierras, central Argentina. Methods:We set up an extensive in situ sowing experiment with a robust split-plot design that integrated spatial scales ranging from 0.4 m 2 subplots at the microsite level (associated with vegetative and microtopographic structures), to livestock exclosure and enclosure plots of several hectares, to an altitudinal gradient of 1,000 m.Components of early life performance were monitored across two subsequent growing seasons. Results: Microsite characteristics played a fundamental role in P. australis establishment, whereby interactions with altitude and/or land use suggested alternate mechanisms: facilitation (likely reduced desiccation) dominated at low altitude while at high altitude, abiotic stress (likely intensive frost and radiation) overruled any microsite effects. At mid-altitude, benefits of competition release prevailed over facilitation and microsite effects gained importance under livestock presence. Inconsistencies between pre-and post-emergence responses illustrated potential trade-offs between beneficial and detrimental effects of microsite conditions upon performance throughout early life: a favourable location for seeds may abruptly turn adverse for seedlings. Main conclusions:We unravel how changes in altitude, anthropogenic disturbances and microsite characteristics jointly modulate P. australis performance across stages of early establishment. Such information is fundamental when categorizing specific

Canopy openness modifies tree seedling distributions along a tropical forest elevation gradient

“…We believe this impacts species distribution. Studies elsewhere have also indicated that both facilitation and competition influence species persistence and elevation ranges (Olsen et al 2016, Ettinger and HilleRisLambers 2017, Cáceres et al 2019). For the tropics, these results are consistent with field observations along the same gradient (Muñoz Mazón et al 2019).…”

Are populations of Polylepis australis locally adapted along their elevation gradient?

“…One month before starting the field experiments (November 2015), grasses and herbs of all common gardens were cut to avoid initial differences in vegetation structure. By cutting the vegetation we also expected to mimic the herbaceous sites with low grazing pressure, where the highest P. australis recruitment is recorded (Figure 2(c-d)) [25,32,43]. Regeneration of this species within woodlands is severely reduced by litter, ferns and moss, and canopy cover [43][44][45][46].…”

“…Polylepis australis is dominant in highland woodlands of central Argentina mountains [20,21], where it is distributed from 900 to 2700 m a.s.l [22]. Previous studies found variation in the species regeneration traits, such as seedling growth and survival, along its elevation range [22][23][24][25]. In particular, these studies suggest optimal conditions for P. australis at midelevations, since its performance decreases towards both elevation extremes in response to the constraining factors typical of semi-arid mountain ecosystems [22][23][24][25][26].…”

“…Previous studies found variation in the species regeneration traits, such as seedling growth and survival, along its elevation range [22][23][24][25]. In particular, these studies suggest optimal conditions for P. australis at midelevations, since its performance decreases towards both elevation extremes in response to the constraining factors typical of semi-arid mountain ecosystems [22][23][24][25][26]. The differences in plant performance between elevations described in those field studies could be a consequence of phenotypic plasticity, considering that populations appear to be connected by effective long-distance pollination [27,28] and that they are not genetically structured throughout elevation gradients [29].…”