A main theme in Alpine ecology is to better explain the position of the tree line. What limits trees from marching up to higher elevations? There are several things that limit upward tree migration including seed dispersal, temperature and wind. Gravity makes seeds fall down mountain sides, temperatures decrease in higher elevations, and wind speeds increase, together limiting the uphill advancement of trees. Coming from the opposite perspective, researchers Katie Becklin, Megan Pallo and Candace Galen wanted to better understand what limits non-woody, higher elevation alpine plants from making a decent to the tree line. Challenging the norms in the field of ecology is a theme I always appreciate, and their paper is everything I thought it would be. I really am a sucker for experimental studies that focus on competition.
The limitations of tree distribution in alpine environments; the tree line.
A species patchiness is explained by many biotic and abiotic interactions. These researchers studied the transitional zone where two ecosystems meet, also known as an ecotone. This willow-meadow ecotone in the central Rocky Mountains (Park County, CO, USA) is where the distinct willow (Salix sp.) tree line ends, and the patchy growth of understory plants and smaller, Salix individuals coincide. This area is a perfect place to utilize multiple competing hypotheses to better understand how aboveground and belowground interactions drive species distributions.
The first thing we need to nail down are the different mutualists associated with these alpine plants. Salix species live symbiotically with ectomycorrhizal fungi (ECM), while the meadow species form mutualisms with arbuscular mycorrhizae (AMF). The first hypothesis was synthesized along with the notion that the taller more robust Salix trees may limit sunlight reaching the understory plants, which reduces their photosynthetic output, ultimately limiting the amount of sugar reaching their nutrient sequestering AMF. With less sugar resources to offer, these plants fungi should return less nutrients to their hosts, and AMF colonization should decrease.
Rocky mountain range.
The second of three competing hypotheses involves the reduction of AMF function through the release of secondary compounds from Salix roots and/or their ECM symbionts. This mechanism is a well-documented species interaction seen throughout the plant and fungal kingdom. This underground chemical warfare takes place when fungi or plants release volatile compounds that specifically evolved to hinder a competitor’s growth directly, or indirectly by reducing the function of their symbionts.
The third and final hypothesis used in this study encompassed the breakdown of leaf litter. If you are familiar with Salix species, then I’m sure you know of the large quantity of detritus they produce. Interestingly, ECM can make better use of leaf litter, while AMF are more limited to other pools of nitrogen and carbon. ECM is generally more enzymatically active and additionally, these fungi have been recorded interacting and exchanging materials with other saprotrophic fungi, ultimately broadening their nutrient niche width. Studies like these with multiple competing hypotheses are mentally and physically demanding because multiple experiments must be carried out.
In the first experiment, meadow plants where covered with shade cloth to stimulate the aboveground environment beneath willow trees without the co-occurrence of the allelochemicals produced by the trees and their fungi. We will call this shade treatment (S). Underground inferences were gained by transplanting both mycorrhizal willows (MW) and non-mycorrhizal willows (NW) to the open meadow. The AMF colonization of meadow plant associated with S, MW and NW treatments where compared to control plants living in unaltered open meadow (OC) and willow understories (WC). In the second experiment, to test leaf litter interactions, AMF and ECM root colonization was compared in plots with and without leaf litter.
There is a lot going on, but it’s generally pretty straight forward especially when you look at the results. With all of the treatment used in the first experiment, AMF colonization was most reduced in WC and MW treatments. The willow control and mycorrhizal willow treatments are the only treatments utilized with intact ECM communities, showing that ECM has a strong negative influence on AMF. Compared to the ECM interactions, the shading treatment wasn’t nearly as detrimental to AMF colonization, which really just highlights the importance of underground competition in these alpine ecosystems.
ECM colonization greatly enhanced by the presence of ECM willow (top). AMF colonization reduced by the presence of ECM willow (bottom). Becklin et al. 2012.
The second experiment had just as compelling results, with willow detritus enhancing ECM colonization in the open meadow, while reducing AMF in both open meadow, and willow understory environments. This interaction drives a negative feedback in AMF and their plant mutualists, as leaf litter enhances ECM, which increases their competitiveness, further reducing AMF.
Leaf litter enhancing ECM colonization in open meadow (top). Leaf litter reducing AMF colonization in open meadow and willow understory (bottom). Becklin et al. 2012.
AMF colonization reduces with enhanced ECM colonization. Becklin et al. 2012.
These series of experiments over the course of two years paint a more descriptive picture of what we see in alpine ecosystems. Personally, I thought aboveground shading would really limit the understory meadow plants. As it turns out, competition between mycorrhizae really drives species distribution in these demanding ecosystems. ECM have a competitive edge, with their ability to utilize leaf litter, interact with saprotrophic fungi and produce noxious allelochemicals. Though, they are probably limited by their hosts physiological limitations, as higher elevations are paired with increasingly demanding factors like lower temperatures and higher wind speeds.