The base of a steep mountain is a foreboding place. Debris — ranging from sand-sized grains to massive boulders — rains down constantly, forming what are known as talus slopes. These slopes are at the edge of stability, prone to failing and sliding. The debris piles hold little water and are seemingly inhospitable to plant life.

Yet on the alpine talus slopes of the Lassen Volcanic Park in the Cascades Range, North America, an abundance of vascular plants, ranging from herbs to the occasional pine tree, make their home on the debris-swept slopes. Francisco Pèrez of the University of Texas, Austin, documented these plants and their interaction with the geomorphology of the talus slopes (Geomorphology 138, 29–48; 2012).

The talus vegetation — which commonly includes buckwheat plants, herbs and low-lying shrubs — shows a number of adaptations for life on a slippery slope. Young, mostly perennial plants typically have shallow root systems that allow them to be carried downslope with the shifting talus. Many of these plants have root systems that trail back up the slope. Only older plants have deep root systems that allow them to resist the downward debris flow. Annual plants are rarely observed in the talus.

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Once the plants establish themselves against the frequent grain and rock slides, snow avalanches and frost creep, they in turn begin to influence the dynamics of the debris flow. Mat-forming plants capture fine-grained debris as it moves downhill, eventually growing up through the accumulating sediment. Compact herbs and shrubs with dense foliage also block debris from moving down the slope. The blockages form mounds, debris wedges and even small terraces. The sheltered areas beneath the blocking plants are often less steeply sloped and contain a higher percentage of fine-grained debris than the surrounding slope. In some cases, the presence of a strong root network even prevents some surface sliding.

This hardy, if uneven, assemblage of plants in such an extreme environment highlights the ability of some plants to cope with the most trying of habitats, and shows how plant tenacity can eventually add some stability to the immediate surroundings.