A novel form of physiologically based plant facilitation results in stable coexistence between two competitive shrubs

Facilitation at early growth stages results in spatial associations and stable coexistence in late growth stages of two long-lived, dominant shrubs

Instead of competitively excluding each other in their resource-limited habitat, we observed two dominant, highly competitive, long-lived shrubs of semi-arid California Coastal Sage Scrub commonly growing in immediate proximity of each other when the plants were in late growth stages (canopies > 1m diameter). Considering the abundance of stress gradients in the seasonally limited precipitation of their Mediterranean Type Ecosystem, we wondered if some form of plant facilitation was occurring. Additionally, it appeared that unlike most other forms of plant facilitation, which break down over time when beneficiaries establish and outcompete the plant that once nursed them, the interactions between the shrub species results in stable coexistence. Based off cultivar cover crops in the buckwheat family, which exude acids to mobilize phosphates otherwise bound up in partially decomposed organic matter, we hypothesized that California buckwheat (Eriogonum fasciculatum) acidifies soils to mobilize nutrients otherwise bound to Ca2+ or CO3 2- leftover from the remains of ancient seashells in the uplifted soils. Any plant that could tolerate E. fasciculatum may then benefit from soil acidification. However, it appeared California sagebrush (Artemisia californica) was one of the few plant species capable of tolerating E. fasciculatum competitive effects. 

We tested our hypothesis among mature plants in the field. Plant facilitation's Stress Gradient Hypothesis predicts that as an alleviated stress becomes more severe, facilitated species will increase their association rate with their facilitator, even if the overall abundance of the beneficiary decreases due to stress. We found a strong positive relationship when the percent deviation of observed from expected association rates of A. californica with E. fascicultum derived from chi-square contingency tests for association were regressed against mean soil pH (R2 = 0.50, p = 0.0224). Further, the most commonly studied forms of facilitation are shading, hydraulic lift, and pollinator attraction, which requires the facilitative species to have larger canopies, deeper roots, and flowers generally found in plants during late growth stages. We wondered if a physiologically based facilitation could begin during early growth stages. In greenhouse experiments, we show that E. fasciculatum begins acidifying soils soon after germination and increases shoot dry mass of A. californica seedlings by 80.2% after 120 days. These results highlight the importance of understanding lifecycle-long interactions among species in evaluating facilitation’s impacts on community structure. Also, less obvious forms of physiologically based facilitation may be more common than the literature indicates.

Schlau et al. 2021 - https://doi.org/10.1111/oik.08603

#competition #coexistence #facilitation #species #interactions

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