Shrub encroachment and persistence: A case of biofeedback leading to stability after state change

Background/Question/Methods

Conversion of grasslands to woody dominated communities has occurred globally over the past 150 years and has been related to various drivers, both exogenic and endogenic. While exogenic factors, often associated with climate change, drive woody encroachment across regions, endogenic factors operate at smaller microenvironmental scales (i.e. < 10 m²). Microenvironmental modification by dominant species leads to emergence of feedbacks as state conversion ensues. To better understand how these bio-feedback mechanisms drive and influence patterns of state change, microenvironmental conditions were measured across a transitional gradient on a barrier island of the North American Atlantic coast. We hypothesized that microenvironmental conditions are ameliorated by woody encroachment. More specifically, increased leaf area reduces soil temperature due to higher light attenuation thereby increasing soil moisture thus, ameliorating conditions that promote shrub establishment in grasslands. Three zones were designated along a coastal grassland-woody transition gradient: grassland, transition zone, and shrub thicket. Microenvironmental conditions (soil and air temperature, soil moisture and salinity) were measured. Leaf Area Index (LAI) was quantified and percent vegetation cover surveyed. Shannon diversity index was calculated for each zone using the canopy-coverage data. Differences in LAI, soil temperature, moisture, and salinity between zones were compared.

Results/Conclusions

Species diversity was highest in the transition zone, followed by the grassland, and lowest in the thicket. LAI was lower in the grassland than the transition zone and thicket, but was not significantly different between transition and thicket. Transition zone air temperatures were more variable due to the heterogeneity of coverage than in the more homogenous thicket and grassland. Temperatures were cooler in summer and warmer in winter in the thicket compared to the grassland and transition zone due to higher LAI.  Soil moisture and salinity were also more variable in the transition than the grassland and thicket, while the thicket and grassland were less variable and not significantly different from one another. During state conversion, microenvironmental variation is greater than an unchanging state due to greater variety of species’ coverage and variable LAI, which in turn affect soil temperature and soil moisture content. Such bio-feedbacks brought about by dominant species lead to stabilization of microenvironment and even function to perpetuate those species; thus, a stable state emerges. Understanding these mechanisms of state change and stabilization will improve predictive ability of ecosystem responses as global changes such as climate change and sea level rise proceed.