Abstract
Drought will increasingly threaten forest ecosystems worldwide. Understanding how competition influences tree growth response to drought is essential for forest management aiming at climate change adaptation. However, published results from individual case studies are heterogeneous and sometimes contradictory. We reviewed 166 cases from the peer-reviewed literature to assess the influence of stand-level competition on tree growth response to drought. We monitored five indicators of tree growth response: mean sensitivity (inter-annual tree ring width variability); association between inter-annual growth variability and water availability; resistance; recovery; and resilience to drought. Vote counting did not indicate a consistent effect of competition on mean sensitivity. Conversely, higher competition for resources strengthened the association between water availability and inter-annual growth rates. Meta-analysis showed that higher competition reduced resistance (p < 0.001) and improved recovery (p < 0.05), but did not consistently affect resilience. Species, site and stand characteristics, and drought intensity were insignificant or poor predictors for the large variability among the investigated cases. Our review and meta-analysis show that competition does not affect the response of tree growth to drought in a unidirectional and universal way. Although density reduction (thinning) can alleviate growth declines during drought, the effects on growth after stress are uncertain. The large variability among investigated cases suggests that local-scale processes play a crucial role in determining such responses and should be explicitly evaluated and integrated into specific strategies for adaptation of forests to climate change.
Highlights
Anthropogenic climate change is responsible for more frequent and intense droughts worldwide (IPCC 2013; Trenberth and others 2014)
We investigated five widely used tree ring-based descriptors of growth response to drought: mean sensitivity, a measure of inter-annual tree ring width variability; association between inter-annual growth variability and water stress; growth resistance (Lloret and others 2011), which quantifies the growth reduction during a specific drought event; recovery, to assess the capacity to recover relative to the drought-induced growth reduction; and resilience, which measures the capacity of growth to return to the pre-disturbance performance
The three resilience components were significantly related to each other
Summary
Anthropogenic climate change is responsible for more frequent and intense droughts worldwide (IPCC 2013; Trenberth and others 2014). Stand density and individual tree size, typically measured as stem diameter at breast height (in lieu of the desirable but more difficult measurement of leaf area, Forrester 2019), determine stand-level competition (Weigelt and Jolliffe 2003) and directly affect water availability and uptake (Krajicek and others 1961; Moreno-Gutierrez and others 2012). A dense canopy (high leaf area index) causes higher stand-level transpiration, which reduces soil water availability (Breda and others 1995), yet it reduces daily maximum soil temperature and evaporation rates, during dry-soil conditions (von Arx and others 2013). Competition exacerbates the effects of drought on tree functional processes, such as stomatal conductance and photosynthetic rate (Breda and others 1995; Moreno and Cubera 2008; Tsamir and others 2019)—even though intrinsic water-use efficiency (that is, the ratio between photosynthetic assimilation and stomatal conductance) was found unaffected (Moreno-Gutierrez and others 2012; Fernandez-de-Una and others, 2016)
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