Abstract
In the southwestern USA, recent large‐scale die‐offs of conifers raise the question of their resilience and mortality under droughts. To date, little is known about the interannual structural response to droughts. We hypothesized that piñon pines (Pinus edulis) respond to drought by reducing the drop of leaf water potential in branches from year to year through needle morphological adjustments. We tested our hypothesis using a 7‐year experiment in central New Mexico with three watering treatments (irrigated, normal, and rain exclusion). We analyzed how variation in “evaporative structure” (needle length, stomatal diameter, stomatal density, stomatal conductance) responded to watering treatment and interannual climate variability. We further analyzed annual functional adjustments by comparing yearly addition of needle area (LA) with yearly addition of sapwood area (SA) and distance to tip (d), defining the yearly ratios SA:LA and SA:LA/d. Needle length (l) increased with increasing winter and monsoon water supply, and showed more interannual variability when the soil was drier. Stomatal density increased with dryness, while stomatal diameter was reduced. As a result, anatomical maximal stomatal conductance was relatively invariant across treatments. SA:LA and SA:LA/d showed significant differences across treatments and contrary to our expectation were lower with reduced water input. Within average precipitation ranges, the response of these ratios to soil moisture was similar across treatments. However, when extreme soil drought was combined with high VPD, needle length, SA:LA and SA:LA/d became highly nonlinear, emphasizing the existence of a response threshold of combined high VPD and dry soil conditions. In new branch tissues, the response of annual functional ratios to water stress was immediate (same year) and does not attempt to reduce the drop of water potential. We suggest that unfavorable evaporative structural response to drought is compensated by dynamic stomatal control to maximize photosynthesis rates.
Highlights
In recent years, widespread forest mortality in response to drought has been documented worldwide (Allen, Breshears, & McDowell, 2015)
In our 7-year experiment, we showed that yearly variation in evaporative structure of piñon pines was driven by changes in soil moisture across treatments
Needle length, area, and stomatal density of needles all decreased with drier soil, whereas neither stomatal diameter nor maximal anatomical stomatal conductance varied significantly
Summary
Widespread forest mortality in response to drought has been documented worldwide (Allen, Breshears, & McDowell, 2015). We hypothesized that in years with low soil moisture and/or high VPD, piñons would (1) reduce needle length (l) and needle area; (2) increase the stomatal density and reduce the stomatal diameter, resulting in a decrease of maximum anatomical stomatal conductance gsmax; and (3) increase the annual SA:LA and annual SA:LA/d To test these hypotheses, we analyzed the drought-induced response of piñon pines over a 7-y ear experiment in New Mexico that artificially modified soil moisture conditions, with three types of treatments: ambient, irrigated, and precipitation exclusion (so-called droughted). Atmospheric VPD was similar across treatments, allowing us to decouple the effects of atmospheric dryness from soil water stress during multiple years, overcoming an issue for understanding long-t erm ecosystems response over long time periods (Novick et al, 2016)
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