Evaluating the effects of a superabsorbent polymer on drought stress in two conifer species

Early seedling death from summer drought stress is a major threat to reforestation success. In bare root seedlings especially, low survival rates can limit potential financial savings compared to container seedlings. Many land managers seek to improve survival by applying superabsorbent polymer root dips to bare root seedlings during planting. In theory, the polymer should absorb and slowly release water, mitigating drought stress, but research on root dipping is limited and has shown mixed results. In response to this research gap, we tested a novel biodegradable superabsorbent polymer on two commonly planted conifers, ponderosa pine (Pinus ponderosa) and douglas fir (Pseudotsuga menziesii), in a controlled greenhouse experiment. Bare root seedlings were planted in heat-treated field soil to simulate post-wildfire conditions. Over nine weeks of biweekly summer watering to induce drought stress, we measured seedling survival and tracked needle chlorophyll fluorescence and stomatal conductance as instantaneous physiological indicators of seedling health. Furthermore, we analyzed the effect of polymer addition on soil water retention with and without seedlings present. Our preliminary results indicate that the polymer increased survival rates for both species, but only improved physiological parameters for the douglas fir seedlings in the first few weeks of drought stress. The polymer increased the field capacity of our test soil, and it extended water retention in the absence of seedlings, but nearly all available water was taken up by seedlings within two weeks of drought regardless of polymer presence. Land managers can incorporate these findings into cost-benefit analyses on superabsorbent polymer usage in reforestation.