Interactions between soil water content and fertilizer on growth characteristics and biomass yield of Chinese white poplar (Populus tomentosa Carr.) seedlings

Chinese white poplar (Populus tomentosa Carr.) is an indigenous tree species in China which plays a key role in the establishment of plantation forests and the conversion of cropland to forestland. By using response surface methodology based on rotatable central composite design, we conducted a test with clone 87 of P. tomentosa seedlings to quantify the interactions among soil water content (W), nitrogen application (N), and phosphorus application (P) on plant stem height (SH), leaf area (LA), net photosynthetic rate (P N) and biomass yield (BY). Our results showed that W, N and P had obvious positive effects on plant SH, LA, P N and BY, and the magnitude of effects in decreasing order was W > N > P. There were synergistic effects of W × N on SH, LA and BY, whose values first increased and then decreased with N increasing under a fixed level of W and also with W increasing under a fixed N. A similar trend was found with the interaction of W × P significantly effecting LA. A multiple target decision model...