Are field crops refuge for woody invaders? Rainfall, crop type and management shaped tree invasion in croplands

Abstract

Woody encroachment represents a global concern which is mostly studied in natural and semi-natural grasslands and savannas. Woody invasion is a growing phenomenon in agricultural landscapes, but it remains elusive how different crop types and their management could shape woody invasion in croplands. Understanding the mechanisms involved in woody establishment in crops may contribute to design strategies to reduce the magnitude and consequences of invasion. We examined to what extent current agricultural practices affect early establishment of the invasive tree Gleditsia triacanthos (honey locust) in maize and soybean crops, during a wet and a dry growing season. In Pampean croplands (Argentina), we selected zero tillage fields of both crops and controlled management (with/without agricultural practices) and crop presence (with/without soybean or maize plants) during two growing seasons. After maize and soybean harvest, we examined survival, growth, and starch reserves of G. triacanthos seedlings. Results showed that G. triacanthos survival decreased from 76% to 34% in the dry year with respect to the wet year. Crop presence enhanced tree survival, likely by protecting seedlings from herbicide and desiccation, but reduced seedling growth. Seedlings were smaller in soybean than in maize, and larger when crop was removed, independent of crop type. Even so, woody survival was on average 61% and 49% after maize and soybean harvest, respectively. Interestingly, seedling starch reserves were modulated by crop type and management, and were positively associated with G. triacanthos survival. In conclusion, we found a hierarchy of controls acting on G. triacanthos establishment in cropping systems, with growing season rainfall variability, crop type and the applied management modifying tree survival and growth. Overall, our study highlights that, under current management, annually cropped fields may represent a refuge for woody plant invaders and a potential front of woody expansion.