Do early‐successional weeds facilitate or compete with seedlings in forest restoration? Disentangling abiotic versus biotic factors

Abstract

Semi‐arid forests need cost‐effective restoration strategies to address their severe degradation. Tree shelters are often used to minimize abiotic and biotic stress during seedling establishment. We asked if early‐successional weeds act as a natural shelter by facilitating native seedlings, contingent on abiotic and biotic stressors and seedling ecological strategy. We conducted a manipulative weed exclusion experiment at a semi‐arid site in South Texas targeted for large‐scale forest restoration to discern the net effect of weeds on the growth and survival of target thornscrub tree and shrub seedlings. We assessed the roles of contrasting seedling ecological strategies (fast vs. slow growth habit), temporal variation in abiotic stress, microclimate and mammalian herbivory in modulating weed–seedling interactions. Ungulate herbivory on seedlings was common, of similar frequency across most species, but not diminished by the presence of weed neighbours. On average, seedlings growing adjacent to weed neighbours experienced modest but non‐significant increases in both height and mortality after 6 months, relative to weed‐excluded areas. However, seedlings without significant herbivory and adjacent to weed neighbours grew more vigorously (increased height and branching) during hot and dry periods, particularly those species with a fast growth habit. Although seedling light‐saturated photosynthetic capacity (Asat) and air temperature were unaffected by weed presence during hot and dry periods, afternoon light levels were reduced by approximately 50%, possibly indicative of lower leaf temperatures and improved seedling water status. Our results show that realizing the facilitative potential of weeds in semi‐arid forest restoration requires minimizing mammalian herbivory and temporally separating competition for resources. Managing for intermediate but not excessive levels of forb canopy cover is likely required to reduce high radiation loads and reduce transpiration without adverse competitive effects. Longer‐term experiments manipulating cover crop identity, cover and mammalian herbivory will inform whether forbs can be effectively exploited to enhance restoration success at large scales.

Winners and losers in dryland reforestation: species survival, growth, and recruitment along a 33‐year planting chronosequence

“…Both soil elemental analysis and factorial experiments manipulating nutrients and herbivores under nurse canopies could disentangle mechanisms limiting species survival and growth (Prober et al 2005). Some data exist on the relative browse intensity on species included here (Dick et al 2016; Arias et al 2021; Mohsin et al 2021).…”

Monitoreo de la supervivencia de una reforestación con especies nativas del matorral espinoso tamaulipeco

“…La superficie total de reforestación fue de 15.43 ha, con una densidad de 816 plantas por hectárea, en un diseño de marco real y una mezcla de especies al azar. La superficie de plantación, cantidad de individuos y selección de taxones se determinaron en función de aquellas más frecuentemente utilizadas, y con altos valores de supervivencia en los monitoreos de reforestación (Foroughbakhch et al, 2001;Jurado et al, 2006;García, 2011;Alexander et al, 2016;Arias et al, 2021). Las plántulas se produjeron en el vivero forestal de la empresa GEMA S.C. en la ciudad de Linares, Nuevo León.…”

“…Con base en los periodos de lluvia de verano en la región (Figura 2), durante los meses de octubre de 2019 y agosto de 2020, se realizaron dos actividades de mantenimiento recomendadas por Conafor (2010): control de maleza y reposición de plantas. El control de la maleza se llevó a cabo mediante la eliminación manual de la especie invasora Cenchrus ciliaris L., para facilitar el crecimiento de las especies nativas deseadas (Arias et al, 2021).…”

“…En el MET, el monitoreo de supervivencia en reforestaciones con especies nativas se ha documentado para siembra directa en campo por semilla y de siembra de planta proveniente del vivero (Foroughbakhch et al, 2001;Jurado et al, 2006;García, 2011;Arias et al, 2021).…”

“…Algunas de las condiciones de reforestación estudiadas incluyen el matorral denso y abierto, diferentes niveles de estrés hídrico y temperaturas bajo 0 °C, en competencia con malezas de temprana sucesión y con el uso de refugios individuales tubulares (Foroughbakhch et al, 2001;Jurado et al, 2006;García, 2011;Alexander et al, 2016;Arias et al, 2021). Los niveles de supervivencia citados corresponden a un intervalo amplio de 0.4 a 99 %, en función de las especies nativas utilizadas y las condiciones naturales o experimentales presentes.…”