Shrubs versus ‘gullivers’: woody species coping with disturbance in grasslands

Abstract

Resprouting of trees and shrubs in forestgrassland ecotones is a key process to understand the dynamics of these systems under different disturbance regimes. This study integrates resprouting of grassland shrubs and pioneer forest trees ('gullivers'), burned in subtropical lowland grassland and cut in temperate highland grassland of Rio Grande do Sul, Brazil. Per grassland site, 20 individuals each of 1-2 grassland shrub species (Asteraceae) and two forest tree species (Myrsinaceae, Myrtaceae) were tagged, and postdisturbance survival and growth monitored for 1 year at 2-4 month intervals. Differences in resprouting vigour (summed-up basal area of resprouted shoots per pre-disturbance summed-up area of basal stems), and in density and allometry of resprouted shoots (allocation mode) were compared between tree and shrub species by linear mixed effects modelling and multiple comparisons, using the Tukey test. All grassland shrub individuals resprouted and regained 73-142 % (species average) of pre-disturbance basal area within one year, as opposed to 14-24 % in trees. All Myrtaceae 'gullivers' resprouted, but up to two-thirds of Myrsine individuals did not survive disturbance. Tree species tended to produce either many slender or few stout shoots, while shrub species were intermediate between these extremes. Forest trees regained 22-46 % of predisturbance height, independent of allocation mode, and grassland shrubs up to 73 %. This suggests that grassland fires allow grassland shrubs but not forest trees to persist and to grow to reproductive size. Differing sprout allocation modes may reflect allometric constraints rather than strategies to outgrow the fire-prone grass matrix.

Why Be a Shrub? A Basic Model and Hypotheses for the Adaptive Values of a Common Growth Form

“…Shrubs may also flower and set seeds earlier than trees (Hypothesis 2); although this seems likely (e.g., Hoffmann and Solbrig, 2003 ; Hermann et al, 2012 ), a review and more empirical studies are desirable. Hypothesis 3 predicts that shrubs should survive stem breakage better than trees, for which there is little evidence (but see Shibata et al, 2014 ; Leonardsson and Götmark, 2015 ).…”

“…Because a shrub does not grow tall, it will reach reproductive size earlier (e.g., Hoffmann and Solbrig, 2003 ), and produce seeds earlier than a tree (e.g., Hermann et al, 2012 ). This gives shrubs an extra fitness benefit after processes that reduce tree dominance.…”

Size Dependency of Post-Disturbance Recovery of Multi-Stemmed Resprouting Trees

“…Production of multiple stems, therefore, does not appear to affect persistence in frequently burned ecosystems in regard to the balance between biomass loss and recovery. Our study species are all trees that resprouted from the root crown; it is not clear how differences in allometric constraints on resprout allocation (e.g., shrubs vs. trees; [57] ) or the belowground structure from which resprouts are produced (e.g., lignotubers or rhizomes; [3] ) influence relationships between maximum and total stem size. Nevertheless, accounting for only the largest pre-fire stem and largest resprout appears to be an adequate predictor of species' equilibrium size and their ability to recovery their pre-fire size and should not lead to misinterpretation of persistence ability over multiple fire cycles.…”

A systematic review of resprouting in woody plants and potential implications for the management of urban plantings

“…The continuous definition was found more broadly across disturbance regimes of lower severity including more types such as herbivory, wind damage, forest clearing and avalanche. Continuous studies often described resprouting response as a scaled weak‐to‐strong response depending on severity with high severity regimes eliminating many ‘weaker’ species (Hermann et al., 2012 ; Kennard et al., 2002 ). Prevailing theories for almost 20 years explain woody plant resprouting response as a continuum that is a function of the disturbance regime (Bond & Midgley, 2001 ; Vesk & Westoby, 2004 ).…”