Plants in most terrestrial environments face the difficulty of maximizing growth while simultaneously defending against potential mortality from disease and animals that want to eat them. This inevitable trade-off requires plants to balance the allocation of resources such as photosynthates (i.e. sugar and starch reserves) and mineral nutrients to growth, reproduction, defence (i.e. secondary metabolites) or internal storage. If all else is equal, plants that allocate a larger proportion of resources to growth must do so at the cost of allocating fewer resources to other areas such as storage. For perennial plants such as trees and woody shrubs, maintaining relatively large storage pools is not only necessary to facilitate the seasonal growth of new tissues, but also provides critical resources to maintain functional metabolism and/or replace tissues after a catastrophic event such as fire or intense herbivory. Therefore, plants that are well adapted to episodic disturbance often ‘bank’ internal resources within complex storage organs, or recycle materials from expendable tissues with high turnover rates (i.e. leaves and fine roots).
Riparian woodlands in the arid regions of North America are highly productive ecosystems with significantly greater plant and animal diversity compared with surrounding landscapes. Species here typically evolved under intense pressure from episodic disturbance caused by frequent flooding, large crown fires and widespread insect herbivory. Adding to the complexity of these systems is a dramatic shift in vegetation cover over the last several decades from native riparian forests to shrublands dominated by the Eurasian tree/shrub tamarisk (Tamarix spp.). Several Tamarix species (also known as saltcedar) were introduced to North America in the early to mid-1800s.
A recent paper in Annals of Botany assesses potential trade-offs between growth and resource storage by comparing radial growth and δ13C in tree-ring cellulose of killed versus surviving Tamarix during years prior to attacks by the Tamarix leaf beetle. The critical balance between growth and storage of resources suggests that in high-resource environments plants that express high growth rates are more susceptible to episodic disturbance than plants that express lower growth rates.
Herbivory-induced mortality increases with radial growth in an invasive riparian phreatophyte. Annals of Botany (2013) 111 (6): 1197-1206 doi: 10.1093/aob/mct077
Under equal conditions, plants that allocate a larger proportion of resources to growth must do so at the expense of investing fewer resources to storage. The critical balance between growth and storage leads to the hypothesis that in high-resource environments, plants that express high growth rates are more susceptible to episodic disturbance than plants that express lower growth rates. This hypothesis was tested by measuring the radial growth, basal area increment (BAI) and carbon isotope ratios (δ13C) in tree-ring α-cellulose of 62 mature tamarisk trees (Tamarix spp.) occurring at three sites in the western USA (n = 31 live and 31 killed trees across all sites, respectively). All of the trees had been subjected to periods of complete foliage loss by episodic herbivory over three or more consecutive growing seasons by the tamarisk leaf beetle (Diorhabda carinulata), resulting in approx. 50 % mortality at each site. Mean annual BAI (measured from annual ring widths) in the 10 years prior to the onset of herbivory was on average 45 % higher in killed trees compared with live trees (P < 0·0001). Killed trees that had higher growth rates also expressed higher (less negative) δ13C ratios compared with live trees. In fact, at one site near Moab, UT, the mean annual BAI was 100 % higher in killed trees despite having about a 0·5 ‰ higher δ13C relative to live trees (P = 0·0008). Patterns of δ13C suggest that the intrinsic water-use efficiency was higher in killed than surviving trees, possibly as a consequence of lower whole-canopy stomatal conductance relative to live trees. The results show that a likely trade-off occurs between radial growth and survival from foliage herbivory in Tamarix spp. that currently dominates riparian areas throughout the western USA and northern Mexico. Thus, herbivory by D. carinulata may reduce the overall net primary productivity of surviving Tamarix trees and may result in a reduction in genetic variability in this dominant invasive tree species if these allocation patterns are adaptive.