The primary goal of my research program is to understand the ecological drivers and
evolutionary processes influencing anthropogenic redistribution of biota, which has
shaped my current research interests in the area of biological invasion. The main
question I ask is: What ecological and evolutionary processes permit or cause changes
through which naturalized and/or exotic invasive plant species achieve their competitive
or general ecological success and influence native biodiversity. My research on
theoretical and empirical analyses of ecological interactions has fueled many of current
research projects, and has resulted in several high impact papers, for example work
published in PNAS, Nature, Ecology, Journal of Ecology, Ecology Letters, Trends in
Ecology & Evolution, Trends in Plant Science, and Plant Physiology.

COMMUNITY AND ECOSYSTEM EFFECTS OF EXOTIC PLANT INVASION

Why some exotic plants, when introduced to a new part of the world, become far more
abundant than in their native range and disproportionally suppress natives is one of the
most puzzling questions in ecology. We made quantitative comparisons in native and
non-native ranges of invaders on local species. Ageratina adenophora is a native of
Mexico and a noxious invasive species in Asia. We found that in the native range
Mexico, presence of Ageratina,was correlated with higher plant species richness than
surrounding plant communities where this species was absent, suggesting facilitation.
But in two non-native ranges, China and India, Ageratina canopies were correlated with
much lower species richness than the surrounding communities, suggesting inhibition
(Ecology 2011, 92: 316-324).We have provided novel empirical evidence that suggest
that Ageratina has evolved a mechanism of increased nitrogen allocation to
photosynthesis (growth) and reduced allocation to cell walls (defense), resulting in an
increase in its growth and vigor in non-native ranges (Proceedings of National Academy
of Sciences USA 2009, 106: 1853-1856; Journal of Ecology 2011, 99: 1116-1123).
These studies provide first evidence of aboveground influences on the commonly
observed enhanced plant growth/vigor in invasives when introduced to non-native
ranges. We have provided a substantial evidence for the biogeographic (native vs nonnative
ranges) variation in the production of volatile organic compounds released by
Ageratina (Ecology 2011, 92: 316-324), which suggest that Ageratina may be
experiencing selection on biochemical composition in its non-native ranges.

Prosopis juliflora, another aggressive invader, exert neutral to facilitative effect on
species in its native range Venezuela but exert strong negative impacts on species
richness in its non-native ranges, India and Hawaii, USA (PLoS One, 2012). Through a
combination of field studies and laboratory experiments, my Ph. D. student has
demonstrated that allelopathic effects of Prosopis juliflora are conditional and soil
properties are important determinant of its allelopathic activities (Soil Biology &
Biochemistry 2014 78: 316-324).Ourwork on Chromolaenaodorata, one of the world’s
most destructive tropical invasive species, revealed that its rhizosphere soils accumulate
high concentrations of a generalist soil pathogen, thus creating a negative feedback for
native plant species which are more sensitive to the pathogens than the invader (Journal
of Ecology 2008, 96: 58-67).

Our research group has contributed toward developing a more inclusive and mechanistic
conceptual framework for invasion that should facilitate quantitative and testable
evaluation of causal factors, and can potentially lead to a better understanding of the
biology of invasions (Ecology Letters 2011, 14: 407-418). We have described major
pathways to identify the ecological roles of soil microbial communities in exotic plant
invasions (Trends in Ecology & Evolution 2010, 25: 512-519). We have recently
discussed invasion risks associated with invasive pasture species and suggested
protocols to lessen such risks in the sustainable agriculture (PNAS 2014, 111: 16627-
16627).

ALLELOCHEMICALS FUNCTIONING IN ECOSYSTEM AND EVOLUTIONARY CONTEXTS

Our research has emphasized that due to the ecosystem complexities, ecological
patterns can rarely be explained by a single cause, and provided a rigorous test of the
allelochemical hypothesis in the invasive success of an exotic species over a range of
abiotic and biotic conditions in its non-native ranges (Plant and Soil 2003 256: 1-11;
Trends in Plant Science 2006, 11: 574-580; Trends in Ecology & Evolution 2011, 26:
655-662; Plant Physiology 2012, 158: 1107-1114).). Our group has advanced allelopathy
in the context of its role in exotic invasions, which consider how evolution might affect
the intensity and importance of allelopathic interactions.

Our research forms the basis of understanding the competitive dominance of weeds and
explains the role of species’ life forms, climatic and edaphic factors in the regulation of
allelochemical activity, and helps to determine species interaction at various levels of
ecological organization. Our work has advanced the research frontiers on allelopathic
mechanisms and methods in laboratory and field settings, and on the role of
allelochemicals in ecological systems (Planta 2009, 229: 569-575; Soil Biology &
Biochemistry 2006, 38: 256-262; Physiologia Plantarum 2002, 114: 422-428; Plant
Physiology & Biochemistry 2005, 43: 77-81; Planta 2003, 217: 529-539; Environmental
& Experimental Botany 2005, 53: 97-104).

Our empirical research has established the role of abiotic and biotic soil factors in
determining allelochemicals in community structuring in both, managed and natural
ecosystems (PLoS One 2009, 4: e4700; Soil Biology & Biochemistry 2006, 38: 256-262;
PLoS One 2010, 9: e12852; American Journal of Botany 1992, 79: 977-981; American
Journal of Botany 1994, 81: 799-804; Weed Science 1996, 44: 393-396; Canadian
Journal of Botany 1996, 74: 1445-1450; Plant and Soil 1995, 173: 251-256; Canadian
Journal of Botany 1999, 76: 1317-1321). We have extensively contributed to the field of
allelopathy particularly in studying allelopathy in contexts of soil chemical ecology and
ecosystems (Botanical Review 1994, 60: 182-196; 1996, 62: 186-202; 1997, 63: 221-
230; 2006, 72: 153-178; Advances in Agronomy 1999, 67: 141-231; Perspectives in
Plant Ecology, Evolution and Systematics 2001, 4: 3-12; Agronomy Journal 2001, 93:
79-84; Forest Ecology & Management 2002; 160: 75-84; Critical Reviews in Plant
Sciences 2003, 22: 221-238).

WEED-CROP INTERACTIONS

My group has demonstrated the influence of habitat, climate and site on the release and
accumulation of phenolics and thus on the phytotoxic potential of cropland weeds (Weed
Science 1996, 44: 393-396; American Journal of Botany 1998, 85: 64-69; Canadian
Journal of Botany 1996, 74: 1445-1450; American Journal of Botany 1998, 85: 64-69).
My earlier workestablished that phenolic allelochemicals released from roots and foliage
of Pluchea lanceolata, a pernicious perennial weed, inhibit the growth of associated crop
species by interfering with their physiological and biochemical pathways (Plant and Soil
1990, 122: 298-302; American Journal of Botany 1992, 79: 977-981; American Journal
of Botany 1994, 81: 799-804; Journal of Chemical Ecology 1991, 17: 1585-1591; Journal
of Chemical Ecology 1992, 18: 713-718; Physiologia Plantarum 1994, 92: 571-576). For
a long time, it is believed that allelopathy is prevalent among perennial weeds. My work
on annual weeds such as Polypogon monspeliensis and Stellaria media has shown that
allelochemicals play more important roles in the competitive success of polycarpic
annuals than monocarpic annuals (Plant and Soil 1995, 173: 251-256; Canadian Journal
of Botany 1999, 76: 1317-1321).

Our studies on Phalaris minor, an exotic weed of wheat fields of northwestern India,
has brought to light some serious environmental and economical consequences.We
found that a nontoxic herbicide, isoxaflutole, could be a better alternate herbicide (Plant
and Soil 2004, 258: 161-168). Farmers often incorporate rice straw (burned or unburned)
prior to sowing wheat. Our research has established that incorporation of unburned rice
straw causes significant damage to seedling growth of next season crop (Canadian
Journal of Botany 2004, 82: 161-168; Biology & Fertility of Soils 2007 43: 557-563). We
have shown how agronomic practices influence efficacy of herbicides (Plant and Soil
2005, 277: 25-30; Paddy & Water Environment 2010, 8: 277-282; Pest Science
Management 2006, 62: 1092-1097). Residues of a number of plant species including the
biomass of certain weeds are used for soil amendment in crop fields. My research has
demonstrated the potential of crop and weed residue to suppress the establishment and
growth of crop plants (Crop Protection 2001, 20: 261-265; Canadian Journal of Botany
1999 77: 1419-1424; Weed Technology 1999, 13: 176-182).

INTERFERENCE SUCCESS OF UNDERSTORY SHRUBS IN CANADIAN BOREAL FORESTS

We have examined the role of Kalmiaangustifolia and Ledumgroenlandicum in the
regeneration failure of black spruce (Picea mariana) stands in eastern Canadian forests.
Kalmia releases phenolic compounds in the rhizosphere of black spruce and thereby
prevents nutrient availability to its seedlings (Canadian Journal of Forest Research 1997,
26: 1899-1904; Plant Ecology 1997, 133: 29-36). Our results suggest that soil
associated with Kalmia suffers from nutrient deficiency particularly nitrogen and
phosphorus but increased concentrations of iron and manganese, and provide basis for
the hypothesis that Kalmia dominated microsites create soil nutrient imbalance
(ActaOecologia 1999, 20: 87-92).